Si7015-A10
14 Rev. 1.0
4.3. Temperature Compensation
The Si7015 relative humidity sensor is calibrated at a temperature of 30 °C; it is at this temperature that the sensor
will give the most accurate relative humidity readings. For relative humidity measurements at other temperatures,
the RH reading from the Si7015 must be compensated for the change in temperature relative to 30 °C.
Temperature-compensated relative humidity readings can be calculated as follows:
Where:
RH
TempCompensated
is the temperature compensated relative humidity value in %RH.
RH
Linear
is the linear corrected relative humidity value in %RH.
Temperature is the ambient temperature in °C as measured by the Si7015 on chip temperature sensor.
Q
1
and Q
0
are unit-less correction coefficients derived through characterization of Si7015s by Silicon
Laboratories.
This temperature compensation is most accurate in the range of 15–50 °C. The values for the correction
coefficients are shown in Table 9.
4.4. 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 top
and bottom traces in Figure 6. 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, the center dashed trace in
Figure 6. In the case of Figure 6, the measurement uncertainty due to the hysteresis effect is ±1.05%RH.
4.5. 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 affect by baking the device (see Section “4.8. Bake/Hydrate Procedure”).
Table 9. Linearization Coefficients
Coefficient Value
Q
0
0.060162
Q
1
0.000508
RH
TempCompensated
RH
Linear
Temperature 30–RH
Linear
Q
1
Q
0
++=
