Any advice on interfacing a K thermocouple?

[gabir]

Dear All,
Using webcontrol, I need to measure the temperature of my chimney stack, say up to 500F. A K-style analog thermocouple would do the job specs-wise but it is notoriously cranky to interface. Any product, advice, experiences or alternatives anyone may suggest would be most welcomed.
Thanks.

 

[JimS]

I have been thinking of doing a very similar thing with an arduino or similar for a wood stove.  I have seen LM358A used as the amp for a production exhaust gas temp for gas engines which used type J or K thermocouples.  Not really precision but seems plenty accurate to use for wood stove or EGT application.  The circuit had a zero and gain adjust.  A more modern/better op amp might eliminate the adjustments.  Or you can buy chips that have cold junction compensation, etc included.  For accuracy of around 10 degrees you can assume the output is linear.  For that accuracy thermocouples are fairly easy to use.

 

[CAI_Support]

Ross made an daughter board can plug on top WebControl8 board, that board has amplifier space and gain adjustment.  Maybe you could send him a message about sending you one of those boards to interface with J or K thermocouples.

 

[Tschmidt]

I'm in the process of updating our wood stove controller and had a similar problem. Needed to measure flue temperature and indicate if it is too low or too high. Due to the limited I/O of the webcontrol I opted to use a PID controler to interface to the thermocouple and control set point and alarm. That way  WebControl only needs to read 2 bits. The down side is WebControl does not have access to actual temerature but that was not an important issue for my application. PID controllers go for short money on ebay.
 
/tom

 

[maltesemanTS1]

RTD and thermocouple signal conditioners with 4-20mA or 0-10VDC output are available from OmegaEngineering.com for about $200.
http://www.omega.com/DAS/pdf/DRF-TC.pdf
 
If you have the ability to program your own lookup table or equation with the proper mV-to-temperature conversion , you can get away with using a miniature ice point reference for a thermocouple (http://www.omega.com/Temperature/pdf/MCJ.pdf) or with simply measuring the resistance of an RTD and applying the conversion equation (http://www.omega.com/Temperature/pdf/F1500_F2000_F4000.pdf)
 
You can also use a DMM with thermocouple input (http://www.omega.com/pptst/HHM29.html)
 
There is also an inexpensive wireless option (http://www.omega.com/ppt/pptsc.asp?ref=UWTC)

 

[JimS]

For a LOT less than $200 you can have a very accurate thermocouple amp.  There are chips out there for around $10 that will do all that.  Should be able to find one on a board for not too much if you don't want to put it together. 
 
Here's the first ad that popped up on google for $78:
http://thesensorconnection.com/signal-conditioners/signal-conditioners/type-k-thermocouple-amplifier-signal-conditioner-0-5-vdc-out?gclid=CLPx-dv0vrYCFdOHMgodBTAAqA
 
Here is a chip
https://www.sparkfun.com/products/307
or
https://www.sparkfun.com/products/306
 
How to build a board for the 595
http://hobbybotics.com/projects/hobbybotics-ad595-thermocouple-breakout-v1-0/
 
Google is your friend.