Uncertainty Estimation in Temperature Measurement by K type Thermocouples

S

sunilgaidhani

#1
Hi

I have to estimate the uncertainty of temperature measurment of reacting chemicals. The experiment is measure the temperature of reaction w.r.t. time of two chemicals mixed together. The thermocouple used is NiCr-Ni (type K) and which will be disposed (can't reuse) as the chemical around it will be cured. The accuracy stated by manufacturer is ±2.5 °C. My measurement range is 30 - 120ºC. As everytime this thermocouple is being disposed i am not calibrating it. Please help me with the inputs on uncertainty estimation.
1) I have the repeat measurements reading from experiments on same material.
2)Accuracy of thermocouple.

Thanks in Advance

Sunil
 
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BradM

Staff member
Admin
#3
Besides the helpful links provided, a couple of thoughts.

You might want to consider using type T. If you get a special limits of error, it has a much tighter tolerance.

I don't see where you have mentioned the tolerance/uncertainty of the device measuring the thermocouple.

I guess you could use the maximum error of the thermocouples as your uncertainty contributor. Not sure if you are making these from a roll of wire. But you could have them calibrate the beginning and end of the roll, and use those values (average them) for all thermocouples made from that roll.

I know you listed that you have measurement readings from the same material. Have you tested the location of the thermocouple in the material? Would someone else be setting up the thermocouple and place it slightly different than you?

Cold junction Temperature compensation (CJC) with thermocouple is important. If it is part of the thermocouple meter, though, it's error contribution would be a component of the meter.

Also, if you are making the beads (measuring junctions) on the thermocouples, you might pick up some error there; although it might be insignificant.

So as contributors that I see:

Thermocouple (and tip)
Thermocouple location
Meter reading thermocouple
CJC (if not part of the meter)
Repeated readings
Error if multiple personnel are involved

:)
 
S

sunilgaidhani

#4
JeantheBigone, Thanks for your Inputs it is really informative.

Hi Brad,

I am using the wire from the roll and as you suggested I will carry out the calibration at start and end of the roll.

The location of thermocouple is taken care as I have fixed the position of thermocouple tip by means of fixture.

I never thought of the contribution from connector. Thanks for raising this.
Also I am using 8 channel data logger to read and record the readings. So how to evaluate uncertanities for these contributors?

:thanx:
 

BradM

Staff member
Admin
#5
Well, you need to pick one standard of measurement to base all the estimated uncertainties. I would think °C would be a good one.

What would be really good is if you had a small probe calibrator that you could use to verify the entire loop (data logger+wire). Then you could use the uncertainty of the probe calibrator as a contributor. Too, I would imagine that this method would lower your uncertainty; not sure if that is a major issue for you.

Not sure how you calibrate the data logger, but there should be an associated uncertainty with it that you can use.

Are you asking how to calculate the uncertainty (in general), or how to estimate the different components of your system? Or... both? :D

No problem either way; we just want to get a feeling of your comfort level here, and how much (and how detailed) you might want to get.

I am by no means an expert with uncertainty calculations and such. But I'd be glad to share what I know. :)
 

tacom

Involved In Discussions
#7
Dear sunil
My thoughts is as given below
Temperature measurment of reacting chemicals with thermocouple has two uncertainty sources . One is from calibration certification of thermocouple ; other is repeatability of measurement.
If you don’t know the measurement uncertainty of the thermocouple used temperature measurment from calibration cerificate , use accuracy stated by manufacturer instead.
Repeatability of measurement (Sr) can be found from technical specification of manufacturer document or fom empirical study.
Suppose your test result 800C
Uacc.=2,5/(3)^0,5 0C
Urepeatablity=Sr 0C
Umeasurement=( U2 acc.+ U2 repeatablity )^2 0C
Expanded uncertainty
800C±2x Umeasurement
TACO

 
T

TamTom

#8
Hello,

I'm not really looking fo MU. But I shall perform a MSA so MU is a point.

And the things you say are a good starting point. But in my process I want to measure a melted alloy.

The measuring system is the same, Thermocouples, meter and data logger, and a cable between.

The problem I have is, that I don't see any chance to get repeated measures. I have a oven with the melting, I have no idea what temperature it have, so after 5 min I measure, I get an result, I dispose the thermocouples and make my casting. Then I melt the next batch, measure, cast,.....

The values I get are not compareable, because it could be that the alloy had 1560°C after 5 min or 1540"C. So any idea?

Regards,

TamTom
 
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