Advantages:

- The industry's fastest response time
- Minimal effects on measured variables
- Operates in environments up to 850°C  (Water-cooled models available for high heat flux applications)
- Measures both heat flux and approximate temperature at the face of the sensor
- Measures heat flux in all 3 modes (radiative, convective, and conductive)

Specification*:
 

	HFM-6 D/H	HFM-7 E/L	HFM-7 E/H	HFM-8 E/L	HFM-8 E/H
Maximum Face Temperature (°C)	850	300	700	300	700
Uncoated Response Time (µs)	17	17	17	17	17
Coated Response Time (µs)	300	300	300	300	300
Minimum Sensitivity (µV/W/cm²)	8	150	150	150	150
HFM Source Independance (Ohms)	1000	3500	3500	4500	4500
Thermopile	Platinum /  Platinum Rodium	Nichrome/ Constantan	Nichrome/ Constantan	Nichrome/ Constantan	Nichrome/ Constantan
RTS Sensitivity (Ohms/ºC)	0.25 - 0.35	0.25 - 0.35	0.25 - 0.35	N/A	N/A
RTS Resistance (Ohms)	100-200	100-200	100-200	N/A	N/A
RTS Metal	Platinum	Platinum	Platinum	N/A	N/A
Thermocouple	N/A	N/A	N/A	Type E	Type E
Housing	Nickel	Copper	Nickel	Copper	Nickel
Wiring	Mineral  Sheath  (350ºC)	Teflon®  (220ºC)	Mineral  Sheath  (350ºC)	Teflon®  (220ºC)	Mineral  Sheath  (350ºC)

*Max. temperature is for uncoated sensors; sensor coating max. temperature is 650°C. Max. temperature for the back end of the sensor is 650ºC and 200ºC for high and low temperature units respectively. Sensitivity numbers are based on coated sensors with a radiant source.

Sensor Coatings:
The standard coating for the face of the HFM sensor is Pyromark 1200, a high temperature black coating with a 0.95 emissivity.  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.

Technical Information:  Heat Flux Measurements with Uncoated HFM - MS Word.doc

Principle of Operation:
This sensor 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 thin film resistance temperature sensor (RTS) in the HFM-6's and HFM-7's and a thermocouple in the HFM-8's.

Calibration:
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.

For further information on Vatell's strict calibration procedures please see:
Vatell Calibrations

Construction:
The heat flux and temperature sensors of the HFM 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.
 

Tempertaure Limits:

Water cooled versions are available for very high temperature applications

Other Technical Information:
HFM Installation MS-Word(700KB)  PDF(32KB)
HFM-6 and HFM-7 Measurements (Manual)    MS-Word(99KB)
HFM-8 Measurements (Manual)    MS-Word

Drawings/Dimensions:

1) Low Temperature HFM-7 E/L, HFM-8 E/L

2) High Temperature HFM-6D/H,  HFM-7E/H, and HFM-8 E/H