WORLD METEOROLOGICAL ORGANIZATION
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COMMISSION FOR INSTRUMENTS AND
METHODS OF OBSERVATION
EXPERT TEAM ON REGIONAL INSTRUMENT CENTRES, CALIBRATION AND TRACEABILITY
First Session
Nairobi, Kenya
23 – 26 September 2013
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CIMO/ET-RIC-1, Doc. 5.1(2)
(13.9.2013)
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ITEM: 5.1
Original: ENGLISH
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Example of MEASUREMENT UNCERTAINTY estimation
TEMPERATURE
(Submitted by Drago Groselj)
This document presents an example of calibration uncertainty for resistance thermometer calibration.
Action Proposed
The meeting is invited to review this document and make recommendations towards the completion of this task.
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Introduction
The measurement uncertainty in the calibration of a thermometer depends on the calibration method used, the uncertainty contribution of the standards, the characteristics of the measuring equipment used and the characteristics of the device under calibration. No general instructions for the measurement uncertainty of certain thermometer types can therefore be given. The examples of measurement uncertainty calculation cannot be directly implemented to any calibration actually carried out but uncertainty contributions must carefully be evaluated in each individual case.
Platinum resistance thermometers are calibrated by the comparison method or in defined fixed points in the appropriate temperature scale. Combination of the two methods is also permissible. Comparison calibration of the resistance thermometers are calibrated in temperature-stabilized baths using reference/working thermometers, suitable electrical measuring devices must be used (ohmmeter, resistance measuring bridge, standard resistors) which must also have been traceably calibrated..
Comparison calibration is performed by measurement of the resistance of the instrument under calibration while it is exposed to a temperature. Fundamentally, four instruments are required as follows:
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Reference standard
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Data acquisition for the reference standard
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Data acquisition for the instrument under calibration
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Temperature source
The technical requirements for the readout are the same for the instruments under calibration and the reference when calibrating PRTs against a reference PRT. If a multiplexing system is available, one readout device can usually be used for both. If the readout is designed for temperature calibration (not just temperature measurement) and has variable settings (current, timing, etc.), then certainly it can be used for both. If the readout is not designed for temperature calibration and/or a switching system is not available, then two or more readouts will probably be required. Before selecting a readout, review the information presented in the readouts section with regard to current settings, timing, multiplexing, etc. Best results will be obtained with readouts designed specifically for thermometer calibration.
A calibration bath/chamber cannot be considered as completely stable in time and homogeneous all over its volume, especially when temperature calibrations by comparison are performed at the best level of uncertainty. This represents a major contribution to the total uncertainty of a calibration procedure. In order to decrease this uncertainty contribution equalizing blocks can be used in calibration baths. The dimension of the block depends on the bath dimension.
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Homogeneity: A gradient is observed as a change of a temperature reading of a thermometer according to a change of its position inside a calibration bath. Basic gradients that can be observed are vertical and horizontal gradient. Because a lot of calibration baths have either a cylindrical shape or equalizing blocks inside it is sometimes more appropriate to define axial and a radial gradient. Uncertainty contribution of an axial gradient is determined as maximum temperature difference between two different positions in axial direction of an equalizing block. The radial gradient is a maximum temperature difference between two different positions in a radial direction.
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Stability: important characteristic of a bath is also short-term stability of a medium temperature. It strongly depends on type of regulation and flow of medium inside the bath. Since the calibration measurements are taken within short time interval, the short-time stability is relevant (cca. 30min). For the time stability of a bath, temperature deviations of a reference thermometer are observed.
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