7.2.2 Design of a leak detector with a quartz window detector
While mass spectrometric detectors separate a gas mix by ionization followed by separation in a magnetic or electrical field, quartz window detectors make use of the different permeation properties of gases.
Figure 7.4: Operating principle of quartz window sensor
The tracer gas mix is conveyed to the quartz surface of a heated diaphragm. The carrier layer for the quartz diaphragm consists of a silicon wafer with several thousand holes through which all incoming gas atoms and molecules can reach the quartz diaphragm. The separation itself takes place at the quartz diaphragm which allows helium, but not other gases, to pass through it. The thickness and temperature of the diaphragm are influencing factors for the permeation of the helium test gas. After the gases have passed through the diaphragm, the tracer gas that has entered is ionized and the ion current is a measure of the leak rate.
Figure 7.5: Vacuum diagram of the MiniTest quartz window leak detector on a system
The unit is connected to the system to be tested (6) with a vacuum connection flange (1). The connection (3) can optionally be connected to an additional vacuum pump. To prepare the leak test, this pump can evacuate the vacuum system of the unit while the shut-off valve (9) is still closed.
The shut-off valve (9) is opened for the test. The optional pump can generate a gas flow which reduces the response time of the sensor at higher system pressures.
The sensor (2) measures the partial pressure of helium in the vacuum. A test leak (7) on the system is used to determine the response time and calibrate the unit.
To protect the sensor and to purge the unit after a strong signal, automatic purging can be carried out. The valve (4) opens the inlet with the throttle (5) and the sensor is purged briefly with atmospheric air.