APPENDIX
C
Theory of Operation
THERMOCOUPLE TEMPERATURE VERSUS OSMOLALITY
The graph on the left is a plot of thermocouple temperature versus
time as the instrument cycles through the program, beginning with
chamber closure (time = 0). The graph depicts the excursion of
thermocouple temperature that typically occurs during each of the
program steps outlined above. T
A
is the ambient temperature in the
chamber. T
D
is the dew point temperature, and ∆T is the dew point
temperature depression. The output is proportional to ∆T.
Assuming that the chamber remains closed while the osmometer
displays the final reading at Step 4, the thermocouple temperature
returns to T
A
after holding at the wet bulb depression temperature
until all of the water has evaporated from the thermocouple. If the
chamber is opened, the water will evaporate almost instantly and
the thermocouple temperature will quickly return to ambient.
The relationship between sample osmolality and the reading
obtained by the osmometer is governed by fundamental considera-
tions. Vapor pressure depression, a linear function of osmolality, has
been identified as one of the colligative properties of a solution. The
relationship between vapor pressure depression and dew point tem-
perature depression is given by
∆T = ∆e/S
where ∆T is the dew point temperature depression in degrees
Celsius, ∆e is the difference between saturation and chamber vapor
pressure and S is the slope of the vapor pressure temperature func-
tion at ambient temperature. The Claussius-Clapeyron equation
gives S as a function of temperature (T), saturation vapor pressure
(e
o
,) and latent heat of vaporization (λ):
e
o
λ
S=
RT
2
where R is the universal gas constant.
The dew point temperature depression, ∆T, is measured as a volt-
age signal from the thermocouple. This voltage is equal to ∆T mul-
tiplied by the thermocouple responsivity which is approximately 62
microvolts per degree Celsius. After voltage amplification by a pre-
amplifier, the signal is processed by the microprocessor to provide
calibrate and compensate functions and display the reading.
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