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

[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...

[Fraser]

@jeffB,

Reference my comment on AC drive. You are correct. I was meaning polarity reversal which effectively produces an alternating current with a period set by the controller to achieve the desired reference temperature and then maintain it. Maybe I need to dig deeper into the driver board to see what it is actually doing with the supply to the Peltier TE assembly.

I have used heated plate type calibration checkers for many years but they are not as good as 'proper' orifice type black bodies, as used in calibration labs. Still flippin expensive at $3500  though !

I forgot to mention. I quickly tested my FLIR E4 on the 988 with it set at 42C. The E4 gave a measurement of the Black Body plate as 42C so is spot on in that part of its temperature coverage. A good confidence check for this sort of temperature but we know that the E4 becomes less accurate as you move away from 30C, especially at lower temperatures approaching 0C. The 988 cannot show this error as its range is limited to 20-45C.

Aurora

[mzzj]

There are some others also, like HGH infrared or Fluke being the most common.

I think most difficult part for the diy-version would be the paint or coating. Having a cavity helps a lot with the emissivity but brings problems with temperature uniformity.

0 cel and 100cel are easy enough to make at any home lab if you are interested checking your camera. Zero is extra easy if you can use snow at winter. Fill a box or a bowl with snow, let it soften at room temperature until it starts to melt and smash drinking glass or a soda bottle in the snow to form a cavity.  Better than 0,99 emissivity and better than 0,1 temperature accuracy.

[Fraser]

Eurotherm 2132 PID controller information attached.

Aurora

[miguelvp]

Related EEVblog video (#403), not the same Black Body calibration, but an Extech IRC350

On top of the Fluke 59 IR thermometer they also use a Flir E60 IR camera and an Extech IR200 thermometer

[helius]

I have a hot air preheater with a similar PID controller.
You can see from the three wires that it uses a RTD, not a thermocouple:

http://en.wikipedia.org/wiki/Resistance_thermometer#Three-wire_configuration

[Fraser]

Agreed, 3 wire RTD used in 2 wire configuration.

Aurora

[Fraser]

@miguelvp,

Yes I have seen that EEVBlog video and I am familiar with the Black body shown. It is sold under many brand names and is not without its issues !

Did you see the time it took to COOL DOWN to the 55C set temperature ? Looks like it uses passive radiation to cool the radiation plate. How would that cope with a set temp of 25C in a lab running at 28C ? Hmmm not great. Also note the poor thermal uniformity on the radiation plate, and the lack of  concentric grooves that improve performance.

I was looking at that model of Black Body and it is actually comparatively cheap at around $1000. It does show in its less than great performance though.

There is no excuse for poor uniformity and the lack of active heating/cooling even at a supposedly cheaper retail price. $1000 is still a lot to pay foe what I suspect is a PID controlled 'hair-dryer' warming a matt metal plate inside a posh box. I would love to see inside one though, to prove/disprove my comments 

Aurora

[SeanB]

Looks like it is a custom part, probably not having a relay output, but having a DC voltage output giving the error magnitude, and the logic output giving the command to heat or cool. That would allow it to arrive at the setpoint quickly, but as the voltage drops it reduces power so that the overshoot is minimal, which makes the loop compensation easier.

I wonder if I could make one, just using some copper plate and them having simply power resistors to heat it, with the emission area being a carbon vacuum pump vane, as I do have some of those around.

[miguelvp]

I was just trying to add a relevant link from the EEVblog that explains one of those calibration system, wasn't implying it was good.

I did notice at the poor thermal distribution when they looked at it with the E80, but they did mention that they calibrate it in house, maybe it's due to contamination from touching the test area with bare hands.

[Fraser]

@miguelvp,

I didn't mean to offend. I just wanted to highlight that even a relatively poor Black Body still costs $1000 

The marketplace for thermal Black Body calibration and check sources is still quite small. I am disappointed at the 'home-brew appearance of some units (including mine) though. It is almost as though they are supplied in such small numbers that they are hand made. At least a check with a thermal camera reveals the limitations of the unit if you are thinking of buying a new one.

Aurora

[Fraser]

@SeanB,

I have experimented with making my own thermal radiator fro a black body. The challenge is to produce a uniform temperature across the plate. Any point or line source of heat such as a power Resistor causes a hot spot visible on the outer radiating surface.

The easiest ways of producing a uniform temperature plate is to choose one of the following

1. Temperature controlled water flowing over the radiators rear surface (not in tubes though)
2. Blow temperature controlled air over the rear of the radiators surface
3. Fit a large Peltier TE heating/cooling module on the rear of the plate. It presents a pretty uniform heat source across its surface. It is important that the Peltier module is larger than the 'viewing' apperture and that a reasonably sized radiator block is used to even out the small non uniformity in the Peltier module.

it is possible to build a simple 'wet' black body but the temperature would normally be set and takes a long time to change.

