Thermocouple Calibration - Digital Display vs. Converting the Millivolt into ?C

N

neoth

#1
Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

hi all,

what is the difference between:

  1. calibrating a thermocouple in a liquid bath and reading the temperature using a digital display.
  2. calibrating a thermocouple in a liquid bath and going thru a ice junction and read out in millivolts using a multimeter. after that, convert the millivolt into °C using reference tables.
regards
 
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BradM

Leader
Admin
#2
Re: regarding thermocouple calibration

  1. calibrating a thermocouple in a liquid bath and reading the temperature using a digital display.
You are using the cold junction compensator (assuming the digital display you are referring to is a thermocouple input) on the display, and accepting whatever error is present in it.


calibrating a thermocouple in a liquid bath and going thru a ice junction and read out in millivolts using a multimeter. after that, convert the millivolt into °C using reference tables.
regards
You are measuring the millivolts of the thermocouple, and essentially "nulling" out the cold junction compensation effects.

Hope that helps.:)
 

Jerry Eldred

Forum Moderator
Super Moderator
#3
Re: regarding thermocouple calibration

If you exclude the implications of the cumulative uncertainties (which based on the question, there are none defined), then the difference is essentially none.

When you connect a thermocouple probe to a digital display (for example, a Type K display - such as a Doric or Fluke 51, etc.), there is built in cold junction compensation and the millivolts output from the thermocouple is measured and converted for you. The difference really is that the digital temp display does the work for you.
 
G

George Weiss

#4
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

I agree with the just mentioned.
In most cases I would expect that having the thermocouple connected directly to a thermocouple accepting & compensated temperature reading digital meter would be done.
In the case where better or sellective method dictates, the thermocouple through ice-bath to DVM for manual mv to deg. F/C conversion could be employed.
The first method could be viewed as easier, and in a vast majority of cases the method used. I could imagine some metrologist really attached to his ice bath, and wanting to keep it around and use it.
 

BradM

Leader
Admin
#5
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

I agree with the just mentioned.
In most cases I would expect that having the thermocouple connected directly to a thermocouple accepting & compensated temperature reading digital meter would be done.
In the case where better or sellective method dictates, the thermocouple through ice-bath to DVM for manual mv to deg. F/C conversion could be employed.
The first method could be viewed as easier, and in a vast majority of cases the method used. I could imagine some metrologist really attached to his ice bath, and wanting to keep it around and use it.
Correct. It all depends on what is required. Testing a thermocouple to 2/ 2 1/2 degrees does not require the rigor that testing one to .5 degree F. :D

If you connect a thermocouple to a thermocouple accomodating indicator, the CJC does affect the accuracy of the temperature. How much it affects, and whether it is significant, is another matter. :)

Here is a link to NIST thermocouple services. At the bottom are a couple of useful attachment links regarding thermocouples

http://ts.nist.gov/MeasurementServices/Calibrations/thermocouples.cfm
 
N

neoth

#6
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

Hi guys,

if i dip the measuring junction into a bath and reading the temperature using a fluke 51, am i calibrating the thermocouple? or i am just measuring the temperature of the bath only.

regards
 
G

George Weiss

#7
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

Once you dip the t/c junction into the bath, (liquid I expect), then you will have a reading on the Fluke 51 meter. The reading will be the temperature of the liquid in the bath +/- [Fluke 51 meter spec/uncertainty] +/- [T/C junction spec/uncertainty]. This assumes that the Fluke meter & thermocouple are within specifications. This generally implies that the Fluke 51 meter and thermocouple junction are calibrated.
If you have set the bath to a specific temperature, then the difference between the setting and the Fluke 51 meter reading is the system error. The system error is the sum of bath, meter and thermal junction errors.
Specs and spec estimates:
Fluke 51 +/-1.4deg.F. +/-0.1%
Standard T/C spec. +/-4.0deg.F. +/-0.75%
Temperature bath. +/-0.5deg.F.
T/C junctions in general are very good about being within +/-0.5deg.F. The meter will normally be within +/-50% of spec. SO, The reading will likely be +/-1deg.F. of the liquid bath digital display.
What has this proved? This is a backwoods calibration, which shows al three items are working.
In this case the thermocouple was likely the item with the largest mftr. Spec.


A calibration of the thermocouple should include:
  1. be calibrated by other calibrated items with a system TAR/TUR of 4:1 for Z540.1
  2. Be calibrated a several points, desirably 2+ to determine offset/bias and gain errors. 0 and 100 deg.C. are strongly recommended, as they are easy to produce and almost all T/C junctions work at these temperatures.
Answer to your question: You did measure the bath temp. and you have performed a partial calibration of the T/C junction if the other system items, (meter/bath), are calibrated. See above.

Another question would be: What did you want to do?
 

ScottBP

Involved In Discussions
#8
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

Time to muddy the waters a bit. Instead of using a bath to calibrate a thermocouple display, how about a thermocouple simulator? Example, a Fluke 744 process calibrator can source a thermocouple voltage. Set it to output 0°C, and connect it to the Fluke 51 using type K wire with type K plugs on both ends, and the Fluke 51 should also display 0°C. But now you're dealing with TWO cold junction compensators, and you have the uncertainty of both to contend with. Also, Fluke 5500A/5520A calibrators can source thermocouple voltages, but there again, they have CJCs in them. Of course on the high end calibrators, the CJCs have lower uncertainties than hand-held TC simulators, but still nothing beats sourcing millivolts (corresponding to the desired temperature reading from a look-up table) into a cold junction made of copper wire connected to thermocouple wire immersed in an ice bath for the lowest uncertainties.
 
G

George Weiss

#9
Re: Thermocouple Calibration - Digital Display vs. Converting the Millivolt into °C

A good amount of muddying, thanks I think !
I bet the question has been lost in time. What do I want to calibrate? might be asked right now. If it is the Fluke 51 meter then the Fluke 5500A or 744A are good choices. I woulder how much is really gained from T/C calibrations when their 4+ deg.F. spec is so wide.
 
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