Fogwell, J. W.

Lorenz, R.

English

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https://ntrs.nasa.gov/search.jsp?R=19700018847 2020-03-12T02:07:02+00:00Z
DIGITAL THERMOMETER
Richard Lorenz
J. W. Fogwell
FINAL REPORT - PART II
Contract No. NAS 9-7852
SwRI Project No. 16-2327
Prepared for
National Aeronautics and Space Administration
Manned Spacecraft Center
Houston, Texas
4
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November 1969
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SOUTHWEST RESEARCH INSTITUTE
SAN ANTONIO	 HOUSTON
SOUTHWEST RESEARCH INSTITUTE
Po,.t Office Drawer 28510, 8500 Culebra Road
San Antonio, Texas 78228
DIGITAL THERMOMETER
Richard Lorenz
J. W. Fogwell
FINAL REPORT - PART II
Contract No. NAS 9-7852
SwRI Project No. 16-2327
Prepared for
National Aeronautics and Space Administration
Manned Spacecraft Center
Houston, Texas
November 1969
Approved:
Douglas N. Travers, Director
Applied Electromagnetics
TABLE OF CONTENTS
Page
I. INTRODUCTION 1
II. GENERAL DESCRIPTION OF OPERATION 2
III. TECHNICAL DISCUSSION 4
A .	 Probe Design and Construction 4
B.	 Electronics 5
IV. CONCLUSIONS AND RECOMMENDATIONS 6
A .	 Conclusior.s 6
B .	 Recommendations 6
APPENDICES 7
APPENDIX A 8
APPENDIX B 10
I. INTRODUCTION
Work completed under Contract NAS 9-7852 produced a prototype
digital thermometer demonstrating accuracies of better than ± 0.2°F for
temperatures measured between 74°F and 115°F. The application of
digital techniques allowed the use of integrated circuitry along with
minimum power requirements. Direct temperature readout by means
of incandescent displays provided precise information without requiring
interpolation of scales. The digital technique allowed the use of minia-
ture sensing devices which yielded rapid thermal-response times. Inter-
changeability of probe devices maintaining the desired accuracy was also
accomplished. The digital thermometer was constructed as a prototype
unit with primary effort devoted to the establishment of high accuracy.
With this success in hand, the miniaturization and refined accuracy
through rigorous ca^ibrating techniques could now proceed.
zII. GENERAL DESCRIPTION OF OPERATION
The digital thermometer is operated on the intervalometer principle
as indicated in Figure 1. A three-stage decade counter stores and displays
the number of cycles received during a time interval. The output of an
oscillator provides the frequency standard while an AND gate provides the
inhibiting and enabling of the oscillator and the counter. An analog to time
interval converter is used to control the AND gate output- This in turn
controls the number of cycles the counter is allowed to count, this repre-
senting the temperature. An important feature of this thermometer is the
method used for the early conversion from analog temperature to time
Most digital circuits provide a calculatabl.e and fixed error, while analog
circuits are subject to drift due to temperature, age, and power supply
variations
Only two analog circuits are used in the system. The first is the
oscillator. This oscillator, being crystal controlled, provides the desired
accuracy for the system for all conditions . The second analog circuit is
the analog to time converter. This converter provides a linear resistance
to tune relationship and requires minimum "analog state" signal operation.
The time constant involved in the temperature measuring operation
is limited only by the sensor or probe. The electronic sampling and time
conversion is an insignificant portion of the overall time constant.
Temperature gradients are obvious for oral temperature measure-
ments (under the tongue) as the sensor is moved from a deep portion of
the mouth toward the front These temperature gradients are apparently
associated with the proximity of the sensor to major blood vessels in the
tongue or in underlying tissue.
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L11. TECHNICAL DISCUSSION
A .	 Probe Design and Construction
In order to insure linearity in temperature measurement for ther-
mistor sensing elements, it is necessary to provide a compensation network
which provides a transfer function creating linear thermistor performance.
The computer program displayed in Appendix A was developed to evaluate
the thermistor performance for various values of resistor variables when
connected as a compensation network. As each value of resistance is tested,
the deviation in temperature from a calculated best fit linear response is
produced If this deviation is smaller than the preceeding deviation the
new resistor values are recorded After all variable values have been
examined, the best calculated set is printed out For this best set of values
a straight line best fit resistance versus temperature and equivalent resis-
tance (of thermistor with compensation network) versus temperature is
calculated and displayed. The maximum temperature deviation from the
straight line resistance versus temperature data is also calculated along
with the equation for the straight line. With this program, any thermistor
or non-linear device can be linearized to a predetermined accuracy
This program was used to insure that all compensated probes provide the
proper resistance-temperature characteristics
When fast thermal response is desired, and accuracy over only a
limited range of temperature or limited accuracy over a wider tempera-
ture range if-, tolerable, single thermistor units can be emploved. When
