DMM thermocouple reading? - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: May 03, 2013, 01:51:42 pm
content: Hey guys, I've got a DMM here that came with a thermocouple. Ever since I've got it though the thermocouple reading has been wrong, always reading over the actual temp by around 20C or so.I have confirmed it's not the thermocouple itself by trying a different one, so something is skewed in the meter itself. I've opened it up, and can't really locate any thermistors, unless it's hidden in a package similar to a SMD resistor or capacitor, or maybe doesn't have one at allSo, my question is, is there somehow I could tweak the thermocouple or meter so it reads correctly? There are 7 potentiometers inside the meter, I'm guessing one of them might have something to do with it, but I don't exactly wanna start twiddling with the potsEDIT: OK, I got bored, so I started playing with the pots, as any good experimenter would doAnd I seem to have actually fixed it. This pot had quite a huge effect on the temp sensor readings, but I tried both resistance, capacitance and voltage measurements, and it didn't change them, so I'm pretty confident the only thing it changed was the temp reading. I'm so glad it works now, and such a simple fix, but it could have gone horribly wrong so easily tooDan

author: mikes
author_rank: Regular Contributor
timestamp: May 03, 2013, 02:27:35 pm
content: A thermocouple doesn't provide a direct reading of temperature. It provides a reading of temperaturedifference. A thermocouple is basically just a connection of two different materials. The common "K type" uses Nickel-Chromium and Nickel-Aluminum. If you have a thermocouple wire, the wire is shorted at one end, you measure at the other. The voltage tells you the temperature difference between the ends.So, for your meter to tell you the actual temperature of the thermocouple, it needs some way to known it's own local temperature (specifically, the temperature of the terminals where the thermocouple is connected). I have no idea how your meter does that. Some may require a second, local thermocouple. Some have a thermistor, or other temperature sensor somewhere. Some just tell you the temperature difference, and you have to figure it out.I'll assume your meter is just showing the difference. If the thermocouple is at roughly the same temp as the meter, it should show close to 0.

author: Things
author_rank: Regular Contributor
timestamp: May 03, 2013, 02:39:58 pm
content: After adjusting the pot to get it back to a reasonable reading at room temp, I held the thermocouple between my fingers, and the reading shot up to around 42C. Putting it on my soldering (Not temp controlled, mind you), it shot up to around 480C.So I'm not sure what's going on here?

author: mikes
author_rank: Regular Contributor
timestamp: May 03, 2013, 03:26:39 pm
content: Why don't you tell us what type of meter and what type of thermocouple you have?

author: Jeff1946
author_rank: Regular Contributor
timestamp: May 03, 2013, 03:32:46 pm
content: Standard way to calibrate thermocouple is to use ice water and boiling water. Put some ice and water in an insulated container like a coffee mug, leave for a few minutes as long as both water and ice are present temperature should be 0 C. Gentlely boil some water, temperature should be about 100 C, if your altitude is significantly different than sea level look up actual temperature. You also could stick the TC under your tongue and temperature should be about 37 C.Note the pot you adjusted may be a gain control which should show up if difference in temps of two points is not about what it should be. Be careful touching TC to soldering iron. Solder could dissolve into TC and affect its performance. Also for uncoated TC salt water is bad.You also can measure the TC on the voltage scale of your DMM. At ambient it should be very close to zero. Just look up TC voltage in a table and see if you get appropriate voltage change at other temperatures.

author: ddavidebor
author_rank: Super Contributor
timestamp: May 03, 2013, 03:38:38 pm
content: You need to boil distiled water twice.One for remove gas, the other is the real test.

author: Things
author_rank: Regular Contributor
timestamp: May 03, 2013, 04:29:26 pm
content: The DMM is a QM1571 from Jaycar, there's a full-res pic here:https://dl.dropboxusercontent.com/u/203420/DSCF1055.JPGVR5 and the other pot to the right near the bodgy diodes are the only 2 that seem to have an effect. I think the one near the diodes may be gain, but really I have no idea. It just seems to be way oversensitive.I can get it to the point where it's right at room temp, say 27C, then if I put it under my tongue, it goes right up to like 42C. So then if I try tune it so the body temp is right, it undershoots the room temp down to like 18C. No idea what's going onDan

author: Galaxyrise
author_rank: Frequent Contributor
timestamp: May 03, 2013, 06:31:35 pm
content: I wonder if you can get a calibration manual from the manufacturer.

author: muvideo
author_rank: Frequent Contributor
timestamp: May 03, 2013, 07:04:15 pm
content: Once I had some headaches from a very cheap dmmwith TC capability. I tried to compare it's readigns withanother DMM and I had some diffences in readings, fewdegrees apart.Then I tried swapping the termocouples between the metersand the readings went all over the place, withoutany meaning anymore. Turns out that the cheaper meter hadit's TC polarity wrong. And it's TC sensor also had polarityinverted. I can only imagine, that once the producer realizedto have the wrong polarity on the PCB (or plastic enclosure maybe),they solved the problem swapping + and - in the thermocouplesensor, so their meter could read it properly... LOL