Answer to Question #10761 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

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

Q

I have to measure occupational exposure from a medical x-ray facility. This particular facility is located at about 5,000 meters over sea level and my survey meter is a typical Victoreen 450P, pressurized ionization chamber. Will this survey meter work for this condition? Do I have to apply any special considerations in regard to the values obtained?

A

The short answer to your question is that the specified instrument's response is not dependent on altitude, and no special considerations apply. The background readings may be higher at the greater elevation than what are observed at sea level, but this would be expected. The pressurized chamber is a sealed cylindrical chamber that is pressurized with air to about six atmospheres. Because it is sealed, the density of the air in the chamber is not affected by altitude. This is different from many unpressurized air ionization chambers that are not sealed and whose responses generally require corrections for changes in air density in the chambers associated with significant changes in altitude from the altitude at which the chamber was calibrated.

Naturally, you must still take into account other possible limitations of the instrument for the measurements you are intending to make. For example, you should be sure that the chamber is capable of measuring the quantity of interest (e.g., µSv h-1) at the photon energies of interest. Good luck in your measurements.

George Chabot, PhD

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