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Moisture Sensor
Quartz Crystal Microbalance (QCM)
The 5920 Moisture Analyzer uses a quartz crystal microbalance sensor to mea-
sure water vapor concentration. This is a piezoelectric device that is capable of
extremely sensitive mass measurement. To make the QCM sensitive to water
vapor the surface is coated with a thin film of hygroscopic material. When gas
flows through the sensor, the coating absorbs or desorbs moisture, changing
the natural resonance frequency of the crystal.
Gas Measurement Cycle
The gas measurement cycle is divided into two periods.
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1-4 | 5920 UHP Moisture Analyzer
During the first period of the cycle, Reference gas flows through the sensor
assembly and the frequency is recorded (Figure 1-2).
During the second period of the cycle (Figure 1-3) , sample gas flows
through the sensor and a second measurement of the sensor frequency is
recorded.
The difference between these two frequency measurements is known as
the delta frequency (∆ ƒ). The ∆ ƒ value is a function of the moisture con-
centration in the sample gas.
PN 592047901 Rev YC
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