So I need to calibrate sample temperature on a small sample heating stage, ~1.5cm diameter. The heater has built-in thermocouple control, but substrates must be mounted on magnetic steel pucks and of course the temperature at the sample surface can be quite different. The obvious answer is thermocouple calibration, but taping the thermocouple to the sample or bonding with epoxy or similar will still leave some significant thermal resistance between sample and thermocouple and the thermal gradients are steep on such a small sample. I was curious whether pyrometery is an accurate option, which sidesteps this issue of thermal resistance to the temperature probe. It appears there are a number of affordable pyrometers that claim accurate readings down to room temperature or below these days. I don't necessarily know the emissivity of my sample, but I can probably figure it out by adjusting the emissivity until the output temperature matches room temperature. I could also probably account for any variation in emissivity with temperature by doing the same thing with the sample on a hotplate. I know the nominal accuracy of pyrometers is like 1% of the scale range, so that implies accuracy is likely only accurate to within a few degree, which maybe is no improvement on the uncertainty due to thermal resistance to a thermocouple temperature probe.
Has anyone tried using a pyrometer for sample temperature calibration in this temperature range? Is thermocouple monitoring still preferable?
Any good ideas for obtaining good thermal contact to a sample with a thermocouple that doesn't change the sample temperature much, with a strong preference for bonding that is non-destructive to the thermocouple?
Someone suggested thermographic phosphors for this, which seems to provide both a very high accuracy and perfect thermal contact with substrate, but I can't find any commercial suppliers of the phosphors and there is some extra equipment involved to excite the phosphors and measure the response. I think we are going to try melting point standards which at least gives perfect thermal contact with the substrate and provides +-1C accuracy, at the expense of only being able to check a few discrete temperatures. Does anyone know where you can buy a full set covering some temperature range, or do you always need to buy standards for every temperature separately? It gets pretty expensive as you check more and more temperatures.
Any attached or surface-contact probe has the downside of local interference: on a small object this effect could dominate the measurement. Pyrometers are very accurate and do not affect the measured area. The decision depends largely upon the need for accuracy or precision. There are surface paints that change colour, but these alter surface radiation. I would recommend using a simple IR pyrometer and, if necessary, doing a furnace-calibration to check the emissivity effect on the measurement (if any).
You could also try a two-colour pyrometer, which measures the temperature using two different wavelengths, such that you don't need to know the emissivity value of the object.
Do you recommend particular pyrometers for this task, in this fairly low temperature range?
There are quite a few articles on the topic - fluorescent thermometry, which has no problems from the category of "parasitic" heat flux or unknown surface emissivity. According to some data, the sensitivity of this method now reaches 0.05 °C up to a temperature of 350 °C, the reproducibility and uniformity are about 0.1 °C. In this connection, I recommend to get a grasp of this method.
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