Editorial Feature

# A Heat Flux Sensor

Article updated on 23 March 2020.

Heat flux sensors are a type of transducer (devices that convert one form of energy to another), converting heat energy into an electrical signal. The intensity of the electrical signal is directly proportional to the heat rate being converted. During exposure of the heat flux sensor to heat energy, the sensor generates an electrical input that allows for the heat flux to be determined.

## How does it work?

A working description of a basic heat flux sensor (also known as a heat flux gauge) is based on the attachment of wires in a thermocouple device to the hot and cold surfaces of an object (Figure 1). The standard measurement of heat flux density is presented as watts per square meter (W/m2).

Figure 1. A basic heat flux sensor.

The principle of this type of heat flux sensor (known as a thermopile) is based on the transfer of heat through a thin film. This thin film is sandwiched between the two thermocouple junctions that both have contact with either the hot or cold surface of an object. As heat passes through the thermocouple junction and penetrates the thin film, the movement forces the thermocouple to initiate a voltage output that is proportional to the amount of heat penetrating the film. Many thermocouple junctions can be grouped together to help amplify the voltage output signal in response to heat energy.

## Calibration

Heat flux sensors need to be calibrated for emissivity to minimize errors. To calculate the emissivity of a surface (how effectively it can emit thermal energy), the voltage output and the temperature of the shell is required. To obtain an accurate reading of heat flux, the voltage output is multiplied by a given calibration constant and, given the temperature of a substrate, you can measure the heat flux in W/m2. Figure 2 illustrates the calculation of emissivity.

Figure 2. Calculation of emissivity.

It is important that there is no voltage when calibrating the heat flux sensor. A common process during calibration involves mounting these sensors to the top and bottom of a heater. The heat power has to be adjusted to zero voltage. There is more control over accurately measuring the output voltage to a heater, and so by using this principle, it is much easier to get an accurate reading of heat flux from the sensor.

## Applications

The most common use for heat flux sensors is in building physics. For example, the use of a heat flux sensor in this discipline will provide a heat flux profile in isolated areas within one building which can be used to estimate the thermal performance of a particular structure. In construction, the application of a heat flux sensor to the warm side of a drying wall provides a measurement of heat flux density as a result of heat energy, moisture content and latent heat flux density.

In building physics, modern-day heat flux measurements involve a bulk auxiliary plate covered with temperature transducers which are then attached to a wall. The wall is penetrated by a heat flux which initiates a temperature difference traveling across the auxiliary plate.

Heat flux sensors are also typically used to detect a spectral range from ultraviolet to infrared to measure the type of radiation. Compared to a basic heat flux sensor as described above, the radiation sensor has a coating above the heat sink that absorbs radiation. Any heat penetrating the heat sensor is transferred to the heat sink, which creates a temperature difference as this heat contacts the hot and cold joints to the coating, making the measurement of heat flux possible. By placing a filter between the coating and radiative source, it is possible to detect a spectral range (figure 3).

Figure 3. Heat flux sensor used to detect radiation.

A pyranometer is a popular sensor used to measure radiative heat flux density in the field of meteorology, climatology and solar energy investigations. This type of sensor has a circular multi-junction thermopile with a series of hot and cold junctions.  A receiver at the edge of a thermocouple is sensitive to radiation with a wavelength of around 300 nm. The output voltage generated by the thermocouples from a pyranometer will be proportional to the irradiation energy absorbed. Radiation data from the pyranometer is usually calculated using planimetry or electronic integrators.

It is important to remember that calibration of any heat flux sensor is fundamental to obtaining the most accurate voltage output reading as a reflection of heat energy. Variables such as temperature and airflow affect the voltage output from a heat flux sensor. The calibration of such sensors can also be affected by altering the boundaries under which the sensor is set to perform. When measuring radiation, it is important to remember that if the sensor is not completely isolated from the substrate (i.e., surface, material thermal properties), the final output can be erroneous. Also, calibrating a cold sensor on a cold surface is not going to affect output voltage measures; however, performing a calibration by using a cold sensor on a warm surface will create a temperature difference and alter baseline measurements. This is why calibration is required to generate an accurate reading of heat flux.

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