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AN ELECTRONIC RESISTANCE
LIMIT INDHZATOR

Thesis for flu Degree of M. S.
MiCHEGAN STATE COLLEGE
Frank Murray Poifon
1948

 

This is to certify that the

thesis entitled

An Electronic Resistance
Limit Indicator

presented by

Frank Murray Pelton

has been accepted towards fulfillment
of the requirements for

ms. degree in E&

Wk
0 Major ro ssor

Date May 21) 1948

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presented by {3 ‘ ‘-.~ J

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AN ELECTRONIC RESISTANCE LIMIT INDICATOR

By
Frank Murray Pelton

A THESIS
Submitted to the School of Graduate Studies of Michigan
State College of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE
Department of Electrical Engineering

1948

 

THESIS

 

S UMIVI ARY

The deWpoint is the temperature at which a vapor
begins to condense as a liquid. If a surface is cool-
ed sufficiently it will in turn cool some of the vapor
surrounding it, which upon reaching its deWpoint con-
denses on the surface. The dewpoint designed by Dr.

G. J. Bouyoucos consists of a special glass tube on
which are placed two or more platinum electrodes. The
passage of a suitable coolant through the tube cools
the surface to the deWpoint of the surrounding vapor.
The presence of the condensate decreases the resistance
between the platinum electrodes. The problem considered
in this paper is the measurement of this resistance and
a method of notifying the Operator when the resistance
decreases to a predetermined value. The temperature of
the cell is measured by means of a thermocouple sealed
in the surface near the electrodes.

The instrument as designed may also be used as a
limit voltmeter for the range from O to 0.4 volts. This
range could be adapted to any greater range with prOper
associated networks.

Essentially, the instrument consists of a d.c.

amplifier preceeded by a low input current stage. The

output of the amplifier controls an electronic switch

which in turn controls an audio oscillator. The audio

203322. 111

signal is applied to a loud speaker.

This paper considers the problems of design and
deve10pement thoroughly and completely, giving analyses
where necessary. In general, this paper could act as
a guide for those contemplating the construction of an
original instrument in the electronic field, showing
the general methods of approach to the varied problems

necessary to be solved.

iv

Preface

The laboratory work presented in this thesis
was done during the development of an original method
of dew point determination due to Dr. G. J. Bouyoucos
of the research staff, Soil Science Department, Michi-
gan State College. The author's chief interest was
in the measurement of the temperature and the resis-
tance of the dew point cell, and to develOp a usable
instrument for dew point determination work. The
entire program of work done on this problem has been
outlined and emphasis placed on the measuring instru-
ments.

Even though the problem has not been completed
to date, workable results have been obtained. Both
instruments described are original with the author
and while they are only partially develOped for this
problem, they have both shown promising results.

Although part of the work was done with compen-
sation from Michigan State College, permission has
been granted by the Department of Soil Science and
the Department of Electrical Engineering that the
material be used for this thesis.

Frank.M.‘Pelton
East Lansing, Michigan
May, 1948

TABLE OF CONTENTS

Summary 111

Preface iv
PART I

Chapter I. Introduction 1

Chapter II.

2.1 General Laboratory Setup

e e-

2.2 Construction of the cell

2.6 Initial measurements

(0

2.4 Correlation of deWpoint & resistance 13

2.5 Dewpoint by recorder 18
Chapter III

3.1 The basic resistance limit indicator 20

3.2 The connecting cable 22
3.5 Necessity for more sensitivity 23
PART II

The Sensitive Resistance Limit Indicator
Chapter IV

4.1 General considerations

24
4.2 Necessary requirements 24
4.3 Desireable features 25
4.4 Block Diagram a26
Chapter V
5.1 General design considerations 29
5.2 The low input current stage 29

vi

Chapter V (continued)

5.2a Electrometer tubes and circuits 31
5.2b The 954 as a space charge grid tube 33
5.2c The hum problem 36
5.3 The direct current amplifier 38
5.4 The electronic switch 46
5.5 The audio oscillator 50
5.6 The audio amplifier 51
5.7 The power supply 51
5.8 The complete circuit diagram 53
5.9 The layout problem 53
Chapter VI Experimental results 65
Appendix I The G.E. PM17 VTVM. 67
Appendix II The Leeds and Northrup potentiometer
Appendix III A temperature measuring device 3:
Appendix IV Analysis of Miller d.c. amp. 72
Appendix V Analysis for one-shot circuit 73
Appendix VI DeWpoint by recorder 74
References 77
Bibliography 78

vii.

CHAPTER I

1.1 Introduction

 

The study of evaporation and moisture control
processes has necessitated the measurement of the
moisture content of gases containing water vapor.

This may be found in terms of the Absolute Humidity,
the Relative Humidity, or the Dew Point Temperature,
all of which are definite related quantities when the
temperature is given. The four most commonly used
methods of determining the moisture content of gasses
are: (l) the chemical absorption method; (2) the
dew point method; (3) the psychrometric method;

(4) the hygroscOpio expansion method.

The chemical absorption method is limited in its
use by the time and care necessary to produce accurate
measurements and for that reason is generally regarded
as a laboratory method and of limited value for mak-
ing field measurements.

The dew point or condensation method has up to
this time made use of a metal mirror which may be
cooled to several degrees below the air temperature.
The mean temperature at which the condensed moisture
forms and evaporates from the mirror surface is re-
corded as the dewpoint.

The sling psychrometer is the method most used

for making measurements of atmospheric moisture

content. By whirling two identical thermometers, one
of which has a moistened muslin wrapping on its bulb
the depression of the wet bulb thermometer gives an
indication of the moisture content of the air. This
method inherently is of little value when the tempera-
ture is at either extreme.

The length of some hygroscopic fibers changes when
they are subjected to a change in relative humidity,
and by means of this property, instruments are built
which indicate humidity. However, these fibers
gradually change their length under steady state con-
ditions and this makes their use subject to the con-
stant recalibration of the instrument.

At the present time in the United States, the
measurement of the atmospheric moisture is generally
restricted to the sling pyschrometer and the hair
hygrometer, despite the inherent disadvantages con-
tained in both.

The method described in this paper is of the dew
point type and is based on the change of resistance
between two electrodes on the surface of glass cooled
to the dew point temperature. It is advantageous in
that only a small sample of air is required, the over—
all cyclic time could be made as low as 1 minute,
and could be entirely automatic. The disadvantage
lies in the care in manufacture of the resistance

unit.

