Isothermal volume measurements for weight computation on monkey

Knepton, J.

Molina, E.

English

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ISOTHERMAL METHOD FOR VOLUME DETERMINATION * 
Efrain Molina and James Knepton 
Bureau of Medicine and Surgery 
MR005,01,01-0097B 
NASA R- 10-009-0 13 
Approved by 
Ashton Graybiel, M, D, 
Head, Research Department 
Released by- 
Captain J, W, Weaver, MC USN 
Commanding Officer 
15 April 1969 
*This research was conducted under the sponsorship of the Office of Advanced Research 
and Technology, National Aeronautics and Space Administration. 
NAVAL AEROSPACE MEDICAL INSTITUTE 
NAVAL AEROSPACE MEDICAL CENTER 
PENSACOLA, FLORIDA 32512 
SUMMARY PAGE * 
THE PROBLEM 
To find an accurate method for measuring weight of humans, animals,and 
objects i n  weightlessness, Present methods used to determine weight cannot be 
applied in weightlessness. 
FINDINGS 
An initial probe to determine the feasibility of measuring volume by an 
isothermal method and applying the method to computation of weight was successful . 
In a study of rhesus monkeys (Macaca mulatta), the method showed great promises 
for further development and su-nt use in space flight. 
* The animals used in  this study were handled in  accordance with the "Principles 
of Laboratory Animal Care" established by the Committee on the Guide for 
Laboratory Animal Resources, National Academy of Sciences, National Research 
Counci I . 
.. 
I I  
INTRODUCTION 
Presently there i s  no device of practical use that can "weigh" objects in 
weightlessness. To surmount this problem, an indirect volumetric weighing method 
based on gas principles and object density should be possible. The present report i s  
an account of our first attempt to perfect such a method using a live anesthetized 
object, the rhesus monkey (Mucaca mulatta). The results indicate that the method 
i s  promising and should be d x p e d  for space flight use. 
MODEL 
The method used here io  measure the volume of an object i s  based on Boyle's 
Law that takes into account the relationship between changes of pressure and volume 
of a gas at constant temperature (isothermal process). The law i s  expressed as 
P,V, = Pa v, 
where 
P, = pressure of gas at condition 1 
V, = volume of gas at condition 1 
Pa = pressure of gas at condition 2 
V2= volume of gas at condition 2 
The model shown in Figure 1 i s  composed of two vessels, Chambers 1 and 2, 
connected by a valve, VA1, which contain the volumes to be measured. Chamber 1 
serves as a resetvoir of given volume of air at a particular pressure above 1 atmosphere 
(atm) and Chamber 2 holds the object to be measured. Valve VA2 connects to a 
compressed air supply and valve VA3 serves as a relief valve for Chamber 2. 
Initially VAl i s  closed and VA2 i s  open until the air pressure inside Chamber 1 
i s  at the desired value, PSI . Valve VA3 i s  open until the inside air pressure equals 
atmospheric pressure, Pat, and then i s  closed, making sure no increase in  air pressure 
occurs during i t s  cut off. After VA2 i s  closed, valve VAl i s  opened slowly to prevent 
abrupt temperature changes, and small transients are allowed to subside. At this point 
both Chambers 1 and 2 are at the same equalization pressure, PE. Conditions before 
and after the opening of VAl can be mathematically related by an equation 
1 
Y 
0 
where 
Val = volume of Chamber 1 
Vcz = volume of Chamber 2 
If an object whose volume i s  not known, V, i s  put into Chamber 2, then 
conditions before and after opening VAl are related in  the following way: 
Solving equation (3) for Vxr we obtain 
or 
After simplication i t  becomes 
Under the assumption that Vcs and Vel are constants in the system, the accuracy 
in measuring V, clearly depends on the accuracy of measuring Psi I PE, and Pato 
For sma l l  errors i n  measuring $ I PE, and Pat, MWIyr At l  r A P ~ ,  and "Pat, 
the error AVx can be obtained from the following equation: 
3 
Equation (5) shows that, for a small value of AVx, the following conditions are desired: 
1) small value of Vc1 
2) a large value of PE - Pat 
Condition (2), above, simply states that the value of V should not be too much 
greater than the expected value of Vx; ;.e., if V, = 1 0  in", K a  should be about 
1500 ins. 
C? 
INSTRUMENTATION 
Operation. Figure 2 i s  a picture of the volumetric device and Figure 3 shows a 
block diagram of the electronic instrumentation. With an object in Chamber 2, 
operation essentially consists of inserting air into Chamber 1 with VAl closed and 
VA2 Open, to a predetermined pressure, P ' , then closing VA2 and VA3. VAl then 
i s  slowly opened and afterward a readout in pounds per square inch i s  made. Upon 
completion of a reading, VAl i s  closed, VA3 opened, and another measurement 
made or the object i s  removed from Chamber 2. 
Transducers. Two pressure transducers from Consolidated Electrodynamics 
Corporation, Model 5-413, are used. This model i s  a strain-gage, unbonded unit, 
having a range of 0 to 50 psia and requiring an excitation or supply voltage of 
approximately 5.0-Volt direct current. The supply voltage used i s  constant within 
i 5  mV. The output voltage from the unit i s  about 20 mV for a pressure equal to full 
