Systems and reliability engineering for automatic pressure transducer calibration syste

Schuler, A. E.

English

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TECHNICAL MEMORANDUM X-53678 
AUTOMATIC PRESSURE TRANSDUCER 
CALI  BRATION SYSTEM 
BY 
Albert E .  Schuler 
George C. Marshall Space Flight Center 
Huntsville, Alabama 
A B STRA CT 
The development of an automatic p re s su re  transducer calibration system 
is discussed in this report .  Evolution from past  practices and systems into 
an automatic calibration system with computerized data handling is described. 
NASA - GEORGE C. MARSHALL SPACE FLIGHT CENTER 
https://ntrs.nasa.gov/search.jsp?R=19680010575 2020-03-24T00:25:19+00:00Z
NASA - GEORGE C.  MARSHALL S P A C E  FLIGHT C E N T E R  
TECHNICAL MEMORANDUM X-53678 
AUTOMATIC PRESSURE TRANSDUCER 
CALI  BRATION SYSTEM 
BY 
A l b e r t  E .  S c h u l e r  
T E S T  LABORATORY 
RESEARCH AND DEVELOPMENT OPERATIONS 
TABLE OF CONTENTS 
Page 
SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  1 
BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  2 
SYSTEM DEVE LOPME NT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  3 
Automatic Field Calibration System . . . . . . . . . . . . . . . . . . .  4 
Automatic Laboratory Calibration System . . . . . . . . . . . . . . .  5 
THECURRENTSYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6 
Figure 
L I S T  OF ILLUSTRATIONS 
Title 
1. Automatic P r e s s u r e  Balance ..................... 
2 .  Block Diagram, Automatic Calibration of Complete 
Measuring System . . . . . . . . . . . . . . . . . . . . . . . . . . .  
3. Block Diagram, 50 Channel Automatic P r e s s u r e  
Calibration System ............................ 
Page 
2 
4 
7 
iii 
TECHNICAL MEMORANDUM X-53678 
AUTOMATIC PRESSURE TRANSDUCER 
CALI  BRATION SYSTEM 
SUMMARY 
This report  describes evolution and principles of automatic p re s su re  
calibration systems. The p res su re  balance as calibration p res su re  source 
and standard, an automatic calibration system for in-place calibration of 
p re s su re  transducers with data acquisition systems a t  the test  stand, and an 
automatic fifty-channel p re s su re  transducer calibration system for  laboratory 
evaluation and calibration of pressure transducers are described. 
I NTRODUCT I ON 
F o r  many years ,  manufacturers have automated their  facilities to  
increase productivity, reduce cost, and perfect manufacturing of components 
to  c loser  tolerances. The Instrument Development Branch of the Test  Laboratory 
at MSFC has extended automation into the field of instrument calibration. This 
report  describes an automatic pressure transducer calibration system. 
Over the past  several  years  the number of pressure transducers required 
f o r  testing missi les  and space vehicles has increased with the increased s i ze  and 
complexity of the test a r t ic les .  
must be highly accurate and reliable under varying and extreme environmental 
conditions, necessitating many thorough and complete tes t s ,  evaluations, cali- 
brations f and recalibrations during manufacture f acceptance, use,  and reuse.  
The transducers generally used in this capacity 
Individual calibration of each transducer at different temperatures,  using 
as a calibration standard the conventional dead weight t e s t e r ,  would require 
many manhours, large amounts of space, and tremendous duplications of equip- 
ment, resulting in great  cost p e r  unit calibrated. Calibration output w a s  
increased by manifolding a l a rge r  number of gages with the s a m e  range, but this 
method required the u s e  of transfer standards, manual p re s su re  regulations, 
and individual readings. 
disadvantage of this method, but manual p re s su re  regulation and the large chance 
f o r  human e r r o r s  are also very undesirable. 
The use of secondary standards is the greatest  
It was evident, that a more basic  standard, automatic operation, and 
manifolding of a large number of gages would be necessary  to  increase quantity 
and accuracy f o r  test, evaluation, and calibration of pressure  t ransducers .  
