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Calibration of thin heat flux transducers

In order to do in-situ measurements, the user must be provided with the correct calibration constant $E_{sen}$. This constant is also called sensitivity. The sensitivity is primarily determined by the sensor construction and operation temperatures, but also by the geometry and material properties of the object that is measured. Therefore the sensor should be calibrated under conditions that are close to the conditions of the intended application. The calibration set-up should also be be properly shielded to limit external influences.

Preparation

To do a calibration measurement, one needs a voltmeter or datalogger with resolution of ±2μV or better. One should avoid air gaps between layers in the test stack. These can be filled with filling materials, like toothpaste, caulk or putty. If need be, thermally conductive gel can be used to improve contact between layers. A temperature sensor should be placed on or near the sensor, and connected to a readout device.

Measuring

The calibration is done by applying a controlled heat flux through the sensor. By varying the hot and cold sides of the stack, and measuring the voltages of the heat flux sensor and temperature sensor, the correct sensitivity can be determined with:

$E_{sen}={\frac {V_{sen}}{\phi _{q}}}$

where $V_{sen}$ is the sensor output and $\phi _{q}$ is the known heat flux through the sensor.

If the sensor is mounted onto a surface and is exposed to convection and radiation during the expected applications, the same conditions should be taken into account during calibration.

Doing measurements at different temperatures allows for determining sensitivity as a function of the temperature.

In-Situ calibration

FHF02SC, a thin self-calibrating heat flux sensor. Sensors that are embedded in construction can sometimes be very troublesome to remove if need to be re-calibrated (in a lab). Some sensors incorporate heaters in order to be able to leave the sensor in place while preforming a re-calibration.

While heat flux sensors are typically supplied with a sensitivity by the manufacturer, there are times and situations that call for a re-calibration of the sensor. Especially in building walls or envelopes the heat flux sensors can not be removed after the initial installation or may be very difficult te reach. In order to calibrate the sensor, some come with an integrated heater with specified characteristics. By applying a known voltage on and current through the heater, a controlled heat flux is provided which can be used to calculate the new sensitivity.