TM-902C K-TYPE THERMOCOUPLE AND THERMOMETER REVIEW AND TEARDOWN

A thermometer designed for scientific and engineering applications. It accepts a standard K-type thermocouple and provide a very simple interface. There are no user-adjustable calibration nor setup capabilities from what I can see. 4 units reviewed to see how they correlate

[@REXXSEVEN]

Thank you for your time and patience in testing this product and providing this video for us. I've always wanted something like this but didn't know that a straightforward version of it existed without having to buy an expensive $80 multimeter that came with a temperature probe. Finally I found something. Much appreciated. I think I may find this especially useful when trying to fix a problem with an oven's internal temperature being being different from what the temperature knob is actually set to. It seems to be a very common annoyance that has occurred with at least three different ovens I have used over the years.

[@RolfRBakke]

Better check the left one for bad connections, it freaks out when you dropped it in the end of the video.

[@theflyinggearman3128]

It would be really interesting to know how acurate those meters are across their specturn, lets say at 800 or even 1000 degree.

[@asdfhun]

Imho when the display shows Lo the low refers to the temperature (out of range) not to the battery voltage (notice the temperature measurement gets inaccurate and displays -3x°C below 2V). The appearing battery symbol on the bottom left is the one that tells you that the battery is about to die.

[@JohnHoranzy]

I was hoping to find jumper pads to convert this to Fahrenheit readout. Still a good deal for $5. I will have to use a better Thermocouple meter for times when I cannot risk a conversion error. Swapping the thermocouple wires and seeing that there is calibration variance between the meters was very helpful. Thanks you for posting.

[@lagman8908]

I also have this meter. It says that it can measure up to 1300°C but look in the product description on ebay: "requires a different temperature probe for over 250°C". I find it very difficult to find a probe on ebay that can go up to 1300°C and that isn't more expensive than the unit itself...

[@yairhayat1400]

Great well done work! Thanks

[@celsiusswatt6021]

The offset of the readings in thermocouple thermometers is usually caused by the accuracy of the cold junction compensation (i.e., measuring the temperature of the contacts on the thermocouple connector and the wires to the point where all are copper PCB). It is easy to check this compensation by shorting the input by a copper wire. In this case the thermometer must display the ambient temperature (or the temperature of the connector). This way one can see whether the problem is in the probe or in the meter electronics. The glance at the board does not show any temp sensor around the connector and on the board, and in combination with poor accuracy of measuring the internal temperature it explains why the readings are bad.

[@davadoff]

That’s brilliant, thanks. I read up on cold junction compensation in the past but now you help me learn & understand it better.

[@freestyla101]

I was lucky enough to get one that will not turn off! Not only that but it also shows all values on the LCD at all times. Couldn’t work it out in the end.

[@REXXSEVEN]

It would be nice if these things came with a small potentiometer (mini adjustment knob) inside that you could use to calibrate the temperature if it's off by a degree or two. I wonder if the thermometer on the left is 2 degrees Lower because it had a defect from the manufacturer when it was made or is it because it's just old.

[@Ni5ei]

The accuracy stated in the manual is wrong. It should read: 0°C to 500°C: ± 0.75% ± 1°C 500°C to 750°C: ± 1% ± 1°C For a 20°C reading that means it could be off by ± 1.15°C. So 2.3°C between the highest and lowest reading.

[@danijel124]

I have the version with the red push button. How do you keep this thermometer always on?

[@superdau]

It's the same with clocks: if you have one of them, you can always tell the exact time. If you have multiple, you are no longer sure, what time it is. ;) Btw. ice water/boiling water would have been easier and will give you an absolute reference point (at least for 0°, the boiling point depends on your altitude). In any case it would have needed less goop ;)

[@LuisHPena-lj2zj]

Good, I have used this TM902C with a 12 point selector at a distance of 30 meters and the reading comes out wrong, sometimes above 10 degrees and minutes with temperature 10 below the actual temperature. How can I solve this problem?

[@bgdwiepp]

Just a note, above the switch, on the board there is a pad labeled "CAL" on the silkscreen, unsure if noticed.

[@electronupdate]

good catch.... I guess one could poke around to see if one could reverse engineer it's function

[@ToddFun]

I noticed the same thing. I'm sure if you short those two CAL pads then power on it will pop up in cal mode. Then shorting a test pad to ground or to another test pad would adjust cal +/- 0.1C per touch. That would be the reverse engineering you speak of, but that is how I would guess the die works. Find the most used most sold "generic" chip datasheet that is dedicated to temp and I'm thinking that will be you die. Such a datasheet should have some cal feature documentation.

[@max1m323]

Hi friends, I need thermometer for my pottery owen (around 1050C). Can it take measure that high?

