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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