Any ideas for low-cost Thermocouple Interfacing methods?

Hi fmilburn

(Replying here to remove the comment indent, since I wanted to share some diagrams)

I've had a shot at doing it, and was close to giving up since I was seeing huge errors!, but it was a mistake in a calculation. I think it's looking good, but reality might be different!

Referring to this circuit (it is similar to before, except I've reduced the filter resistance further), I first closed the switch and tried seeing what the expected values should be. The switch closed is when the thermistor (cold junction) temperature needs to be measured:

I picked a NTC thermistor with 1 kohm resistance at 25 deg C, and used a Beta value of 3650. In the screenshot below, the Excel spreadheet boxed section labeled "Standalone Thermistor" was used to check that the formula was working for conversions from resistance to temperature, and vice-versa. Green cells are input values, pink cells are outputs from formulas.

The spreadsheet is very clunky, it isn't all automated, some values are currently looked up.

Using the normal potential divider formula (i.e. Ohms law) I worked out what the voltage across the thermistor would be, and that is in the box labelled "Thermistor in Potential Divider".

Once I had this, I used a Spice simulation to see what it thought I would get, with the switch closed. The "Thermistor error" box is the area where I tried simulating from -30 to +50 degrees C, which is likely to be the max range that the cold junction will encounter.

I varied the thermistor voltage between the limits of -5mV and +20mV since that will cover more than 400 degrees C of interest area, and the thermocouple voltage will have a slight impact when reading the thermistor voltage. It turns out, the error is small, mostly smaller than 1 deg C apart from at the 50 deg C extreme. The green cells at the right side of the "Thermistor error" box show the Spice simulation results, and I'm really happy with those. The thermistor measurement looks good with the current layout.

Next, I moved to the thermocouple measurements, i.e. with the switch open. I didn't use a formula, I just used a lookup table from a datasheet for a Type-K thermocouple. It was a similar idea to the thermistor, i.e. just trying to model in Excel what is expected, and comparing with Spice. In the case of the thermocouple, since the measurement of that includes the thermistor resistance, the circuit looks like a potential divider circuit where the source voltage is the thermocouple voltage (Vthermocouple), and the resistors in the potential divider are R2, RADC1, and the thermistor. Taking them into account, I can work out what the voltage across the ADC should be (Vadc), and vice-versa, i.e. with a given Vadc measurement, to convert back from that to determine what Vthermocouple is. 

I used the "Vadc for Thermocouple measurements" boxed section for this, while varying the thermistor resistance across the range -30 degC to 50 deg C, for thermocouple voltages of -5mV and +20mV, which correspond to -154 degC and 485 degC if the cold junction were at 0 degC.

The Spice simulation agrees very closely, so this is good confidence that the formulas are now good, and that if this circuit was built, then it may work!

The one major caveat is, that the resistances in series with the thermocouple will cause huge error, if the ADC internal resistance changes. because that ADC resistance is part of the potential divider. The spice simulation may be useless, because of this assumption (I don't have a spice model of an ADC, I just modeled using the resistance in the datasheet, which is 2.25 Mohm divided by the gain, which will likely need to be set to the maximum which is 8). Thats 280 kohm, although I used a value of 250 kohm, the concept is the same, I would just need to change that param in the spreadsheet.

I can reduce the effect of the ADC input, by reducing the value of the thermistor, so I've chosen a small value of 1 kohm at 25 deg C, and perhaps reduce further by paralleling it with another resistor, although that may be unnecessary. I don't know though, it's probably time to try the circuit out for real! 

It will be possible to test the performance by applying a voltage in place of the thermocouple, this will simulate any temperature, and using a worst-case resistance for the thermistor, which would be at the coldest temperature, e.g. 15.94kohm for -30 deg C. 

If that RADC1 value doesn't change much, then the measurement might be good. From the simulation, a 10% change in the value of RADC1 barely makes a difference to the measurement, so that's a good sign I guess, but I really don't know in practice if it will change even more. 

In summary, there are positive signs, but it could all go wrong in practice.

Very clever :-).  I find that making a working Excel model can be very useful as it requires one to understand the problem in a way that a canned model does not. 

Very clever :-).  I find that making a working Excel model can be very useful as it requires one to understand the problem in a way that a canned model does not.