In a previous blog, I described the standard method for measuring emissivity. It can be found here http://www.thermalvision.ie/2012/01/measuring-emissivity/
Since posting this I have had several communications from people for whom this method just doesn’t work. There are various reasons why this can be unsuitable, including not being able to heat the object, or the object temperature being very high, and burning the pvc tape. Here are some suggestions;
Firstly we must look at the reason why we need to heat the object. The reason for this is simple, the object must be at a different temperature than the reflected environment, in order to have a thermal contrast. In other words so as we can see the tape.
Suggestion 1, bring the object into a cold environment, this removes the necessity to heat the object, once it has a higher temperature than the surroundings it should be ok. From a practical point of view there are some difficulties with this. The camera should be setup and running for a while so as it can stabilize in the cold environment. you would need to have the sample ready, camera running, and then bring in the sample and begin working immediately. It would also create problems if the ambient temperature was too cold as the object would start cooling too fast, and ideally it should not be in a transient state.
Suggestion 2, cool the object. This has problems with the formation of condensation which can cause difficulties. But it is workable provided steps have been taken to minimize this.
Suggestion 3, forget about emissivity measurements. Measure the reflectivity, and calculate the emissivity by using 1-reflectivity=emissivity. This is often the only option, particularly for low emissivity materials.
Suggestion 4, forget about the tape. Use a thermocouple to determine the true temperature. Don’t forget that thermocouples often underestimate the true object temperature, as they do not display the temperature of the object that they are in contact with, but rather display their own temperature. this is particularly problematic with objects that are in a transient state.