Thermocouple Question - Page 1
Forum section: electronics / projects

author: eerocketman
author_rank: Contributor
timestamp: June 19, 2018, 06:20:24 am
content: Hi everyone I was wondering what sort of output I would get if I flipped a thermocouples polarity at a connector that was far away from the main thermal junction. Please advise. See illustration below_————————— | | ============\| | >=========== | |——————————-/—Thanks!Ali

author: Amper
author_rank: Frequent Contributor
timestamp: June 19, 2018, 06:30:31 am
content: If i understand your question correctly, you will still not get any changed output, if your connector doesnt have any thermal gradient. The specific materials are only important when you want to be sure not to generate anything IF you have gradients. Otherwise you can also just use any connector you come across, no special materials required.

author: snarkysparky
author_rank: Frequent Contributor
timestamp: June 19, 2018, 12:58:05 pm
content: You would get a meaningless signal possible. Thermocouple voltage is developed along the line proportional to temperature gradient. What you would get is determined by the temperature at the switching point.I assume you have a situation where a mistake has been made and you can't fix it ?If the temperature at the switch point is near the temperature at the measuring instrument and the measured temperature is much different then it might work.

author: mzzj
author_rank: Super Contributor
timestamp: June 19, 2018, 01:28:11 pm
content: You get f*cked up outputPretty common problem in industrial thermocouple installations: extension cable is connected with wrong polarity on both ends."output" error depends on temperature differences between main junction(CJC) and the extension junction. The amount of error is roughly two times the temperature difference between extension junction and cold junction for the base metal thermocouples!(B-type would be the special case where polarity errors on extension cables are largely irrelevant)

author: wraper
author_rank: Unknown
timestamp: June 19, 2018, 01:34:32 pm
content: The result will be that the cable from such connector till the measuring device will produce EMF opposite to the thermocouple if a temperature gradient along the cable is not 0. Wires used for thermocouple extension are made either from the same materials as thermocouple or materials which perform similarly at relatively narrow temperature region such cable is exposed to. This is important as almost all thermal EMF is produced along the wires, not at the junction.

author: wraper
author_rank: Unknown
timestamp: June 19, 2018, 01:41:31 pm
content: Quote from: mzzj on June 19, 2018, 01:28:11 pmWhat? It should be among the most affected. As thermocouple is made from platinum/rhodium and therefore is very expensive and is used for very high temperatures, extension cable normally will be connected at a very hot place. Somewhere just outside the furnace where temperature willeasily exceed100oC. EDIT: it won't exceed 100oC as this is the max I see for B type extension cables I can find but still usually will be a very hot place.

author: mzzj
author_rank: Super Contributor
timestamp: June 19, 2018, 02:08:16 pm
content: Quote from: wraper on June 19, 2018, 01:41:31 pmHint: take a look at B-type thermocouple sensitivity (uV/C°) or total thermovoltage around 0 to 100C°https://www.thermocoupleinfo.com/type-b-thermocouple.htmB -type extension cable is just copper-copper wire and measurement relies on B-type "near-zero" sensitivity on extension cable and cold junction at low temps.Cold junction temperature measurement is also not necessarily needed with B-type thermocouple if the cold junction is within reasonable temperature span like say 0...100cSame reason as why on R and S type thermocouples 1 degree cold junction error contributes only about 0.5 degrees to final result.( and at -196 C° cryogenic temps 1 degree error on cold junction causes ~3 degrees error on K-type measurement)

author: helius
author_rank: Super Contributor
timestamp: June 19, 2018, 02:32:54 pm
content: Quote from: eerocketman on June 19, 2018, 06:20:24 amThe answer is that the output will be unpredictable, based on temperature differences that can't be compensated in the usual way.Normal thermocouple practice is that there are exactly two points at which temperature affects the measurement: the connection of the two wires at the probe end (the "hot junction" by convention, although it could be any temperature), and the connection of each wire to the measuring circuit (the "cold junction" by convention). Each wire produces a voltage by the Seebeck effect according to its Absolute Thermoelectric Power and the temperature difference across its length. The difference of these voltages in the two wires is the measured quantity.To know the temperature of the probe end, you must know the cold junction temperature because they can only be measured together. That is the reason "cold junction compensation" is required. If you introduce a third thermojunction midspan by "flipping the polarity" of the wires, you must now know its temperature as well; this could be provided by a thermistor or RTD at that point, but it needlessly complicates the apparatus, so this design is avoided. Instead, the same alloy wires are used all the way back to the measurement circuit (thermocouple extension wire).

author: wraper
author_rank: Unknown
timestamp: June 19, 2018, 02:35:28 pm
content: QuoteOk it basically has no thermal EMF in 0-50oC region. I did not see them in person, only type R and S among platinum thermocouples.