Temperature measurement - Page 1
Forum section: electronics / projects

author: Omar
author_rank: Newbie
timestamp: October 21, 2022, 07:08:26 am
content: I wandered what would happen if I use different materials to measure the temperature instead of using the K type thermocouple included with my multimeter.....So I tried random metals around and i got different behaviour as i expected.....I tried my skin as a test object and got very strange behaviour 😂😂I can't upload the video here so here is a linkhttps://twitter.com/Omar___matar/status/1583024761625796611?t=LgVAmR2_iAzw1Q8oEynXaA&s=19Can anyone explain this ??

author: Abdurrahman YAMAN
author_rank: Newbie
timestamp: October 21, 2022, 09:21:58 am
content: You need to understand how thermocouples work.

author: Berni
author_rank: Super Contributor
timestamp: October 21, 2022, 09:28:57 am
content: The multimeter is just picking up some stray voltage from your hands. Since thermocouples produce very tiny voltages it doesn't take much to show a large reading.

author: Omar
author_rank: Newbie
timestamp: October 21, 2022, 11:15:39 am
content: I know that but the main question is where this voltage came from

author: todorp
author_rank: Regular Contributor
timestamp: October 21, 2022, 12:38:39 pm
content: The bad jinn put the voltage there...

author: james_s
author_rank: Super Contributor
timestamp: October 21, 2022, 06:34:50 pm
content: A junction between any two dissimilar metals will produce a voltage. Proper thermocouples are made of specific metals that do this particularly well, while being durable under the conditions they are intended to work in.

author: Coordonnée_chromatique
author_rank: Frequent Contributor
timestamp: October 21, 2022, 08:19:05 pm
content: The Seebeck effect refers to the development of an electromotive force across two points of an electrically conducting material when there is a temperature difference between those two points. Under open-circuit conditions where there is no internal current flow, the gradient of voltage is directly proportional to the gradient in temperature.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: October 21, 2022, 09:13:20 pm
content: The simple answer is that your body acted as an antenna and brought the stray voltage in. The details of how this happened will depend on your environment, the thermocouple and your meter. Most likely is that a tiny alternating current was capacitively coupled into the thermocouple circuit which was then rectified by a junction in the circuit. The junction may be intentional, in the input circuitry of the meter or it may be an accidental junction (a point contact junction) somewhere in the chain. In thermocouples not properly bonded this is not uncommon.It would pay to google Seebeck effect and thermocouples to find the full story on how they work. There are some details and shortcuts used in DMMs which affect operation and accuracy.

author: james_s
author_rank: Super Contributor
timestamp: October 22, 2022, 02:04:05 am
content: There's the antenna effect, and then if you have dissimilar metals you can also form a battery with salty skin moisture as the electrolyte.

author: Johnny B Good
author_rank: Frequent Contributor
timestamp: October 29, 2022, 02:30:38 am
content: Quote from: james_s on October 22, 2022, 02:04:05 amAnd, finally, we have "our winner"!

author: bidrohini
author_rank: Regular Contributor
timestamp: October 30, 2022, 10:40:31 am
content: Thermocouples are not meant for body temperature measurement. For body temperature, try IR thermometer ICs like mlx90614

author: Terry Bites
author_rank: Super Contributor
timestamp: October 31, 2022, 07:17:29 pm
content: True. A thermistor is usually stuck up your a***, at least in my medical experience. YSI of course.