09.02.2022How does a Temperature controlled Quartz Crystal Microbalance (TQCM) work?
Modern crystals are typically small in size and encased in a protective package to preserve their long-term stability. In contrast the crystals used in a TQCM sensor are significantly larger in size to increase collecting area and are directly exposed to the environment to monitor the bake out.
The fundamental resonant frequency of the quartz crystal is dependent on how it is manufactured, with frequencies between 30 kHz and 100 MHz. When exposed to environmental conditions the actual resonant frequency will be affected by changes in pressure, temperature, and mass.
How does a TQCM work?
Since a TQCM sensor is designed to detect changes in mass deposited onto the crystal, the effects of pressure and temperature need to be removed so an accurate measurement of the changes due to deposited mass can be taken. This poses a difficult problem if only a single crystal is used inside the thermal vacuum chamber as the change in resonant frequency due to the vacuum and elevated temperature would make the change due to deposited mass impossible to isolate.
The solution to this is to introduce a second matched reference quartz crystal that operates identically to the first, however is placed in an enclosure within the vacuum chamber that allows the crystal to operate at the same vacuum and temperature conditions as the first but is shielded from any deposited mass. Through analysis of the frequency response of the two crystals it is possible to isolate the effect of deposited mass from any environmental factors.
Fundamental to the monitoring of the bakeout process is the knowledge of when the rate of outgassing of volatile molecules has reduced to an acceptably low level. This can be determined directly by observing the rate of change of frequency over a period of time.
To aid in the volatile molecules depositing onto the exposed crystals surface, the TQCM sensor is cooled to -20°C. This quickly removes kinetic energy from the molecules when they contact the crystals surface, improving the likelihood that they ‘stick’ to the surface and add to the crystals mass.
What is a TQCM?
AEON have a separate article for 'What is Thermal controlled Quartz Crystal Microbalance?' this article is particularly helpful with gathering some general understanding of what a TQCM is should you wish to read it.
AEON’s TQCM sensors have been designed from fundamentals to ensure measurement accuracy and reliability. Having a modular design also means we have the flexibility to position and orient the TQCM sensor within the vacuum chamber. Therefore, if required a TQCM sensor could be placed near a critical location on the equipment being baked out. Alternatively, a novel solution using multiple TQCM sensors could be implemented to locally monitor outgassing at multiple locations along a piece of equipment.
This information can then be fed back into the thermal vacuum chamber control system to increase or decrease heating in a particular region to ensure uniform and complete bakeout of the equipment to a level previously not possible.
None of this would be possible without AEON's in-house one of a kind facility which has allowed AEON to design, build and commission this thermal vacuum chamber dedicated to performing the bakeout process for flight hardware to ECSS standards.
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For further details on AEON’s bakeout facility or TQCM systems you can discover more in our brochure.
AEON supply TQCM's for thermal bakeout chambers, enquire here about purchasing your own.