Type testing, calibration and routine checks
The three activities that establish whether a radiation protection instrument reads correctly — type test, calibration and routine check — the question each answers, what each document states, and why none of them substitutes for another.
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Three activities establish that a radiation protection instrument gives a trustworthy indication: the type test, the calibration and the routine check. They are often confused, because all three expose an instrument to a known radiation field and compare its indication with a reference value. They answer different questions, they are performed on different objects at different intervals, and the documents they produce support different conclusions.
Three questions
| Activity | Question answered | Object | Frequency | Document |
|---|---|---|---|---|
| Type test | Does this design meet the performance requirements of its standard over the stated ranges of conditions? | One or a few units representative of a model | Once per design, repeated after design changes | Type-test report |
| Calibration | What is the response of this unit at a specified reference quality, with what uncertainty, traceable to which standard? | Each individual unit | Periodically, and after repair | Calibration certificate |
| Routine check | Has this unit changed since it was last calibrated? | Each individual unit | Frequently, often before use | Local record |
A type test establishes what a design can do. A calibration establishes what one unit of that design does at one set of conditions. A routine check establishes only that the unit still behaves as it did at calibration. Each depends on the one before it.
Type test
A type test is a conformity test made on one or more items representative of a production design. For photon dose rate meters the requirements are those of the applicable standard — for portable workplace and environmental instruments, IEC 60846-1 — and the tests vary each influence quantity in turn: a quantity that is not the one being measured but changes the indication. Photon energy, angle of incidence, dose rate, ambient temperature, relative humidity, supply voltage, electromagnetic fields and mechanical stress are all influence quantities for a dose rate meter.
Rated ranges
The result of a type test is expressed as a set of rated ranges: the ranges of each influence quantity over which the instrument has been shown to meet its requirements. An instrument with a rated energy range beginning well above the lowest energies in the field where it is used has no demonstrated performance in that field, even if its calibration certificate is current. Selecting an instrument is therefore primarily a comparison of rated ranges with the conditions of use — photon energies, dose rates, temperatures, whether the field is pulsed — and only secondarily a question of calibration.
Single samples
A type test is performed on few units because it examines the design. The consistency of production is assured by the manufacturer’s quality system and, unit by unit, by calibration. A type-test report does not state that every unit of the model behaves identically; it states that the design is capable of meeting the requirements.
Calibration
A calibration determines the relationship between the indication of one instrument and the conventional true value of the quantity — the value attributed to the quantity at the point of test by the calibration laboratory, with a stated uncertainty. For an ambient dose equivalent rate meter the reference quality is normally the ¹³⁷Cs gamma radiation described in Reference radiation qualities. The result is usually reported as a calibration factor — the conventional true value divided by the indication — or as its reciprocal, the response.
Traceability
The conventional true value is traceable through an unbroken chain of comparisons to a primary standard. In most countries the chain runs from a primary standards laboratory, through a secondary standards dosimetry laboratory, to the laboratory that calibrates field instruments. The IAEA and the WHO maintain a network of secondary standards dosimetry laboratories for countries without a primary standard. Accreditation of the calibrating laboratory to ISO/IEC 17025 is the usual evidence that the chain and the stated uncertainty are sound.
Content of a certificate
A calibration certificate is complete only if it states the conditions under which the factor applies: the radiation quality, the conventional true value or values, the orientation of the instrument and its reference point, the distance, the ambient conditions, the uncertainty with its coverage factor, and whether any adjustment was made. The last item matters for retrospective use. A result recorded as found, before adjustment, shows how far the instrument had drifted since the previous calibration, and therefore whether measurements made during that interval are still valid. A result recorded only as left, after adjustment, does not show this.
Scope of a single calibration
A calibration at one quality does not re-establish the energy response. The response at other energies follows from the type test, scaled by the calibration at the reference quality. This is the reason the two activities do not substitute for each other: a calibration certificate cannot show that a design responds adequately at low energies, and a type-test report cannot show that a particular unit is correctly adjusted. Laboratories commonly calibrate at more than one dose rate, so that the linearity of the individual unit across its ranges is also confirmed. Guidance on calibration practice for radiation protection instruments is given in IAEA Safety Reports Series No. 16, Calibration of Radiation Protection Monitoring Instruments.
Interval
The calibration interval is set by the applicable regulation or by the user’s quality system; one year is common. An instrument is also recalibrated after repair, after any event that could have changed its response, and when a routine check falls outside its tolerance.
Routine check
A routine check confirms constancy. A check source is placed in a fixed, reproducible position relative to the detector — usually in a jig — and the indication is compared with a reference reading taken immediately after the last calibration, corrected for the decay of the source. A tolerance band around the reference reading is set in advance, and a reading outside it removes the instrument from service until it is investigated.
Functional checks are made at the same time: battery condition, display segments, audible and visual alarms, and the plausibility of the background indication.
Scope of a routine check
A routine check is made at one energy and one dose rate. It detects gross faults and changes in overall sensitivity. It does not detect a change confined to part of the energy range. Damage to the energy-compensation filter of a Geiger–Müller tube, for example, alters the response mainly at low photon energies, and a check with a ¹³⁷Cs source can remain within tolerance while the low-energy response has changed substantially. Recording each check result, rather than only pass or fail, allows a slow drift to be seen before it reaches the tolerance limit.
Relationship to this tool
The characteristic evaluation tools on this site reproduce the calculations of the type tests they cover from readings entered by the user: relative responses, coefficients of variation, response times and the corresponding verdicts. The output is neither a type-test report nor a calibration certificate. It does not establish traceability, it does not assess the irradiation facility, and it does not replace either activity — see the disclaimer. It is useful for checking the arithmetic of an evaluation, for comparing an instrument’s results with the requirements of its standard, and for understanding what a type-test report contains.