Thermocouples - National Physical · PDF filepresentation as the identity had to be ......

Thermocouples

UKAS

Mark Stevens

• Gaffs

• Homogeneity

• Reference junctions

• Uncertainties

Outline…

Unfortunately there are few photos in this

presentation as the identity had to be

removed to protect the GUILTY

But

You know who you are

Feed-through

thermocouples

• This is the common practice of using feed-

through thermocouples as part of the

measuring system

• Often used in high air pressure heat

treatment chamber. Referred to in the

industry as ‘autoclaves’, but with pressurised

air rather than steam.

fixed-wing aerospace

applications

• Calibrated thermocouples are connected to a bank of terminals mounted inside the chamber.

• The thermocouples are placed around the chamber for a typical thermal survey.

• The measuring system is connected to bank of terminals on the outside of the chamber.

• The feed-through thermocouples are the main part of the system generating the measured emf, but they are not calibrated.

EMF

generated

here

A witnessed example

• An array of around 250 thermocouples mounted on a wing-section former weighing about 20 tonnes. Each thermocouple is spring loaded to keep it in contact with the wing. The tips of the thermocouples were being calibrated in a shallow hot-block.

• In use, the thermocouples are connected to the monitoring systems using un-calibrated feed-through thermocouples

EMF

generated

here

Calibrated thermocouples

Un-calibrated extension cable

Dry Block

Calibrator

Reference

Thermometer

Dry Block

Calibrator

EMF

generated

here in use EMF

generated

here in the

calibration

the need for pre and post

checks• When un-calibrated thermocouples are used

for chamber calibration, and they are

calibrated ‘on-the-day’ before use

• There must be a programme of re-checking

the thermocouples after the calibration to

demonstrate that the measuring system has

remained stable.

– Test lab - Not changing old

base metal thermocouples,

simply because the calibration

doesn’t seem to have changed• Here, the tip of the thermocouple is being abused

regularly in order to carry out the test procedure. Only a few millimetres of the thermocouple are used in the measurement, and looks truly beaten up.

• The calibration results with 250 mm immersion look very stable.

• Other abuse includes using the thermocouple itself to tie in position causing mechanical strain

Reluctance to employ batch

calibration of thermocouples• Thermocouples can rarely be calibrated with

the major temperature gradient at the same place as when they are used. In these circumstances, a well-designed batch calibration process is better and often cheaper than calibrating every thermocouple

• With this approach there is a link with the uniformity of the batch which helps to estimate the error of using a difference part of the thermocouple to generate the EMF

EMF

generated

here in the

calibration

EMF

generated

here in use

Inhomogeneity

• It is generally agreed that inhomogeneity of the UUT should be included as a component of uncertainty because of the undefined temperature gradients which will occur around the immersion depth stated in the calibration certificate.

• However, there may be an exception where the purpose of the measurements is to identify inhomogeneity (as is the case with the UKAS audit batch calibrated thermocouples).

• In such cases, the lab should state the value of the component obtained

Assessor Witnessed event

• Inhomogeneous ageing of a type S reference thermocouple amounted to 25 °C at 1050 °C.

• Type S thermocouples that are in continuous use at 1000°C in air can age at about 10°C per annum.

• But they can be effectively annealed from time to time. This significantly reduces any inhomogeneity caused by localised gradients etc.

• They are generally recalibrated, whereas base metal thermocouples are often discarded.

Reporting the results of a

calibration

ILAC document P14:01/2013 states in 5.4

• In the formulation of CMC, laboratories shall take notice of the performance of the “best existing device” which is available for a specific category of calibrations.

Also contribution for the item under calibration 6.4 it states:

• Contributions to the uncertainty stated on the calibration certificate shall include relevant short-term contributions during calibration and contributions that can reasonably be attributed to the customer’s device

Thermocouple Homogeneity

Issues (Euramet cg-08).

• Basis of Issue: Euramet cg-08 update in November 2011 includes

• “it is recommended to take at least 20% of the Class 2 tolerance value for the corresponding type of thermocouple according to EN IEC 60584-2 [7] as contribution (k = 1) to the uncertainty.”

