Temperature calibration practicalities - help! - Page 1
Forum section: electronics / projects

author: DTJ
author_rank: Super Contributor
timestamp: October 07, 2017, 10:03:42 am
content: I've got a temperature sensor based on a digital temperature sensor IC that I am trying to accuracy test.It's housed in a 1/2" (12.5mm) stainless tube and potted in polyurethane. The sensor is close to the end of the tube housing and is specified to be accurate to <0.1°CI'm trying to check its accuracy over the range of say 0°C to 90°C.The sensor is placed in an aluminium block along with a calibrated PT-100 reference sensor.This is immersed in hot water inside an insulated cooler. The top of the cooler is covered to slow cooling.I have a propeller on the end of a plastic shaft spinning at around 1.5Hz to stir the water.The set up is shown in the photo below (with the aluminium block shown lifted clear of the water).I'm still getting a temperature difference between the two sensors of around 0.3°C or 0.4°C.The error seems fairly constant - at 80°, 65° and 45° it's around 0.3°C.I'm still waiting for it to cool to see what it does at ambient but I suspect it will still be off by ~0.3°.If I get an un-potted sensor and thermally bond it to the reference sensor using some machined brass rod and some silicon heat sink compound they track within about 0.05°.This assembly is wrapped in bubble wrap and buried in foam inside a cardboard box and left to stabilise at room temperature.If I simply put the sensors against one another and again insulate them well I will see an error of say 0.3C or 0.4°C, the thermal bonding seems to work.So why can't I get the sensor under test to match the reference sensor when immersed in water?I guess it could be that there really is an error of 0.3° at elevated temperatures but I doubt it as I expect the sensors to comply reasonably well with the data sheet specs.I can't bond an un-potted sensor IC to the reference sensor and immerse it in water for obvious reasons.What are people experiences in trying to do this??

author: Ian.M
author_rank: Super Contributor
timestamp: October 07, 2017, 11:26:01 am
content: Self-heating + poor thermal conductivity of the polyurethane potting compound?Try a constant temperature bath e.g an melting ice-bath, and also a well insulated water bath at ambient to eliminate errors due to the thermal lag between the isothermal block and the sensor and the decreasing bath temperature.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 07, 2017, 11:49:33 am
content: Both sensors look like they can conduct quit a lot of heat through there connection / tube. So usually the sensor should be emersed quite deep. Just a few inches in the AL block might not be enough.With a water bath at higher temperature, there can be quite some colder temperature at the top because of evaporation cooling. This is why higher temperature baths prefer oil.

author: DTJ
author_rank: Super Contributor
timestamp: October 07, 2017, 12:07:57 pm
content: Quote from: Ian.M on October 07, 2017, 11:26:01 amSelf heating isn't an issue - it's asleep pretty much all of the time and only wakes for 10's of mS before sleeping again.The PU potting delays response but I figure as long at the rate of cooling of the whole bath is much much slower than the heating delay through the potting I should get reasonable results. The PU isn't too bad, the step response time constant is around 30 seconds to 63%.I'm happy the sensor is accurate at room temperature, I just want to confirm it at higher and lower temps.I'll try an ice bath tomorrow and see how it goes.Cheers.

author: DTJ
author_rank: Super Contributor
timestamp: October 07, 2017, 12:11:56 pm
content: Quote from: Kleinstein on October 07, 2017, 11:49:33 amWhen it's under test the aluminium block is about 50mm below the water surface. I handn't considered evaporation cooling - I'd sort of hoped having the water mixed constantly might minimise any thermoclines.I should try floating an 20mm styrofoam sheet on the water surface.Oil might be the way to go - very messy!

author: DTJ
author_rank: Super Contributor
timestamp: October 08, 2017, 04:56:01 am
content: OK - progress!After all this it seems I have a faulty sensor that under-reads ~ 0.3°C.I tried the ice-point test as per Ian's suggestion and it read 0.3°C low so I put a bare PCB prototype in some heat-shrink and tested that and it read within 0.02° of the reference sensor.On the plus side I found a great video on creating a good ice-point reference:and reference sheet:https://www.measurement.govt.nz/sites/all/files/training-manuals/TG01-July-2009.pdfI'm not sure why the sensor reads low. All I can think of is that I damaged it during the soldering process.