As it is now, the plate contains three K type thermocouples. Measuring temperatures with thermocouple requires appropriate amplification, and I like the AD8495 for that. I already have a breakout board for it (see my other project), but it's quite expensive - about € 4 for the less accurate A grade chip.

So I had a look at multiplexers. Various sources recommended one or the other device, some recommended against multiplexing at all. So I just gave it a go with the ADG409, which is a dual 4:1 multiplexer. Only one ADC channel and two digital outputs are needed for four thermocouples, which is really neat!

Here's a picture of my test circuit (ignore the display and the green PCB to the right)

It consists of

• a Teensy 3.0
• two AD8495 breakout boards
• an ADG409 multilpexer
• and 5 K-type thermocouples:

One thermocouple is used as a reference and directly connected to one of the AD8495s. The other four thermocouples are connected to the multiplexer, which enables one of them according to the address inputs.

Regarding cable end sleeves on thermocouple wires: I prefer not to crap my pants about this. They are part of the cold junction and A->B->C is just as fine as A->C is.

Long story short, here are the readings in tenths of a degree at room temperature and in steam above boiling water:

  +------  first AD8495
  v   |<- second AD8495  ->|
  218  224   222   222   224
 1015 1027  1025  1023  1025

The first column is the reference reading (no multiplexer) and the other four are multiplexed. Given that the AD8495A has an initial accuracy of max 3 °C I'd say that these are close enough to each other and to the reference reading - I'm quite satisfied with this result.

For 3.3V systems there's also the ADG1609 and probably a bunch of others. The green PCB in the first picture is the upcoming Teensy LC, which unfortunately is not 5V tolerant. The AD8495 outputs its positive supply voltage when no thermocouple is connected, or when the connection is broken, so that combo wouldn't have worked.