Heat flux sensors works to convert 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. To understand the working principle to a heat flux sensor, lets first look at this transducer in more detail.
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 thermocouple junctions aligned between an object making sure that these junctions are in contact with the hot and cold surface to this object (Figure 1). A standard measurement of heat flux density is presented as watts per square meter (W/m2).
Figure 1. A basic heat flux sensor.
The principle to this thermopile concept 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 to 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.
Heat flux sensors need to be calibrated for emissivity to minimize error. To calculate the emissivity of a surface, 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 depicts calculation of emissivity.
Figure 2. Calculation of emissivity.
It is important that there is a zero 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.
The most common use for heat flux sensors is in building physics. For example, use of a heat flux sensor in this discipline will provide a heat flux profile in isolated areas within one building which can estimate the thermal performance of a particular structure. In construction, application of a heat flux sensor to the warm side of a drying wall provides a measure of heat flux density as 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 is then attached to a wall. This wall is penetrated by a heat flux which initiates a temperature difference travelling 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 earlier in this article, 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 and so a measurement of heat flux is 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 a 300nm. 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 obtaining the most accurate voltage output reading as a reflection of heat energy. Variables such as temperature and air flow will affect the voltage output from a heat flux sensor. 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 fundamental 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 by 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, which is why calibration is required to generate an accurate reading of heat flux.
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