It should be noted that this method is original
in that no buffer solution or substance is used to
bring the resistance to a lower value. The conduc-
tivity being provided purely by the glass, the impur-
ities in the condensate, and the water condensate
itself. With this method, the resulting cell has a

fixed calibration for given conditions.

CHAPTER II
2.1 General Laboratory_Setup

During the experimental work done in the develop~
ment of this method, a system was used similar to
that shown in Figure 1. A suitable coolant was
passed through the cell "A“ which when the outside
surface temperature reached the deWpoint of the sur-
rounding gas, caused condensation of moisture between
the electrodes "B”, resulting in a change of resistance
between the electrodes. This was measured by a suit-
able instrument "C". The temperature of the thermo-
couple “D” was measured by the potentiometer "E".
Resistance vs temperature was plotted from this data.
A calibration of the cell was made in terms of resis-
tance at the dew point temperature determined by the
sling psychrometer. Initial work was restricted to

the range where the latter is accurate.
2.2 Construction of the Cell

Basically the cell consists of a glass tube which
has been bent into the form of a ”U“ with the bottom
of the “U“ flattened. On this flattened portion of
the tube the electrodes, which were several individual
100ps of platinum wire, were placed. Platinum was
used to prevent corrosion difficulties. An example

of the cell is shown in Figures 2 and 3.

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Extreme care was necessary in the construction
of the cells. After the “U” tube was formed, the

lOOps of platinum wire were tightened in place by
twisting the ends of the wires. The tubes were then

heated and a slight pressure applied to the inside
of the tubes, causing the glass to expand and hold

the loOps in place. A capper-constantine thermocouple
was then sealed on the surface of the tube as near

the electrodes (100ps) as feasible. It was necessary
that care be exercised so that the thermocouple was
not “burned." After the cell was completed, it was
annealed to remove strains which tend to cause
breakage.

Throughout the entire program of develOpment of
this method, only the one basic design for the resis-
tance cell has been used, although several electrode
spacings and tube types have been investigated.

The glass tubing which was used to best advan-
tage was Corning G-Sl 5 mm I.D. x 1 mm wall thickness.
The use of extra thin walled tubing while it had the
advantage of requiring less coolant and a shorter
running time, resulted in considerable loss due to
breakage. Thicker walled tubes physically strengthened
the cell but resulted in unnecessarily long cycles and
required excessive amounts of coolant.

The major source of trouble with the cells

resulted from foreign matter lodging between the

electrodes. This caused erratic Operation of the
cells and caused several of them to be useless.

The cells were cleaned by successive treatments
with aqua regia, soap and water, acetone or ether,
and then with alternate washing with distilled water
and drying in an oven. Extreme care was exercised
in keeping the cells clean after such a treatment
was completed.

The spacings between electrodes was varied from
about 1/64 to 1/4 inch. The very close spaced elec-
trodes generally resulted in unstable Operation, due
to the inability of the above method to remove all
foreign matter. When a close spaced cell was obtained
which was not erratic, the Operation was reliable and
constant.

The wider spaced cells, while easier to clean,
gave such high resistances that the change in resis-
tance was almost immeasurable.

The choice necessitated a cell whose electrodes
were close enough to give a measurable change in
resistance and yet far enough that they could be
easily cleaned. Practically, this was a hard medium
to obtain, although several cells were obtained which
had both of these characteristics. Data is shown

for one of these in the following section.

—s—

2.3 Initial Measurements

The first step in the estimation of this method
for the determination of the dew point was the measure-
ment of the resistance of the cell.

An R.C.A. Voltohmist Jr. with the highest cali-
bration of 109 ohms, gave only a very slight deflec-
tion of the indicator when connected to the cell in
the dry state.

Recognizing that the resistance of the cell
was in the order of 1010 ohms, the circuit of Figure 4
was assembled and measurements made. If r1 is the
resistance of the cell to be measured, R the value of
a series resistance, and E is the voltage applied to
the series combination (neglecting the internal re—

sistance of the vacuum tube voltmeter) r1 becomes

r1 ' R( E-V2

V

where v is the voltage measured across the resistance R.

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The original assumption was found to be correct and

was verified as follows:

with a: - 90 volts, R a 2.0 x 107 Ohms, and with
the cell in the dry condition, v was found to be 0.01

volts and

r I 90(20§106 = 1.8 x 1010 ohms.
1 .0

Upon cooling until the moisture from the air condensed
between the electrodes, the resistance decreased to
about one third of that value. These readings were
made on a 0-3 volt scale and were therefore only
approximate.

Obviously it was possible to obtain more accuracy
in these measurements by increasing both E and R, and
by using a more sensitive instrument to measure the
voltage v. This was accomplished by increasing E to
290 volts, using 6-20 megohm resistors in series for
R and using a General Electric Model PM-l? vacuum
tube voltmeter to measure the voltage v. This setup
is shown in Figure 5. With the electrodes at room
temperature, the VTVM read 1.13 volts on the 3 volt
range. Checks on the other ranges showed the same
voltage. When the resistance R was removed, the Opera—
tion of the meter was erratic on all ranges, showing
that it was the external resistance R and not the
internal resistance of the meter which caused the single
valued readings on the different scales of the instru-
ment. The value of R was then decreased to 20 megohms
whereupon the voltage v drOpped to 0.34 volts. This

was measured on the 0—1 volt range. There is no

-10-

 

internal grid resistor incorporated in the instrument
on this range. The variation from linearity was due

to the leakage resistance of the leads making the
connections to the system. Upon cooling, the voltage

v had a maximum of 7.6 volts, but returned to 0.34
volts when the cell was allowed to warm and the moisture
evaporated increasing the resistance of the cell to

- the original dry reading previously obtained. Using
the data Just given, the cell resistance was again

computed to be

r1 = 290 (20) 106 = 1.65 x 1010 ohms with
0.34

the electrodes in the dry state. These results were
quite promising in that they correlated reasonably
well with the data taken with the Volt-Ohmist, which
resulted in a value of r1 8 1.8 x 1010 ohms. This
meant that for this cell which had been washed several
times and subjected to many different conditions
between the two sets of measurements, the dry resis-
tance was approximately constant.‘

The problem of constructing an instrument which
would measure and give an alarm at a desired voltage
of the magnitude obtained in the above data was not
particularly difficult in theory and was set aside
for future consideration; meanwhile, correlations of
voltage against the actual dew point as determined

by a sling psychrometer were investigated.

