EEVblog Electronics Community Forum
Products => Thermal Imaging => Topic started by: Fraser on September 22, 2020, 09:46:43 pm
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DIAS CS 400-M1 Blackbody
Temperature Range : +50C to +530C
Aperture Diameter : 30mm
Emissivity : 0.97 +/- 0.01
Uncertainty +/- 2K
Control Stability +/- 0.5K
Controller type : PID (Omron E5CN-Q - SSR output)
Temperature Sensor : 3 Wire PT100 Class A
Heat-Up time : 30 Minutes
Datasheet url: http://www.dias-infrared.de/pdf/pyrotherm_cs400_eng.pdf (http://www.dias-infrared.de/pdf/pyrotherm_cs400_eng.pdf)
The DIAS CS 400-M1 Blackbody is a custom unit designed and built by DIAS (Germany) for use with their range of IR temperature measuring and imaging systems. The CS 400 comes in two versions. The standard unit provides a temperature coverage of +50C to +400C and the CS 400-M1 model has extended temperature coverage of +50C to +530C. Both units appear identical in appearance but the "M1" version uses an enhanced heater and thermal insulation to cope with the additional thermal energy within the head. I have the "M1" version :-+
The CS 400 system comprises two units that are connected by a pair of silicone insulated cables. The larger of the two units is the Temperature Controller and the thermal emission plate is housed inside a smaller cast aluminium box and emits its thermal energy through a 30mm diameter aperture. Detail of the two units follows.
The Controller:
This unit contains a large 50V 225W Torroidal transformer that provides the drive voltage for the systems 14.8 Ohm heater that is located in the head unit. To control heater output, and so the emission plate temperature, the controller uses an OMRON E5CN-Q PID temperature controller. The E5CN-Q version is designed to drive a Solid State Relay using PWM. This makes for an excellent closed loop PID temperature control system. A Crydon CKRD2510 10A a.c. SSR provides the switching of the 50V supply to the heater. The sensing element in the emission plate is a PT100 Class A RTD.
An unusual, but useful feature provided on the CS 400 system is an Emissivity adjustment control to adjust the output of the emission plate to match IR temperature measurement systems which cannot be set to the 0.97 emissivity of the Blackbody. The analogue Emissivity control uses a precision 10 turn potentiometer that acts directly upon the PT100 elements single "A" leg.
The Emission plate head
The CS 400 Blackbody uses a separate compact head unit that contains the Emission plate, heater and PT100 RTD sensor. The energy output aperture is 30mm but the actual emission plate located behind is 50mm diameter. The Emission plate uses a pyramid profile surface that is covered in high emissivity paint. The Blackbody is capable of temperatures as high as 530C so the interior of the head unit is filled with ceramic wool insulation. Two 3 core screened Silicone insulated cables are used to connect the head unit to the Controller unit. One cable is used for the PT100 sensor and the other drives the heater. Keeping the PT100 cabling separate from the potentially noisy heater drive avoids noise induced errors in the temperature control loop.
The unit appears a little "home made" and this is no great surprise to me. Such units are made in small numbers and are constructed using common COTS components. The only specialist components used in this unit are the Emission plate and the heater that drives it. The emission plate surface can be a flat type or one with a 'profile'. Common surface profiles are concentric rings, a fine pitch spiral or a collection of miniature pyramids. The CS 400 uses the pyramid profile. The important high temperature, high emissivity paint is applied to the emission plate to provide a known and predictable emissivity value across the whole operating temperature range of the Blackbody.
So what is the story of the this particular DIAS CS 400 Blackbody ?
I had seen the unit advertised on eBay some time ago, but the asking price and its location (Korea) stopped me buying it and I moved on. Last week I returned to the unit and negotiated the price down to something more palatable to my risk appetite. I could see that the unit needed two expensive connectors and would likely need repair work as well. Once working, the unit would make a very useful Blackbody so it was well worth buying.