• This appears to be based tests carried out in Brazil

• This is equivalent to 0.3 %T (k=2) at higher temperatures.

• We would like to unify the approach amongst UKAS

and other EA assessors and the values stated in

cg-08 are not consistent with the observed

measurements.

Suggested approach

• New thermocouples, annealed noble metal thermocouples and thermocouples that have been used below 50 % of the EN standard range

• The following minimum inhomogeneity components should be used:

• Base metal: 0.05 °C + 0.05 % of temperature (a lab may opt for 0.1 % of temperature for simplicity)

• This estimate is supported by “years of assessments” and the results obtained for UKAS’ audit batch of heat treated type N thermocouples.

• Noble metal: 0.02 % of temperature

Suggested approach

• If the maximum temperature is MORE

than 50 % of the EN standard range of the

thermocouple, then one lower temperature

point must be repeated in the calibration.

• Labs may choose to demonstrate that

lower figures can be used for certain types

of thermocouple.

Suggested approach• Thermocouples which have been used above 50 % of

the EN standard range for the type of thermocouple, including noble metal thermocouples which have not been annealed

• The same figure is used as above, and a suitable ‘buyer beware’ statement to be included in the certificate. Something like,

• “Used thermocouples can have significant inhomogeneity if they are subjected to mechanical stress or high temperatures. This can cause significant changes in thermocouple performance if the thermocouple is immersed to a depth different to that stated in this certificate.”

• It is also reasonable for the lab to assume that the thermocouple is not used outside of the calibration range requested by the customer.

Suggested approach

• Batch calibration of base metal thermocouples

• Labs can apply to be assessed for batch calibration of base metal thermocouples. They need to have procedures for sampling and a suitable method of presenting individual and averaged results. The uncertainty (and CMC) of individual results does not need to include a component for inhomogeneity. The uncertainty stated for the average result must include a component derived from the variations observed for the batch. A repeated point may be required as above.

• This will appear as a separate entry in the schedule of accreditation.

Suggested approach

• Justification

• For the calibration of batch/characterising of

thermocouples, usually sampled at the beginning, middle

and end of a reel, the variation in the measurements is

mostly due to the inhomogeneity. So to have a

component for the inhomogeneity while measuring

inhomogeneity has not correct. But the measured

inhomogeneity should be included into the uncertainty of

the batch calibration. So the CMC could assume zero

inhomogeneity (perfect material) but have a comment

that the actual measured inhomogeneity will be added to

the uncertainty.

Suggested approach

• For Calibration labs who don’t know what

inhomogeneity is we hit them with the full

Euramet cg-08 value.

• Or don’t accredit them at all

CJC

Cold Junction Compensation

• What if you are measuring sub zero

temperatures then which end is the cold

junction

We should probably be using the term

Reference junction

Un-calibrated cold junctions

• No cold junction measurement assumed to

be 22 °C

The Manual states:

Ambient temperature away from 22 ºC will

incur errors

Un-calibrated cold junctions

• No cold junction measurement assumed to

be 22 °C

• No calibration of the CJC

• Calibrated using the front terminals, but

connected in practice using the rear

terminals

• Calibrated on channel 1 only (out of 16

possible channels), but there are separate

CJCs for every 4 channels

inappropriately positioned

CJC• Only covered by a thin layer of plastic, so very

susceptible to fluctuations in ambient temperature

• CJC affected by heat generated from the electronics within the measuring unit. This can cause differences between the channels of 0.5 °C or more.

• Vertical run multi-channel thermocouple connection strip with a central CJC, mounted close to the floor with no draft proofing. Progressive variations of more than 2 °C were seen from top to bottom, and these were not repeatable.

• Thermocouple connections placed in direct sunlight, in cold/hot drafts (e.g. the vent from a laptop), or close to heat sources (exposed steam pipes in an autoclave)

Insufficient time for

stabilisation of CJC

• Thermocouple calibrator moved from a boot of a cold car to a furnace room at about 28 °C then to a temperature controlled area at 20 °C, and calibrations carried out within a few minutes. The errors proved to be significantly greater than the claimed uncertainty.