-12-

Several sets of data were taken on a number of
cells. Data for only one of the best cells will be
presented in the following section. Only a selected
few carefully made cells produced as good results,
but it should be possible to manufacture cells of this

standard commercially.
2.4 Correlation of the Dew Point with Resistance

As the voltage is indirectly a measure of the
resistance and is easily measured and recorded without
computation, it was used as a reference to be plotted
against the temperature.

The temperature was measured by the use of a
Leeds and Northrup hand potentiometer' connected to
the thermocouple in the cell. This method, although
inaccurate due to lag in reading of the potentiometer,
proved to be sufficiently accurate for the results
needed to show that a definite correlation existed.

An instrument designed by the author to make an
accurate instantaneous reading of this temperature is
described in Appendix III.

The measurements shown in the data on the follow-
ing pages were taken over a period of several months
and under various conditions. Only a very few of the
runs have been omitted because of obvious errors noted

at the time of making the runs (such as loose connec-

~---“-----—-‘-’---

° A description and circuit diagram for this potentiometer
is given in Appendix II.

~13~

tions, dirty electrodes, etc.).

A typical run consisted of (1) measuring the dry
resistance and room temperature, and then (2) passing
a small flow of carbon dioxide to cool the cell, and

(3) recording simultaneously the voltage v and the

temperature T in (degrees F.) as read on the instru-
ments. After the run was completed (i.e. frost had
formed on the surface of the glass) a determination

of the dew point was made with a Tycos sling psychro-
meter. A plot of the voltage as a function of tempera-
ture was made for each run and the voltage marked‘which
corresponded to the temperature indicated as the dew
point by the sling psychrometer. A weighted average
was chosen to be the calibration point as being the
voltage at which the dew point was reached. (This
voltage was of the same order of magnitude for all

of the cells but a distinct calibration for each was
necessary) Twenty-five runs were made on Cell #1,

and that data is shown on the following pages.

A graph of a typical run showing the gradual
decrease of resistance with the decrease of tempera—
ture and the characteristic knee of the curve is shown
in Figure 6. Note that the resistance increases as
the temperature is further decreased. This is due to
the formation of ice or frost on the surface of the

glass which has a lower conductivity than that of the

water.

~14-

Table I shows the data for 25 runs obtained from
cell #1. As each run was completed, a graph was drawn
of the temperature vs. voltage, and the dew point
temperature as found from the sling psychrometer was
marked and the voltage at this point noted. The aver-
age of these voltages was taken as the voltage at the
dew point for calibration of the cell.

The readings of the psychrometer were interpreted
with the use of the standard Psychrometric tables,

published by the U.S. Weather Bureau, U.S. Department
of Agriculture.

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Room IDEw Pt. ’Dew‘Pt. Relative Error
temp. gpsychr. Resis. Humidity

74.5 50.5 50.6 44.0 +0.1
72.8 47.75 47.5 40.8 -0.45
75.1 47.55 45.5 40.0 -1.75
75.0 46.75 46.0 40.0 -0.75
71.0 45.0 45.25 56.0 +0.25
70.5 45.0 45.5 55.5 +0.50
72.5 46.0 44.8 40.5 -1.2
72,5 47.5 46.5 42.5 ‘-1.0
71.8 49.0 47.6 45.5 -1.4
72.5 49.5 49.0 44.5 "0'5
70.5 50.0 49.4 48°5 -O.6
70.5 50.0 49.5 48's -0.5
70.0 48.7 48.5 48.5 -0.2
69.5 47.5 47.0 45.5 -O.5
69.0 47.2 46.7 45.0 -0.5
75.0 54.0 55.6 48,0 +1.6
75.2 54.0 56.0 47.0 +2.0
75.2 54.0 54.0 48,0 0.0
75.6 55.0 55.6 49,0 +0.6
75.6 55.0 55.6 49.0 +0-6
75.6 55.0 55.5 49.0 +0-5
75.7 55.0 55.1 49.0 +0-1
75.7 55.0 56.0 49.0 +1-0
74.0 54.0 55.0 49.0 +1-0

The air is considered positive if the dew point
is greater by resistance than that by the psychrometric

method.

 

 

From the foregoing data, it is obvious that the

error is considerable in some cases. Seeking a

correlation with some of the factors involved, it was

found that there was a definite relationship between

the error between the dew point by the two methods

and the dew point by the resistance method. This was

plotted and is shown in Figure 7. No satisfactory

explanation for this has yet been given.
With this correction factor the average error

was reduced to only 0.1%. This small error is well

within the experimental error assumed for this work.
2.5 Dew Point by a Recorder

Although not pertinent to the rest of the problem,
an investigation of the use of a recorder to plot both

temperature and resistance as a function of time was

made. The preliminary work is outlined in Appendix

VII-.

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CHAPTER III
5.1 The Basic Resistance Limit Indicator

It was necessary to take precautions in the
gathering of the data as previously described. The
work required two peeple to make the simultaneous
readings of temperature and voltage. Thus, rather than
to make a plot of the temperature vs. voltage for
each run, it was desirable to make an instrument
indicate when the dew point temperature was reached.
It seemed reasonable that a vacuumtube voltmeter with
a relay in the plate circuit of the tube would accom—

plish the purpose, with the relay controlling a buzzer
or hell. Such a unit was constructed and is shown in
Figures 8 and 9. The bias was controlable with R2
and the relay could be made to close at voltages of
0 to 4.5 volts. This unit gave fiarly satisfactory
results for about one month when it was found that the
tube characteristics had changed greatly. It was
found that this was due to severe positive voltages
having been applied to the grid of the 1T4 vacuum
tube, caused by allowing the leads to the cell to
become shorted while the high voltage was on. After
the replacement of the tube, the unit functioned
normally.

At times when relative humidity increased to high

values for a period of time, the leakage resistance

~20—

 

 

 

 

 

 

 

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of the instrument and its associated lead cable became
less than that of the cell and the resultant measure-
ments were of no value.

