Thermocouple reading backwards - Page 1
Forum section: electronics / beginners

author: Peabody
author_rank: Super Contributor
timestamp: November 24, 2018, 12:02:53 am
content: I ordered a cheapass K-type thermocouple with digital readout - a TM-902C from Ebay - and it arrived today. The problem is that it reads cold objects as hot and vice versa. I've taken it apart, and there's no way anything could be reversed on the board, so my guess is that the two wires are reversed inside the thermocouple connector. Does that make sense? If I put my digital voltmeter on the connector, with the red lead on the plus pin, and turn on my hair drier, I should get a positive voltage, right? And if it's negative, which it is, that means the wires are reversed?Will I be able to reverse the wires using regular solder, or do they have to be welded or something?Of course I can send it back, but if it's a simple fix, I'd rather avoid the delay in getting something that works.

author: ratio
author_rank: Regular Contributor
timestamp: November 24, 2018, 12:47:52 am
content: I ordered an eBay Special two thermocouple thermometer a few years ago. Seems it operated on °C internally; when displaying T1-T2 it would read the temperature of the thermocouples, convert the temp to °F, subtract T2 from T1, convert the temp to °F, & display the temp.

author: den
author_rank: Contributor
timestamp: November 25, 2018, 01:07:22 pm
content: Ideally they should be welded or screwed together, because ideally even the connector pins are made of the same material, as wires (very questionable, if you bought it for a dollar). When you solder it, you make additional thermocouples in the connection, but since the soldered connection is small and localized we can assume it has a uniform T, so the voltages cancel out.

author: Nusa
author_rank: Super Contributor
timestamp: November 25, 2018, 01:15:41 pm
content: Have you opened the connector body? I suspect the answer to the OP's question will be obvious once you do that.

author: Peabody
author_rank: Super Contributor
timestamp: November 25, 2018, 05:39:33 pm
content: The wires are connected inside the connector body using little screw terminals. So I was able to reverse the wires without soldering. And now it works properly. Well, it's probably not the most accurate thermometer, but reacts to changes quite rapidly. The ambient reading is 22.2C, which is about right, and holding the tip between my fingers, I get 33.6C, which may be just a bit high. The operator's manual is in Chinese.What I don't know yet is whether the thermocouple will work up to 220C for reflow use with leaded solder paste. The meter goes up to 1300C, but the included thermocouple is a TP01, which the package says is "suitable" for measuring air temperature from -50C to 204C. I don't know what happens if you exceed 204C, and measure a PC board. Banggood sells one that looks exactly the same, and they say it will go to 650C. But feedback says otherwise - the shrinkwrap at the end starts to melt at 210C. So I suspect it may be necessary to spend a bit more for a thermocouple that works. Adafruit has one for about $10 that is used in a number of the toaster oven projects on Youtube.

author: amyk
author_rank: Super Contributor
timestamp: November 25, 2018, 05:45:29 pm
content: 33.6 sounds about right for the surface temperature of a human body, given that the inside is ~37C.The thermocouple itself will continue working up until it melts; it's usually the materials beside the junction which limit its maximum continuous working temperature.

author: Peabody
author_rank: Super Contributor
timestamp: November 25, 2018, 06:13:11 pm
content: Quote from: amyk on November 25, 2018, 05:45:29 pmThe outer insulation of the thermocouple wire is fiberglass, which should easily handle 220C, but the black shrink-wrap-looking stuff near the junction may be another matter. When I get time, I'll pre-heat my oven to 425F, stick the thermocouple in, and see what happens.I should mention that rocketscream.com also sells a high-temp type-K for use with their reflow controller, so presumably we know it will work. It's only about $5.

author: Peabody
author_rank: Super Contributor
timestamp: November 25, 2018, 09:41:55 pm
content: I tried this thermocouple in a 425F oven, and it seemed to do fine. The black shrink stuff got a little sticky, but that's all. Also, the Andreas Spiess Youtube video on using an Aldi toaster oven for reflow used the same thermocouple as mine, and it appeared to do ok. So maybe there's a bit of extra room in the spec.