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Meter requirements

Radiation meters must be properly calibrated. When selecting a meter, it must be ascertained that the meter is capable of measuring the radiation quality and dose rate in question.

The key aspects when selecting a meter are the suitability of the meter for the purpose, or that it is intended for the measuring of the radiation quality that is being measured, and that the meter has been calibrated and is sufficiently accurate. The requirements set for the meters vary depending on the purpose for which the measurement results are used. Attention must also be paid to the correct use of the meter so that the measurement result is sufficiently representative of the prevailing exposure conditions.

Meter selection

When selecting a meter, it must be ascertained that the meter is capable of measuring the radiation quality and dose rate in question, the energy range is suitable, the qualities of the meter are compatible with the situation of use and that the prevailing environmental conditions do not affect the measurement result in an uncontrollable fashion. When measuring pulse-shaped radiation or very high dose rates, the operability of the meter must be ensured particularly.

Calibration and monitoring of meter operating condition

Radiation meters must be properly calibrated. Calibration refers to the determination of the relationship between the values indicated by a measuring instrument and the values of the measured quantity, that means the determination of the calibration factor and its estimated uncertainty. Radiation meters are calibrated in a laboratory with a traceability to international radiation metrological standards. Calibration must be performed before the implementation of the meter and after this at least once every five years unless otherwise required during the approval of the measurement method or activity, or otherwise. The initial calibration done by the manufacturer is often sufficient for the taking into use of a radiation meter.

Calibration must be performed for the energy range within which the meter will be used. In some situations, additional requirements have been set for calibration. The requirements are presented in Guide ST 1.9 or separately during the approval decision process.

The operating condition of the meter must be checked at regular intervals between calibrations. This includes checking the general condition of the meter and a functional test using a suitable radiation source.

The reading of the meter is not usually adjusted in connection with the calibration. In order to find out the actual value of the radiation quantity, the reading of the meter must be multiplied with the calibration factor. The calibration factor may depend on the radiation quality measured.

Accuracy of measurements in radiation and dose measurements

All measurements include uncertainty factors, the impact of which is estimated in the uncertainty of measurement assessment. The requirements concerning measurement result uncertainty are presented in appendix C to Guide ST 1.9.

The concepts related to uncertainty of measurement are listed in the table below. All concepts defined have been reported as relative, meaning that they indicate the percentage that the value of the concept is of the measurement result (uncertainty, error and intrinsic error), probability 1 (level of confidence) or average value (standard deviation of the mean).

Concept Explanation
Uncertainty of measurement Error limits within which the true value of the quantity measured is most likely to be.
Expanded uncertainty Uncertainty multiplied with the coverage factor.
Coverage factor The factor by which uncertainty is multiplied to obtain the expanded uncertainty. The coverage factor indicates the level of confidence: factor 1 = level on confidence 68%, factor 2 = level of confidence 95%.
Level of confidence The probability that the true value of the quantity measured is within the error limits.
Standard deviation of the mean The positive square root of the arithmetic mean of the squares of the remainders of the results of measurement and their mean value.
Intrinsic error of measuring instrument An error which has been determined in basic conditions.
Error The difference between the result of measurement and the true value of the quantity measured.



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