The entire unit was enclosed in a small cabinet,
and was thus adaptable to portable construction.
Miniature Eveready type 467 "B" batteries were used to
supply the high voltage to be placed in series with
the measured cell. A Burgess type 230 "B“ battery
was used as the plate supply for the 1T4 vacuum tube.
A Burgess 4FH 1.5 volt "A“ battery was used for the
filament supply. No appreciable lowering of voltages
was found even after several months of intermittent

use.
5.2 The Connecting Cable

A cable which would allow a leakage resistance no
less than about lolsohms was necessary for this instru—
ment and it was obtained by using a very good grade
of waterproof twin cable originally meant for outside
use on light circuits. After cutting the necessary
length from the stock roll, it was placed in an oven
whose temperature was Just below that of the softening
temperature of the rubber covering. After 24 hours
in the oven repeated applications of G. E. Glyptal
#1201 Red were made over the entire cable and care
was taken to insure complete impregnation of the

insulating materials which were exposed at the termina

-22.

tions of the cable. Small alligator clips were fastened
to one end to permit temporary connections to the dew
point cell. The other end was connected to terminals
on the front of the indicator panel. After a long
period of use, this cable develOped surface cracks and
the leakage conductance again became too great for this
application.

Leakage resistances internal to the indicator
were cut to a minimum by suSpending all wiring in the
initial circuit and paraffining all wiring and sus-

pensions.

3.3 Necessity for More Sensitive Instrument

During the eXperimentation with the wider spaced
electrodes, it was necessary to measure much higher
values of resistance and this was tried by several
methods.

The method using the G. E. vacuum tube voltmeter
was again assembled with Ra-ZO x 107 ohms. Indications
were that the resistance was of the order of 1012
ohms. This was barely distinguishable on this setup.
Thus if these cells were to be used, a new instrument
would have to be built which would respond to roughly

.01 volt change across the resistor R. The design of
this instrument is the major problem to be considered

in this paper.

~25—

PART II The Sensitive Resistance Limit Indicator
CHAPTER IV

4.1 General Considerations

As mentioned in the previous section, the need
was apparent for an instrument which would respond to
a given change in the voltage across the resistance
R of about 0.01 volts. (Figure 4) With a 2.0 x 107
ohm resistor in series with the cell and batteries of
500 volts, this would correspond to a minimum resis-

tance of about 1012 ohms. With this in mind, it

seemed probable that an all electronic instrument could
be built which could eliminate all mechanical com-
ponents such as relays which are sensitive to physical
changes, shocks, etc., the following were compiled as

a set of prerequisites for such an instrument:

4.2 Necessary Reguirements

 

(1) With a versatile instrument in consideration,
it was necessary that a switching system be employed
to change the resistor R and thus change the range
and sensitivity of the instrument. This would require

that the grid current be in the order of one-one

hundredth of the current to be measured. If the

resistance at the limiting point is to be 1012

ohms,
using 500 volts in series, the resultant measured
current would be

I - 0.01 = 5.0 x l010 am eres.
“1°” T271"? p

0

~24»

Thus the grid current to cause no error greater
than about 1% should be less than 10‘12amperes.

(2) The elimination of the relay and the sub-
stitution of a suitable electronic switching circuit
would be desirable. This should control an oscillator
which would make a very distinct keying with a minimum
of grid bias change.

(5) Although after the dew point cell has proven
reliable, a portable instrument using batteries will
be desirable, but one which could be used from 117
volt A.C. power source would be satisfactory. The
unit should be so designed that it would be reasonably
independent of line voltage variations and other

physical changes.

4.3 Desirable Features

 

(4) To measure the resistance on both the warming
and cooling cycles, a switch circuit would be necessary
to start and stop keying the oscillator at the same
value of input voltage.

(5) To obtain an indication of the stage of the
cycle, a meter showing the value of the resistance
should also be included and should be roughly calibra-
ted from infinite to any desired point depending on
the range switch. This calibration should be fairly
accurate by multiples of ten so that only one meter

scale would be necessary for all sensitivity ranges.

(6) Some means for limiting the voltage applied
to the input grid should be included so the input
tube could not be destroyed by allowing the cell leads
to become shorted while the voltage was connected.

(7) A means for varying both the pitch and volume
of the audio oscillator should be included to apply
for varying external hearing conditions.

(8) The unit should if possible, be entirely
self-containing so that the least amount of trouble
would be necessary in transporting it.

(9) Common design practices and layout principles
should be followed, and in addition no "special" tubes
or associated equipment should be included unless
absolutely necessary. The cost should be kept as
low as feasible without sacrificing too much on the
high standard of construction desired.

(10) Operation and alignment should be possible
with a common vacuum tube voltmeter and should not

be unnecessarily complicated.
4.4 Block Diagram

With the above considerations in mind, the block
diagram in Figure 10 was drawn and each of its parts
then considered in detail. It consists of an input

stage which will fulfill the need for the very low
grid current, a direct current amplifier controlling

an electronic switch which will key at the same point

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at which it unkeys and with no intervening manual
Operation. This is to control a suitable audio
oscillator and the output of this amplified by con-
ventional audio amplifier feeding a speaker. The power
supply should be well regulated and be of heavy enough

current capacity so that the unit would be amply supplied.

~28~

 

CHAPTER V

5.1 General Design Considerations

The nature of the application of the resistance

limit indicator being develOped, required that extreme

care be exercised to incorporate the utmost reliabil-

ity in the instrument. At the same time the cost was

an important factor and wherever possible, it was held

to a minimum. Yet versatility was also desired and

some things were included which would not be required

in a production instrument but were desirable for

eXperimental purposes. For example, the use of volume

and pitch controls on the oscillator, while not entirely

necessary were quite convenient in use.

5.2 The Low Input Current Stage

The measurement of currents in the order of 1012

amperes requires that the grid current of the input

tube be less than that value and generally about at

least one one-hundredth of that value. The grid current

in the normal Operation of most receiving type tubes

1
is caused by one or more of the following:

(a)
(b)
(c)
(d)

electrons from the filament thermionic
electrons emitted by the grid
positive ions given off by the filament

photoelectric electrons given off by the
grid due to the light from the filament

\_ he

(e) electrons emitted by the grid due to the
soft X-rays given off by the plate caused
by the plate current

(f) ionization by collision with gaseous
molecules

(3) leakage currents in the tube socket connections

With ordinary receiving type tubes the flow Of

electrons to the grid due to the thermionic emission
by the filament may be reduced by a suitable negative
bias voltage applied to the control grid. The elec-
trostatic field produced around the grid repels the

electrons and reduces that component of the grid current.

