Omega CMDP Series, Руководство пользователя

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Chilled mirror hygrometers (CMH) use a primary fundamental principal measurement which is a change in resistance.
This principle works when a sample of process gas flows across a special polished coated mirror. The mirror
temperature is lowered by thermal electric coolers (located beneath the mirror block/mirror) until the water vapor of gas
condenses out on to the mirror in the form of either dew or frost. There is an optical circuit which constantly monitors
the mirror surface and looks for equilibrium between the optical LED detectors (a reference and the dew detector). The
temperature of the mirror surface is measure by a highly accurate Platinum Resistance Thermometer (PRT) which is
embedded and sealed in the mirror block which is thermally connected to the mirror. The PRT resistance measurement is
what makes this a fundamental dew point measurement.
In more detail, a fundamental measurement is such that the parameter being measured, is the same as the
Standard International definition of that parameter. In the case of the Chilled Mirror Hygrometer, that measurement is
the temperature of the mirror surface covered by a water layer that is in equilibrium between the liquid and gaseous
phases of water. If that temperature measurement is accomplished by a platinum resistance thermometer, the
measurement is equivalent to the definition of dew point. Tables constructed by NIST, notably by Wexler and
Greenspan, establish the equivalence of dew point and the vapor pressure of water. Using this equivalence and a second,
fundamental, temperature measurement, realizes the definition of Relative humidity: Vapor pressure of water/saturation
vapor pressure (temperature) of the gas being measured, X100(%) i.e. %RH =100*P
H2Ov
/P
SAT
. Addition of a 3
rd
measurement, the pressure of the gas being sampled, allows measurement of almost every other humidity parameter.
Some further unit examples: absolute Humidity (g
H2Ov
/m
3
), mixing ratio (g
H2Ov
/kg
DRY
), Parts per million (P
H2Ov
/(P
Tot
-
P
H20
)), enthalpy(h), to name a few.
Chilled mirrors hygrometers are used where accuracy, typ-0.2°C or better, repeatability, stability and no
drift are paramount. CMH are the preferred reference standard in most calibration labs. Also, CMH work well due to
their ruggedness, their inertness, their simple maintenance and ability to accurately measure sample gasses remotely
from process, i.e. ovens, dryers, furnaces, etc. Dew points above ambient can be measured by heating the chilled mirror
and associated components. Chilled mirrors can be, and are, constructed as an insertion probe, provided the temperature
of the process is < ~95 °C.
In closing, the upside of the chilled mirror hygrometer is that it uses a fundamental principal, it can be designed to be
inert to many corrosive gases, it is a repeatable in its measurement, there is no measurable hysteresis, it is very accurate
and has a very long operating life. The downside is that it is expensive and is typically slower in response time when
compared to other secondary principal humidity measurements.