more accuracy is required for a wide temperature range, multiple ther-
mistors are required. The process of bonding or encapsulating the several
thermistors together raises the thermal response time; thus. acc;urac, and
range can be achieved at the cost of size and thermal response time . Two
probes have been developed and tested by SwRI; one having a 10-second
therma'. responsr time with a -0.2F° accuracy for a range of 94'F to 104°F.
The second probe exhibited a three-minute thermal response time with
^0 3F" accuracy for a range of 74'F to 110 °F. Additional c.alcolations are
required in order to determine the decline in ac.•uracy for ti-e first probe
with increased temperature range
5B.	 Electron- cs
The electronics module contains a timing gate which is directly
dependent on the resistance value of the thermistor.
 . This gate allows a
backward counter :o determine the number of cycles of a 10 MHz oscil-
lator occurring during the gated. period The timing gate can be adjusted
to provide a difference in gating time ('T' 2 -'r ) which allows a count
change, as indicated by the digital counter, of temperature 2 - tempera-
ture 1	 Thus . for a temperature change of 10.2F * .,, the counter will
display a change in the number counted of 102. In order to count the
exact and correct number, it is necessary to begin the count at a number
other than zero. This number is calculated when the compensation
network and probe values have been determined. Readout of the temper-
ature is ac( omptished by a pinlite 7 bar incandescent display
	 This,
along with the digital system. allows direct. reading of the temperature
without interpolation of scales or the changing of scale values to obt;^ in
the desired ac(:uracy. Appendix B gives the mathematical development_
Construction of the electronics is highly adaptable to medium
scale integration because of the use of digital integrated circuits . The
system as con,tructed used integrated circuits but was not built for
minimum package density. The prime objective was to design for future
dense packaging techniques but construct circuits in the most economical
manner sufficient to evaluate the system.
6W. CONCLUSIONS AND RECOMMENDA T IONS
A. Conclusions
Testing of the digital thermometer provided results better than
the desired ac .0 racy of -+ 0.2F* over the temperature range of 74 °F to
115°F. Time constants of approximately 10 seconds were accomplished.
The system provided a stable temperature measurement and desired
ac(:uracy with interchangeable probes which attached to the measuring
unit by a quick- disconnect plug. Probe cost in large quantities would
be held to a minimum of approximately $2.00 each.
B. Recommendations
Recommendations are as follows:
1 .	 Evaluation of various probe or sensor temperature ranges
vs ac(:uracy in order to expand the overall measuring
capability of the thermometer should be made.
2. Probe configurations would be designed which produce
the fastest temperature time constants possible.
3. A miniaturized version of the digital thermometer using
medium scale integrated circuits should be constructed
4. The total current requirements should be reduced t.o not
greater than 700 milliamps at 5 volts DC.
5. Rechargeable battery packages should be designed for
continuous operation of at least 10 minutes duration before
requiring recharging TY-is would provide at least 4 hours
ncrma', routine clinical or hospital service life before
recharging.
APPENDICES
APPENDIX A
COMPUTER PROGRAM
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APPENDIX B
INVENTION DISCLOSURE
Date
Date
DIGITAL THERMOMETER
The purpose of the digital thermometer is to measure temperature
with a temperature sensitive resistor and display the actual temperature
with an optical readout. The conversion step from the analog temperature
sensing element to digital logic circuits can be accomplished by two
obvious methods:
(1) Count the number of cycles from an oscillator for a fixed
sample time, allowing the temperature sensitive resistor
to control the oscillator frequency.
(2) Count the number of cycles from a fixed oscillator, allowing
the temperature sensing resistor to vary the sample time.
The second method is most easily employed, using fewer components and
providing more stable operation. A one-shot multivibrator is used to
vary the sample time of an oscillator as shown in Figure 1. The number
of cycles occurring during the sample time is counted by a binary counter,
converted to decimal quantities and displayed by an optical readout.
The uniqueness of this system is the method of acquiring direct
temperature to count ratio. The "on" or sample time of a one-shot multi-
vibrator is given by the "RC" time constant. The RC components are
shown in Figure 2. It is anticipated that this system will operate with a
linear resistance vs temperature element. If this temperature sensitive
element is allowed to replace the R in the RC equation, then the sample
time is directly proportional to R.
Thus
T = RC where T = sample time
Signature of fnventor	 Date
/ 71yov G ^,
Signature of Inventor	 Date
W 2
if R varies according to
Rx = Ro + RoN& t
where the subscripts denote the temperature and p t = change in temperature
from to
Tx = RxC
For some change in temperature
Tx - To = C [ (Rs + Rx) - (Rs + Ro)] where Rs might be some series
resistance
Rx = Ro + Ro( N) ( tx - to)
Tx - To = C [RoN (tx - to)]
Then
C =
	 Tx - To	 (1.1)
RoN (tx - to)
We also know
	