The reduction of the Operating voltage and thus the
Operating temperature of the filament or heater will

reduce the heating Of the neighboring electrodes, and
if they are made from special materials, a negligable
amount of thermions are emitted. The flow of positive

ions may be reduced greatly by inserting a positive
space charge grid between the hot cathode and the
control grid. This compels the positive ions to be

bent back and returned to the cathode. This space
charge grid also serves to collect any negative space
charge which may be present in the tube. The lowering
of the filament temperature will also reduce the pro-
duction Of photoelectrons from the grid. The use Of
plate and anode voltages less than about 6 volts will
gliminate even the softest X—rays produced at the plate.

This will also greatly reduce the effect of ionization

~30—

 

by collision and thus reduce that portion of the grid

current.
5.2a Electrometer Tubes and Circuits

Special vacuum electrometer tubes which eliminate
as much as possible the causes listed above, are avail-
able on the market. The best known among these is the
General Electric Company Pliotron type FPSefi With a
very sensitive galvonometer and other associated equip-

ment one Of these tubes may be made to detect currents

as small as 1017 amperes. This tube could be used for
the detection of the currents in the order of 1012such
as those produced in the dew point cell, with the
circuit previously described (Part I) however, the
price Of $55. is a prohibiting factor.

The Victoreen Instrument Company produces their
type VX413electrometer tube which will measure currents
of 10’12amperes and would also be suitable for this
application. The price of $12.50 is more reasonable,
but the use of a receiving tube if possible would further
reduce the cost to about $2.00 maximum and if it is
possible to measure the current necessary with this
type tube, it would definitely be preferable.

A circuit has been described+using a type 1A6

‘receiving tube, and reports good results in measuring
currents down to lO‘lzamps. Beyond this point the "

-31-

 

system becomes erratic and therefore unusable. This
type tube requires a very high input resistance which
is not possible in the dew point system because Of the
leakage resistance of the input cable connecting the
cell to the measuring instrument.

The General Electric Company has recently pro-
duced a direct current amplifier which uses in the input
stage a #954 acorn type receiving tube. It is Opera-
ted as a space charge grid type tube, and is also used
with an input resistance of a high value. (in this
instance, 4.0 x 1010 Ohms.) Due to the construction
of this tube and its physical size, experiments were
run with the lower values of input resistance, and the
tube was found to perform satisfactorily when the
conditions were such that the effects listed previously
in this section were minimized.

Following is the data taken on several #954 type

receiving tubes picked at random from general stock.

.. 32..

5.2b The 954 Qperating Characteristics

To the author's knowledge, the Operating
characteristics of the 954 as a space charge grid
type tube are not available commercially, thus the
Optimum conditions were to be found by experiment
only. The circuit shown in Figure 11 was assembled

and the characteristic curves Obtained for this tube.

 

 

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The grid current was determined by changing
the external grid resistors and thereby calculating
the voltage across these resistors after the
characteristic curves were known.

Figure 12 gives the common curves and Figure 15

gives the variables as a function Of filament

voltage (a.c.)

~33~

5.2 (c) The Hum Problem

The problem Of 60 cycle pick up hum was contem-
plated presented itself immediately upon putting the
unit into Operation. Thus it was necessary to put in
some sort Of filtering circuit to eliminate reaponse

much above that Of direct current with fairly slow

variation of the order of 1 cycle per second. The
construction Of an L-C filter for this frequency was
deemed to be unnecessary as the inclusion of a shunt
condenser would satisfactorily limit the frequency
response of the amplifier. The logical place to
supress the hum was in the low-level stage and pre-
ferably in the plate so as to minimize leakage resis~
tances in the condenser from affecting the calibration
of the instrument. Assuming that a 10,000 Ohm plate
resistor in the first stage and a time constant Of

1 cps. would be suitable a condenser Of

C8 t = 10"4 farads or 100 micro farads would
r

suffice. As the plate Of this tube is run at the fair—
ly low voltage of 8 volts, this size capacitor was
easily available. NO serious trouble in lag due to

the condenser charge was encountered. The power for
this stage is small and as the regulation should be
very good, it was desirable to isolate this stage from

the large bleeder supply and obtain the necessary

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voltages from a separate bleeder system fed from the
regulated voltage from the power supply. This was
formed completely from wire wound resistors and 4-watt
wire~wou1d potentiometers as shown in the circuit,
Figure 14, Of the resultant first stage, and its

associated bleeder system.

5.5 The Direct Current Amplifier

 

The problem of the direct current amplifier to
be used for such an application is always difficult.
The inclusion Of batteries was definitely undesirable,
and this required the use of a common power source for
all stages. Several circuits are in use for this type
application, and are considered here.

Perhaps the most used type is that shown in
Figure 15 in which the power for each stage is taken
from a common bleeder. This has a serious disadvan-
tage in the variation of the voltages on the several
taps along the bleeder resistor. This causes drift
and thus might result in a quite unstable instrument.
If, however, the current in the bleeder circuit is
much greater than that in the circuits Obtaining power
from the bleeder, this variation is minimized and the

resultant drift is reduced.

The change in resistance of the bleeder due to
changes in temperature is also a common cause of drift,

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be allowed to come to the steady state condition as
soon as possible. This can be done by keeping the
temperature Of the bleeders constant, by ventilation.
In general this type of amplifier should not be used
when it is possible to avoid it.

Another method Of stabilizing the above type
circuit is to use a balanced type amplifier. This
requires twice the number of tubes and associated
parts but results in an amplifier which is fairly
independent of supply voltage variations. A typical
circuit is shown in Figure 16.

A second type Of circuit commonly used in D.C.
amplifiers is that shown in Figure 17. It makes use
of the voltage divider network which is returned to
the high negative voltage V2, to get a resultant grid
voltage which may be applied to the following stage.
The main disadvantages of this type circuit are that
the gain is reduced by the previously mentioned voltage
divider network and also that a quite high voltage is
needed for a power source.