Air currents also need to be considered when building a Black Body source as the radiating plate has a very high emissivity acting much like a heat-sink. Any significant air movement across the plate will create a temperature gradient and non uniformity across the surface. This is why my unit has a protruding tube in front of the radiating plate.


I use car VHT matt black paint (header paint) on my test pieces and it works well with emissivity of around 98% Candle soot is also a good radiating surface but it is very fragile. The proper high emissivity 3M paint for Black Body applications has eluded me to date. The 3M paint has stable emissivity across a broad range of temperatures which common matt paints sometime do not.

Emissivity does not need to be perfect, just of known value and predictable at different temperatures. A PT100 sensor can be used with a thermal camera to determine the true emissivity of a surface coating. Uniformity is not such an issue for such tests so you could use a large aluminium power resistor as a test heat source for the tests.

Hours of fun to be had 

Aurora

[-jeffB]

I was thinking of treating a copper block with sulfide (either solution or gas-phase) to get a good coating of copper sulfide. Silver might be even better -- better thermal conductivity, a nice black sulfide coating -- but of course it wouldn't be cheap.

I suppose I'd want to look up emissivity statistics for sulfide thin-films, though, and I don't know how readily that info is available.

What about this:

Silver the inside of a (metal?) bottle with a neck large enough to make a good aperture.

Tarnish that silver layer.

Immerse the whole bottle in a well-agitated water bath, with its temperature tightly controlled. (Substitute alcohol or oil for temperatures below or above water's freezing point.)

It wouldn't be beautiful, but it just... might... work...

[mzzj]

Emissivity does not need to be perfect, just of known value and predictable at different temperatures. A PT100 sensor can be used with a thermal camera to determine the true emissivity of a surface coating. Uniformity is not such an issue for such tests so you could use a large aluminium power resistor as a test heat source for the tests.

Hours of fun to be had 

Aurora

And there you got your chicken and egg-problem 
Many thermal cameras  are pretty much crap for absolute temperature measurement so you should have "known ideal" blackbody to compare the readings. There is some very good ones too, but often manufacturer specifications seem like a wet dream compared to real life.

And yes, high temperature matt black header paint is best what I have been able to get my hands on.  Been too lazy/busy to actually try and measure the emissivity despite working in a temperature calibration lab  with a dozen of blackbodies available

[Fraser]

@mzzj,

Indeed, chicken and egg 

I am fortunate to own many thermal cameras including FLIR PM695's but all are rated at +-2%, or 2C.
I have a new AGEMA Thermapoint 8200 system with high accuracy thermal head. I can achieve +-1% or 1C with the Thermopoint. I would use that system for checking paint emissivity. The method is simple. Accurate measurement of the surface temperature using a contact thermometer. The Emissivity setting on the Thermopoint 8200 is then changed until it matches the reading on the accurate contact thermometer. Good enough for my needs 

http://www.flir.com/legacy/view/?id=51509

One thing I have learnt over the years though is that Emissivity has a dramatic effect on any IR non contact measurement accuracy. Before people get too excited about absolute accuracy, they should consider the emissivity of the targets that they will be surveying. If the user does not know the exact emissivity of the radiant surface, there will be an error introduced into the measurement, and that is before we consider distance to target and ambient temperature ! 

Thermography is full of variables  If possible, direct contact temperature measurement using a precision sensor is likely to be a more accurate approach.

For me, I wanted a Black body thermal source for testing cameras after repair. For that it will be fine.

Now calibration of thermal cameras..... I leave that to those with the deep pockets for lots of appropriate calibration sources and OEM proprietary software. 

Aurora

[ebeall]

Great info, thanks Aurora!

I've also been looking for the 3M high emissivity paint, if anyone can find it or a replacement, please let us all know.

These kinds of blackbodies are also how the sensitivity of thermopiles are calibrated (easier, just a single higher temperature calibration). I don't have a great blackbody built. It was to be deferred til the Kickstarter funded, which of course didn't happen. There is also a die calibration at several setpoints, but this is much easier, just a milled aluminum orifice the thermopile body can seat fully into, along with several DS18S20's and an arduino.

I have to mention that I learned water is not always a great calibration object - on average it can appear to be pretty emissive, but the absorbance spectrum is all over the place. It absorbs at several frequencies most cores are sensitive to and that throws it off. It depends on the spectrum your imager is sensitive to.