IR Thermometers F vs C

The little hand-held digital infrared thermometers can be quite handy for a lot of things. I have a little Actron unit that my son gave me for a present. A discussion on the LabRats facebook page about the resolution of Fahrenheit vs Celsius go me to wondering about how the unit converts and displays the two temperature scales. The display is capable of showing degrees and tenths in either the F or C scale. So while watching the latest episode of Warehouse 13 on Hulu I did a little experiment. With the thermometer set to display F I took multiple readings of the surroundings and recorded the number on the display by making a check mark by each tenth degree if it showed up on the display. I did this for temperatures of 70.X, 71.X, 72.X and 73.X. After several hundred readings including continuous scanning there were several numbers that never showed up. I won’t reproduce the entire chart here but I was unable to get 70.2, 70.5, 71.2, 72.0, 72.2, 72.8, 73.0 and 73.7.  In analog to digital converter parlance this is called missing codes. The experiment was repeated with the thermometer set to read out in C. In Celsius I was able to get every tenth degree. What I suspect is happening here is that the sensor is being read out in some totally arbitrary binary scale that has enough bits to get the tenth degree resolution in Celsius but not in Fahrenheit which has roughly twice the resolution of Celsius. It is also possible that the internal calculations are all made to get a Celsius number and then converted to Fahrenheit for display. It was always drilled into us in school that when doing lab work, know the limitations of your instruments. Two things come to mind here. With this thermometer you are not getting tenth degree resolution when using the Fahrenheit display and the missing codes could possibly introduce artifacts into data taken with the thermometer.

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2 Responses to IR Thermometers F vs C

  1. Jim,
    Fascinating finding. I’ll check my instruments to see if they also have this issue.
    Thanks, Forrest

  2. Jim, I checked the most important IR thermometer in my 2-year study. It shows no missing units after the decimal place.

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