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I'm working on a project, which is focused on taking frequent (250ms interval or smaller) measurements of extreme temperatures (0-700 C) in 16 points. I need to take measurements of multiple thermocouples, and I'm having trouble deciding on the best approach.

One of my ideas was to use 16 thermocouple amplifier ICs that are I2C enabled, putting all of them on a PCB with STM32F4 and taking measurements every 250ms. However, most thermocouple amplifier ICs like MCP96RL00 allow for only 8 devices per bus, which creates the need to use two I2C buses for 16 thermocouples.

Then, I was thinking of using a single, good thermocouple amplifier IC, with cold junction compensation and all the fancy features, connecting all thermocouple negative terminals to a common piece of copper on a PCB and using a ultiplexer to switch between which thermocouple's positive terminal goes to the IC. However, I have very poor experience with analog electronics, and I'm not sure if there even exists a multiplexer, that allows voltages as small as those of thermocouples (milivolts), has low resistance, and has 16 channels. I wasn't able to find one such device.

Electromechanical relays could do the job, but they might be too slow for going through 16 or more thermocouples every 250ms.

How to approach this problem? Is any of my ideas viable? Would you suggest something more optimal? If the multiplexer way is viable, can someone point me in the right direction of a multiplexer, which can be used this way? I don't need extreme precision - 2-5 degrees Celsius of precision would be acceptable. Worst-case scenario, 16 MCP96RL00 will do the job, but I'd like to optimize the costs wherever possible, to stay within the budget - it's a student project with a very tight one. In case of expanding to 64 or 128 thermocouples, the costs get insane.

PineLel
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  • Do cost considerations prevent using multiple good thermocouple amplifier ICs? – Andrew Morton Jun 26 '21 at 21:35
  • If that was my precision and I had that number of thermocouples I certainly would try to mux them to save on the cost of amplifiers ICs. THere are a few gotchas though about the mux. I don't remember what they are but I know there are articles about it which you can Google. At worst, you can forego on solid state muxes and go with electromechanical relays but remember...you are dry switching those relays. Really dry switching so select and use them well and don't ever run currents through them at any time over their life. – DKNguyen Jun 26 '21 at 21:36
  • @AndrewMorton Unfortunately, yes. It also lacks scallability, and in case of expanding the system, to, say, 64 or 128 thermocouples, which is a possible turn of events in the future, the cost gets way too high. – PineLel Jun 26 '21 at 21:44
  • @DKNguyen I'm worried that electromechanical relays may be too slow for going through all 16 inputs every 250ms – PineLel Jun 26 '21 at 21:49
  • @PineLel Your profile doesn't say what country you're in. What parts are available to you? Do you have access to all of DigiKey, Mouser, samples, etc? Or, do you have regional limitations? [Not taking into account the siliconocalypse which we all suddenly found ourselves in.] – Nick Alexeev Jun 26 '21 at 22:07
  • MAX31850 comes to mind. It has a onewire connection, not I²C, so you can hook up as many devices to the bus as you want. – Janka Jun 26 '21 at 22:08
  • @NickAlexeev Poland - Mouser and DigiKey are fine, probably no regional limitations - business as in the entire EU and USA – PineLel Jun 26 '21 at 22:15
  • I'd argue that multiplexing 16 thermocouples is too risky or expensive for a student project. – DKNguyen Jun 26 '21 at 22:17
  • @DKNguyen How so? A single, good thermocouple IC and a MUX will probably be cheaper, than 16 thermocouple ICs. – PineLel Jun 26 '21 at 22:23
  • @PineLel Are the thermocouples grounded or floating? More generally, could you give us an idea about the end use of this device? That's going to put some of your requirements into a perspective. 250ms refresh rate is fairly fast for temperature measurements, potential scaling to 64 and 128 channels, cost considerations. [By the way, I don't think that this is too risky for a student summer project. Especially if you've got a decent mentor.] – Nick Alexeev Jun 26 '21 at 22:24
  • @PineLel Let me rephrase: I would argue that any project involving 16 thermocouples, individually amplified or muxed is overly ambitous for what I assume is an undergrad capstone project. I say muxing is risky because there are gotchas there and as a student project with everyone doing this for the first time, you are unlikely to have the time to resolve things or have board re-spins if it doesn't work the first time around. – DKNguyen Jun 26 '21 at 22:24
  • @NickAlexeev The purspose of the device is to probe the temperature of many points of a hybrid rocket motor during a static fire test. The outer case, in extreme conditions, can get from room temp to 700C in mere seconds, and the temp data is very valuable for evaluating any design flaws. The more measurement points, the better our analysis can be. Yes, the best way here is to use a thermal camera - but that's waaay outside of the budget. – PineLel Jun 26 '21 at 22:32
  • Are the thermocouples floating or grounded junction? What means will you use to connect to them? Any "chip" with built-in CJC sensing is not great by instrumentation standards. – Spehro Pefhany Jun 26 '21 at 22:54
  • Do not forget about cold junction compensation. There are commercial systems that do exactly what you are trying to build. Look up a DTS4050 16 channel Thermocouple Scanner as an example. – Gil Jun 26 '21 at 23:11
  • A quick Google for 'k type multiplexer' gained some good hits. There's a number of examples using a mux chip ahead of the thermocouple amp. There's even an open source project for the RasPi that would be easily adapted. – Kartman Jun 27 '21 at 04:54
  • +1 for actually putting your accurayc tolerance in the original question! – Neil_UK Jun 27 '21 at 06:35
  • Many MCUs have more than one I2C peripherals. Even if they don't, you can software-bitbang any amount of buses you want. And I2C bus multiplexers do exist, so a single 8 channel multiplexer allows to connect 64 chips to single MCU. You can chain up to three levels of multiplexers too, so having a single I2C bus or 8 chips per bus is really not an issue. – Justme Jun 27 '21 at 08:53

