Thermocouple + DMM = 4C Below True - Page 1
Forum section: electronics / beginners

author: cobalt232
author_rank: Contributor
timestamp: April 13, 2017, 04:05:35 am
content: hi,i am trying to use a k-type thermocouple with the siglent sdm3045x multimeter.i keep getting a reading that’s about 4C less than the actual room temperature.the thermocouple is accurate with my agilent handheld.i was under the impression that the siglent was factory setup to work with k-type ones.can someone shed some light?thanks

author: ebclr
author_rank: Super Contributor
timestamp: April 13, 2017, 04:27:53 am
content: The thermocouple is a differential sensor, who generate a mv based on the difference of temperature between, Hot junction and the cold junction, the Hot junction is the probe itself.At room temperature, thermocouple output will be always 0 because hot and cold junction will be at same temperature, then Thot - tcold = 0, and the instrument will show only reading based on the internal cold junction compensation sensor.Just put a short circuit and the room temperature must be displayed, if not internal sensor is out of calibration or damaged

author: TheAmmoniacal
author_rank: Unknown
timestamp: April 13, 2017, 05:35:44 am
content: Sure you have the orientation correct?

author: Rerouter
author_rank: Super Contributor
timestamp: April 13, 2017, 09:18:59 am
content: thermocouples need cold junction compensation, if it reads correctly with another meter i would suspect its internal temp sensor is reading slightly off, and this is causing the difference in your readings,

author: nfmax
author_rank: Super Contributor
timestamp: April 13, 2017, 09:52:01 am
content: This instrument claims to include cold-junction compensation, and also multiple different types of thermocouple. Check you have selected the correct thermocouple type 'KITS90', and also check that relative mode is off (see the manual for this).Also, check that you really do have a K-type thermocouple! You may also find a glass of ice water useful as a temperature reference.

author: cobalt232
author_rank: Contributor
timestamp: April 13, 2017, 05:34:00 pm
content: well i set the siglent to kits90 and made sure that relative mode is off.still no joy.then i shorted the circuit and it read 4C ABOVE the correct temp.so the amprobe tpk-59 causes the dmm to read 4C below and the shorted circuit reads 4C above.any further suggestions before i send off an email to siglent that probably won't be answered?thanks

author: nfmax
author_rank: Super Contributor
timestamp: April 13, 2017, 08:38:15 pm
content: Try setting a different thermocouple type - is it still 4?C off? Or some other temperature error

author: cobalt232
author_rank: Contributor
timestamp: April 13, 2017, 10:00:20 pm
content: shouldn't it just read room temperature with a shorted circuit though, as someone previously mentioned?there are several different types of thermocouples to choose from. i guess i could try . . .

author: dacman
author_rank: Frequent Contributor
timestamp: April 14, 2017, 12:36:19 am
content: SDM3045X Type K: 0.5 degrees CelsiusSDM3045X Cold Junction: 2.0 degrees CelsiusStandard Type K Thermocouple: 2.2 degrees CelsiusTotal Error if added: 4.7 degrees CelsiusNote: Apparent error just using the DMM 600 mVdc specification, converting 23 degrees Celsius to Type K mVdc, applying the upper specification limit, and converting it back to temperature is about 1.2 degrees (vice the specified 0.5). Doing the same thing at -260 degrees results in an error of over 12 degrees (vice the specified 0.5).

author: cobalt232
author_rank: Contributor
timestamp: April 16, 2017, 03:47:52 am
content: well siglent got back to email quickly. that's good.i was asked to check the thermocouple in an ice bath . . . it read 0C correctly.then i was asked for a raw voltage measurement while the dmm was shorted in the temperature mode.that gave a value of about 5 micro volts.the thermocouple works accurately at room temp. with the fluke and agilent.maybe i was expecting too much and it is functioning within acceptable parameters.