Quick Comparison Check of Multiple Thermocouples on one Datalogger

H

HikerLT

#1
I have a question:

Using an Agilent Datalogger, with approximately 40 thermcouples placed directly onto the units we are testing in a thermal chamber from -40C to +70 C . We also have thermocouples plcaed in free air positions ( low, middle, top) of the racks holding the test units, and also at the air intake and air exhaust of the chamber.

The question I have is :

What is a quick method to check that all the thermocouples are reading the same temperature ? My reason for asking is that on the last test run, a couple of the channels apeared to have temperture reading results that were significantly different from the other sensors.

Would it make sense to place all the sensors close together on a metal plate, inside the chamber, and get readings at cold, ambient, and high temperatures ?

Thanks for any suggestions and advice !
 
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Ninja

Looking for Reality
Staff member
Super Moderator
#2
Sorry to "answer" a question with a question(s)...

What temperature range are you measuring, and with what type of thermocouple?

The temperature and type can make all the difference in the world.
Are you measuring any temperatures in excess of ~700C ?

+++++++++++++++++++++++++++++++++++++++++++++++++++++
Edit: Silly me...you already stated your temperature range in the OP, sorry.
Good advice down below...I second the recirculating temp bath as a reality check and for setting offsets and span.
 
Last edited:

Mikishots

Trusted Information Resource
#3
I have a question:

Using an Agilent Datalogger, with approximately 40 thermcouples placed directly onto the units we are testing in a thermal chamber from -40C to +70 C . We also have thermocouples plcaed in free air positions ( low, middle, top) of the racks holding the test units, and also at the air intake and air exhaust of the chamber.

The question I have is :

What is a quick method to check that all the thermocouples are reading the same temperature ? My reason for asking is that on the last test run, a couple of the channels apeared to have temperture reading results that were significantly different from the other sensors.

Would it make sense to place all the sensors close together on a metal plate, inside the chamber, and get readings at cold, ambient, and high temperatures ?

Thanks for any suggestions and advice !
Not sure I follow your intent; the thermocouples should NOT be reading the same temperatures. Based on your stated locations of the probe tips, you should be getting differing values returned. Are you using calibrated probes?

We use multiple thermocouples in a large solution heat treating oven - the probes returen different values depending on where they are situated in the chamber. We depend on the spread to tell us that degree of variation within the chamber.

If you could provide more detail on what exactly it is that you're measuring, and why, it would be very helpful.
 

BradM

Staff member
Admin
#4
It sounds to me like the first thing you need to do is assure your measuring device is working correctly. The best way is to have a high uniform environment (recirculating bath or something) and have all the probes in there at once.

You really need to have that system calibrated. It's been a while since I have used Agilent datalogger, but I recall there being a way to assign an offset, assure the scaling is correct, etc.

Do you have access to a recirculating bath or a calibrator block or something? At least you should verify they are all reading consistently.

You can build a proper ice bath and possibly boil some water to verify accuracy at 0 C and 100 C, but there are issues with them. Just bundle the wires together where the tips are together (but try to keep them from touching).
 
Z

zamclachia

#5
What we have done is to send the data logger out for calibration / verification and included a single TC to be used during the cal / val stage. We then use this TC as a reference. We place the other TC's alongside the ref. and compare the temperatures. Each TC is individually numbered to keep control.
 
H

HikerLT

#6
Thanks Everyone for the tips and suggestions
- the Agilent Datalogger IS calibrated
- yes,I understand the distributed placement of the thermocouples means they will not all read the same temperature - it's just that ONE of the thermocouples has readings that are significantly different from others in close proximity to it - which is why I question it's data.
-
 

Jerry Eldred

Forum Moderator
Super Moderator
#7
Such a quick bevy (sp?) of responses. A pretty common quick check (as zero offset is one of the more common issues with thermocouple probes/sensors, is to put them all in crushed ice slurry (and for this purpose, it doesn't HAVE to be pure distilled water - I'll explain), as you are just looking for if one of them has an offset.

Put them all together in the crushed ice slurry (read my article on the topic in Oct/Nov/Dec 2011 if you want a good reference on the subject). They should all read approximately zero celsius. The reason it does not HAVE to be distilled is that this is a relative offsete check.

If you want a true zero degrees celsius, use finely crushed distilled ice per one of the procedures referenced in my article.

One of the mitigators is that each metal to metal interface is theoretically another thermocouple junction, and can (and sometimes does) cause an offset. A bad, corroded or loosely connected thermocouple plug can cause offset. So if you get a zero offset on one of the thermocouples, try cleaning and/or re-tightening the plug. Also clean the pins of the plug if they look corroded.

A significant offset is defined by the specs for the type thermocouple you are using. I believe type K for example is +/-2.2 C (don't have docs in front of me). If offset is less than the specs of the thermocouple, but more than your process can tolerate, you may need to pre-characterize the individual sensors at zero for offset, and perhaps check them in an accurate bath or drywell at approximately your critical temperature. End user processes often are tight enough that just being "calibrated" regarding the data logger isn't enough. The data logger may be calibrated, but that does not take into account some other system variables (such as what I wrote above).

Hope that is of some help.
 
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