2 Answers2

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Use case

The purpose of the device is to probe the temperature of many points of a hybrid rocket motor during a static fire test. [From comments]

Requirements

K-type 0 to 700°C
accuracy ±5°C or better
16 thermocouples
250ms round-robin refresh rate somewhat cost-sensitive

Assumptions

I'm making a working assumption that the thermocouples are floating. The O.P. should check this assumption from the application perspective, because it has a great effect on the architecture and cost of the thermocouple front end.

I'm making a working assumption that the O.P. knows about cold junction compensation (CJC), and will take care of it.

Approaches (menu)

Individual amplifier for each thermocouple

This is the most capable approach. No issues associated with multiplexing, such as settling time of the filters.

Multiplex the thermocouples with mechanical relays

Mechanical relays are a good method for switching thermocouples, because they don't introduce offsets. On the downside, the relays switch more slowly than solid state electronics. The relays also have a finite number of cycles; they will wear out over time.

A small signal relay takes about 5ms to settle. So, it's possible to poll 16 temperatures with a 250ms period.

Use a A/D converter with a built-in mux

For an example, see ADS114S08. These chips combine a mux, an amplifier, an A/D. Thermocouple measurements is one of their standard use cases. There are plenty A/Ds like this from various manufacturers.

This may be the cheapest approach: slightly under $1 per thermocouple (for the chip).

If you will be selecting a A/D like this, make sure that you actually can switch from one channel to another, and make the conversion fast enough.

If the thermocouples didn't have to be polled fast, these A/Ds would be an obvious choice.

Suggestion

First, look for integrated A/D converters with a mux and an amplifier.

If you can't find one that can poll fast enough, then do a individual front end for each thermocouple. (Look for one with an SPI interface, perhaps.)

Nick Alexeev
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I have very poor experience with analogue electronics, and I'm not sure if there even exists a multiplexer, that allows voltages as small as those of thermocouples (millivolts), has low resistance, and has 16 channels. I wasn't able to find one such device.

I was involved in designing test equipment for measuring vibration and temperature on aero engines for Rolls Royce and others. The thermocouple design used DG409 multiplexers on both thermocouple leads and, it worked very successfully. The multiplexer power supplies cannot be earthed i.e. the "electronics" needed to be floating to avoid common-mode voltage problems (more on this below).

After the multiplexers, we used a single high-quality instrumentation op-amp with an appropriate gain and that fed to a serial ADC which was controlled via a galvanically isolated barrier (to avoid the common mode voltage problems hinted at above).

We also had dummy channels that measured local 0 volts and a known accurate reference voltage so that calibration could be performed. The thermocouples were connected to the PCB in groups so that cold junction compensation could be achieved on several thermocouples. Hence, if the circuit could attach to (say) 32 thermocouples, we grouped 8 thermocouples and had an RTC for cold junction measurement for each group. The RTC measurement was also fed through the multiplexer.

We could multiplex at hundreds of hertz.

Andy aka
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  • Did you introduce any offset voltage to the thermocouple leads before the signal entered DG309, or did you pass the signal through the MUX directly? Does the 60 ohm Ron resistance of the MUX impact the measurements? – PineLel Jun 27 '21 at 11:10
  • @PineLel no, yes and no (providing you use a high impedance input amplifier after the MUX aka an instrumentation amplifier. But, +5 volt and -5 volt power rails are needed with the thermocouples generally orientated towards 0 volts/mid-rail. If any of the thermocouples are biased at some ungodly voltage then it becomes trickier. This was for aero engine testing in a facility. Thermocouples were on the blades of the turbine/fan. The electronics was mounted on the rotating part. Data/signals were taken off optically at the centre of rotation. – Andy aka Jun 27 '21 at 12:27
  • Actually, I've just checked an [older post of mine](https://electronics.stackexchange.com/questions/399565/what-options-are-available-for-measuring-grounded-thermocouples/399569#399569) and I used the DG409 not the 309. I'm altering my answer to suit. Sorry about that slight error (although it makes little difference). – Andy aka Jun 27 '21 at 12:31