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[on:]

Hi all!

I have project where I need to log temperature around 600 Celsius degrees in few spots and control 220v "oven". I'm looking forward to use arduino and K type thermocouple probe's like this: http://www.ebay.com/itm/180937152179
As I understand for arduino I also need thermocouple amplifier probably like this: https://www.adafruit.com/products/269 (digital quite expensive 17.5$ per peace)

Questions:
1. Is there cheaper/alternative way to go? I need 6 - 8 spots to log so it's go around 150$ for thermometers  
2. There is IC AD595 10$ per peace (analog). Still ~ 100$. For this IC do I really need ice point reference?
3. What would be cheapest "industrial" solution for logging stuff up to 1000C degrees?

May be You have some experiences with these or similar products and can share about reliability & accuracy & "What to do or not to do"...

Many thanks.

[Reply #1 on:]

You could use relays to switch each thermocouple in turn to one thermocouple amplifier.  This will introduce cold-end error due to the metal missmatch at the thermocouple connections.  You can either choose to ignore the error (20degC or so) or it can be compensated for by having and RTD measure ambient temperature, which is then subtracted from the measured temperature.

[Reply #2 on:]

You can use analog multiplexer chips to switch multiple thermocouples to a single thermocouple amplifier. See page 7 of http://www.analog.com/static/imported-files/application_notes/AN-369.pdf.

[Reply #3 on:]

You could use relays to switch each thermocouple in turn to one thermocouple amplifier.  This will introduce cold-end error due to the metal missmatch at the thermocouple connections.  You can either choose to ignore the error (20degC or so) or it can be compensated for by having and RTD measure ambient temperature, which is then subtracted from the measured temperature.

If you place the relays close to the thermocouple interface IC the ambient temperature at that point should be the same and so the compensation performed by the TC interface chip should still be valid. Only if the relays are placed externally and are at different ambient temperature then the TC interface chip is at would there be a compensation problem.

Lefty

[Reply #4 on:]

With no cold junction compensation you should not get more than ~10C error according to that AD-369 datasheet, but that is including ambient temperatures well outside of human comfort.

NIST publishes tables/formulas for calculating temperature from thermocouples if you use a plain A/D converter. An MCP3424 would give you four channels for ~$4 and you can have up to nine on an I2C bus. But it's a SOIC and the 18bit conversion rate is a bit slow (3/second).

[Reply #5 on:]

Thanks all! 

For now my research end up with MAX6675 setup.
There is also simple tutorial: http://mbed.org/cookbook/MAX6675-Thermocouple.
It costs about 3.23$ per peace so this is cheaper than relay + transistor for each thermometer.

[Reply #6 on:]

Why not MAX6675 + analog multiplexer?

[Reply #7 on:]

Why not MAX6675 + analog multiplexer?

Yes, I will do so.
And it will look probably something like this: http://code.google.com/p/multidisplay/wiki/TypKThermocouples

[Reply #8 on:]

FYI, MAX6675 has been replaced by MAX31855, which has wider temperature sensing range, better resolution, and is available in versions for several different thermocouple types. Still has an SPI interface, but the data format is changed from the MAX6675, so libraries for the earlier part will not work for the MAX31855.

[Reply #9 on:]

I made a break out board for the MAX31855 chip because I wanted to play with that chip too.

I have the bare board up on lish.com with the caps inductors and protector. https://dcande.lish.com/

[Reply #10 on:]

Thanks for activity.

FYI, MAX6675 has been replaced by MAX31855, which has wider temperature sensing range, better resolution, and is available in versions for several different thermocouple types. Still has an SPI interface, but the data format is changed from the MAX6675, so libraries for the earlier part will not work for the MAX31855.

I can't find MAX31855 for convenient price - around 3$.
IMHO buying
https://tindie.com/shops/TAUTIC/max31855-thermocouple-to-digital-converter/ 15$+ or
https://www.adafruit.com/products/269 17$+
is kind of wasting money...
In my case temperature range will not be higer than 800C and MAX6675 should do fine with in 1024C.

I made a break out board for the MAX31855 chip because I wanted to play with that chip too.
I have the bare board up on lish.com with the caps inductors and protector. https://dcande.lish.com/

This is almost same situation - financially disadvantageous for me:
8$ Your product + MAX31855 + shipping and I need at least 6 or 8 sets of these.

Come on guys! Do You would buy 6-8 thermometer for 120$+ ? 

[Reply #11 on:]

You can also get a couple as a Sample from Maxim when I did it I got 3 of them for free. https://shop.maximintegrated.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX31855KASA%2B

[Reply #12 on:]

You can also get a couple as a Sample from Maxim when I did it I got 3 of them for free. https://shop.maximintegrated.com/storefront/searchsample.do?event=Sample&menuitem=Sample&Partnumber=MAX31855KASA%2B

Yes, that's an option!