Temperature control for calibration - Page 1
Forum section: electronics / projects

author: jmaja
author_rank: Frequent Contributor
timestamp: October 31, 2013, 01:15:24 pm
content: I need to calibrate a measuring device at different temperatures (-20 - + 70C). I can't have the device inside an oven etc. while measuring due to the nature of the measurement. The tempererature doesn't need to be very accurate (+- 2-5 degrees is OK).Could this be done with a peltier element? It would be 10x10x10 cm aluminium box on a table. Can I cool it down to -20 C with peltier elements without building insulating box around it? What kind of element would be sufficient and what kind of cooling it would need. Blower is not an option due to vibrations.

author: SeanB
author_rank: Super Contributor
timestamp: October 31, 2013, 07:05:33 pm
content: How about a freezer unit with an insulated box inside containing the DUT, with the peltier unit doing either heating or being a passive plate for cooling ( it really is good at being a heater, but -20 will be a dream you will never reach) which then will do the job. Mopst peltier elements are severely limited in thermal difference, typically they can manage 15C at max current with only a low heat flow through them with a massive heatsink attached to both sides. you will need an insulated box though.

author: dfmischler
author_rank: Frequent Contributor
timestamp: October 31, 2013, 07:12:53 pm
content: How about somedry icein an insulated box with baffles to control temperature in the test chamber. Might take a little playing to get to the target temperature and hold it. What are you going to use as a reference?I once observed a co-worker doing environmental testing in an insulated box with a heat gun for high temps and a dewar flask ofLN2for cold. It worked fine and was a good rehearsal before going to a real testing lab.

author: jmaja
author_rank: Frequent Contributor
timestamp: November 01, 2013, 11:12:31 pm
content: Measuring temperature is not a problem, since I have several temperature sensors on the PCB and I only need to calibrate against them. Thus I just want to get a rather stable temperature somewhere around -20 C and somewhere around 70 C. The calibration will be referenced to the PCB temperature, which doesn't have to relate to any real temperature as long as it is repeatable.I could just put the DUT into freezer/oven, take it out and do the calibration, but I'm worried that such a dynamic situation may not correlate well to the actual measuring environment, which has stable temperature in short term (stationary device, with big mass outdoors). Eg. there may be severe temperature gradients on the PCB or between the PCB and it's supporting structure affecthing the actual measurent and also the temperature measurement in a different way than in real world.For doing the actual calibration I need to attach (with screws) the DUT to the calibration system. That's why I can't do it in an calibration oven or insulated box. Maybe I could do it in a Sauna and in a very big freezer? Hmm.... I have a Sauna at home and winter is on it's way and may give -20C.

author: codeboy2k
author_rank: Super Contributor
timestamp: November 02, 2013, 06:26:43 am
content: sounds like you need to build up an insulated box that can have the DUT inside, the calibration system outside, screwed together through an access port in one wall of the chamber. Perhaps that even needs to be an insulated access port.Is that possible?

author: jmaja
author_rank: Frequent Contributor
timestamp: November 06, 2013, 11:14:30 am
content: First version is now done and tested. I got an idea to use frozen water glycol mixture as cold source and then heat with a resistor. I have 3-6 cm polyurethane insulation with 12x12x12 cm inner space. M4 fixing screws go through the insulation and are made of 316L, which has much lower heat conductance than normal steel. Total heat capacity is about 1.8 kJ/C (based on BOM including insulation). I have 0.18 l of 39% vol ethylene glycol in an aluminum case with good heat transfer to DUT. That should have 49 kJ latent heat at -23C and heat capacity of 1 kJ/C (including the case and included to the 1.8 kJ/C total).So based on calculations (assuming no heat flux through the insulation) I should get a bit under -20C when all the glycol water has melted when starting at +30C. In practice I got only a few degrees below zero, but I'm not absolutely sure that all the glycol water had time to freeze before the experiment. The DUT cooled down over 20C in the first 3 minutes and reached zero in 12 minutes. The peak was at 26 minutes and T was less than 1C from the peak for more than 20 minutes.After all the glycol water had melted (~40 min after inserting it) the DUT began to warm up at ~1C in 10 minutes. Then I started to heat it up with a 50 W halogen bulb inside the glycol water. First at 5 W and then 10 W, 20 W and 40 W. I had 40 W on from ~16C for 54 minutes. At ~49 minutes I could hear boiling, probably only at the surface of the bulb. The DUT peaked at 89C (10-15 minutes after heating was turned of) and then started to cool down at ~1 C in 5 min. Now, 8.5 hours after I stopped heating, DUT is at 42C while ambient is about 23C. DUT is on using 0.8W. According to calculation the balance T at 0.8W should be about 8C above ambient, but DUT has also local gradients, thus I'd expect the balance T measured on the PCB to be around 13C above ambient.Looks very promising! I need a bit more to get all the way to -20C, maybe it is enough to properly freeze the glycol water, I need more of it or I need to cool down DUT a bit with other methods (e.g. another patch of glycol water) before inserting the glycol water. Then I can sweep the whole temperature range at 1C in more than 5 minutes or up to 1.5 C/min with 50W heating.