The Blackbody arrived this week and was all that I had hoped for. First impressions of its condition were good and it just needs a clean. Without the connectors I was not able to test the units functional state but I would not apply power before inspecting it anyway. Any equipment that has traveled by post should really be inspected for internal damage and that is my standard M.O.
The Controller inspection and repair
Upon opening the case of the Controller unit I could clearly see transit damage, caused either during its journey from Korea or when it was disposed of by units previous owner. The controller contains a very heavy 225W torroidal transformer and these can cause serious chassis deformity in a drop scenario if not mounted on a reinforced hard point in the casing. In the CS 400 the transformer is mounted on an aluminium chassis plate that is suspended between the lower side bars. The aluminium chassis plate was seriously distorted as it had no hard point for the single bolt that held the transformer in place. Fortunately the movement of the transformer had not caused serious damage and only some gentle reshaping of the chassis was needed. It is amazing what you can do with your heel, a piece of wood and a tack hammer :) I added large repair washers and ripple washers to the transformer mounting bolt to improve the installation. A large area ribbed steel load spreader is really needed but as the unit is not going to get dropped again any time soon I will live with the current solution. No other repair work was need on the controller so I got on with cleaning its casing. It cleaned up pretty well with just soapy water and IPA.
The Head Unit inspection and repair
The Head Unit is a solid beast thanks to its epoxy coated cast aluminium enclosure. The enclosure is just a common COTS product but meets the needs of the head design well.
The Silicone cables enter the head through cable glands but the soft nature of silicone insulation make the job of the cable glands compression cable grip difficult and one cable had escaped the cable gland completely. Thankfully the cable cores are undamaged. The two cables looked very marked and I considered replacing them but I soon discovered that screened 3 core silicone insulated cable is both hard to source and expensive ! The application of IPA to the cables removed the marks with relative ease so replacement is not required :)
Access to the head units internal parts is via the front panel. Once removed, the 50mm emission plate is revealed. The emission plate is surrounded on all sides except the front with high temperature ceramic wool. The emission plate is mounted on a inverted 'U' shaped support bracket that is secured to the rear of the head casing . Two small ceramic spaces separate the hot emission plate from the support bracket but a thermal path remains via the two mounting screws that lass through the bracket and ceramic spacers and into the emission plate. Full thermal isolation of the emission plate is desirable but not always easily achieved so some compromises have to be made.
Upon disassembling the internal parts of the head unit some minor issues were discovered that will require rectification work. Both of the screws securing the emission plate to its 'U' bracket were very loose. One ceramic spacer is cracked in half and will need to be replaced. The high temperature cement that had locked the two screw heads in place had failed and fallen off. This allowed the lightly tightened screws to unwind from the emission plate through thermal cycling. Wiring the screw heads is a better solution to thread creep. When reassembling the head I will wire the screw heads to prevent movement. The high temperature cement securing the PT100 RTD probe in the emission plate has failed and the probe was easily pulled out of the plate. Whilst not a serious issue as the probe remained in the head, I will apply new fire cement to the probe at the point where it leaves the emission plate. The cables will both be re-seated in their respective cable glands as changing the cable securing method is not justified for my use scenario.
The head was cleaned with IPA to remove some of the dirt but there were marks that were too engrained to come off. I elected to use metal polish past to remove the marks and then applied T-Cut car paint restorer to the grey epoxy paint to restore its semi gloss finish. Only the front panel required this treatment as the rear section of the
enclosure was easily cleaned with just IPA.
No further issues were found with either the Controller or Head unit so it is just a case of reassembling the head and terminating the two cables with their respective connectors. I have yet to order the connectors but both are available from Radio Spares.
Once everything is reassembled and terminated I hope to have a very neat little Blackbody thermal source for temperature measurement test and calibration tasks. The 30mm aperture and pyramid profile emission plate do not lens themselves to thermal camera flat field calibration work. A large area flat emission plate type Blackbody is the preferred tool for that task.
I attach the CS 400-M1 datasheet and the pictures of my unit taken from the eBay auction. Posts that follow will show the disassembly of the unit and points of interest.
Fraser
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The Controller Unit Teardown Pictures .......