Tx = (C) [Rs +	 Ro + RoN (tx - to)]	 (1-2)
Substitute for (C)
Tx = ( Tx - To) [ Rs + Ro(1 + N (tx- to))]RoN (tx - to)
	
Signature of Inventor	 Date
/ 7Akv 66'
	Signature of Inventor Date
	
Sign uTe of Witne 's) 	 Date
	
tore f Wit ess	 Date
.	 4
3An ideal situation would exist where Rs = 0.
For this condition:
TxRs + TXRo ( 1 + Ntx - Nto) - T O R S - ToRo ( 1 + Ntx - Nto)
TX	 RoNtX - RoNto
TX = TO R S + TxR.o + T
XRoNtx - ToRoNto - T O RS - TO RO - ToRoNtx + ToRoNto
RoNtx - RoNto
0 = TXRo- TORO- ToRoNtx + ToRoNto
0 = Tx - To - ToNtx + ToNto
0 = TX - To + TON (to - tx)
T ON (to - tx) = To - Tx
To - Tx
N = To(to - tx)
N
 = T^t 
T 
t
	
where N = change/ °F
o x' o)
(Tx - To) = K (tx - to) ; To = Kto
We want to force the linear relationship such that
K = 10 
in 
wheren = 1, 2, 3, etc.
Aignature of'Inventor 	 Date
 
Signature of Inventor	 Date
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Sign tune of Wit ess 	 I	 Date
	
`
	
ndtu r e o i e as	 Date
4For a 10 MHz oscillator, the sample time for a four place readout would
correspond to K = 10-6.
-6
	Thus N= T	 N= 10	 N = KK so that N= tl
	
0	 0	 0	 o
This is the value of N which will allow a zero series resistance. A value of
N less than that valued would require a -Rs which would be undesirable. In
fact, it would be desirable to work with any value of N independent of any
series resistance. A silicon resistor is presently marketed produced an
N = .00389 / °F. This N does not meet the minimum value requirement for
zero Rs, thus the system must be adjusted in some manner to obtain the
desired direct readout.
The value of C must first be calculated to produce a desired change
in sample time ( Tx - To) for a desired change in temperature (tx - to) and
a given N and Ro .
Next, evaluating Tx for the calculated C and using Rs = 0, it can be
shown that the count produced by sampling a fixed oscillator for Tx differs
from the actual temperature tx by an additive constant. This amount of
reading is offset by pre-setting the counter to a starting number less than
zero, thus producing -an accurate readout of the temperature between the
limits tx and to .
An example follows:
Temperature values: train = 90 °F; tmax = 110°F
Frequency of oscillation: 10 MHz
Desired to read: 5 places
R90 = 10452— N - .00389/ °F
The numbers to be read are:
^-Q9000 to 11000
/- ^	 In	 11
Signature of Inventor
-r
Signature of Inventor
7 Z:	/ ^' ,G
Date
/ 
Date
Signat, of Witne
n3tux'e o Wit e
	
s 	 Date
f /
	
si(	 Date
Signat j're of W' e s s
i atur` o	 itnefss
r
Date
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Date
5
For the 09000 representing 90.00 °F
T90 = .9 x 10-3
For the 11000 representing 110.00°F
T 110 = 1 .1 x 10-3
	
tmin = 90 OF	 tmax = 110 °F
	
T 110 ' T90	 .15 x 10'3	 _5
	
tmax ' tmin	 15
= 1 x 10 = K
K	 10'5	 10-5
	
R90N	 (104) (.00389)	 38.9
C = .0257 Y. 10' 5 = 0.257 µf
T90 = (C) (R90 ) = (.0257 x 10' 5 ) (104) = 25.7 x 10'4
T 100 = (C) (R90 + R90NA t) = (. 0257 x 10' 5 ) (104 + 104 x .00389 x 10)
T 100 = 26. 6997 x 10'4
T 110 = ( C) (R90 + R90NAt) = (.0257 x 10' 5 ) (104 + (104)) (.00389)(20) 1.0778 x 104
T110 27.699 x 10'4
Count for T 90 = (T) (f) = 25.7 x 10" 4 x 10 7 = 25700
Signattu1re o£'Inventor	 Date
gnature of Inventor	 Date
6Count for T 100 = 26699.7 = 26700
Count for T 110 = 27699.5 = 27700
For a temperature of 90 ° we read	 25700
We want to read	 9000
The difference is 	 16700
For a temperature of 100* we read	 26700
We want to read	 10000
The difference is	 16700
For a temperature of 110 * we read	 27700
We want to read	 11000
The difference is	 16700
Note that in each case the decimal reading differs from the actual
reading by 16700 counts. Thus, by setting the counter to 83300, the counter
will read the correct values of temperature after the proper sample time
occurs.
Signature ,of Inventor
Signature of Inventor
YDate
/I
Date
Z r N'4.1 	 k-^'
Signat ro/of Wit	 s	 Date	
f
ig fifre o Wit b 	 Date
TRIGGER IN
OUTPUT (SAMPLE TIME)
OSCILLATO
7
 DECADE	 DECADE
p}"^ COUNTER ^'—"'{ COUNTER
READOUT 1	 I READOUT 1-- — — — -
FIGURE 1
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I
TRIGGER IN
FIGURE 2
}c, nature of Inventor	 Signature, o Witness
Si- nature of Inventor	 ^Signat re, ' Witness
Date


Digital thermometer, part 2 Final report

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