What is Heat Flux? What are Heat Flux Sensors?


Heat flux is the flow of thermal energy per unit area. It can be measured in units of Watts per square meter (W/m^2) or in the English unit equivalent of BTU per square foot-hour (BTU/(ft^2-hr)). Heat flux sensors are instruments that can be mounted on or into a material in order to directly measure the amount of thermal energy that is flowing through it due to all three modes of heat transfer; conduction, convection & radiation.




What is the difference between Heat Flux and Temperature?


Heat flux is the amount of thermal energy that is moving through a material while temperature is more of a measure of the amount of thermal energy contained at a certain point.




How does a Heat Flux Sensor Work?


Most heat flux sensors that are available on the market are differential-temperature thermopiles. They operate by creating a relatively small temperature difference across a thermal resistance layer (TRL) which is the core material of the heat flux sensor. Using Fourier's law of heat conduction with a one-dimensional heat transfer assumption under steady-state conditions, the temperature difference across the TRL is proportional to the heat flux through the sensor shown in the equation below and diagram to the right. Where q" is the heat flux, k is the thermal conductivity of the sensors, d is the thickness of the sensor, and T is the temperatures on the top and bottom surfaces of the sensor. Thermopile based heat flux sensors utilize thermoelectric elements that form thermocouple junctions on either side of the thermal resistance layer. The thermocouple junction pairs induce an analog DC voltage that is actually directly proportional to the temperature difference. This is much easier than accurately measuring each surface temperature individually. Multiple thermocouple junction pairs are placed in electrical series so that the output analog DC voltage from the heat flux sensor is amplified to increase overall sensitivity of the sensor. Calibrating the heat flux sensors can then relate this output voltage signal directly to the heat flux that is flowing through the sensor using a sensor sensitivity value that is provided with every heat flux sensor from FluxTeq.




Why is measuring Heat Flux important compared to Temperature?


Heat flux is a valuable measurement to make in a wide variety of applications. While temperature typically can only tell you about the current state of a thermal system, heat flux can be used to predict the future behavior of a system.




What is the Temperature and Heat Flux Range of FluxTeq's Heat Flux Sensors?


FluxTeq's PHFS heat flux sensor product line is specified to endure temperatures within the range of -50 to 120 degrees Celsius (-58 to 248 degrees Fahrenheit). Heat flux range for the PHFS sensors is approximately +/- 150 kW/m^2 or +/- 47,500 BTU/(ft^2-hr). FluxTeq's specialty HTHFS-01 heat flux sensor was specifically designed to withstand extreme temperatures up to 1000 degrees Celsius (1832 degrees Fahrenheit). Heat flux range for the HTHFS-01 sensor is approximately +/- 1000 kW/m^2 or +/- 317,000 BTU/(ft^2-hr).




Is anything (besides the sensor itself) required to take Heat Flux measurements?


Heat flux sensors, similar to thermocouples, output an analog DC voltage signal that has been calibrated to directly correlate to the amount of heat flux that is flowing through the sensor. Each sensor is provided with a sensitivity constant value that is used to make this conversion calculation. In order to measure heat flux you must​ be able to measure the voltage signal that is outputted from the sensor. Typically these voltages can be relatively small (in the microVolt range) so it is necessary to have some sort of precision voltmeter or data acquisition device that can resolve these signals. FluxTeq recommends using a device that is capable of resolving around 1 microVolt in order to get accurate measurements from the heat flux sensors. FluxTeq sells data acquisition devices, the FluxDAQ and FluxDAQ+, that are fully capable of accurately measuring signals from multiple heat flux sensors and thermocouples.




Can I order Heat Flux Sensors in custom sizes or shapes?


Yes! FluxTeq has the unique capability of manufacturing custom sized heat flux sensors for your application. Such custom work does require initial tooling costs and longer lead times but we are willing to work with you to design the best sensor size/layout for your testing. Please inquire about custom jobs at info@fluxteq.com or call us at +1-540-257-3735.




What is the Lead Time for Heat Flux Sensor and FluxDAQ Measurement Systems?


FluxTeq prides itself on very short leads times compared to competitor heat flux sensor companies. Typically, small orders for our PHFS heat flux sensors, FluxDAQ and FluxDAQ+ measurement systems only have lead time of 1 - 2 business days before they are shipped out to you. Larger orders may have slightly longer lead times depending on the quantity. Orders for FluxTeq's specialty HTHFS-01 high temperature heat flux sensors typically have lead times of 2 - 4 weeks but can also depend on the order quantity. Inquire about lead times at info@fluxteq.com to find out exactly what they would be for your order.




What's the advantage to FluxTeq's Heat Flux Sensors over its competitors?


There are a number of advantages to FluxTeq's sensors over other heat flux sensor manufacturer's products. Not only is the PHFS product line offered at a fraction of the retail price but the PHFS sensors maintains similar or better performance characteristics of high sensitivity, small thermal resistance and overall durability of the sensor. The PHFS-01 heat flux sensor model is also thin and flexible which is ideal for mounting to curved surfaces. Also, the HTHFS-01 high temperature heat flux sensor pushes the limits by being able to withstand the highest temperatures of any commercially available heat flux sensor at a maximum of approximately 1000 degrees Celsius (1832 degrees Fahrenheit).




What is the difference between the PHFS-01, PHFS-01e, PHFS-09e and HTHFS-01 Heat Flux Sensors?


Each of the different sensor models has its own set of characteristics that were designed for different applications. The table below compares some of their attributes and current applications.




Are there instructions for the Heat Flux Sensors?


Yes, below are links to PDF documents that explain how to use PHFS heat flux sensors. The first document explains general use of the sensors while the second document details how to use the sensors to measure R-value of a material. General PHFS Heat Flux Sensor Instruction Manual: http://docs.wixstatic.com/ugd/b0a63e_6df4f2d55dde448ca21df5e32063f9ad.pdf Instruction Manual for Using PHFS Heat Flux Sensors to Measure R-Value: http://docs.wixstatic.com/ugd/b0a63e_92e6cf4572124f68ae9a89541b75a4a1.pdf





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