Products > Thermal Imaging

Thermal Camera Black Body calibration check source - inside the IsoTech 988

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Fraser:
What do you find inside a $3500 Black Body thermal calibration check source for IR thermometers and Thermal Imaging cameras?

Well you are about to find out :-)

I purchased an IsoTech 988 Black Body source after Mike gave me a 'heads-up' on an auction. Such units do not come up for sale often and normally command high prices when they do. Thanks Mike  :-+

I repair and collect thermal cameras so a Black Body thermal source is something that will be very useful for checking measurement accuracy and possibly thermal gradient issues. So what is a Black Body temperature source? Well in simple terms it is a plate or orifice that presents an IR measurement device with an accurate source of thermal energy at a set temperature and known emissivity. This is not as simple as it may sound. Providing a thermal radiator that does not have a thermal gradient across its surface and that is accurate in terms of surface temperature requires some clever design and control. As such Black Bodies are not the most common or cheap of devices. Any industrial or medical user of thermal measurement technology knows that it is important to regularly test the measurement equipment against a known good Black Body thermal source. Annual calibration does not help if the measurement equipment drifts out of specification between calibration dates.

IsoTech(UK) (not to be confused with Radio Spares Brand IsoTech) provides a range of Black Body thermal sources to suit many situations. The 988 is designed for equipment that works at temperatures around ambient. It has a range of 20C to 45C. Perfect for my purposes as many camera specifications are stated with a 30C target temperature. For a Calibration facility, this unit would not be suitable and many different Black Bodies are used, each set at a temperature to calibrate the measurement device at a specific point in its temperature coverage.
The 988 is a confidence check device for thermal measurement technology but that does not make it cheap. The unit normally sells for over $3500 ! The accuracy specification is better than that of my cameras so it is suitable for my accuracy checks.

How do you know a Black Body is actually accurate at any point in time ? Well the unit contains a PID with associated thermal sensor and it also has a test port to the centre of the Black Body radiator plate. Into this an accurate thermocouple or PT100 probe is inserted. Such probes are far more accurate than a thermal cameras nominal +-2C / 2% tolerance. I checked my unit and it was spot on calibration. It is only a year old so that was not unexpected.
OK now to the units details:

1. A 12V 5.5A external SMPSU provides power to the 988 unit. This means that the unit may also be powered from a car/van if required.

2. The Black Body radiator plate is made from aluminium in the form of a 'Puck'. It is around 100mm in diameter x 20mm thick, and has concentric rings cut into its outer face. The cut surface is then coated with a high emissivity matt paint that is predictable in terms of emissivity at different temperatures.

3. Behind the radiator plate a Peltier thermo-electric heater/cooler is attached. The Peltier unit and Aluminium 'Puck' is surrounded by insulation material to assist in stabilising the metals temperature whilst in use and slow the Delta T.

4. A large heatsink is mounted on the rear of the Peltier TE module. This is a requirement for correct operation of a Peltier TE heater/cooler assembly. A thermal gradient is created across the assembly and the heatsink radiates heat or cold depending upon the Peltier TE operating mode.

5. A Peltier TE generates heating or cooling on the Black Body radiator plate dependent upon the polarity and voltage applied to its two terminals. The unit has to heat or cool the radiator plate as it is possible that the selected reference temperature is below ambient. i.e. 25C in a 33C ambient room. A Driver PCB is controlled by a smart PID unit into which a set of operating rules have been programmed for the specific application.

6. The Smart PID receives radiator plate temperature from an RTD that' is buried inside the aluminium 'Puck'. It is able to carefully control the amount of heating or cooling applied to the radiator plate by the Peltier TE assembly and so maintains the plates temperature within the tight tolerances detailed in the specifications.

7. The temperature control within the 988 is closed loop and will do its best to maintain the correct radiator plate reference temperature over a wide range of ambient temperatures.

8. External air movement in front of the Black Body radiator plate can introduce thermal gradients so a tubular 'hood' is fitted to isolate the air movement inside the tube from that outside when taking measurements. My unit has also been fitted with four guide pins to assist in positioning IR thermometers within the tube. I may well remove these as they are not needed in my application.

9. Finally, a fan is fitted in the case to push air past the heatsink and out of an exit port on the opposite side of the case. The fan has its own speed controller and temperature sensor to manage its speed.

Well that is what you get in Black Bodies these days for $3500. Nothing particularly complex. The expensive part is the smart PID and decent Peltier heater/cooler assembly. This is a classic case of supply and demand. Black Bodies are specialist test instruments and are not produced in numbers that encourages discounts or low prices.

Could you make your own ? This type of Black Body is relatively easy to build. The only unusual part will be the Peltier TE driver board. I have not investigated its operation yet but it contains a 555 timer and some basic logic driving four MOSFETS (?) so appears to be an AC driver (?). As stated, the Peltier TE assembly must heat or cool the radiator plate in order to meet the set temperature, no matter what ambient temperature exists around it. (within specs of course) I may investigate that driver board further but not right now as I have no available time until March.

Sorry the pictures are not better but I have done this in a hurry as I thought it may be of interest to those, like me, who wanted to know how a plate based black body was constructed. I have a draft of the units interconnections and modules but it is too untidy to upload so that will have to wait till another day. I may upload it 'as-is' if there is interest.

Aurora

Updated - original text stated PID input was a thermocouple. It is in fact a PT100 RTD.

Fraser:
Internal Pictures 1

Fraser:
Internal pictures 2

Fraser:
IsoTech 988 datasheet and manual

IsoTech manuals and tech documents are to be found here:

http://www.isotechna.com/Isotech_Tech_Library_s/57.htm

An interesting IsoTech introduction to Black Body sources is to be found here:

http://www.meter.hu/adatlap/labor/isotech/feketetest_kalibratorok.pdf

The 988 appears in the document. Worth a read if you own an IR thermometer or thermal camera.

-jeffB:
Very nice. I didn't know you could buy commercial black-body sources that didn't use a cavity as the target. I can imagine that those would be much more expensive.

Wouldn't the quad-MOSFET driver just provide current to the TEC with reversible polarity, one direction for cooling, the other for heating? I don't think you'd ever drive a TEC with actual AC...

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