Thermocouple - Page 1
Forum section: electronics / projects

author: bifrost1107
author_rank: Newbie
timestamp: August 13, 2019, 05:11:08 pm
content: Hello, I am having some difficulties with thermal readings. I am trying to build arduino based desoldering gun control. I am using max31855 (with level shifter) to arduino for measuring temerature inside heat element. Readings are fine until it hit's ~150C+, then I start getting "something is wrong with thermocouple" error on arduino adafrut 31855 example sketch. Sometime it read temperature and over 150C few times, sometime just give me error message.... I am having some strange resistance measuring on it, if I measure it + to - I get ~10 Ohms (on ~100C) and other way around - to + i get 0 Ohms, on room temperature it's 1.4 Ohms + to - and 1.2 Ohms - to +. I don't know witch type it is, but one end is magnetic (negative end) so i am pretty sure it is thermocouple. Can any1 confirm me thermocouple should not act like this, so i know it is failed thermocouple? And can it be something else if it's not thermocouple to act like this?Thank you.I am using this heat elementhttps://www.pcb-soldering.co.uk/c005a-heating-element-soldering-iron.htmlEDIT:I tested with another k type thermocouple, everything is fine with with max31855 measuring and it, yet it's also act same as the problematic one with resistance measuring... And if one in heating element is broken it would not giving me any resistance....

author: magic
author_rank: Super Contributor
timestamp: August 13, 2019, 08:00:05 pm
content: Resistance of a few ohms seems normal and yes, accurate measurement is difficult because it generates voltage if the end isn't kept at exactly the same temperature as the plug, that's sorta the whole point of a thermocouple.Negative is magnetic in type K.I'm not quite sure why a thermocouple would work up to 150°C and then stop. Maybe some mechanical problem, loss of connection when it heats up? Is it accurate when it appears to work? Test it in boiling water or whatever.

author: bifrost1107
author_rank: Newbie
timestamp: August 13, 2019, 08:26:38 pm
content: Yes it's accurate when return temperature value, I measure tip temerature with other thermocouple on multimeter... It's does no have to be 150C exactly but around it... sometimes it's works correctly for short time, and on higher temps. I added readError code to sketch, and I am getting 2 & 4 witch is shot to gnd and short to vcc, I don't think it's fault sensor in heat element, as i sad before I remove it from circuit when it start problems and measure resistance and it's not open.... Yet when it start bugging I remove it and connect one from multimeter its reading fine.... So my guess is cold joints maybe, I could not solder it on wires so i crimp it, and probalby some more passive components... I forgot to mention i am using 0.1uF ceramic cap on TC terminals and 2 ferrite coild one on each end of TC.Edit: found problem. So I have a full circuit there... I am driving Triac (with AC) it's opto isolated... Funny thing I have a transformer hooked inside ATX PSU enclosure with switch to turn it on.... Moment i put power cable from mains, transformer is off but thermal readings start to bug.... What is it? If i unplug cable everything works fine. Transformer is off could it be Earth wire it's ESD....?EDIT2: Yes definitely! It's Earth wire. Solved. Thank you.