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Moisture Sensor
Quartz Crystal Microbalance (QCM)
The 3050-AMS analyzer uses a quartz crystal microbalance sensor to
measure water vapor concentration. This is a piezoelectric device that is
capable of extremely sensitive mass measurement. To make the QCM sen-
sitive to water vapor the surface is coated with a thin film of hygroscopic
material. When gas flows through the sensor, the coating absorbs or de-
sorbs moisture, changing the natural resonance frequency of the crystal.
Gas measurement cycle
The gas measurement cycle is divided into two periods.
During the first period of the cycle, dry reference gas flows through
the sensor assembly and the frequency is recorded.
During the second period of the cycle, 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
concentration in the sample gas.
Gas Flow
Sample system
The 3050-AMS Moisture Analyzer sample system enables the analyzer to
make reliable water vapor measurements at concentrations down to 0.1
PPMV. The sample system serves several purposes.
1.
It controls the sample conditions at the sensor assembly (e.g., flow
rate, temperature, pressure). Hence, the QCM-based sensor is isolated
from variations in the ambient conditions that would otherwise alter
the response characteristics of the sensor.
2.
The sample system is designed to continuously switch the sensor as-
sembly between the sample gas and a dry reference gas. This sample
switching scheme yields both higher sensitivity and reduced drift in
the sensor readings.
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Overview
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