AN607

14

Rev. 1.7

4.7. Hysteresis

The moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure

history, particularly its recent or extreme exposure history. A sensor exposed to relatively low humidity will carry a

negative offset relative to the factory calibration, and a sensor exposed to relatively high humidity will carry a

positive offset relative to the factory calibration. This factor causes a hysteresis effect illustrated by the solid trace

in Figure 10. The hysteresis value is the difference in %RH between the maximum absolute error on the

decreasing humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative

humidity setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). In the

example of Figure 10, the measurement uncertainty due to the hysteresis effect is ±1.0%RH.

4.8. Prolonged Exposure to High Humidity

Prolonged exposure to high humidity will result in a gradual upward drift of the RH reading. The shift in sensor

reading resulting from this drift will generally disappear slowly under normal ambient conditions. The amount of

shift is proportional to the magnitude of relative humidity and the length of exposure. In the case of lengthy

exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. It is generally

possible to substantially reverse this effect by baking the device as described in the following section.

4.9. 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:

≥ 12 hours

≥ 10 hours

Following this cycle, the sensor will return to normal operation in typical ambient conditions after 48 hours.