DonPancoe (Customer) asked a question.
I have been evaluating thermocouples, thermistors and RTDs for performance. Thermocouples seem to have the fastest response, but I consistently notice the readings to ~4 degrees higher than what I am measuring. For example, and ice bath will read about 4 deg C.
I know that CJC is included in the P1-04THM module and is supposed to be applied automatically, so I don't know why this offset appears 100% of the time. Also, since the CJC is supposed to be automatic and the P1-04THM module only returns actual temperature as float 32, I can think of no way of correcting this offset other than in the ladder logic or other upstream programming.
Different thermocouples have different offset values, so it's almost like I'd have to "calibrate" each one to get accurate absolute temperature readings.
Should I just go with another sensor type as long as the response time is fast enough? Any suggestions appreciated.
I do have the latest firmware BTW.
have you checked this -> www.automationdirect.com/support/question-answer?techId=1107
Interesting. The loopback connection was definitely reading high at about 31 deg C in about a 22 deg C room.
This is a simple bench top setup, so there wasn't much more I could do in terms of improving the grounding, but I did make sure to connect the Earth/chassis ground of both PLC racks to the green wire of the 120V cable feeding the 24V DC power supply.
It got better, at least initially, with the loopback connection dropping from about 31 C to about 26 C, and the thermocouple moving to within about 2 degrees of the other temperature sensors.
I'd be curious if there was anything else I could do to get it even closer, but here's what I'm seeing right now. In the time I've been typing this, the CJC has drifted up and the thermocouple seems to have drifted farther away from the other sensors.
We had a project controlling/ monitoring/ alarming temperatures at a blood plasma storage facility. They had a third-party testing company routinely measure/ document temperatures. There was always a small offset between the PLC temperatures and the temperatures they measured with their test equipment. Don't know why: difference in: measuring locations? measurement technology? air flows? tolerances? accuracies? cable runs? ambient/ cable temperature compensations? They required a (pass-code-protected) means on the C-more so they could "calibrate" the PLC temperatures to match their test equipment temperatures, and everyone was happy.
The temperature reading from an ice bath will always be higher than 0 degrees Celsius. This is because heat conduction along the TC wires will heat the tip of the TC.
To get closer to 0 Celsius TC reading, you need to stir the ice bath or gently create a flow of water around the TC.
You didn't mention the type of thermocouple you are using. There are many types such as K, J, E, etc. The type you are using must match the type that is selected in the software. The default is type J, but the most common type is K. If the software setting and the type don't match, you will get an erroneous reading.
If the loop back is off that means the CJC is off. If CJC is off, all readings will be off.