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Black Body Calibration check source - Exergen Calibration Master

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Fraser:
Dear Thermal imaging fans,

I recently purchased a pair of Exergen calibration master portable Black Body thermal sources.

Exergen are a well known OEM of high performance non contact IR thermometers and they produce a range of industrial and medical non contact IR detection and measurement devices.

In the medical profession there is a need for a confidence check source against which to test the IR thermometers that are used on patients. To this end Exergen produce a Master calibration check kit.

The kit is unusual in that it contains a Black body thermal source with pre-set temperature, and a master IR thermometer. The Master IR thermometer is used only for checking the temperature of the Black Body aperture and is not used on patients. Once the temperature of the Black Body is confirmed, other IR Thermometers may be checked against it. This is a little like a transfer standard.

The Master IR thermometer is self calibrating and the manufacturer claims that it does not need recalibration except after repair. This claim is attributable to the thermal sensor that Exergen deploy in their devices.

So I have a pair of mini Black Body thermal sources that should be pretty accurate and will be useful as an accuracy check for IR thermometers and thermal cameras. The diameter of the black body is only around 1” but this is enough for many applications.

Description of the unit:

The Black Body consists of a small plastic enclosure that may be powered from a rechargeable battery or the supplied 240Vplug pack. The unit is designed to be highly portable for use on hospital wards as a quick confidence check device.

The Black Body chamber is a conical well design that forms a Black Body environment for the IR thermometers inserted into it, and so avoids measurement errors.

The case is easily opened after the removal of two self tapping screws in the battery compartment.

Inside the case there is a small horseshoe shaped PCB surrounding the Black Body chamber. The PCB is not secured in place by any fixings. There is a power transistor attached to the bottom of the Black Body chamber but this is not the primary heat source. The chamber is heated by a heating element would around its exterior. A thermistor is used to monitor the chamber temperature and this is buried in the aluminium casting. The value is 14K Ohms at approximately 20C so it is likely a 10K type.

The small PCB contains through-hole components and includes the following major semiconductors

LM358          Op-Amp                           Qty 1
MJE 521       NPN Power Transistor       Qty 2
2N3904        NPN transistor                  Qty 2
2931Z-5.0   5V 100mA LDO Regulator Qty 1

I quickly sketched the schematic of the unit and attach it for information. I make no apology for its crude appearance. It is just a sketch and done at speed.

The 9V battery or plug pack supply is used to power the heating element and also feeds a 5V Low Dropout (LDO) Regulator that can provide up to 100mA of current. Both op-amps within the LM358 are used. They form a two stage heating element temperature controller. One quickly heats the Black Body to a pre-set temperature and the other is the fine control of the heater to bring it up to the correct operating temperature.  The two op-amps each have their own Darlington pair for control of the heating element.  The pre-set temperature may be adjusted via a 10 turn potentiometer.

The units have been briefly tested with a FLIR E4 and appear to work well. There is room for improvement in the current thermistor based design and modifications may be undertaken to improve temperature stability.

Fraser

Fraser:
Internal pictures

Fraser:
Flir E4 images

The first image is without the addition of a close-up lens. The second image is with the Inframetrics 6" close-up lens fitted.

The small temperature error introduced by the auxiliary lens is clearly visible.

Fraser

SeanB:
Nice little black body there, and quite small, as well.

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