Bad reading of thermocouple on SCXI-1102

[gperron]

Hi,

Some of the thermocouple plugged in the SCXI-1102 give bad reading(unusual value). There is maybe an issue with the use of grounded and non-grounded thermocouple? Is the system need to be calibrated?

Thanks.

[Laura F.]

Hello gperron,

When you say some thermocouples give bad values, does this mean that you have some that give the correct value?  If so, try switching the ones that give you a correct reading with the ones that are giving a bad reading.  If the bad readings follow the thermocouples, you can conclude that those thermocouples are bad.  Are you using the built-in cold junction compensation on the SCXI-1303 to make sure you are getting the most accurate readings?

In general, thermocouples are floating sources and you measure them differentially with a bias resistor connected between the negative terminal and ground.  Page 3-4 of the 1102 User Manual describes connecting signals to the 1102.  The 1303 Installation Guide describes how to replace bias resistors, if needed.   

Thanks,

Laura

[gperron]

Laura,

Thanks for the info. It's took a while but...

I do not have any bad thermocouple, just some of them giving bad reading from time to time.

My problem was that i used the SCXI-1300 module, which has not the resistor system on it. I switched to the 1303 module with 10Mohm for the bias and pull-up resistors and everything seem to work fine.

Another problem i got is related to the ambient temperature. In our labs, we recently switched from climatize system to the heater system for the winter, then, i notice a gap of about 4-5°C in the reading(With 1300 module). The gap correspond to the temperature variation in the lab due to the system switch.

Do you thing that the 1303 module will solve this problem as well? I mean, avoid ambient temperature dependency?

Thanks,

Guillaume

[Laura F.]

Hello Guillaume,

The SCXI-1300 and 1303 both have built-in cold junction compensation which should minimize the effects of the changing room temperature if you are utilizing it.  These knowledgebase(1) articles(2) include some information about the changing temperature at the CJC affecting readings. 

What are you using your thermocouples to measure the temperature of, and what is the temperature that you are measuring?

Thanks,

Laura

[Logan_K]

The SCXI-1303 will be more accurate that the SCXI-1300 for quick temperature swings in the ambient room temperature. The reason is because the SCXI-1303 is designed to be isothermal, which means that it holds the temperature of the CJC sensor and all of the screw-terminal junctions at roughly the same temperature even when there is a large temperature gradient in the room. Although, if your room temperature is very stable, then you will still get better CJC accuracy with the SCXI-1303, but a 4-degree temperature difference seems like a large number to me for the SCXI-1300 if you are using the CJC sensor properly.

-Logan

[gperron]

Laura,

I'm using about 12 thermocouples to monitor a water boiler and the hot water flow in differents point in a coffe machine, so that make the measured temperature going up and down.

I did not use the CJC as built-in, so that explain why i got 4 degree offset. I set it up to built-in and everything work as needed.

Thanks you both Laura and Logan.

Guillaume

P.S. It was the first time i used this forum and find it very efficient.