“Old” and “new” measured variables in radiation protection
Note on spelling: We avoid indexes because they interfere with the line spacing. In the continuous text we therefore write e.g. Hx instead of Hx.
A very old measured quantity that is still used today in some countries is the standard ion dose Js, which is measured in R (X-ray). As its successor, the photon equivalent dose Hx was introduced in Germany in 1980, which is measured in Sv (Sievert) and became a legal measurement on 01.01.1986. From this point on, initial calibrations of R measuring instruments were no longer possible. Independent of the photon energy, the conversion Hx [Sv] = 0.01 Sv/R × Js [R] applies. At that time, it was ultimately only a change of unit and not a change to a really new measured quantity. For instruments, this meant that although the display had to be changed (divide all values by 100 and replace “R” by “Sv”), the detector itself could remain unchanged. The practical benefit of this change may well be doubted. In fact, the measured quantity Hx remained a German stand-alone, which was not accepted internationally (except in Austria).
Based on the European Directive 96/29/EURATOM, which should have been implemented into national law by 13.03.2000, the Radiation Protection Ordinance (StrlSchV) of 2001 again introduced new measured quantities in Germany. By the way, these new measured quantities were already not that new at that time. They go back to the ICRU Report 39 from 1985. In Germany, the recommendation was already made in 1993 to introduce the new measured quantities on 01.01.1995. This took a little longer, namely until the new StrlSchV came into force on 01.08.2001. The old measured quantities were granted a ten-year transition period, which expired on 01.08.2011. Since 01.08.2006, measuring instruments could only be calibrated in the new measured quantities. Although no longer so new, we will nevertheless continue to talk about the “new” measured quantities, since they are the currently valid measured quantities.
What is new about the new measured quantity?
The new parameters, as far as they concern our products, are the ambient dose equivalent H*(10) (for local dosimetry) and the depth personal dose Hp(10) (for personal dosimetry), both measured in Sv. The main difference to the old measured quantities is that the new measured quantities are defined in phantoms that simulate the influence of the human body on the radiation field. For local dosimeters (our dose rate meter 6150AD is in the official language a “local dosimeter”), which are not worn on the body, this means that they must reproduce the influence of the phantom. For personal dosimeters, this means that they must record the influence of the phantom in a suitable manner. For this purpose, a modification of the energy dependence is necessary, especially in the case of local dosimeters, i.e. modification of the detector. A device for H*(10) or Hp(10) is therefore constructed slightly differently from a device for Hx and must always use Sv as the unit. In contrast, an instrument designed for the old measured quantity Hx can display both Hx in Sv and Js in R. Our international Hx versions of the 6150AD dose rate meter also make use of this possibility. There the user can switch between the units Sv and R.
It should not be concealed that the new measured variables are by no means recognised worldwide. In the USA, for example, the new measured variables still play a subordinate role in practical radiation protection today (status 2016). That is why we still produce instruments for the old measured variables and still list them on the English version of our homepage.
Physical backgrounds to the measured variables in radiation protection can be found in the category Measured quantities in radiation protection.
What do “calibrate” and “gauge” mean?
From a technical point of view, both terms are equivalent; they mean the adjustment of a measuring instrument to the smallest possible measuring error. Legally speaking, however, there is a big difference. A calibration is the official statement that a measuring instrument does not exceed certain “calibration error limits”. Thus, legally speaking, calibration cannot be carried out by private individuals, but only by the calibration offices and only on equipment that meets certain requirements. Successfully calibrated devices receive a seal (calibration mark) from the weights and measures office. Calibration (at least for our instruments) does not involve any adjustment of the measuring instrument by the weights and measures office but only checking the instrument for compliance with the calibration error limits. If the device does not comply with the calibration error limits, it will not be “readjusted” by the weights and measures office, but rejected.
How large are the calibration error limits?
The Measurement and Calibration Ordinance specifies the calibration error limits or refers to the documents and guidelines in which the calibration error limits are stated. For radiation measuring instruments, reference is currently (2016) still made to Annex 23 of the old calibration regulations. For personal dosimeters and portable local dosimeters a calibration error limit of 20% is specified there. This means that even under standard conditions, a device of this type may display incorrectly by up to 20%. This may seem like a lot to you. However, it certainly corresponds to the current state of technology as even complex secondary standards still have uncertainties of a few per cent. In principle, radiation cannot be measured as accurately as length or time.
Are calibrated devices particularly accurate?
No, at least as far as our instruments are concerned. At Automess, all instruments are calibrated with equal care, regardless of whether or not they are subsequently submitted to an official inspection. A calibrated instrument is therefore not a particularly accurate instrument, but an instrument whose measuring accuracy is checked and certified by an independent official body.
What do “calibratable” and “PTB certified” mean?
Before 01.01.2015, “calibratable” designated the property of a measuring instrument to be approved for verification on the basis of a national type approval of the Physikalisch-Technische Bundesanstalt (PTB). Verifiable instruments were allowed to bear an approval mark consisting of a stylised “Z” with the approval number. The calibration offices were only allowed to calibrate calibratable devices. For some measurement purposes, the calibration regulations prescribe the use of calibrated measuring instruments. The calibrations had to be repeated at certain intervals.
Since the Measurement and Verification Act (MessEG) came into force on 01.01.2015, the term “calibratable” no longer exists in official language use. This term now reads approximately “in accordance with the provisions of the Measurement and Verification Act”. Because of this change in terminology, even things like the pre-packaging regulation and the road traffic licensing regulation had to be changed. Also the term “type approval” (for calibration) no longer exists in metrology. Instead, a successful type examination by PTB now ends with a “Type Examination Certificate”.
Even under the new regulations, there are calibrations that are repeated at certain intervals. Only the very first calibration in the life of a measuring instrument no longer exists. It is replaced by the conformity assessment, which we will discuss in detail in a moment.
According to the new regulations, as a new device is only assessed for conformity once at the beginning but can in principle be calibrated as often as required afterwards, the term calibratable is actually still applicable. Deviating from the official use of language, we will therefore not generally avoid this term. At the same time, we will also use the more general term PTB-certified, which is compatible with both the old and the new regulations. By both terms, we mean those instruments that meet all the requirements for conformity assessment with subsequent calibrations.
The energy range of radiation meters is often only specified to 3 MeV or less. How do the devices behave at higher photon energies?
This question is particularly raised because of the 6 MeV radiation of N-16 occurring in nuclear facilities. Energy compensated Geiger-Müller counters always indicate too much for such energy. We have created an information sheet on this subject.
I measured the dose rate at various distances from the source and found deviations from the 1/r² law. Why is that?
The 1/r² law only applies under certain conditions. We have created an information sheet on this subject.