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Thin film, ultra fast, very high temperature heat flux sensor
The HFM series sensors are the fastest heat flux sensors available today. They measures two thermal variables at its front surface simultaneously- the rate of thermal energy flow per unit area (heat flux) and temperature. The polarity of the heat flux signal indicates the direction of heat flow; its magnitude is proportional to heat flux. Surface temperature is indicated by a thermocouple in HFM-8
The standard coating for the face of the HFM sensor is Krylon 1602 Ultra-Flat Black, a high temperature black coating with a 0.97 emissivity at 5 microns. The coating can effect the sensitivity and time response of the HFM sensors. It is recommended that applications involving radiative heat transfer use a coated HFM because the absorption properties are better. Uncoated HFM's used in radiative applications have a lower sensitivity than the information contained in the above table. The time constant is 300 microseconds for coated HFM's. Uncoated sensors can be used for applications involving convection and conduction without effecting the sensitivity of the instrument. The uncoated sensors have a 17 microsecond time constant.
Each sensor comes with a NIST traceable calibration certificate that provides all of the information necessary to convert the signals from the sensor to their heat flux and temperature values.
All the HFM series heat flux sensors have a maximum temperature above which damage is likely to occur to the sensor. In order to keep the sensor within the allowable operating temperature range, it is possible to water cool the sensor. This can be achieved in one of two ways:
It is not recommended to connect water cooling circuits in series because of the pressure drop.
The heat flux and temperature sensors of the HFM sensors are thin films deposited by proprietary techniques onto a substrate. The heat flux sensor is a differential thermopile, deposited as a precisely registered composite pattern of three materials. When heat flows into or out of the substrate surface, a small temperature difference develops across the thermal resistance elements of the thermopile, and each thermocouple pair produces a voltage proportional to the heat flux. The total voltage across the thermopile is the sum of these voltages, and indicates the direction and magnitude of heat flux. The temperature sensor of the HFM-6's and HFM-7's is a platinum resistor which surrounds the heat flux sensor. Its resistance value changes with the substrate surface temperature. Temperature is usually indicated by passing a small constant current through the resistor and measuring the resulting voltage. The HFM-8's use a Type E thermocouple for temperature measurements on the face of the sensor. Substrate surface temperature may be used to correct the output signal of the heat flux sensor for variation in conductivity of the thermal resistance element as a function of temperature. It may also be used to detect changes in the calibration of the heat flux element and measure the heat transfer coefficient in some applications. The total thickness of the thin films which make up the HFM is less than 2 microns. As a result the response time of the HFM is fastest in the industry. In order to reap the full advantage of the thin film sensors' small temperature drop, the HFM substrate is designed to have the highest possible heat transfer coefficient to the surrounding material. High temperature HFM's have nickel housings which mechanically grip the substrates with a high force. Low temperature HFM's are swaged into copper housings. Both types of housings use capture nuts to clamp them to the mounting surface, producing a low resistance thermal path between the sensor surface and the surrounding material. An HFM may be mounted in a surface or on the end of a probe. Electrical connections are made to a single Lemo connector at the end of a short wire exiting the rear of the sensor. Cold junction compensation is not required.
|Max. Face Temp (°C)||700|
|Uncoated Response Time (micro sec)||17|
|Accuracy||±3% of sensitivity|
|HFM Source Impedance (Ohms)||3500|
|Thermopile||nichrome / constantan|
|RTS Sensitivity (Ohms/°C)||0.25 - 0.35|
|RTS Resistance (Ohms)||100-200|
|Wiring||Mineral Sheath (350°C)|
Available with water cooled mounting adaptors. Additionally, custom housings can be made to suit customer requirements. Please call.
This unit is shipped in a protective plastic tube 485mm long, 40mm diameter weighing 95g