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Author Topic: $4 Thermocouple TM902C quality and accuracy?  (Read 1485 times)

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Online kalel

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$4 Thermocouple TM902C quality and accuracy?
« on: June 19, 2017, 09:27:25 AM »
Likely the cheapest temperature meter is the TM90C with K type Thermocouple.

Price is around $4 (slightly less) with free shipping and probe.

Some mentioned points:
Specifications:
Measurement range: -50°C to 1300°C
Resolution: 1°C
Accuracy:
0°C to 500°C: +(0.75%+1°C)
500°C to 750°C: +(1%+1°C)
0°C to 20°C: +2°C

I found some images of the inside by searching it on this forum, but not much of a discussion or review (by searching the name). Since now it's an old thing already, that model at least, I guess more people have had it and used it, and perhaps even tested the accuracy/calibration (I guess calibration can be vary from unit to unit however).

If you have used it, I'd like your opinion about the quality and accuracy. It is the cheapest unit I could find, but I'm just wondering what can be expected (is it one worth purchasing for general use).
« Last Edit: June 19, 2017, 10:53:33 PM by kalel »
 

Offline MosherIV

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Re: TM902C quality and accuracy?
« Reply #1 on: June 19, 2017, 10:35:47 PM »
Quote
If you have used it, I'd like your opinion about the quality and accuracy.
Quality - poor. The on/off switch actually does not disconnect the battery. Search the web and you will find someone has posted instruction and video on how to do a modification.

I have no complaints, it matches all my other thermacouples (mostly DMM).
 
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Online kalel

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Re: TM902C quality and accuracy?
« Reply #2 on: June 19, 2017, 10:50:16 PM »
Quote
If you have used it, I'd like your opinion about the quality and accuracy.
Quality - poor. The on/off switch actually does not disconnect the battery. Search the web and you will find someone has posted instruction and video on how to do a modification.

I have no complaints, it matches all my other thermacouples (mostly DMM).

A little bit ouch about the battery. Although it depends on the current drawn, perhaps it's something that makes it last forever. Anyhow, if the switch is the biggest issue, I guess it might be well worth the money.

Is this the right video?

- It seems a bit complicated, but as some have proposed in comments there might be a simpler solution

Here's one comment from YouTube:
Quote
Based on your video I immediately tested it with my Fluke 87 IV and measured the standby current of the TM-902C in the off position.   It was in the micro amps draw range.    I have left the battery in and there seems to be no issue of excessive battery drainage

I guess this might be different for different units, but... hopefully it isn't.
 

Offline MosherIV

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #3 on: June 19, 2017, 11:53:05 PM »
Yes, that is the video.

I think the mod is tricky because the switch is left being held in place by just 1 pair of solder lugs.
When I did it, I was scared I knacked my one because the readings were all over the place just after I did the mod. I realised that the circuit was still hot from the soldering.
I had to wait a couple of hours before the unit went back to normal and in cal.

The mod changes the switch from just un-biasing the transistor to actually isolating the battery - it is worth doing.
Or simply remove the battery when you are not going to use it for a while.
 
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Online Gyro

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #4 on: June 20, 2017, 03:01:51 AM »
Haha, the first time I've seen that video. Unfortunately it misses a fairly fundamental finding - the IC in the thermometer is actually designed for a 3V battery supply!  ;D

I found this out by accident when the first one I bought smoked when I put the battery in (dead fake regulator). After a bit of investigation I found that the main IC needs 3V. It even has a low battery indicator on the display which cuts in as a pair of AAA alkaline batteries go flat - and before accuracy fails. The battery compartment is also perfectly sized to accept a 2xAAA battery holder.

One other point - there are two versions around, one with 1'C resolution and one with 0.1'C resolution. You want the latter of course. I did a writeup on here somewhere, I'll take a look later.

EDITS:

- Ah, ok. It wasn't much of a writeup, it's the last post in this thread, which includes a rather useful review and comparison test of multiple units:
http://www.eevblog.com/forum/testgear/tm-902c-cheapest-dedicated-k-type-thermocouple-thermometer-review/

- I have a pair of these units which I have calibrated to consistently match within 0.1'C (after decent settling time). The PCB includes a footprint for an adjustment pot as you can see in the PCB photos in this thread:
http://www.eevblog.com/forum/reviews/tm902c-digital-thermometer-info/msg184850/#msg184850

You can also see the SOT-223 regulator that I removed and bypassed on my units for 2xAAA battery operation (note the 'idiot diode' across the battery leads too). The SOT-23 package transistor, Q1 is used as the temperature sensor for the cold junction compensation. On mine I have also added adhesive copper foil around the thermocouple connector and back of the PCB to try to keep temperature differential between the connector and Q1 as low as possible.