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The Chassis damage repair......
The aluminium chassis 'deck' is made from relatively soft aluminium plate so it was possible to remove most of the distortion in it using just a thin piece of wood, my carefully applied heel and a small tack hammer. I deliberately left a slight dome around teh transformer mounting hole as this aids transformer centering and lateral stability. The action of the securing bolt helps to create this mild dome shape during the original installation.
The side mounting points were slightly distorted but were easily restored to their normal position with the tack hammer and a block of wood acting as a drift.
Fraser
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Controller reassembly........
Hole repair washers were used on both sides if the transformer mounting hole to spread the load a little. Wavy washers were used to maintain tension on the bolt and a double locked nut arrangement was used to stop the transformer securing nut becoming loose through vibration etc. I replaced both large neoprene washers that sit either side of the transformer as the originals were looking tired. The replacements were made using a common card cutting compass.
The controller was reassembled without difficulty.
Fraser
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The Head Unit teardown
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Cleaning the head casing and cables.......
The cleaners used were
First : IPA
Second : Metal Polish (only on the faceplate)
Third: Car paint restorer (only on faceplate)
The cables cleaned up well using just IPA and the metal polish tidied up the stainless steel aperture trim nicely
Fraser
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I have been looking at the way the Emissivity correction is applied to the PT100 circuit and that still has me puzzled. The control comprises a 100 Ohm potentiometer wired ‘single ended’ to act a variable 100 Ohm resistor. There is a 100 Ohm 1% resistor wired across the two wires connected to the potentiometer, so in parallel with whatever resistance is set on the pot. This adjustment control is wired in series with the 3 wire PT100 single wire leg. This means a resistance of 0 to 50 Ohms can be inserted into the PT100 sensor loop. The adjustment uses a 10 turn 0-10 mechanical indicating knob. With the adjustment set to ‘0’ no Emissivity adjustment is inserted as the potentiometer is set to Zero Ohms.
I have written to DIAS asking for the user manual as I hope that will make the use of the Emissivity adjustment control a little clearer.
Fraser
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There is a slightly different model for sale at ebay.
Unfortunately they want 1000€, this fit's my budget not at all :--
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The pot and resistor allows you to put in a false temperature by fooling the measuring circuit into thinking the Pt100 element is hotter than it actually is, and thus the control loop will drop the black body temperature down, lowering it's rated emission of heat to match what a lower emissivity surface would be showing to the camera. Likely there is a table of values for the dial, so that you can set up a predetermined emissivity for any desired setpoint, though this likely will only really be effective at higher temperatures, as you really can not adjust the heater to below ambient, and it will probably tap out at the 50R added resistance mark, so will give at most roughly a 130C lower temperature on the plate from the shown temperature on the controller, limiting the band of emissivity you can handle.
Likely will be around 0.7 at the highest temperatures being the lowest it will go, though around 150C it probably will be able to go down to 0.4 emissivity, though those figures are very rough off of the top of my head ones. Depends on the emissivity spec of the actual blackbody paint in there, so there should have been a calibration sheet in the unit giving the actual values for the specific unit, but probably tossed away as just scrap paper.
I think you will probably have to buy a few Pt100 probes extra for it, and have a calibrated meter, to set it up properly, though likely it will be pretty accurate with the sensor it comes with, as the Omron controller keeps the sensor calibration and linearising tables in it, you might want to read them out and write them down to back up the unit, at least then if you change the sensor or controller you have a basis to start from.
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SeanB,
Thank you for your helpful comments. You have highlighted exactly what was puzzling me.
Whilst I understood the idea of inserting additional resistance into the PT100 loop to ‘fool’ the PID controller into thinking the emission plate is hotter than it actually is, forcing a temperature reduction response, I could not see how the 10 turn pot mechanical Indexing dial could be calibrated in Emissivity. As you say, it is likely that there was a Emissivity Control reference table or chart provided with the CS 400 that was used to set a value at a particular temperature.
No response from DIAS regarding the user manual yet, but I must be patient ;D
Fraser