A more satisfactory circuit has been described
by Millergand is shown in Figure 18. It is similar to
the foregoing circuit with the exception that a voltage
regulator tube has been substituted for the resistor
R1 in Figure 17. By returning R2 to ground and prOpor-

tioning the currents correctly, it is possible to

produce the correct bias voltage Eccz shown in Figure 18.

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The gain of this amplifier is that normally Obtained
from the usual resistance coupled amplifier and may

be shown to be

Gain - K= R]
§1+rp

Obviously this is the most satisfactory of those
herein described.

The electronic switch in this unit necessarily
must change in either direction at very close to the
same absolute voltage. If it was possible to make
the unit sufficiently stable, no intervening amplifier
would be necessary, but hum etc., picked up in the
initial stage and that present in the power supply
output limits this stability.

To Obtain the approximate hum voltage normally
Obtained from a few feet of exposed unshielded wiring
such as might be used near the cell of this unit, the
following circuit was assembled and rough measurements

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It was found that a voltage of approximately

.4 volts was impressed upon the wire when about 2

feet were eXposed or unshielded, to the average varying
electromagnetic and electrostatic fields present in

the laboratory. Upon touching the wire with the hand,
a voltage Of about 11.6 volts was found. Assuming that
the cell is to be Operated with a shielded cable except
for the terminations at the cell and at the instrument,
and that the cell is equivalent to the two feet of

wire used in the above measurement, it is seen that

the first stage will have at all times the observed
.4 volts A.C. impressed on its grid. When the first

stage has a gain of K, the 100 micro-micro farad con—
denser placed across the plate Of this tube will
reduce this to about .0026 K-volts.

Further reduction of the A.C. component of the

signal voltage may be Obtained through the use Of more

shunt condensers on the following stages, the only
limitations on these being that the time constant for
the associated circuit be Of the order of 1 second.
Thus the gain needed for the amplifier stage is small
and is further dependent only on the stability Obtain—
able in the rest of the system.

In all respects, a gain of 100 would be more than
sufficient to Operate the electronic switch without
serious trouble from either hum or power supply vari-

ations.

 

 

Considering the design of the type amplifier

described by Miller, and previously mentioned in this
section, as applied to a 6AC7 type tube, it is found

that the gain obtainable is about 200. (Appendix IV)
A modification of the first d.c. amplifier de-
scribed was constructed and used in this instrument,
using 2 triode 6SJ7 stages. The Miller circuit was
then inserted to make a comparison with the above, and

the results are shown in Chapter VI.

5.4 The Electronic Switch

 

Recalling the necessity for the electronic switch
in the normal cooling and warming cycle to produce a
change in either direction at the same voltage with~
out any intervening Operation, manual or otherwise,
it was necessary that some type of temporaryswitching
mechanism be utilized.

A modification Of the familiar Eccles Jordan
flip-flOp circuit is the one shot circuit shown in
Figure 20. The Operation of the switch may be de-
scribed as follows:

Assume that tube #1 is drawing current and has a
bias as prescribed by the normal Operating character-
istics for the tube. The voltage divider system con-
sisting of R3 and‘R4 in series, is connected to the
#1 plate. If the current drain through R1 is con—
siderably greater than the bleeder current through

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R3 and R4, the potential of the #2 grid will be a func-

tion of the potential of the #1 plate. Thus, if the
potential Of the #1 grid is decreased sufficiently,

and thus increasing the potential of the #1 plate, the
#2 grid voltage will also be raised thereby causing
an increase of current and an increased voltage drOp
in R2. This decreased potential is fed back to the

#1 grid by the condenser C. This causes complete cut-
off Of the #1 tube and the #2 tube assumes complete
control.

This is a temporary condition however, as the
charge is then discharged through the series circuit
R,.R2 and voltage source Ebb° This returned the grid
of #1 tube to above the curoff point and the switching
Operation takes place in the other direction. When the
voltage applied to #1 grid is Just slightly below the
switching potential, the circuit will oscillate with
a keyed period of

t1 = C (R+Rz)
the unkeyed period Of the oscillatory cycle will
depend on the difference of the keying potential and
that of the applied potential and is

t . 1 1.1L...
2 RC n R(ei‘€1)

A mathematical analysis of the above circuit is given

in Appendix V.

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5.5 The Audio Oscillator

 

It was necessary that the audio oscillator to be
used in this application go from s.non-Oscillatory to
an oscillatory state with a minimum change of grid
potential. Preliminary eXperiments showed that a
conventional tuned plate audio oscillator required
about 5 volts change in grid bias to pass through this
transient condition, and that the change was not
abrupt but very irregular and erratic. Thus it was
necessary to find an oscillator which would be more
suitable for this application. This was found in the
multivibrator type audio oscillator as used in the
Army's tape keyer TG—lOB. The circuit for this
oscillator is shown in Figure 21. It was found that
only about 1 volt was required to start the oscilla-
tions and that the frequency changed very little when
the circuit was oscillating.

The pitch of the oscillator was made variable
through the use of several condensers. These were
switched into the resonant circuit formed by L and C
shown in the above figure. Although this was not
necessary to the Operation of the dew point work, it
was thought feasible because Of the advantage Obtained
in that the Operator might change the pitch and thus
eliminate partial fatigue when using the instrument
for long periods of time. This change in the L-C
stabilizing circuit did not appreciably affect the

-50..

voltage at which the circuit broke into Oscillation.

5.6 The Audio Amplifier

 

As it was necessary to hear the audio tone, the
output Of the oscillator was amplified by a two stage
conventional RrC coupled amplifier. This allowed a
high enough level to be fed to a remote position if it
became necessary to do so. A volume control was in-
cluded so the Operator might adjust the sound to
a suitable level. The output was fed to a.6" PM
speaker.

5.7 The Power Supply

In order that the instrument hold its calibration
over a reasonable period of time, it was necessary
that the voltage supply be very well regulated. In
general there are two methods of regulating a d.c.
voltage electronically. (1) By shunting a gas diode
regulator tube across the poorly regulated supply,
and (2) by inserting either a series or shunt high
vacuum triode controlled by a pentode tube amplifier.
The former is the simplest, but does not afford quite
as good regulating characteristics as the latter.
However, due to the number of tubes and the extra
equipment for the latter, the gas diode type regulator
was chosen.