I can't remember exactly, but I think I calculated that, by changing from a 9V battery to 2xAAA the battery life extended to something like 500 hours.

You can buy thermometers with dual thermocouple inputs fairly cheaply too but I find that having two of these is more useful... You don't have to worry about ground loops or how far apart the measurement points are.
« Last Edit: June 20, 2017, 03:51:48 AM by Gyro »
Chris

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Online kalel

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #5 on: June 20, 2017, 04:10:36 AM »
I read your posts there, and wondering, what kind of battery holder for the AAA that fits inside did you use?

I have to contemplate the possibility of having to change it as well if I get this unit - because as you mentioned, not only is it less efficient (might be something I can live with to spend less time on modding), but the 9v battery can even burn it out, which is not so great.
 

Online Gyro

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #6 on: June 20, 2017, 04:28:31 AM »
Hi kalel,

Just a basic ebay wire-ended one like this (many sellers):

http://www.ebay.co.uk/itm/1X-Battery-Storage-Case-Box-Holder-Leads-Fr-2xAAA-3V-Battery-with-Wire-EA79-/332221431192?hash=item4d59f0bd98:g:s8sAAOSw~FJZGWQK

With a little bit of foam behind it to stop it rattling.

The logic of whoever designed these things really eludes me! Why put an expensive 9V battery and a regulator in when the thermometer chip is actually designed for a 3V battery supply. I couldn't believe my eyes the first time I wound the voltage down and the Low Battery symbol popped up on the display (Edit: at the correct threshold)!  :palm:

Measuring against a lab grade total imersion mercury thermometer they're actually pretty linear, even at 0.1'C resolution. Most of the time I use mine for monitoring temperature rise rather than absolute temperature, so having two that track accurately is nice. You don't have to bother of course. Bypassing the regulator is easy, just snip its legs and bridge the appropriate pads. There's no need to mess around with the switch at all as the leakage current disappears along with the regulator.
« Last Edit: June 20, 2017, 04:50:07 AM by Gyro »
Chris

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Online kalel

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #7 on: June 20, 2017, 05:19:47 AM »
Hi kalel,

Just a basic ebay wire-ended one like this (many sellers):

http://www.ebay.co.uk/itm/1X-Battery-Storage-Case-Box-Holder-Leads-Fr-2xAAA-3V-Battery-with-Wire-EA79-/332221431192?hash=item4d59f0bd98:g:s8sAAOSw~FJZGWQK

With a little bit of foam behind it to stop it rattling.

The logic of whoever designed these things really eludes me! Why put an expensive 9V battery and a regulator in when the thermometer chip is actually designed for a 3V battery supply. I couldn't believe my eyes the first time I wound the voltage down and the Low Battery symbol popped up on the display (Edit: at the correct threshold)!  :palm:

Measuring against a lab grade total imersion mercury thermometer they're actually pretty linear, even at 0.1'C resolution. Most of the time I use mine for monitoring temperature rise rather than absolute temperature, so having two that track accurately is nice. You don't have to bother of course. Bypassing the regulator is easy, just snip its legs and bridge the appropriate pads. There's no need to mess around with the switch at all as the leakage current disappears along with the regulator.

Thank you for that information, it's seems worth doing that mod.
 

Offline Nusa

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #8 on: June 20, 2017, 05:20:55 AM »
I had two of those tucked away, so I dug em out and sure enough the battery was dead in the one that had a battery in it.

I put one on the power supply and it was happy down to 2.3V input to the regulator. I will be modding them to the AAA configuration.
 

Online Gyro

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #9 on: June 20, 2017, 05:28:17 AM »
While you've got it apart to solder in the new battery leads, you may as well chop the regulator too - then you'll get full Alkaline battery life (down to 0.8V/cell) and lose the off state leakage current too.