These problems were solved with the automatic p re s su re  t ransducer  calibration 
system described in this report .  
BACKGROUND 
Around 1950 the  author developed a dead weight p re s su re  balance which 
regulates automatically any desired pneumatic p re s su re  within an accuracy of 
0.05 percent of the measured value (F ig .  1) . A rotating piston converts the 
Pis ton 
FIGURE 1. AUTOMATIC PRESSURE BALANCE 
pres su re  into force and a mechanical balance compares this force  with the force 
of dead weight. When these forces  a r e  unbalanced, an accurate  switch sys tem 
operates solenoid valves and regulates inlet or  outlet of compressed air until 
p re s su re  t imes effective a r e a  equals the force of dead weight on the other a r m  
of the balance. Fast, medium, and slow air flow in or  out, and a t ime delay 
feature for temperature changes and pneumatic shocks provide a smooth and 
fast control mechanism. 
2 
The advantage of the p re s su re  balance is its ability t o  achieve and main- 
tain without constant attention a desired p res su re  with the accuracy of a p re s su re  
standard,  and to  keep the transducers under test relatively clean by using 
compressed gas r a the r  than hydraulic fluid as is normally done with dead 
weight calibrators.  
The idea of the automatic pressure balance was originally conceived to  
provide an accurate reference pressure when p res su res  are known approximately 
and have to  be measured with extreme accuracy. The p res su re  balance is set 
to  regulate i ts  p re s su re  to  the expected value, and the difference between this 
reference p res su re  and the unknown pressure is measured with a differential 
p re s su re  gage of much sma l l e r  range, which increases  the accuracy accordingly. 
SYSTEM DEVELOPMENT 
Successful development of the p re s su re  balance for the reference p res su re  
application led to  the realization that the p re s su re  balance was an excellent 
calibration standard and could be the key to  automation in the field of p re s su re  
gage calibration. By electrically controlling the placement of weights on the 
balance pan, the usefulness of the pressure balance could be further expanded 
since the selection of pressure could then be automatically programmed for an 
ent i re  calibration operation. 
Development specifications were written and a contract w a s  awarded to 
Gilmore Industries to  design a simple device to  provide automatic application 
o r  removal of the weights and a remote electrical  selection of weights required 
fo r  a certain p re s su re  calibration program. The resulting devices are called 
"Weight Handler" and "Programmer.  
The principle of the weight handler can  be seen in  Figure 1. Springs 
hold the weights up above the weight pan. To lower a particular weight onto the 
pan, the appropriate solenoid valve is energized and compressed air is applied 
to  the proper pis todcyl inder  assembly, compressing the spring and lowering the 
weight onto the balance pan. A hook-and-eye arrangement between the weight 
and piston insures  that there is no interaction between weight and weight handler 
while the weight is i n  place on the balance pan. 
o r  binary-coded decimal system of weight combinations , a relatively small  
number of individual weights provides large p re s su re  ranges in increments of 
1 psi ( 0.69 N/cm2). Thirteen weights provide 8191 p res su re  values in the 
binary system and 16 weights provide 4 decades o r  9999 values in the binary 
coded decimal system. 
Through the use  of a binary 
The programmer provides for the automatic operation 
3 
af the entire calibration. 
particular program and by a tape reader  which converts information on the 
tape into electric signals to control all required operations. 
This is accomplished by a punched tape s tor ing the 
Automat ic  Field Cal ibrat ion System 
Figure 2 shows the block diagram for automatic calibration of a com- 
plete pressure measuring and data acquisition system while installed in actual 
place of operation. First, the programmer initiates operation of the proper  
solenoid valves to connect a selected group of t ransducers  via manifold to  the 
p re s su re  balance. Then it commands the weight handler to place the proper 
OTHER PRE5SlRE 
RAhCES 
~ &+-~~2>* I 
10 P S I  20 PSI  50 psi 100 psi 500 psi 1000 psi  
(6.89 N/cm2)(13,78 N/cm 2)(34.48 N/cm2 )(68,95 N/cm 2)(344.74 N/cm 2)(689u48 N/cm 2, 
i 
I - - - -  - _  L- - --.  - 
I 
TYPICAL OTHER PRESSURE MANIFOLDS A S  PER DETAIL 
OTHER P R E S S W  IRANSDUCERS 
PROGRAMMER 
MEASURING POINT 
FIGURE 2. BLOCK DIAGRAM, AUTOMATIC CALIBRATION O F  
COMPLETE MEASURING SYSTEM 
4 
set of weights on the balance pan. 
by the p re s su re  balance, and the regulated p res su re  is applied to  the manifold 
connecting to the transducers t o  be calibrated. 
