Calibrating a J thermocouple

Lost_Man

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Join Date
Apr 2005
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I have a J thermocouple connected to 1746-NI8. I am getting some very high readings on it (237 when it s only 70 degrees). Is there a way to calibrate it? and if so, how do I go by doing that?
Are the readings from the 1746 the actual temperature or should I scale it?
 
Readings like that do not appear to be a calibration issue. It looks like
a configuration problem. Card can be configured for C or F degrees as well as type of thermocouple. ( check these ) From the readings you gave
it looks like it is configured for C deg. Card will show temp with an implied decimal point 237 or 23.7 deg C would be about 70 deg F.

EDIT: Just noticed you said 1746-NI8 this is not a thermocouple card.
Do you have a thermocouple connected to some converter?

If you meant 1746-NT8 than above would apply
 
Last edited:
Thanks Mickey for ur reply

The 1746 Universal analog does accept a thermocouple in the first three channels.
So it s compatible with the J thermocouple. I did setup for both C and F but still was getting very high results.
If the J thermocouple is linear, I could just use a formula to substract the difference. But, I am not sure if it s linear or not.
 
Note what Mickey said about the implied decimal point.

Most TC cards that I've used return the temperature in tenths of degrees. (C or F depending on configuration).
 
Lost_Man said:
Thanks Mickey for ur reply
Click to expand...
Lost_Man said:


The 1746 Universal analog does accept a thermocouple in the first three channels.

So it s compatible with the J thermocouple. I did setup for both C and F but still was getting very high results.

If the J thermocouple is linear, I could just use a formula to substract the difference. But, I am not sure if it s linear or not.
Click to expand...




I checked the specs for the NI8 and it says nothing about accepting thermocouple inputs, let alone doing the linearization that is required to make the reading worth anything. Thermocouples output a mV signal that is referenced to 32Deg. F or 0 Deg. C. Without the proper input conditioning the reading is worthless.

I was going to try and attach a copy of the specs, but there is always a reason that my attachments/pic uploads will not suit the site. I know, the list of acceptable file types and such is clearly stated, but, it just will not work for me so maybe we can start a separate thread for the attachment deficient, like me.
 
Last edited:
Years ago I had a problem with thermocouples right about when the first TC card for the SLC 500 came out.

Ironically it was a project I did for an OEM for Allen-Bradley at their main HQ in Milwaukee. To get the answer, we called the engineer who designed the card who was within that building (now that's service!).

He determined that the thermocouple was unshielded and needed to be shielded (or vice versa - it's been a while). Anyway, getting the proper TC fixed the problem. However, that was back then using a different card. If the card you ARE using is a 1746-NT8, it supports either grounded or un-grounded TC's, so be sure to wire them up as the manual shows. (I just checked A-B's website about this.)

For the record, I also agree that it could be a configuration problem...
 
Then it appears to be working correctly. It's reading room temp as 23.7 degrees C. That is a little high for 70F. The ballpark rule I keep in my head is 22=72 (22C=71.6F).
To convert C to F in your head. Double it, subtract 10%, add 32.
Scaling is done in the Module. You most likely will need an offset. 1 or 2 degrees is nothing to get alarmed about.
Are you using type J T/C wire all the way to the module? If not, you will have a problem with 'Cold Junction Compensation".
 
so why it s giving me 237 instead of just 23 or 24 degrees. I am reading my input from N7:0, so why it s not rounding to the nearest integer?
 
Mickey said:
...Card will show temp with an implied decimal point 237 or 23.7 deg C would be about 70 deg F.
Click to expand...
You need to insert your own decimal point in the display. This is typical when sending BCD numbers to a HMI for example. No need to mess with floating point or decimals. Just put a decimal point in the display, and send the number to it.
 

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