TH02

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2.8. Protecting the Sensor

Because the sensor operates on the principal of measuring a change in capacitance, any changes to the dielectric

constant of the polymer film will be detected as a change in relative humidity. Therefore, it is important to minimize

the probability of contaminants coming into contact with the sensor. Dust and other particles as well as liquids can

affect the RH reading. It is recommended that a filter cover is employed in the end system that blocks contaminants

but allows water vapor to pass through. Depending on the needs of the application, this can be as simple as plastic

or metallic gauze for basic protection against particulates or something more sophisticated such as a hydrophobic

membrane providing up to IP67 compliant protection.

TH02s may be ordered with a factory fitted, solder-resistant cover, which can be left in place for the lifetime of the

product. It is very low-profile, hydrophobic and oleophobic, and excludes particulates down to 0.35 microns in size.

SOrdering Guide” for a list of ordering part numbers that include the cover. A dimensioned drawing of the IC with

the cover is included in section. The sensor should be protected from direct sunlight to prevent heating

effects as well as possible material degradation.

2.9. Bake/Hydrate Procedure

After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can

become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under

normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after

shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g., 48

hours of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate

recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of the

following steps:

Baking the sensor at 125 °C for ≥ 12 hours

Hydration at 30 °C in 75 %RH for ≥ 10 hours

Following this cycle, the sensor will return to normal operation in typical ambient conditions after a few days.

2.10. Long Term Drift/Aging

Over long periods of time, the sensor readings may drift due to aging of the device. Standard accelerated life

testing of the TH02 has resulted in the specifications for long-term drift. This contribution to the overall sensor

accuracy accounts only for the long-term aging of the device in an otherwise benign operating environment and

does not include the affects of damage, contamination, or exposure to extreme environmental conditions.