Answer to Question #13808 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements

The following question was answered by an expert in the appropriate field:

Q

Why are we using standard room temperature of 20° C and pressure of 101.3 kPascal (kPa) in dosimetric measurements in primary standard dosimetry laboratory (PSDL) units? Please explain to me the exact reason.

A

When instruments are sensitive to possible changes in ambient temperature and atmospheric pressure it is common and desirable to calibrate them under standardized conditions. Such procedures are typically followed by any PSDL. Ionization chambers that are open to atmosphere are the most common of instruments that are so treated. What this means is that, for a given instrument exposed to a radiation field of known intensity, the readings are adjusted to give the expected result at the specified calibration temperature and atmospheric pressure.

In the United States, the National Institute of Standards and Technology (NIST) is the lead calibration laboratory, and it typically uses a temperature of 220 C and a pressure of 1 atmosphere (101.325 KPa ) for calibration of such instruments. These values have been selected as acceptably representative of indoor laboratory conditions. Using fixed temperature and pressure values allows the user of the calibrated instrument to normalize the reading to the calibration temperature and pressure to obtain an interpretation of dose under specified conditions. For example, when a user obtains a current reading of A at a temperature of 23.5 degrees C and a pressure of 102.0 kPa, the value corrected to the reference temperature of 220 C and 101.325 kPa would be I [(273.15+23.5)/(273.15+22)](101.325/102.0)] = 0.99843A. (Note that temperatures are converted to absolute values). This makes for consistency in comparisons of results, allowing not only for internal consistency, but also providing a framework for comparing results among different facilities. You have cited a calibration temperature of 200 C, which may be the case for some laboratories outside of the United States (or inside the United States if the user has a requirement for the lower calibration temperature). This is perfectly acceptable as long as everyone involved is using the same criteria.

I hope this addresses your concerns.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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