The necessity for a low ripple voltage is Obvious

as the direct current supply is used tO supply the

-51-

bias voltages all through the amplifier, and as any
change in the grid voltages of the first stages is
amplified and results in a correspondingly larger
change on the grid of the switch tube. To lower this
ripple voltage to a usable value, it was necessary

to employ a two section filter. The voltage regulator
provides additional filtering. The two section filter
causes the inclusion Of additional resistance and
therefore reduces the overall regulation. Also the
necessity for a high voltage (500 volts) at fairly high
current for this type Of instrument requires either
that an extra high voltage power transformer be used,
or that condenser input be used on the filter. The
former is considerably more costly and would require
more space on the chassis, while the latter affords
still poorer regulation. Some Of the disadvantages

of the condenser input filter would be overcome

through the use of an excessively high bleeder current,
thus improving the regulation and Obtaining the neces—
sary high voltage. Since the overall regulation was

the important feature, and this being Obtained by using
the high bleeder current and the gas diode regulator
tubes, it was necessary that the regulation of the

power supply prOper be only sufficient to maintain

the output voltage which supplied the VR tubes slightly

greater than that required to keep them in continuous

discharge. The change of the currents being drawn from

~52»

the bleeder and power supply was calculated to be less
than about 10 milliamperes throughout the completion
of one cycle, and for very small voltage settings,
almost negligible, thus the change in voltage due to
the slant characteristic of the gas diode regulator
tubes will not appreciably affect the output voltage.

In addition to the above, it was desirable that
the instrument be fused to prevent damage in event of
parts failure, or higher than normal input line voltage.
The fuse was to be mounted in the back of the cabinet.

The complete power supply and regulator circuit
diagram is given in Figure 22.

5.8 The Complete Circuit Diagpam

 

Having chosen each component part and arrived
at a suitable circuit for each, the problem remained
to combine the individual parts into one workable
unit. The details of the small calculations neces-
sary are not included but are Obvious upon reference
to the complete circuit diagram shown in Figure 25.
A parts list is shown in Table II.

5.9 The Lgyout Problem

 

Certain parts, such as the transformer, chokes
and filter condensers, require a large percentage of

the tOp of any chassis on which a unit of this type is
to be built. Thesewnre placed along the back side of

the chassis so as to allow the remaining circuits

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Table II. Parts List for the Sensitive Resistance
Limit Indicator.

Resistors:

 

The following resistance values are in Ohms and
are 4 or 1 watt resistors unless otherwise noted.

# value wts. # value wts.
1 1M 24 27K
2 10M 25 27K
5 20m 26 27K
4 50 4 28 1M
5 15 50 1.4K 4
6 200 51 2M
7 500 52 .5M
8 10K 55 .1M
9 see note 54 400 10
10 .1M 55 2.5K 25
12 .lM NOTE: resistors 9,11,
14, 27, 29, are com-
15 20K posed of 5—2500 Ohm
100 wt. resistors in
15 .lM series.
16 .lM Tubes:
954
17 .2M 2 6SJ7
5 6SJ7
18 .5M 4 68N7
5 6SN7
19 50K 6 68J7
7 6L6
20 68K 8 5U4
9 VR150
21 10K . 10 VR150
11 P- 47
22 .59M 12 NE—2
25 27K

-56....

 

 

Condensers;

Values are in micro-farads, 600 v. wkg unless other-
wise noted.
# value v.

1 100 50
2 .25
5 .05
4 .05
5 .05
6 .01
7 .04
8 .1
9 8.0 50
10 .05
11 .05
12 8.0 50
15 .002
14 20.
16 8.

17 20.

Other Parts:

T1 Stancor P-4081
T2 Stancor A-5825
L1& L2 8 hy. @ lOOma.
L5 Thordarson T20052

Spkr. Jensen PM6C

~57—

which would require controls to be placed along the
front panel. Care was taken to avoid stray coupling
between the power transformer and the input stages.
Also the cables which carried the raw do from the
rectifier were run separately from the cables carrying
only signal, dc supply, and filament supply voltages.
The individual parts were mounted in a fixed position
as much as possible by using Jones mounting strips
throughout. Mounted in this way, there was very little
instability due to vibration.

The electronic switch stage was not included
when the instrument was first said out, but was later
found necessary and included. A preferred arrange-
ment is shown in Figures 24 and 25. The change in-
dicated would further decrease the length of wiring
and simplify the wiring procedure considerably. The
unit, while quite compact, has been designed so as to
make each part have its own place. The large bleeder
resistors were mounted external to the unit both for
space considerations and to allow them to be better
ventilated. The 6" loud speaker was mounted on the
front panel and was protected by a copper screening.
The indicator meter was mounted as shown in the panel
view, Figure 24.

The wiring was done with the standard color coding
procedure: black for negative or chassis; red for high

voltages from the d.c. supply; yellow for the bias

 

 

 

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, u 4 _
_ _
.... a
_ a
_
(l... . C.

 

“.3402... 5353

voltages; green for the a.c. filament supply voltages;
and blue for those carrying the signal voltages. The
general layout of the wiring is shown in Figure 25.
Photographs of the completed instrument are shown
in Figures 26, 27, and 28. These too, will show much

Of the general type Of construction used.

 

 

 

 

CHAPTER VI

6.1 Experimental Results

 

After construction was finished and the unit
had been properly aligned, the Operation was satis-
factory except for considerable drift. In view of the
type of circuit employed, a check was made of the
voltages internal to the bleeders. Both were checked
with the potentiometer circuit and after only a fifteen
minute warmup period, the voltages were found to re-
main within o.1% thenceforth. It was found that with
the plate lead removed from the #954 input tube, the
amplifier and switch circuits became very stable and
over a two hour test run varied less than 0.01 volts
refered to the input. The trouble was finally located
in a poor filament drOpping resistor in the first
stage. On replacement with a higher wattage resistor,
the Operation was much better. A small amount of drift
would be tolerable as the unit could be recalibrated at
any time.

No particular difficulty was encountered with any
of the circuits used.

With the heavy bleeder current Of 40 ma., the common
d.c. amplifier was very stable and the Operation was not
improved by the insertion Of the Miller circuit; how-

ever, if the regulation of the voltage along the bleeder

resistance had been poor, this would not be true.

-55-

The input stage was run as shown in section
5.2b and resulted in about unity gain. The d.c.
amplifier had an overall gain Of 167, and was very
sufficient.

