APPENDIX
C
Theory of Operation
The Vapro osmometer embodies newer technology. It is based upon
a measurement of vapor pressure depression made possible by
thermocouple hygrometry. The vapor pressure method enjoys a sig-
nificant intrinsic advantage over the measurement of either freezing
point depression or boiling point elevation in that it can be per-
formed without the necessity for a change in the physical state of
the specimen. It is thus a passive technique of measurement that is
free from measurement artifacts that often occur when the speci-
men to be tested must be altered physically. This fundamental dif-
ference in methodology gives rise to the many advantages of the
vapor pressure osmometer over the older method.
In the Vapro vapor pressure osmometer, a 10 microliter sample of
the solution to be tested is pipetted onto a small, solute-free paper
disc which is then inserted into a sample chamber and sealed. A
thermocouple hygrometer is incorporated integrally within the cham-
ber. This sensitive temperature sensor operates on the basis of a
unique thermal energy balancing principle to measure the dew point
temperature depression within the chamber. This parameter, in itself
a colligative property of the solution, is an explicit function of solu-
tion vapor pressure.
PROGRAM STEP 1, EQUILIBRATION AND ZERO SET
The sample is introduced into the chamber and the chamber is
closed. Simultaneously, “In Process” and a countdown by seconds
is displayed. (This remains until the end of sequence at Program
Step 4.)
At this point, there will generally be some difference between the
temperature of the specimen and the temperature of the sample
chamber. Temperature equilibrium occurs within a few seconds. The
vapor pressure may also reach equilibrium during this interval. The
microvoltmeter reads the amplifier voltage to establish the reference
for the measurement.
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