The resultant force is automatically balanced 
After the programmed pressure is stabilized, a "read" command is 
given to r eco rd  transducer outputs. Then the programmer initiates the applica- 
tion of other weights for a new pressure step. When all calibration s teps  for 
the connected transducers are completed, the programmer operates the solenoid 
valves to  reconnect the calibrated transducer to  the system where they are 
needed and all transducers with another p re s su re  range are connected via mani- 
fold to  the p re s su re  balance fo r  calibration. Through a 'Istart" command, the 
calibration console performs all the programmed functions in  an unattended 
operation. 
A uto ma t ic La bo rato ry Ca I i b r a  t ion System 
Throughout the years  of rocket development it has been an accepted 
technique to calibrate pressure transducers in actual position at the test 
stands together with the data acquisition systems. The automatic p re s su re  
calibration system just described increased economy and accuracy of these 
calibrations. However, the increasing numbers of p re s su re  transducers 
required in testing space vehicles and components resulted in a new policy to 
use at the test stands. Instead of "end-to-end" p res su re  calibration, an 
electrical calibration was used at the test stand and the electric signal/pressure 
relation was determined in the laboratory. 
transducer had become the leading pressure transducer at MSFC's test stands 
and the shunt calibration of s t ra in  gages is the best and most familiar electrical  
calibration technique. It consists of switching well determined r e s i s to r s  as 
shunts parallel  to certain a r m s  of the s t r a in  gage bridge to produce the s a m e  
bridge output as the respective pressure would. 
By that t ime the s t ra in  gage pressure 
Usually shunt calibrations a re  made at 25, 50, 75, and 100 percent of 
the p re s su re  range. 
r e s i s t o r s  installed in their  data acquisition network, and the laboratory has 
identical shunt r e s i s to r s  i n  its calibration network. Specifications for newly 
purchased s t ra in  gage pressure transducers require built-in shunt r e s i s to r s  for 
20 and 80 percent of the range, and sufficient accuracy between these two points 
is a s su red  by linearity specifications which are checked in  the laboratory. F o r  
this evaluation, test, and calibration work, various modifications and improve- 
ments were made on automatic pressure calibration systems. A major improve- 
ment was automatic punch c a r d  data recording and automatic computer data 
computation and tabulation. 
For these steps the tes t  stands have the proper shunt 
Other major improvements were a programmer 
5 
with switches instead of punched tape, additional shunt mode capability, auto- 
matic shunt-to-pressure calibration, transition between different modes, and 
automatic programmed application of preset  shunts and pressures .  
Most of these modifications and improvements resulted from combined 
efforts of the Instrument Development Branch, Test  Laboratory, MSFC, and 
Vitro Services, prime support contractor of the Test Laboratory. 
THE CURRENT SYSTEM 
In 1965 and 1966, Vitro Services personnel developed a new and most 
advanced fully automatic calibration system for simultaneous calibration of 
50 p res su re  transducers. This convenient, accurate,  and highly productive 
50 channel automatic pressure calibration system is  shown in Figure 3. 
The pressure transducers to be calibrated are installed in an environ- 
mental test  chamber and manifolded to  connect them to  the p re s su re  balance 
which is the calibration p res su re  source and standard. 
provides a temperature range of -120' F to +350" F ( 188.7" K to 449.8' K), and 
it takes up to two hours to  reach temperature stability and equilibrium when 
the temperature setting is changed. 
