The measurement uncertainty in the calibration of a thermometer depends on the calibration method used, the uncertainty contri

WORLD METEOROLOGICAL ORGANIZATION __________________ COMMISSION FOR INSTRUMENTS AND METHODS OF OBSERVATION EXPERT TEAM ON REGIONAL INSTRUMENT CENTRES, CALIBRATION AND TRACEABILITY First Session Nairobi, Kenya 23 – 26 September 2013

CIMO/ET-RIC-1, Doc. 5.1(2) (13.9.2013) _______ ITEM: 5.1 Original: ENGLISH

Example of MEASUREMENT UNCERTAINTY estimation

TEMPERATURE
(Submitted by Drago Groselj)

Summary and purpose of document

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.

1. 
   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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