A radiometer is a thermogage heat flux sensor designed to block convective heat flux such that it only measures the incident heat flux striking its sensing face. It does this by means of a window that is held in front of the measurement face to prevent convective heat flux from reaching the measurement surface. In other respects, the heat flux sensor is a normal circular foil Gardon Gage type. For low heat flux levels or brief measurements it may be conduction cooled, for larger heat flux levels or longer term measurements, water cooled.
The sensor requires no power, and will output 10mV when its full scale input range is reached. The transfer function is entirely linear. A NIST traceable calibration certificate supplied with the product details the exact sensitivity of the head flux sensor. Over-ranging the unit by up to 50% is allowed and the transfer function is still linear in this condition. Running at greater than 50% over-range is not recommended and doing so will shorten the life of the sensor. At 100% over-range permanent damage is likely to occur. The heat flux measurement is not dependant on temperature.
The window that blocks the convective heat flux is held in place by a copper adapter that screws to the copper body of the sensor. This allows good thermal contact with the body of the sensor to keep the window cool. Two different window materials are offered, Sapphire (Al2O3) and ZnSe.
| Material | Transmission Range | Thermal Conductivity | Specific Heat Capacity | Knoop Hardness |
|---|---|---|---|---|
| Sapphire | 0.17 – 5.5µm | 0.2721W/cm/degC | 0.763J/g/degC | 2000 |
| ZnSe | 0.6 – 21.0µm | 0.18W/cm/degC | 0.339J/g/degC | 120 |
Sapphire is the most commonly used window because of its chemical resistance and hardness, but ZnSe is also used for its wide transmission range. More detailed information on the characteristics of Sapphire can be found here, and ZnSe here. It does need to be highlighted that while the thermal conductivity of the window is low and it is well coupled to the body of the heat flux sensor thermally to keep it cool, it will still heat up over time. As the window warms, it will re-radiate onto the sensor and cause the measured heat flux to increase with time even if the energy output of the primary heat flux source itself is static. Removing the heat flux source at the end of the measurement will allow any re-radiated heat flux from the window to be measured and used to correct measurements taken.
The presence of the window will restrict the field of view of the heat flux sensor which would otherwise have been 180°. The TG9000 is offered with three different viewing angles, 60° (the narrowest), 90° and 120° (the widest). Other viewing angles are available as a custom product. When calibrated, the reference heat flux source will fill the entire field of view, and the supplied calibration certificate reflects this. Normally the viewing angle of the radiometer should be selected such that the target being measured fills its field of view. Failure to fill the field of view means that the View Factor needs to be calculated and taken into account to correct the measurement.
The ideal heat flux sensor will have very high emissivity such that it absorbs all the incident radiation and converts it to heat. Vatell use one of two different black coatings, dependant on the range. For heat flux levels lower than 340 W/cm², Pyromark1200 (which has an emissivity of 0.95) is used. For greater ranges, Colloidal Graphite (emissivity 0.82) is used. The supplied calibration certificate takes into account the emissivity of the sensor.
The body of the Radiometer must always be kept at 200 Celsius or less. Generally, the cooler the radiometer, the less that it will age and therefore the less that the calibration will drift with time. At no time should the heat flux sensor body exceed 200 Celsius otherwise damage will occur. A water cooled TG9000 package should be selected if if the temperature would otherwise exceed 200 Celsius. As a general recommendation, if the level of heat flux is greater than 5 W/cm² and the exposure time is greater than 5 minutes then a water cooled unit should be used. If the level of heat flux is less than 5 W/cm² then a conduction cooled radiometer may be used. Measurements of greater than 5 W/cm² and for durations of less than 5 minutes may or may not need to be water cooled depending how the radiometer is mounted (consider additional conduction heatsinking).
For a water cooled radiometer, the water flow rate depends on the package used. For a standard sensor with 1″ diameter body it should be possible to pass about 4 liters/minute through the sensor as a maximum. Ensure that water is flowing before the heat source is applied, and is left on for sufficiently long after the removal of the heat source to prevent damage. If the water starts pulsating / chugging, this is an indication that boiling is occurring and the flow rate should be increased otherwise damage will occur. The use of water (tap water is OK) is generally recommended as coolant as anti-freezes and other liquids could react with the copper and cause fouling.
It should be pointed out that the heat flux measurement and accuracy is not in any way dependant on the body temperature of the radiometer. Keeping the radiometer cool prevents damage and minimises aging.
All Vatell TG9000 heat flux sensors are built to order with the range, package, viewing angle, window and cable all specified by the customer. When enquiring about or ordering a radiometer, please specify the following:
This is full scale range of the sensor for which the output will be 10mV. Allowed values are between 10 W/cm² and 500 W/cm², although smaller ranges are possible when the radiometer is used with an amplifier. When manufactured, this is always considered a target value and the actual full scale range will only be revealed after calibration. Actual values up to about 35% away from the target are considered acceptable for a TG9000, but usually much tighter results are achieved. The Gardon Gage technology underlying the sensor is linear up to 50% over-range, so even with the manufacturing tolerance the resultant radiometer will always have a linear output in the target measurement range.
Sapphire and ZnSe are standard choices. Other materials are possible, but this will add considerably to the cost as a reference standard for the calibration will be required.
Standard choices are 60°, 90° and 120°. Other angles are possible, but this will add considerably to the cost as a reference standard for the calibration will be required.
There are many package options for the TG9000. The popular packages are summarised in the table. There are many other packages for which there are drawings which offer gas purging, threaded bodies, different water cooling pipe lengths, Swagelok connectors for the water cooling pipes etc. Then there are custom options. If none of the popular packages are suitable for the application, please call to discuss package options.
| Package | Window Bezel Diameter | Body Diameter | Features | Water Cooled? |
|---|---|---|---|---|
| TG9000-9 | 1.25″ | 1.0″ | Yes | |
| TG9000-9F | 1.25″ | 1.0″ | Mounting Flange | Yes |
| TG9000-30 | 1.25″ | 1.0″ | No |
As standard, the TG9000 will be supplied with a 6′ Belden cable rated to 200 Celsius. A 2′ glass fibre cable may be substituted for this at no additional cost. Custom cable lengths are possible.
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