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OVERVIEW
Theory of Operation
The Model 2850 Moisture Analyzer (Figure 1) measures low level moisture concentrations in gases
such as oxygen, hydrogen, helium, argon, and nitrogen. Sample moisture data is normally output in
the form of concentration in parts per million volume (ppmv). The software can also be configured to
output in pounds H
O per million standard cubic feet (lbs/MMscf), dew point temperature (°C or °F), or
2
water vapor pressure (psia) of the sample.
Calibrated range is 1 to 100 ppmv; full operational range is 0.1 to 1000 ppmv. The analyzer provides a
trend indication for measured concentrations above or below the calibrated range. If concentration is
below 0.1 ppmv, display will read < 0.1 ppm. An external dryer capable of drying the reference gas to
<10 ppb is required for optimum performance.
The analyzer compares the oscillation frequencies of two crystal oscillators: an oscillator with a constant
frequency (nominal ƒ = 9.0 MHz) and a measuring oscillator (nominal ƒ = 8.98 MHz). The difference
between the constant and measurement oscillation frequencies is known as the cell frequency (ƒ
The
cell).
measuring oscillator contains a crystal which is coated with a thin film of hygroscopic material and en-
closed in a measuring cell. When gas flows through the cell, the crystal coating sorbs or desorbs mois-
ture, thereby changing the natural oscillation frequency of the measuring crystal.
Internal gas flow diagram for the Model 2850 is shown in Figure 2. Concentration measurement cycle
is typically 60 seconds, divided into two 30 second periods. During the first period of the cycle, sample
gas flows through the measuring cell. At the end of the sample period, flow stops momentarily while
Figure 1: AMETEK Model 2850 Moisture Analyzer
Overview 1-1
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