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Any ideas for low-cost Thermocouple Interfacing methods?

shabaz

Note: See here for a project that resulted from this discussion: BLE EasyTempProbe 

Hi,

Since thermocouple measurement ICs are getting expensive/hard to find, I wished to use a single channel ADC, for thermocouple measurements (actually, I want to use a dual channel ADC for two thermocouples, but it's likely the same problem just doubled!).

The trouble is, the cold end of the thermocouple needs measuring too, and I was thinking of using a thermistor for that because that's easier to obtain (and cheaper) than an IC sensor. In summary, I wished to multiplex a thermistor and a thermocouple.

I've come up with the diagram below so far and wish to use it in an environment where the cold junction might be in the range of -40 to +50 deg C, and the thermocouple might be in the range of -40 to +400 deg C (maybe a Type J thermocouple). I think it will have an error of a few deg C. The ADC is 16-bit, and I likely won't be using the whole range of it. The ADC has a PGA of up to 8X, so it will be set to 8X when measuring the thermocouple.

The main benefit of the proposal below is that it is cheap since it just needs a couple of transistors and a few resistors. I could think of more complex circuits for a more accurate measurement, but it would be nice to see if this is good enough, or if it could be tweaked to be good enough unless anyone has other suggestions.

If it works, this would be a cheap way (under $5, ADC included) to have two thermocouples each with their own compensation, with the drawbacks that accuracy might be a few degrees at best, and no isolation either, unfortunately. 

Any ideas would be gratefully appreciated, since I'm sure I may be missing some great techniques, missing the wood for the trees, etc! Has anyone come across any low-cost methods to do such a thing? Any mistakes I'm making?

image

  • in reply to scottiebabe

    Wow that's awesome! Thanks for testing this, it is great to see the scope trace. 

    I made some good progress yesterday, and the code is functional although I'm using a lot of floating-point for now. Anyway it made my prototyping time shorter, the code can always be refined later. I still need to order the 1k thermistor, and order/find a thermocouple probe. I might have a Type-K probe lying around somewhere.

    I still need to do the PCB design.

  • in reply to shabaz

    I do think the input impedance is a bit low, but fortunately for your application it shouldn't be a big deal. Its only for the thermocouple that you need the frontend PGA. I only wrote a very basic uPython script, it isn't a full driver but for interest here is the code I used:

      

    from machine import Pin, I2C
import time
import struct
import ctypes

pwr = Pin(9,Pin.OUT)
pwr.value(1)

i2c = I2C(1, scl=Pin(11), sda=Pin(10), freq=400_000)

# MCP3425 has a 7-bit address of 0b1101xxx
# which is 0x68 -> 0x6F (104 -> 111)

i2cDevs = i2c.scan()

PossibleDevs = set(i2cDevs).intersection(range(104,112))
print( 'Found potential i2c devices: {}'.format(PossibleDevs) )

# Assume the MCP3425 is the first one!
MCP3425ADDR = PossibleDevs.pop()

i2c.readfrom(MCP3425ADDR,3)

GAIN_1 = 0b00
GAIN_2 = 0b01
GAIN_4 = 0b10
GAIN_8 = 0b11

SRATE_240 = 0b00
SRATE_60  = 0b01
SRATE_15 = 0b10

SRdef = {
        "RDYn": 7 << uctypes.BF_POS | 1 << uctypes.BF_LEN | uctypes.BFUINT8,
        "CMODE": 4 << uctypes.BF_POS | 1 << uctypes.BF_LEN | uctypes.BFUINT8,
        "SRATE": 2 << uctypes.BF_POS | 2 << uctypes.BF_LEN | uctypes.BFUINT8,
        "GAIN": 0 << uctypes.BF_POS | 2 << uctypes.BF_LEN | uctypes.BFUINT8,
}


dat = bytearray(1)
StatContReg = ctypes.struct(ctypes.addressof(dat),SRdef)
StatContReg.GAIN = GAIN_8
StatContReg.SRATE = SRATE_15
StatContReg.CMODE = 1
StatContReg.RDYn = 1
i2c.writeto(MCP3425ADDR,dat)

for i in range(100):
    time.sleep(0.1)
    dat = i2c.readfrom(MCP3425ADDR,3)
    sample = struct.unpack('>h',dat[0:2])[0]
    vmeas = sample*2.048/(2**15)/8
    print(vmeas)

    Its an interesting part for its cost, simplicity (i2c), and package sot23-6 for the 3425...

  • in reply to scottiebabe

    It's definitely an easy-to-use part with just a single register and simple read/write scheme : ) The single-channel version of the ADC is not currently available at UK Farnell, it has an ETA of next year, but there are lots of the dual-channel in-stock, for very little cost.

    I ran a quick test on the circuit, using a resistor to simulate the thermistor at zero degrees C, and using a 'process calibrator' (which is incidentally not calibrated) to simulate the thermocouple.

    Across the range of -100 deg C to +400 degC, the discrepancy between the calibrator and the measurement with this project is under 1 degree C.

    At -200 and +700 degrees C, the error reaches 2 degrees C so it's less usable at those sorts of temperature ranges at present, but I need to explore further, I've done no investigation/troubleshooting yet. I'm really happy it's usable for a decent temperature range, I was not expecting much more accuracy than this, but I'll investigate further to see where the error is coming from. It's partially likely to be the resistance of the ADC, since I have hard-coded it in my formula to be 2.25Mohm/8, and that won't be very exact; I could tweak the hard-coded value of course, but I don't know it's really worth it, since it is good enough to be usable provided I'm not at a steelworks or building my own kiln etc!

    Also the interpolation in the code is partially causing a bit of error, since it's every 10 degrees C currently.

  • in reply to shabaz

    The burr-brown ADS1100 looks awfully similar to these microchip parts, I wonder which came first :) https://www.ti.com/lit/ds/symlink/ads1100.pdf 

  • I've created a separate post to document things as a practical project with code etc, so that this thread remains focussed on the ADC circuit.

    BLE EasyTempProbe