INFICON Crystal 12 Sensor - Manual (Page 45)

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Crystal12 Sensor Operating Manual
Chapter 4
Maintenance
4.1 General Precautions
4.1.1 Maintain the Temperature of the Crystal
Periodically measure the water flow rate through the crystal sensor to verify that it
meets or exceeds the value specified in chapter one. Depending upon the
condition of the cooling water used, the addition of an inline water filtering cartridge
system may be necessary to prevent flow obstructions. Many coating systems use
parallel water supply taps that provide high total flows. An obstruction or closed
valve in the pipe that supplies water to the sensor head would not result in a
noticeable reduction of total flow. The best test is to directly monitor the flow leaving
the sensor.
The crystal requires sufficient water cooling to sustain proper operational and
temperature stability. Ideally, a constant heat load is balanced by a constant flow of
water at a constant temperature. INFICON quartz crystals are designed to provide
the best possible stability under normal operating conditions. No crystal can
completely eliminate the effects of varying heat loads. Sources of heat variation
include radiated energy emanating from the evaporant source and from substrate
heaters.
4.1.2 Use the Optimum Crystal Type
Certain materials, especially dielectrics, may not adhere strongly to the crystal
surface and may cause erratic readings. For many dielectrics, adhesion is
improved by using crystals with silver coated electrodes. Gold is preferred for other
applications.
4.1.3 Crystal Concerns when Opening the Chamber
Thick deposits of some materials, such as SiO, Si and Ni will normally peel off the
crystal when it is exposed to air, due to changes in film stress caused by gas
absorption. When peeling material is observed, replace the crystal.