![]() #3 subtract the voltage of the known thermocouple from the voltage of the unknown temp thermocouple. #2 locate the associated voltage on the chart. #1 take the known thermocouples temperature. If you were to use 2 thermocouples, you would The thermistor is highly accurate, but is more expensive, and has a much smaller temperature range. Thermocouples have a wide temperature range, low cost, and many different types for many different applications. There are advantages and disadvantages to both options but which is better is dependent on the use case. 2 of the most common are to either, use another thermocouple in the same circuit with a known temperature using a thermeter or use a thermistor. Now in order to measure the temperature of the thermocouple you need another way to measure temperature. Rather than a voltage correlating to the temperature of the thermocouple. Hence the confusion on different temperatures producing the same voltage, but also explains why the voltages are mirrored over 0☌, because you should really think of it as the difference in temperature between the 2 metals in the thermocouple. Otherwise you are just producing a meaningless voltage reading. Remember that a thermocouple produces a voltage because of a difference in temperature, and so because we are looking for a difference, we must have something to reference against. ![]() The confusion is coming from the fact that you cannot get the information you want with the information you are using. However at the bare minimum to clear things up you must know that thermocouples operate on the seebek effect, and essentially produces a voltage based on a difference in temperature between 2 different metals. I won't go into a lot of depth about how thermocouples work because it involves quantum mechanics and would sort of stray from the original point of the question. I realise this thread is kinda old but in case anyone else in the future is curious I will explain why there is confusion and how these charts work. The reference book I have available to me was in Fahrenheit, but 68F -> 1.306mV:Īll of the values seem to differ by 10C (1.019 mV vs 1.537 mV), except the Fahrenheit value seems to be about halfway, so I'm sure it's no coincidence that my interpretations of the charts are wrong, but using the calculators I just punched it in. Is looking up a cold junction offset value in any way different than looking up a regular value? In my example problem, I went to my chart to find the mV for 20C, should I be looking at a different chart for cold junctions?įrom my sample problem textbook, they found 20C -> 1.537 mV:įrom an online chart, I also found 20C -> 1.537 mV:įrom another chart online, I found 20C -> 1.019 mV:Īn online calculator I found says 20C -> 1.019 mV:įluke's online calculator agrees 20C -> 1.019mV: Related to #1, is there a case to use one or the other above? These appear the same, but the value that I add to or subtract from is always a mystery, specifically because all charts seem to have a far left value and far right value that are the same, and for me it would be intuitive if the right value was always +10. Some charts number from -10 to 0 in columns, others number from 0 to +10 in columns. I seem to also have found multiple charts and online calculators that contradict eachother, so if somebody could please help me work this out it would be greatly appreciated.Īs far as I can tell, my sources of confusion come from these questions: I know I have to account for the junction temperature by adding the voltage at 20C to my measured voltage, then looking up the result in the chart. ![]() How to solve the problem is straight forward. Specifically, I have a sample problem in a study guide where the measured voltage is 21.57 mV with a 20C junction and a Type J thermocouple. Studying for the PE exam, everything seems to be a breeze, but I cannot for the life of me read a TC chart.
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