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Author Topic: Multiplexing a Thermocouple Input  (Read 6739 times)
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Does anyone have a suggestion on what multiplexer I should use for 4 thermocouples into one MAX6675?

I worked with thermocouples for decades in a refinery. They are a real pain to interface with correctly and are not all that accurate to begin with. They were cheap and could be run many hundreds of feet in length, so if you needed to have hundreds of temperature measurement points in a chemical plant they were the preferred method of times bygone. Now they use mostly RTD sensors for new construction.

 Unless one needs to utilize their high temperature capabilities (above 500f) then there are cheaper more accurate and easier to use temperature sensors avalible. RTD are nice and several IC temp sensors are good also. Thermistors can be inexpensive and useful, but calibrating can be a pain the ass.

Lefty


« Last Edit: February 24, 2011, 02:09:58 pm by retrolefty » Logged

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Do you have any links to RDT's that might be useful in a smoker project? 3-4' long braided cover pointy tips for meat, not so pointy for cabinet. As I have found out thermistors are a PIA to try and get right... thermocouples (the kind I need) are easy to get and the MAX6675 gives you a SPI output.

As always I'm open to sugesstions.

John
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What temperature range are you needing to measure?  That is one of the most important specifications for selecting a sensor.

And don't just tell us cold to HOT.  smiley-grin
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If he's smoking meats like I am, then 50-200F would be interesting.  50-400F would be great for also measuring the smoker temp.  I too would love to find a cheap, accurate & food safe alternative to thermistors if available.
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http://cgi.ebay.com/ET-73-Maverick-ORIGINAL-Food-3ft-1m-Replacement-Probe-/290359161273    US$ 11.85

If you want to use thermocouples, then you must live with the limitations (the finicky wiring, and ice-point interface in particular). You will have to make the trade-off decision whether using cheap thermocouple probes is worth the higher overhead for your project.

I think that's the thermistor that JT said he tried.  Actually reading his comments again, looks like he tried the above thermistor *and* thermocouples (hence the title of this post).  Honestly, sorta surprised that I haven't been able to find the Steinhart-Hart coefficents listed anywhere for that part since it seems everyone points at it as the solution for tracking food temperature (pretty sure it's the same used by Thermoworks)... actually come to think of it, I own one of the Mavericks- I should test their resistance values to make sure.

Anyways, looks like I won't be finishing building my race bike this weekend afterall, so looks like I'll have time to figure out the S/H coefficents myself.
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I can't speak for JH, but I can say for a complete newbie such as myself, figuring out how to properly use this thermistor has been bit of a challenge.  There's a lot of general documentation about thermistors, but I haven't found a start to finish "how to" for how to setup an unknown thermistor.  Also, for these thermistors, there is NO technical information on them (B values, R @ 25C, etc) included and unless you get lucky and find the Amwei website and do a little digging you're completely on your own, and even then the specs don't seem complete (no Alpha value for example).  Of course, I'm just assuming the parts are the same!

Or you can be like me and miss-understand how to calculate the S/H coefficents and rather then measuring resistance at different temps, you measure the analog voltage via the Ardiuno and then wonder why the numbers don't make any sense. smiley  I'm still not sure about what resistor I should use in the divider circuit... 10K seems to work pretty well, but the listed R value at 25C is ~98K which I thought meant using a 100K resistor.

Anyways, I've got all day saturday to figure this out.  Sunday I'm going to smoke a brisket, so hopefully I can do a little real world testing.
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From what I've read and the testing I've done, thermistors are extremely non-linear (significantly more so then other temp probes) so taking a few reference measurements will only really work if you care about a very small temp range.  In my case at least, where I'm tracking the temp of the meat & smoker temp, I'm looking at a real world range of about 50-350F which is a bit much from what I can tell without taking many many measurements.  Or take fewer measurements and apply the S/H equation.

Seriously though, now that I understand what needs to happen to make the S/H equation work, it's pretty easy.  Take a few reference measurements, process that data with some code available on thermistor.sf.net and then plug the resulting coefficents into the Arduino Thermistor4 library.  Of course, I still have to read up on the whole AREF thing to make sure my analog reads are accurate, but that's required no matter what.

Either way, I'm happy to admit I'm over engineering this- do I really need data logging to generate RRD graphs?  Prolly not.  3 meat/smoker probes and an ambient temp sensor?  Total over kill.  But considering how much work this whole project is, not really a big deal and as long as I'm learning something I'm having fun. smiley
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FYI, I can now say that the Thermoworks and Maverick/RediCheck probes are not the same.  After comparing them side-by-side there are a few visible cues to indicate they are different- the Thermoworks has a clear silicone strain relief on the probe end for the wire and the connector is grey rather then black being the two easiest.  The Thermoworks wire is thinner and the probe is slightly different shape as well.

Also, comparing the resistance values at a number of temps shows that they're different enough that the S/H coefficents would not be usable.  Anyways, I'll be posting the table of R values vs. Temp and my calculated coefficents for the Thermoworks probes in a different thread if anyone else would like to use them.
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Anyways, I'll be posting the table of R values vs. Temp and my calculated coefficents for the Thermoworks probes in a different thread if anyone else would like to use them.

Great, I'll take mine medium rare, thanks.  smiley-wink


Lefty
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What temperature range are you needing to measure?  That is one of the most important specifications for selecting a sensor.

Yes, I'm smoking meats and other foods so 50F to 350F would be fine.

John
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JT did not mention WHY he thought he couldn't use that replacement probe. If they can use that probe for a dozen different ultra-cheap thermometer products, I cannot see why it would be difficult to read with Arduino.
Thermocouples would be my LAST resort as a solution for this project.  For all of the reasons JT has observed.

I have not been able to generate the proper Steinhart-Hart coefficients so far to get the ET-73's to work. I've gotten within 40F of actual...

John
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FYI, I can now say that the Thermoworks and Maverick/RediCheck probes are not the same.  After comparing them side-by-side there are a few visible cues to indicate they are different- the Thermoworks has a clear silicone strain relief on the probe end for the wire and the connector is grey rather then black being the two easiest.  The Thermoworks wire is thinner and the probe is slightly different shape as well.

Also, comparing the resistance values at a number of temps shows that they're different enough that the S/H coefficents would not be usable.  Anyways, I'll be posting the table of R values vs. Temp and my calculated coefficents for the Thermoworks probes in a different thread if anyone else would like to use them.

Let me know when you do if you don't mind.

I've also found several examples of thermistors that use a look up table, but I've not figured out how that works yet.

John
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Relays are the safe bet.

Quite the contrary - a junction of dissimilar metals sat next to a heating coil?  And multiplexing might be at kHz rates, relays switch slowly and wear out.  I'd go for high-performace CMOS analog multiplexer chip.
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Let me know when you do if you don't mind.

I've also found several examples of thermistors that use a look up table, but I've not figured out how that works yet.

John

Will do.  Do you have a link of a food-safe thermistor w/ lookup table?  Should just be able to plug in the values into the S/H coefficient calculator and save the effort of manually measuring a bunch of temps.  FWIW, Amwei has some look up tables, but unfortunately it doesn't seem valid for the meat probe on their website (b values don't match).
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I used the Maverick ET-73 thermistors but have not found a lookup table for them only examples of using a thermistor with a lookup table.

John
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