I'm trying to solve a problem on a project for one of my customer, 'cause we are having hard times to find an appropriate solution.
He needs something really simple (more on this later) and as we are both not much into electronics, all we can do is to call "companies with a name" for offer on their hardware, and end up with things that are really perfect to land on the moon but really overpriced for our scenario.
I'm trying to find a different alternative, thus this question.
Scenario:
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40 thermocouples to be sampled (brand unknown, as they are 20 years old and nobody remember were they come from, but this is not the problem 'cause we can always change them all)
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Required sampling precision:
+- 0.5 degrees2 Celsius degrees. -
Required sampling speed: a whopping once every two minutes. That is, 120 seconds divided 40, an astonishing sampling rate of one thermocouple every three seconds 😀
So, it's obvious we don't need 20k euros equipment, but it seems that there is nothing simple out there and given that we need 40 input, prices are going up really quickly.
On top of this we still need a few digital outputs (3 or 4, still unclear) and some analog inputs to sample some voltages, and all this stuff will be driven by a custom software on a Windows PC. So to the question (finally)…
In this scenario, can buying a simple I/O board and somehow multiplexing the thermocouples to a single analog input feasible? Do Multiplexer with 40 inputs exists at all? (P.S. the multiplexer should be controlled by the PC, ideally)
Update after gathering some more information with the customer
Ok, at the end I did a lot of work and I've been be able to get all the info I needed. Seems like there was some misunderstanding with the customer, now everything is clear and I can give more info.
First of all, the required precision is 2 Celsius degrees. As I've noticed someone pointed, it the temperature range is between 100 degrees: That is between room temperature room temperature+100
Currently used thermocouples are standard T type, from Tersid. As of now the 20 years old thermocouples are connected to a 20 years old I/O board, by 20 years old, 10-15 meters long wires, in an environment with a lot of electromagnetic noise, and work like a charm. Don't ask me how, I have no idea 😀
Best Answer
I've recently done some PCB design with thermocouples, and have learned about how finicky they can be if they aren't treated exactly as they're supposed to be. If you want a precision of 0.5 degrees, you must be able to measure 20uV of precision across the leads of the thermocouple! Do you really trust whatever multiplexing solution you use to introduce less than 20uV of noise? Not to mention that you can no longer perform proper cold junction compensation.
I know that usual precise thermocouple equipment is very expensive, but you can spend a lot less than 20k euros and still measure each thermocouple individually. I have used the MAX31855 in the past, which works very well. It has a precision of 0.25 degrees, cold junction compensation, simple SPI interface, and it costs less than 5 euros per IC. For the sake of accuracy and simplicity, I suggest you spend less than $200 on 40 of these thermocouple IC's instead of multiplexing the thermocouples.