AN607

6

Rev. 1.7

3.2. Temperature and Humidity Sensor Placement

The following are general guidelines for sensor placement:

0.2 °C, or compensate for heating.

and insulate the sensor from larger system thermal masses.

comparable in size to the cavity in which the sensor resides.

can absorb or emit moisture as well as give off fumes that affect the senor reading.

use the ePTFE filter cover to prevent ESD directly into the sensor area. Use conformal coating material on

the leads, or use ESD diodes on all pins. Unused pins may be connected to VDD for ESD protection.

For situations in which it is not possible to completely insulate the sensor for the system, insulate as much as

possible and use a temperature sensor connected to the system to allow compensation of residual heating. If a

thermal model can be developed for the overall system, it is also possible to apply an inverse filter and speed up

the response of the overall system to changes in the ambient. These are discussed in more detail below.

3.2.1. Place the Sensor Away from Heat Sources

As air is heated or cooled, humidity will be reduced or increased by approximately 5% per degree. That is,

increasing temperature by 1 °C will reduce the relative humidity of 100% humid air to 95%RH or 50% humid air to

47.5% RH. This “rule of thumb” is useful for estimating the effect of small temperature increases.

The first consideration in trying to measure humidity or temperature outside of an enclosure is to place the sensor

away from any heat sources internal to the enclosure or to thermally insulate the sensor from the internal heat

source so it is better connected to the ambient environment than internal heat sources. For accurate humidity

measurement, heating from other sources should be limited to no more than 0.2 °C.

In some cases, where the amount of heating is known or can be measured, it is possible to compensate for the

heating or cooling. For larger temperature increases, the “Magnus” equation can be used to more accurately

calculate the change of humidity for a change in temperature (see also appendix B).

3.2.2. Thermal Mass and Thermal Resistance

When the IC is soldered down, it becomes thermally connected to the printed circuit board on which it is mounted.

The printed circuit board is, in turn, thermally connected to the system it is mounted within. The time constant to

respond to changes in ambient air temperature depends on the effective thermal mass the device is connected to

as well as the effective thermal resistance.

If the entire system (such as a thermostat) can be expected to go up or down in temperature along with the ambient

it is measuring, then separating the sensor from the system is not necessary, but response time will generally be

slow. A general model for this is shown in Figure 7.