That's unless you plan to really bodge it by just cutting off the 9V battery clip and soldering to the wire ends that is!  :P
Chris

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Offline Nusa

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #10 on: June 20, 2017, 05:47:23 AM »
Never fear, I planned to do it properly. I just didn't have a clue it was needed until I saw this thread.
 
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Offline MosherIV

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #11 on: June 20, 2017, 06:20:13 AM »
there are two versions around, one with 1'C resolution and one with 0.1'C resolution.
Really :palm:

There is little point in 0.1'C resolution when the absolute accuracy of K-Type thermocouple is 1'C
https://en.wikipedia.org/wiki/Thermocouple
Look for the table near the bottom of page.
That means that any temp reading is + or - 1'C from the actual ! That is worst case. Unless you have determined the actual deviation, you will never be sure if it is actual +0.8'C or some other number.
The best resolution thermocouples are T-Types according to the table with 0.4'C.

The only use the fraction of 'C is - is if you want to know relative temp change.

FYI these cheap chinese copies are of an old 1980s model, check out this old thread :
http://www.eevblog.com/forum/reviews/how-well-do-cheap-thermocouples-work/
Look for this picture :


This original contains a lot more circuitry because it has the proper 'cold junction conpensation' circuit. There are modern Cold Junction conpensation chips around but they cost more than the whole cheap circuit!
I suspect they are using a transitor to monitor ambient temperature, hence when I did the mod the temperature read out was screwy for an hour until the SOT23 transistor returned to ambient temp.
Again read all about it on the wikipeia page or search for how thermocouples work and what the proper circuit should be.

(Having cold junction compensation still does not fix the absolute temperature accuracy of the thermocouple itself)

Saying that, I am amazed how well the bodged circuit works!  ;)
 
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Online Gyro

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #12 on: June 20, 2017, 06:53:05 AM »
Wow, they have, err, cost reduced it a bit in the past few decades!  :D (I thought that case looked reminiscent of something I saw in Practical Electronics!). It's a shame you can't get hold of the originals any more, but as you say the bodged ones do work very well, considering. Actually It would be possible to move Q1 off the board and mount it on thin flying leads against the connector pins (with a bit of Kapton for insulation), I might try that.

True enough, you're not going to get 0.1'C accuracy but with two matched thermometers you can certainly resolve fractions of a degree... and get a reasonable calibration of an individual thermocouple over your range of interest, as long as it isn't too wide (assuming that you calibrate it). The original looks to have had adjustments for spot and slope though.

The one you show could well be the origin of the 9V battery supply (I don't see a regulator), but you'd think they would know the supply characteristics of the chip they are putting on the board now - it's a COB after all! The battery holder is an easy fit in the existing battery compartment, maybe they just never tried.  :-//

It's impressive how many variants you can fit into one model number isn't it!
« Last Edit: June 20, 2017, 07:15:13 AM by Gyro »
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Offline Nusa

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #13 on: June 21, 2017, 10:23:52 PM »
Modification complete on my two units. All I did was:
1) removed the 9V connector
2) removed the 3V regulator and cleaned up pads
3) solder-bridged the regulator input & output pads
4) added wires from the 2xAAA battery holder
5) soldered the switch mounting tabs for better mechanical support

And that's all there is to it. I did the power supply test again. The battery indicator comes on at 2.3V. The display changes to "Lo" at 2.2V. New AAA batterys provide 3.3V, and that seems to be fine too. Obviously, much more than that would probably kill it with no regulator to protect it.

The regulator that was in it was rated up to 24V input...so the original configuration would've been great to hardwire into a 12-15V vehicle system, or similar scenarios.
 
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Online Gyro

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Re: $4 Thermocouple TM902C quality and accuracy?
« Reply #14 on: June 22, 2017, 04:52:07 AM »
Glad you did it properly, not that I ever doubted you.  ;)

Quote
The regulator that was in it was rated up to 24V input...so the original configuration would've been great to hardwire into a 12-15V vehicle system, or similar scenarios.

Assuming it was a genuine regulator! I suspect that the thermocouple is elevated from the battery negative terminal too. Your battery life should be so long now that it won't matter if you forget to turn it off for the odd week or so.


P.S. I'm pretty sure I tested mine up to about 3.6V but I didn't want to push it too far.
« Last Edit: June 22, 2017, 04:57:07 AM by Gyro »
Chris

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