The reader is asked to note that this instrument
has not been completely develOped as applicable to
the dewpoint problem, and therefore a few of the
mechanical refinements such as the cabling of the

internal wiring has not been completed.

-66..

APPENDIX I

For those wishing additional information about
the General Electric Model PM-17 vacuum tube volt-
meter, the circuit diagram is shown in Figure 29,
as obtained from the Operating instruction booklet.

This instrument was particularly useful in that
it has an input impedance of over 200 megohms on the
0-1 d.c. volt range. This assured the Operator that
there was no possibility of the internal resistance
affecting the measured voltage.

A photograph of this instrument is shown in
Figure 50.

 

 

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l -..; i . . 5 . w -. .

 

 

 

APPENDIX II

The Leeds and Northrup potentiometer used in
these experiments was a very good instrument, but it
did not lend itself to Obtaining an instantaneous
reading Of the temperature. The lag in time required
to read the instrument limited its accuracy in this
problem.

A potentiometer measures the temperature by meas-
uring the output voltage from a thermocouple, and
calibrating it in terms of temperature. This is done
with an arrangement similar to the circuit shown in
Figure 52. A battery is placed in series with a
variable resistance and a calibrated slidewire. By
inserting a standard cell in series with a galvono-
meter and placing these across a known portion of the
slidewire, the voltage may be adjusted precisely by
varying the resistor R until the galvonometer is
zeroed. Then the circuit may be connected to a
thermocouple and the temperature read by again zeroing
the galvonometer and reading the calibrations on the
slidewire. The time lag occurs in this Operation.

A photograph showing the L and N potentiometer

used is shown in Figure 51.

-69—

 

 

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APPENDIX III

It was necessary to measure the temperature of
the dew point cell. This was accomplished by measur—
ing the voltage produced by a copper—constantine thermo-
couple which was sealed in the surface of the cell
near the electrodes. Originally it was planned to do
this with a Leeds and Northrup hand potentiometer
calibrated for that purpose. It was found, however,
that there was considerable error due to the time lag
in reading the temperature after the signal was given
by the alarm circuit already discussed. This was due
to the Operation of adjusting the potentiometer

manually to Obtain a zero reading on the galvonometer.

It was therefore necessary to acquire an instru-
ment which would measure and give an indication of the
temperature Of the thermocouple instantly. Cost again
was an important factor, and thus, as all of the
commercial instruments designed for this purpose were
too expensive, the problem Of design and construction
of such an instrument was undertaken.

Essentially the problem consisted of measuring
a potential in the order Of a few millivolts and to
have the value of the voltage instantly available.

An instrument has been designed by the author to
measure this temperature to 0.2°F. instantaneously.
This instrument is now in the latter stages of develOp-

ment and will be released at a later date.

-71-

 

APPENDIX IV

Considering the circuit in figure 18, let
the amplification factor of the tube be u, and the

associated plate and grid resistors R1 and R re—

2
spectively. The tube T2 is a gas diode voltage regu-
lator type whose voltage drOp is assumed constant.

The load presented to the plate of tube T1 is

 

 

then,
R R
a: 12
R1+R2
and the gain becomes
URI
K = R
-—--——fiR12 +r

where rp is the plate resistance of the tube.

Normally, R1 is much less than R2
Rl

R1+rp

which is the gain for an ordinary resistance coupled

so, ' K =

amplifier for mid-range frequencies.

—72—

 

APPENDIX V
Considering the modified one-shot circuit

shown in figure 20, assuming the applied voltage v1
volts above the voltage necessary to cause the switch-
ing action, the condenser C will charge through the
resistance circuit formed by R and R2 according to the
equation R'd +

8’“

t
whose solution is

V1 ~(t1-t2)
91~92‘*§T(1 ~ 6 “sir—"l
where R'= R + R2, and el-e2 are the limiting voltages
for the switching Operation. The quatity (tl-tg) is

the time of duration of the keyed pulse and is equal to

T as 1 v1
" “ Riel-e27

 

~73—

APPENDIX VI

An attempt was made to plot both temperature
and conductivity as a function of time through the
use Of a Brown Instrument Company, 12 point recorder
originally intended for use with cOpper-constantine
thermocouples. Alternate points were connected in
parallel and the recorder was used as a two point
instrument.

The range Of the instrument was -100°F to +250°F,
and was quite hard to read within about two degrees.

The resistance element Of the deWpoint cell was
connected to point 1 through a battery which caused
the voltage develOped across the internal resistance
of the recorder to be recorded. Thus the voltage read
was prOportional to the conductivity of the resistance
cell.

The alternate set of points were used to measure
the temperature of the thermocouple on the dew point
cell.

The results were not entirely satisfactory due to
an inherent drift present in the recorder when measur-

ing currents in the order of 10'"8 amperes. A sample run

is shown in figure 55.

474-

 

 

 

 

 

 

 

 

I

..J. .Ill--"¢' *0 5|
. . . .

 

 

.

 

L.

 

Data as gathered by a recorder showing

55

Figure

~75-

 

 

 

 

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temperature and conductivity as a fuction of time.

 

2.
5.
4.

REFERENCES

Hund, High Frequency Measurements. ( see also

 

references given by Hund pg. 104 ff.)
G. E. Bulletin # GET 484B
Electrometer tube catalogJ Victoreen Instrument Co.
Rev. Sci. Inst., 9:229

S.E. Miller, Electronics, Nov. ~41.

 

-76-

BIBLIOGRAPHY

Everitt, Communications Engineering

 

Eastman, Fundamentals of Vacuum Tubes
Michaela, Advanced Electrical Measurements
Laws,.Electrical Measurements

Applied Electronics, MIT Staff.

Hund, High Frequency Measurements

Hund, High Frequency Phenomena

Brainerd et al, U.H.F. Techniques
Skilling, Transient Electric Currents
Terman, Radio Eng. Handbggk

F. T. & T., Reference Data for Radio Eng.

Army publication, TMll-452

 

Army publication, TM11-466

 

R.C.A. publication, RC-15

Laboratory notes from EE 454, Michigan Ctate Colleve

Also General Reading in the following Periodicals:
Rev. Sci. Inst.

Jour. Sci. Inst.

Physics Rev.

Proc. I.R.E.

QST

Electronics

Radio News

 

-77-

 

 

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