I think thermocouples are the only sensible way to handle 400C.
You'll need a shield - see eg the MAX6675, and probably an analog mux. Better than relays, which can give a contact EMF.
I've use the MAX6675 shields, and they work fine - but a word of caution. In strong electric fields - ie close to mains - they can give erratic readings. Add a 100uF capacitor between the thermocouple -ve input and ground to cure this.
If you buy 'bare' themocouples - normally type K for this sort of job -they're not too expensive. and have a low thermal mass and hence fast response. If you want to do this on the cheap, buy a length of thermocouple extension lead, cut to length, and melt an end together with oxy-acetylene. I've done this out of curiosity, and it worked well.
And, unlike cheap thermistors, thermocouples are very reproducable from unit to unit.
Cheap reproducability is a good thing, thanks. Regarding that matter, i've also considered buying 200-300 meters of T type thermocouple wire and weld/crimp them together.
For the analog mux thing, i can buy seperate max6675 shields for each channel so i dont need to rely on an analog mux.
+1 for thermocouples.
For 1° accuracy 0-400 look up table is a lot of SRAM.
What controller are you using?
I'll probably use an arduino mega or uno. Yes 0-400 look up table would take lots of SRAM but i can also put the lookup table in the progmem.
I'd put lookup tables in flash - why waste scarce SRAM?
If you're taking a lot of readings I suggest you stream them as raw data to a PC running a terminal program eg RealTerm using serial.Write().
Save to a file and import into eg Excel for further processing .
+1. I plan to save the data in .csv files on an SD card and later process with Excel or similiar software, to make the system more portable. The hydraulic press that will be used in the experiment is not something that you would like to keep your computer near.
Arduino programming part is not a big matter in this project as far as i've been able to predict.
Had a quick look on ebay....
0-500c Temperature Range 3mm Width Thermocouple Extension Wire 33ft for sale online | eBay
4 quid for 10 metres. Good for 500C
So if you cut it into 10 that's about 40p a pop plus a few seconds of oxy-acetylene.
If you haven't access to the kit, visit a local backstreet car repair garage.
Or you could always cut off the embedded bit after your experiment and re-melt the new end.
Thanks for that but the type of the pair isn't certain on the listing.. If i fail on achieving good welds, could i just solder or use small aluminum clamps/crimps at the ends?
Also on a quick look on aliexpress, i might just switch to a proper multichannel thermocouple temperature logger device, but my uncertainities on thermocouples continue:
Am i right that thermocouples generate a potential difference which is a function of almost only the connector side junction temperature and the sensing side junction temperature? I mean, the parts of the thermocouple cables between junctions will probably be passing from other places inside the specimen with different temperatures, including other measurement points. Would i still get a reading that is only dependent on the temperature of the end joint?
Can i just buy hundreds of meters of wire pairs of certain thermocouple types(for example i have T type in my mind), cut them in desired length, make a junction by crimping/welding/soldering(soldering is a bit controversial but i've read somewhere that as long as the solder bead isnt exposed to a temperature gradient it would be okay.) and expect it to generate voltage differences equal to other ready made thermocouples of the same material pairs?
3)Let's say i've bought 100 thermocouples of extra pure grade materials with no measurable inhomogeneities, and i have a zero noise system that can read the voltage in a thermocouple with 100% accuracy+precision. Would all these thermocouples give exact same temperature readings when given enough time to settle or would the junction geometry and some other factors affect readings?