Theory Of Operation; Sample Gas Path; Figure 1 Measurement System - Michell Instruments QMA401 User Manual

INTRODUCTION
1.2

Theory of Operation

The Quartz Crystal Microbalance (QCM) technology for moisture measurement is
based on monitoring the frequency of a hygroscopic-coated quartz crystal with specific
sensitivity to water vapor.
Bulk adsorption of water vapor onto the coated crystal causes an increase in the crystals
effective mass, modifying its oscillation frequency in a very precise and repeatable
manner. The frequency change is in proportion to the water vapor pressure in the
sample gas, providing a direct measurement of moisture content.
The sorption process is fully reversible with no long-term drift effect, giving a highly
reliable and repeatable measurement.
1.3

Sample Gas Path

The QMA401 measurement system must be supplied with gas at the required pressure
(to match that of its calibration) via the VCR connection on the rear of the instrument.
The flow is controlled automatically.
The sensor cell is located at the end of the sensor block and contains the sensor and
reference oscillators. Figure 1 shows a schematic diagram of this sampling system:
DD
MFC
QCH
PS
2
DD
HE1
MG
V1
FC1
QCH
Desiccant column
Mass flow controller
Sensor cell
Pressure sensor
Figure 1
QMA401 User's Manual
Sample In
HE2
V2 V3
FC2 FC3
MFC
PS
Key
Moisture generator
MG
Solenoid Valves
V1, V2, V3
Heat exchanger
HE1/HE2
Flow control
FC1
Measurement System
97450 Issue 2, March 2018
Sample Out