• Issues have also been seen when the measuring instrument is battery powered, if then connected to the mains to charge, the battery warms up significantly affecting the CJC by several °C

Calibration options for process calibrators

Calibrate complete system including CJC

Calibrate voltage generation & CJC separately

Calibration options for process calibrators

Calibrate complete system including CJC

Calibrated in the way it is used

Requires external cold junction (or CJC)

which may increase complexity of calibration and

uncertainty

Requires more lead changes

Calibration options for process calibrators

Calibrate voltage generation & CJC separately

CJC can be checked on one thermocouple type

Easier to compare with manufacturer’s specification

Needs to be understood by lab and customer

Control/measure

Thermocouple

Electrical Simulation of temperature

of an oven controller

Cold junction calibration - measure mode

20.53 °C

Thermocouple

indicatorMetal block

or stirred liquid bath

Effect of thermocouple characteristic should be

negligible because temperature gradients

are insignificant, but temperature must be

measured accurately.

PRT

20.57 °C

Cold junction calibration - source mode

20.57 °C

Thermocouple calibrator

set to PRT temperature

Metal block

or stirred liquid bath

PRT

20.57 °C

0.008 mV

Millivoltmeterreading close to zero

Copper/copper cableconnected to each thermocouple leg

• Voltmeter

• Temperature indicator and probe

• Liquid bath / block temperature gradients

• Thermocouple extension cable

• Setability of process calibrator

• Repeatability

Components of uncertainty

Combination of components

From M3003

Uncertainty

• Reference probe

• Environment

• Artefact being measured

Uncertainty

• Reference probe

– Calibration

• Uncorrected error

• Fit to the reference function

• Resolution/ uncertainty of the EMF measurement

• Temperature effects on the Reference junction

• inhomogeneity

– Drift

• From difference between calibrations

Uncertainty

• Reference probe

– Calibration

– Drift

• Environment

– Gradients

– Stability

• Artefact being measured

– Resolution

– Stability

An example of a Measurement

Description of measurementFridge/incubator/chamber using type T thermocouples from 0°C to 50 °C

Note

number

(below)

Source of uncertaintyValue

±Unit

Probability

distributionDivisor

Sensitivity

ci

Standard

uncertainty

ui (°C)

Degrees of

freedom

vi or vf

1 Calibration of thermcouples and logger 0.30°C normal (k=2) 2.00 1 0.15 i

2 Chamber stability (from results sheet) 0.15°C normal (k=1) 1.00 1 0.15 9

3 Maximum uncorrected calibration error 0.25°C rectangular 1.73 1 0.14 i

4 Drift of thermocouple system between calibrations 0.20°C rectangular 1.73 1 0.12 i

5 Ambient temperature effects on logger 0.10°C rectangular 1.73 1 0.06 i

6 Temperature changes/gradients at cold junction 0.20°C rectangular 1.73 1 0.12 i

7 0.00 rectangular 1.73 1 0.00 i

8 0.00 rectangular 1.73 1 0.00 i

9 0.00 rectangular 1.73 1 0.00 i

uc Combined standard uncertainty normal (k=1) 0.31 172

U Expanded uncertainty normal k for vf 2.00 0.62 172

Notes

1 From certificate 123456789 dated dd mmm yy. Uncertainty stated as ±(0.2 °C + 1 digit), taken to be ±0.3 °C.

2 The measured stability of the chamber is taken to include the repeatability of the temperature measuring system.

3 Maximum error in calibration certificate, over the range 0 °C to 50 °C is 0.25 °C for any individual thermocouple.

4 Estimated by comparing previous calibration certificates. Confirmed by ice-point checks every 3 months.

5 Manufacturer's specification is ±1 °C for an ambient temperature range of 0 °C to 35 °C. System is calibrated and used at 20 °C ±3 °C. Ambient temperature effect estimated as ±0.1 °C for ambient temperatures from 17 °C to 23 °C.

6 This was evaluated by placing all thermocouples in an ice/water mixture and the results logged over 3 hours with the lab in normal

use. Variations of up to ±0.2 °C were observed.

That’s it

• It looks easy

• But its not

• To ensure consistency at an appropriate

level we need more published data

• Reference junction

• Uncertainty

That’s it

• Any Questions