The tes t  chamber 
The programmer controls the binary coded signals for all steps of the 
operation and it has the following four sub-units: 
1. P r e s s u r e  control matrix,  
2 .  Shunt control matrix,  
3 .  System o r  logic control matrix, 
4. Computer. 
Four  selector switches s e r v e  to  dial the full-scale p re s su re  desired f o r  
the calibration, and additional push-button switches program the computer t o  
perform calibrations in the desired s teps  and increments. 
tion, the programmer connects the transducer bridges ei ther  with the shunt 
calibration resis tors  in the programmer box o r  with the shunt r e s i s to r s  built 
into the transducers.  
F o r  shunt calibra- 
The scanner sequentially connects all t ransducers  t o  the ratiometer,  
which includes analog-todigital conversion. The digital volt ratiometer com- 
pares  the output signals of the connected transducer with the voltage of a 
reference power supply and gives the millivolt/volt output-input ratio in digital 
form to  the card punch o r  magnetic tape r eco rde r ,  A standby power supply 
6 
provides normal  excitation of <ne traxlsblicers, ax! wher? t,-e scanner  connects a 
t ransducer  t o  the ratiometer, the excitation of the t ransducer  is switched t o  the 
reference voltage. The reference power supply is periodically checked when 
the scanner  connects it to a calibration standard which provides precision dc 
voltage. 
AUTOMATIC PRESSURE BALANCE 
I--- - - - - - --  1 
I WEIGHT I BALANCE 
SIGNAL 
BALANCE 
CALIBRATION I 
PROGRAMMER PRESSURE - - - - - - - 
I I 1 
I f 
SHUNT 
CALIBRATION + r r -  
CARD PUNCH OMPUTER T C  
A -  D 
RATIO METER 
I 
SCANNER I I 
I I I 
PRESSuBe TRANSDUCERS :I 
I 
I I 1 
I I 
ENVIRONMENTAL CHAMBER 
FIGURE 3.  BLOCK DIAGRAM, 50 CHANNEL AUTOMATIC 
PRESSURE CALIBRATION SYSTEM 
The output of the digital ratiometer is recorded on punch cards  together 
with temperature,  barometr ic  pressure,  and programming instructions. A 
coded formula identifies channel numbers and p res su re  s teps  o r  shunt calibra- 
tion steps.  The IBM 526 printing summary punch is used. This machine has  a 
programming drum and is a multipurpose machine suitable fo r  various 
recording and controlling applications. It is  planned to use magnetic tapes for 
recording in the future to reduce storage problems. 
The punched cards  a r e  sorted and printed out locally for  checks and raw 
data  files. The Computation Laboratory performs calculations to determine 
sensitivity, linearity, hysteresis ,  temperature characterist ics,  and deviation 
limits. A summary  sheet of these data provides quick-look information about 
the usefulness of a t ransducer  fo r  a special application. Then shunt calibrations 
are made with the real data acquisition systems at the test stands or wherever 
7 
the transducers a r e  used. The combination of p re s su re  and shunt calibrations 
in the laboratory, and the shunt calibrations in the field satisfy all calibration 
field measurement. 
I requirements and provide very  reliable calibration data for  evaluation of the 
APPROVAL 
AUTOMATIC PRES SURE TRANSDUCER 
CALIBRATION SYSTEM 
By Albert E.  Schuler 
The information in this report has  been reviewed for security classifi- 
cation. Review of any information concerning Department of Defense o r  Atomic 
Energy Commission programs h a s  been made by the MSFC Security Classifica- 
tion Officer. This report ,  in its entirety, has been determined to be unclassi- 
fied. 
This document has a l so  been reviewed and approved for technical 
accuracy. 
W. H. SIEBER 
Chief, Test Instrumentation and Control Division 
Director ,  Test  Laboratory 
9 
D I STR I BUT I ON 
DIR 
DEP-T 
cc -P 
I-RM-M 
MS -H 
Ms -1P 
MS-IL ( 8) 
MS-T ( 6 )  
Author ( 100)  
Scientific and Technical Information Facility ( 25) 
P. 0. Box 33 
College Park,  Maryland 20740 
Attn: NASA Representative (S-AK/RKT) 
10  


Automatic pressure transducer calibration system

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