A thermogage (also called Gardon gage) heat flux sensor produces a voltage output when exposed to heat flux. The heat flux transducer is a differential thermocouple that measures the temperature difference between the centre and the circumference of a thin constantan foil disk mounted into the Oxygen Free High Conductivity copper (OFHC) body of the heat flux gage. The reference thermocouple is the junction between the sensing foil and the copper water cooling body. A differential thermocouple arrangement is created by a second thermocouple being created by the connection of a fine copper wire to the centre of the constantan sensing disk. When the sensing disk has a heat flux source applied to it, the heat energy absorbed by the foil is conducted down to the copper heat sink (the energy loss conducted down the fine copper wire attached to the centre of the sensing disk being considered negliagible). The centre to edge temperature difference produces an output voltage directly proportional to the applied heat flux.
When using high quality materials, the differential thermocouple produces a linear response over the temperature range of -40 to +200 Celsius as errors due to thermal conductivity changes with temperature are essentially concelled by the Seebeck coefficient of the Constantan. No cold junction compensation is required with the heat flux measurement system as the thermocouple formed between the sensing disk and the copper body is the reference for the thermocouple at the centre of the sensing disk.