[@reverendnant6496]

These are nice little units. My board (same part number) is a little different from yours but if you look under that switch, there should be a pad where the unit can be set up for a momentary push on/off. I actually have use for this in a Fahrenheit version. I was hoping there might be an easy way to do this as I did on a different kind of unit by soldering over two pads. I tried T1 (the left one) but that didn't appear to do anything. I am wondering about S. I do suspect that such an option would be somewhere it could be easily connected to a switch or button but that does not appear to be the case and all of these pads being hidden under the display makes it tricky to mess with. Interesting to see it works all the way down to 2V. That might present some interesting options for alternative power sources. 9V batteries are not cheap. Though I don't think this is likely to burn through them too quickly.

[@reverendnant6496]

I have identified a unit, the MS6501. It has a different button layout but the LCD has all the same symbols on it. Probably not many are interested in the Fahrenheit function but it does also have a HOLD function. I bet these are the same units internally. It would be interesting to crack one open and see if these units can be modified to have similar functionality.

[@danijel124]

Is this probe ok to test my soldering iron???

[@discoHR]

Wanted to buy one after watching this review. There are plenty of TM-902C on eBay but the description says the device measures only up to 750°C and the included probe only to 204°C. 750°C would be more than enough but 204°C is way too low for my needs.

[@electronupdate]

+discoHR buy a higher temp range k type : search for "high temperature themocouple k type"

[@discoHR]

Will do, thanks.

[@dick7540]

Could you tell me what brand it is or who did you buy it from. Thank you.

[@electronupdate]

Dick Bromberg Ebay: www.ebay.com/itm/TM-902C-Digital-Thermometer-new-version-0-1-degree-resolution-50-to-1300C-/200902258791

[@XOIIOXOIIO]

Man, I built something like this using a one wire temp sensor and arduino because I couldn't afford a thermocouple device, and now they are five bucks on ebay. I'll have to get one of these.

[@umarmushtaq7575]

Jesse Can you please Guide me in bying yours one i also need this

[@ericgee6585]

I forgot. Did you get anywhere with calibration?

[@electronupdate48]

No, i have not figured out the cal mode

[@al_kaloid]

I'm a little late, but apparently the exact version or PCB layout you present in this video (QD-902C, with the 9V instead of the newer 2xAAA 3V) can be easily calibrated by adding a trim pot to the UR3 pad (see EEVblog forum post about it). I just bought a two pack on eBay, but unfortunately the PCB layout changed quite a bit (says QD902D now and uses aforementioned 3V instead of 9V). One is around 2 degrees low and I really wanted to calibrate/adjust them properly. I don't think there is anything similar on this iteration, which is a shame tbh.

[@johnmij5828]

Nice tear down, looks just like the meter i got. Only mine is 24 years old ! Your thermometer has no temperature sensor on the thermocouple connector, looks like its on the pcb. So the cold junction compensation is rather poor. If you touch the thermocouple connector the temperature reading will change.

[@ericgee6585]

Wow I've just bought this for £2.96 inc! 14/09/2015 Thanks

[@DeeegerD]

+Eric Gee I picked mine up a couple years ago on eBay for $5 Cdn. Works well.

[@MrA754]

Do u recommend it?

[@electronupdate]

Alqumra.com For the money I paid, yes. There's better.... but, as always, that's always more expensive.

[@Ghlargh]

I have a couple of this model of meter, Comparing to other types of meters they seem to be within specifications. One thing i noticed (after attempting to test the meter's maximum readout using a gas torch) was that severely overheating the probe tip will cause the probe to permanently change it's characteristics, that particular probe measured something like 3 degrees lower after heating, i swapped the probe and the measurement was ok again. I wonder if you tried to swap the probe on that meter that measured 2 degrees below the others?

[@asanzafatimallagas3420]

Would that works for measuring flame heat?

[@Ghlargh]

@@asanzafatimallagas3420 The standard K type probes seem to be damaged by extreme heat for more than a couple of seconds, likely because the metal oxidizes, hanging it's characteristics. You might be able to coat the probe end with something that prevents oxidizing but that would also add a layer of thermal insulation and thermal mass. Not sure if it would work in practice.

[@asanzafatimallagas3420]

@@Ghlargh Can you suggest for a thermometer that can measure up to 1300C?

[@Ghlargh]

@@asanzafatimallagas3420 I haven't had the need to measure anything that high so i don't know of any. The main thing is going to be the probe, you can always find a compatible readout device but the probe tip needs to survive the temperature you need to measure. It's possible that there aren't any direct probes available for temperatures that high in an oxidizing environment (air), so there might also be some completely different methods for indirectly measuring flame temperature that i'm not aware of.

[@narayanchandilkar8154]

How to buy it.

[@electronupdate]

ebay

[@user-dc8em3ou2z]

No you get what you pay for! Mine is 30 degrees + more than all the other thermometers I compare it with!

[@user-dc8em3ou2z]

Junk!