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View Full Version : Parallax Thermocouple kit (#28022)



Bean
08-18-2006, 02:01 AM
I'm writing some code for the Parallax thermocouple kit, and with the whole mess just sitting on my bench I consistantly get a thermocouple reading of about -11 (which is -11 * 15.625uVolts = 172uVolts / 40uVolts per degree = about -4C.

So I'm wondering why that would be ? I realize the DS2760 will have SOME self-heating, but 4C ??? I find that hard to believe. Everything seems to work fine other than this 4 degree offset.

Any ideas ? Does anyone have this kit that can confirm or deny my readings ?

P.S. I'm using the K-Type thermocouple if that matters...

Bean.


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Cheap 4-digit LED display with driver ICwww.hc4led.com (http://www.hc4led.com)

Low power SD Data Logger www.sddatalogger.com (http://www.sddatalogger.com)

"You're braver than you believe, stronger than you seem, and smarter than you think" Christopher Robin to Pooh

John R.
08-18-2006, 03:22 AM
Bean;

I'm not familiar with the Parallax kit, but do you have the thermocouple wires by chance "reversed". A thermocouple does have a "polarity".

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John R.

8 + 8 = 10

Bean
08-18-2006, 03:31 AM
John,
No the thermocouple works fine if I touch the end, the temperature goes up like it should. It just seems like there is a 4C offset. Which is no big deal, I can just offset the temperature by 4C I guess.

Bean.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Cheap 4-digit LED display with driver ICwww.hc4led.com (http://www.hc4led.com)

Low power SD Data Logger www.sddatalogger.com (http://www.sddatalogger.com)

"You're braver than you believe, stronger than you seem, and smarter than you think" Christopher Robin to Pooh

Bean
08-18-2006, 08:51 AM
I think I found the problem. I guess it's some kind of ground loop problem.
When I disconnect the SX-Key the reading is normal.
I'm not surprised, when your measuring microvolts anything can affect the reading.

Bean.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Cheap 4-digit LED display with driver ICwww.hc4led.com (http://www.hc4led.com)

Low power SD Data Logger www.sddatalogger.com (http://www.sddatalogger.com)

"You're braver than you believe, stronger than you seem, and smarter than you think" Christopher Robin to Pooh

Chris Savage
08-18-2006, 10:04 PM
Bean,

I'm glad you got it figured out. I too have some experience with ground loops affecting the thermocouple. The solder pot controller we did had an interesting side-effect from this since the solder pot is grounded and the probe is touching the solder. Turns out we had to isolate the control board from being grounded. This is usually a rare issue...Only affects you when what you're measuring is grounded...usually.

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Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)