EEVblog Electronics Community Forum

Products => Test Equipment => Topic started by: splin on February 18, 2020, 02:40:56 pm

Title: Recommend low cost digital thermocouple thermometer
Post by: splin on February 18, 2020, 02:40:56 pm
I'm looking for a K-type thermocouple meter that's a bit better than the cheapy TM902 models. I don't want to spend much - £20 max ideally, but would like better resolution - 10mK would be good as well as better update rates. 2 or 3 readings per second would be good.

Absolute accuracy isn't too important, its more a question of being able to estimate rates of change of temperature and especially when temperature has stabilised, hence the resolution and speed requirement. 1mK would be excellent if somewhat unlikely in my price range!

I'd consider paying more for better absolute accurarcy and/or more channels. Datalogging would be nice but that would likely take it considerably further from my desired price point.

Any suggestions?
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: mariush on February 18, 2020, 02:54:36 pm
How about an evaluation board for a chip like MAX31850, for ex. 14$ for this: https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/101020555/1597-1682-ND/9369935 (https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/101020555/1597-1682-ND/9369935)

Add an arduino or custom program a microcontroller, and a cheap 1-2 line display and a battery and you're done.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: splin on February 18, 2020, 05:42:17 pm
How about an evaluation board for a chip like MAX31850, for ex. 14$ for this: https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/101020555/1597-1682-ND/9369935 (https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/101020555/1597-1682-ND/9369935)

Add an arduino or custom program a microcontroller, and a cheap 1-2 line display and a battery and you're done.

Yes, I had considered that but it would likely result in me buying the parts and then adding them to my long queue of to-do projects and never quite get round to building it!

I'd prefer a ready built solution.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: maxwell3e10 on February 19, 2020, 12:15:12 am
I don't think there are any cheap thermocouple meters that have 0.01 degree resolution. With 40 muV/C Seebeck coefficient you would need  nV voltage resolution. The best sensor for measuring changes in temperature without high absolute accuracy is a thermistor. And you can measure it with just an ohmmeter. Better get a high-resistance thermistor (100 k-1 MOhm) to minimize self-heating. If you want 1 mK resolution without a fancy DMM, you can put it into a bridge circuit.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: splin on February 19, 2020, 01:38:48 am
I don't think there are any cheap thermocouple meters that have 0.01 degree resolution. With 40 muV/C Seebeck coefficient you would need  nV voltage resolution. The best sensor for measuring changes in temperature without high absolute accuracy is a thermistor. And you can measure it with just an ohmmeter. Better get a high-resistance thermistor (100 k-1 MOhm) to minimize self-heating. If you want 1 mK resolution without a fancy DMM, you can put it into a bridge circuit.

Yes your righ, but I want a thermocouple meter for rapid response - I know there are some very small thermistors available but still not as fast as a thermocouple.

I guess I'm going to have to increase my budget, but looking on ebay, there seems to be a dearth of meters at all price points. I guess that's down to coronavirus? There used to be cheap two channel meters (fancier cases but no better specs than the cheap TM-902C meters) but even those seem to have disappeared.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: maxwell3e10 on February 19, 2020, 03:42:36 am
I like this kind because they can measure a bunch of different thermocouple types
https://www.ebay.com/itm/Digital-K-Type-Dual-Channel-LCD-Display-Thermometer-Thermocouple-Sensor/153703397334 (https://www.ebay.com/itm/Digital-K-Type-Dual-Channel-LCD-Display-Thermometer-Thermocouple-Sensor/153703397334)

You could hook up a thermocouple to a high resolution DMM to get 0.1 uV resolution. Or maybe one of those Aneng8008 DMMs with 1 uV resolution would be good enough.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: Tom45 on February 19, 2020, 03:56:57 am
I think you would be hard pressed to find a thermocouple meter with 0.01C resolution at any price.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: maxwell3e10 on February 19, 2020, 04:06:27 am
Out of curiosity, I switched the meter linked above to S type thermocouple, so it thinks the Seebeck coefficient is 7 times smaller than K type that is actually plugged into it. The temperature readings are wrong, of course, but the  fluctuations are about 0.2F . So, in principle its capable of reading about 0.01 C. Maybe with some hacking it could display another digit if one made a Seebeck lookup table 10 times smaller than K type.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: splin on February 19, 2020, 02:02:19 pm
I think you would be hard pressed to find a thermocouple meter with 0.01C resolution at any price.

Unfortunately it seems your right. Technically it shouldn't be a problem (<400nV p-p noise maximum) but I guess that given the inherent inaccuracy of thermocouples it simply wouldn't make sense to provide .01C resolution for most users.

I guess I'm going to have to make my own after all.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: splin on February 19, 2020, 02:03:59 pm
Out of curiosity, I switched the meter linked above to S type thermocouple, so it thinks the Seebeck coefficient is 7 times smaller than K type that is actually plugged into it. The temperature readings are wrong, of course, but the  fluctuations are about 0.2F . So, in principle its capable of reading about 0.01 C. Maybe with some hacking it could display another digit if one made a Seebeck lookup table 10 times smaller than K type.

Thanks, interesting - I might just buy one of those anyway. Datalogging would be nice but it seems that only comes at much higher price points.
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: maxwell3e10 on February 19, 2020, 05:40:12 pm
I think you would be hard pressed to find a thermocouple meter with 0.01C resolution at any price.
If you look at DMM7510 meter specs, it shows thermocouple reading with 0.001C resolution
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: Tom45 on February 19, 2020, 06:57:20 pm
If you look at DMM7510 meter specs, it shows thermocouple reading with 0.001C resolution

That's amazing. 4 wire RTD resolution is "only" 0.01C. And it will satisfy the wish for data logging too.

Spec sheet for temperature is below.

Title: Re: Recommend low cost digital thermocouple thermometer
Post by: splin on February 19, 2020, 10:39:03 pm
If you look at DMM7510 meter specs, it shows thermocouple reading with 0.001C resolution

That's amazing. 4 wire RTD resolution is "only" 0.01C. And it will satisfy the wish for data logging too.

Spec sheet for temperature is below.

Nice,  but it's quite a long way from my original £20 price limit though!  :-DD
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: Tom45 on February 20, 2020, 11:57:43 am
Nice,  but it's quite a long way from my original £20 price limit though!  :-DD

I just tried my Keysight 34465a with a K thermocouple and it displays to 0.001C too. The Keysight 34461a is quite likely the same at just over $1000. That would be less than one third the price of the DMM7510. Still quite a bit over your desired price point though.

And finally the Rigol bench multimeters display a silly 5(!) digits after the decimal point. And that can be had for well under $1000.

So there are options for 0.001C  thermocouple resolution after all. I was wrong about there being few options "at any price". But at your price point? Nope.



Title: Re: Recommend low cost digital thermocouple thermometer
Post by: NoisyBoy on February 21, 2020, 06:29:49 am
Unlike the 34465A, 34461A only supports RTD and Thermistor, K type Thermocouple is not supported.

In any event, as you said, even the Rigol option is way over the target price point. 
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: thatguy1988 on April 05, 2020, 05:52:51 pm
Ive bought on Ebay for about $20. 4 Channle reader, comes with 4 thermocouples. Excellent considering the price
Title: Re: Recommend low cost digital thermocouple thermometer
Post by: MegaVolt on April 06, 2020, 11:17:31 am
If you look at DMM7510 meter specs, it shows thermocouple reading with 0.001C resolution
Be careful with the DMM7510. To measure with a thermocouple, you need to measure the temperature of the cold junction with that accuracy. DMM7510 does not provide this :( Therefore, this becomes your new headache.