scale value, i .e. , 50 psia. The transducers are accurate within h0.25 psia. 
Both pressure transducers were calibrated to atmospheric pressure obtained from a 
barometer just prior to the experiment and maintained during all measurements. 
D-C Amplifiers. The output voltage from each pressure transducer was amplified 
by a factor of 500 by means of an operational amplifier used in a noninverting mode. 
This i s  an analog device operational amplifier, Model 183 J ,  with a very small voltage 
offset, small drift, and high input impedance 
In order to filter out fluctuations which may be caused by the animal's breathing, 
a capacitor was placed in  the feedback circuit of the operational amplifier. The 
amplifier has provision for zero suppression signals so that differential readings can be 
made; this feature was seldom used, however. 
Output Readout. Each amplifier has i t s  own meter circuit with provisions for the 
following: 
a) Amplifier balance readout scale. 
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b) A calibration scale to calibrate the pressure transducer (zero offset 
The full scale indication i s  equivalent to ambient atmospheric pressure (usu 
psia for this experiment). 
c) Four output scales: 5,10,25, and 50 psia. 
Since the accuracy of needle deflection-type meters i s  only 1 t, a 
digital voltmeter, Model 6653, was tried to improve the accuracy. This demonstrated 
that this latter type of meter should always be used. 
TEST ANIMALS 
Several rhesus monkeys (Macaca mulatta) of different scale weights and sex 
were subjected to volumetric measurements in  Chamber 2. Prior to insertion in the 
Chamber the primates were sedated at the Vivarium with a drug, Inovar-Vet. For 
each measurement, a rhesus was subjected to equalization pressures of not more than 
25 psia for about 5 minutes before returning to 1 atmosphere. A small ventilation fan 
provided air to the rhesus during nonpressurizing periods. Usually three measurements 
were done on each monkey. The veterinarian in attendance at the measuring sessions 
found no signs of aftereffects in  the animals. 
D ISCU SSlON 
- Error. A limited search of the literature revealed a method similar to the one 
reported herein but designed to measure the volume of liquids and using servosystems 
for obtaining the desired measurement (1). Another method used by the Northrop 
Systems Laboratories, but not fully described, showed an accuracy in the order of 
f 18 per cent (2) . 
Ours i s  truly an isothermal method, and careful steps were taken to make sure 
this condition was maintained. There were apparently three sources of error in our 
results: 1) accumcy of pressure transducer, 2) expansion of chamber walls due to 
increase in pressure, and 3) accuracy in  obtaining measurements of the two chambers. 
The first source i s  treated i n  the preceding INSTRUMENTATION section. The other 
two kinds of errors were allowed for in calculating the animal's volume by taking the 
difference between the values of V, of equation 4(c), with and without the animal . 
These errors were detected when i t  was discovered that equation 4(c) did not yield 
a zero value without the animal 
The overall inaccuracy of the system was found to be 0.5 pound. 
Temperature . A mercury thermometer taped to the inside wall of Chamber 2 showed 
no indication of significant variations i n  temperatuk of the chamber during any of the 
measurements . 
7 
Weight. To convert volumetric data to weight, density of the animal (volume/ 
w e i g h t ) t o  be known. To give some idea of the way in  which the isothermal volume 
method could be used to determine an animal's weight, i t  was assumed that the density 
of M. mulatta i s  1 .O. This results in  1000 cc/kg or 27.67 in3/lb. In the two right 
handcblumnsof Table I it can be seen how nearly the weights from the ma 
volumes approximate their mechanical scale weights . Exact density measuremen 
scale weights are planned for more accurate comparison of the two measures. 
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REFERENCES 
1 . Cohn, I . H., Gas laws provide new method of zero-g gaging. Space/ 
Aeronautics, _I 45: 102+, 1966 . 
2. Hesse, 0. M., Orbiting experiment for study of extended weightlessness. 
NSL 67-302-2. Palos Verdes, Calif .: Northrop Systems Labomtories, 
1967. 
10 
UNCLASSIFIED 
Secnritv Classification 
I DOCUMENTCONTROLDATA- R 8 D  
ISOTHERMAL METHOD FOR VOLUME DETERMINATION 
4 DESCRIPTIVE N O T E S  (Type  of report and inclusive da te s )  
5 A u T H O R G I  (First  name, mtddle initial, l a s t  n a p e )  
Efrain Molina and James Knepton 
78.  T O T A L  NO O F  PAGES 7b .  N O  O F  REFS 6 R E P O R T D A T E  
15April 1969 11 2 
8a. C O N T R A C T  OR G R A N T  NO 98. ORIGINATOR’S R E P O R T  NUMBERIS1 
NASA R-10-009-013 
b. PROJECT ~ c )  BuMed N AM! -1 067 
MROO5.01.01-0097B 
C .  96. O T H E R  R E P O R T  NOIS1 (Any other numbers that may be assigned 
this report) 
1 I d. 
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mulatta) were used as test animals. 
L I N K  A 
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LIP 
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Isothermal method for volume determination

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