Environmental and electrical influence quantities
Ambient temperature, relative humidity, atmospheric pressure, supply voltage, electromagnetic fields and mechanical stress as influence quantities for radiation protection instruments — what each does to the indication, how a type test examines it, and which of these tests this tool covers.
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The radiological type tests vary the radiation field: photon energy, angle of incidence, dose rate. An instrument used outside a calibration laboratory is also exposed to conditions that have nothing to do with radiation — cold, heat, damp, low batteries, radio transmitters, being dropped — and each of these can change its indication. A type test examines them in the same way as the radiological quantities, and the results are reported in the same form. They are easy to overlook when choosing an instrument because they are rarely the headline of a specification sheet.
Terms
An influence quantity is a quantity that is not the one being measured but affects the relation between the indication and the measured value. Reference conditions are the values of the influence quantities at which the instrument’s response is defined — a stated temperature, humidity, pressure, orientation and so on. A rated range is the range of an influence quantity over which the instrument is required, and has been shown, to stay within a stated limit of variation. The test result for each influence quantity is a relative response: the response at the test condition divided by the response at reference conditions, with everything else held fixed.
Only one influence quantity is varied at a time. The radiation field during an environmental test is therefore the reference quality at a fixed dose rate, so that any change in indication can be attributed to the condition being varied.
Ambient temperature
Temperature acts on the detector and on the electronics. In a scintillation detector the light output of the crystal and the gain of the photomultiplier both change with temperature. In a vented ionization chamber the mass of air in the sensitive volume falls as temperature rises. In a GM tube the operating characteristics shift, and in all instruments the electronic thresholds, the reference voltages and the display can drift. Batteries lose capacity at low temperatures, so the supply voltage test and the temperature test interact in field use.
The test establishes the response at reference temperature, then at the extremes of the rated range after the instrument has reached thermal equilibrium. Equilibrium matters: an instrument read too soon after a change of temperature reports a transient, not a steady-state response. This tool’s evaluation workflow includes the ambient temperature test of IEC 60846-1 (§11.2) as an optional item, with default rows at the reference temperature and at the two extremes of the rated range.
Relative humidity
Humidity acts mainly on insulation. In an ionization chamber the signal current is small, and moisture on the insulators between electrodes creates a leakage current that adds to it or drifts with time. Condensation, which occurs when a cold instrument is brought into a warm room, can short a high-voltage supply and produce spurious counts in a GM instrument. The test holds the instrument at an elevated temperature and high relative humidity until equilibrium and compares its response with that at reference humidity. It is included in this tool as an optional item (§11.3).
Atmospheric pressure
Pressure affects only detectors whose gas is in contact with the atmosphere — principally vented ionization chambers — through the mass of gas in the sensitive volume. The effect is the one that matters when an instrument calibrated near sea level is used at altitude or in an aircraft. Sealed chambers, GM tubes, scintillators and semiconductor detectors are insensitive to it, apart from mechanical effects on thin windows.
Supply voltage
A battery-powered instrument must either measure correctly across the whole discharge of its battery or indicate clearly that the battery is too low before the indication degrades. The test therefore runs the instrument at reduced supply voltage and checks both the response and the low-battery indication. A dose rate meter whose reading falls quietly as its battery weakens is a more serious failure than one that switches itself off.
Electromagnetic compatibility
Radio-frequency fields from transmitters, mobile telephones and nearby electrical equipment can couple into the detector circuit and produce spurious counts, a false current or a corrupted display. Electrostatic discharge from the operator can reset or disturb an instrument. Static magnetic fields change the gain of photomultiplier tubes. The type test exposes the instrument to defined levels of each and requires that the indication remain within limits, or at least that the instrument does not indicate a dose rate it is not experiencing.
Electromagnetic disturbance is a particular concern for instruments used near accelerators, radar, industrial radio-frequency heaters and medical imaging equipment, where both strong fields and radiation can be present together.
Mechanical stress
Portable instruments are carried, dropped and vibrated. A GM tube’s thin window can rupture, a sodium iodide crystal can crack, a photomultiplier can lose alignment with its crystal and a connector can loosen. The mechanical tests apply drops, vibration and shock and then repeat a functional check and a response measurement. A crack in a scintillator does not necessarily stop the instrument working; it changes the light collection and therefore the response, which is why a response measurement after the mechanical test matters more than a functional check alone.
Light
Detectors that convert radiation into light, and photodiodes that detect it, respond to visible light if their housing is not light-tight. A small light leak produces an indication that changes with ambient illumination. The fault is easily missed in a laboratory with constant lighting and appears in sunlight.
Other radiations
For a photon dose rate meter, radiations other than photons are also influence quantities. A thin-window instrument used in a field containing beta radiation can indicate a dose rate that includes a beta contribution the instrument was not designed to measure. In mixed fields with neutrons, a photon instrument’s response to neutrons is a further source of error. The type test documentation states which of these have been examined.
Use of the results
Each environmental result is reported as the variation of response over the rated range, compared with a permitted limit. The practical use of these results is a comparison with the conditions of use. An instrument rated for a temperature range that does not include an unheated store in winter, or for a humidity that excludes a tropical site, has no demonstrated performance there. The individual variations also feed an uncertainty budget: where an instrument is used away from reference conditions, the variation measured in the type test is the usual basis for a Type B component for that influence quantity. The uncertainty this tool computes does not include such components, as Measurement uncertainty in the evaluation explains.
Coverage in this tool
The tool computes the relative responses and verdicts for the ambient temperature and relative humidity tests from the readings entered. Atmospheric pressure, supply voltage, electromagnetic compatibility, mechanical and light tests are outside its scope; they are recorded in the type-test report of the laboratory that performs them.