Answer to Question #1366 Submitted to "Ask the Experts"

Q

Do I need to account for the expansion of mercury (Hg) as a function of temperature when I am pressure correcting? Some people take the barometric reading and subtract 2.5 mm Hg to get the current temperature [Editor's note: I assume you mean, "corrected pressure" and not "current temperature."]. They stated that the actual pressure should be reduced by this much as a result in room temperature (22 C) expansion. Is this right? What should I do? They say: C(T,P) = [(273.15+T)/295.15]×[760/(P-2.5)] I know that the AAPM (Association of Physicists in Medicine) Task Group-51 and Task Group-21 says nothing about this, but some say it is assumed. How?

A

If an ionization chamber is an "open" type where it can communicate with the environment, observed measurements (that is, charge collected per absorbed dose) will need to be corrected for any possible change in internal air mass. This is because the chamber (with specific electrometer) correction factors (for example, gray to air/coulomb) generated at the time of calibration are most often calculated to "normal" room temperature and pressure (T&P) conditions (that is, 22 degrees C and 760 mm Hg). Any deviation in temperature or pressure from these "normal" conditions will change in internal chamber air mass and therefore affect the charge generated and collected. Thus, all external beam radiation therapy dosimetry protocols (for example, Task Group 21 or 51) call for the correction of ionization chamber measurements for ambient T&P conditions while a measurement is being made. If you are using a mercury (Hg) barometer to take the ambient pressure reading at or near your radiation measurement (for example, in a radiation therapy clinic), one does need to consider the volumetric expansion of the Hg with temperature. This expansion may be a factor of 2 to 6 greater than the metal and glass used in the construction of the barometer scale and tube and may be translated into a pressure measurement error. A quality Hg barometer will have an accurate thermometer mounted on the instrument, and a table or graph will have been provided of correction factors. I would consult the barometer manufacturer and would NOT recommend a simple subtraction of 2.5 mm Hg from the "observed pressure" reading to obtain the "actual pressure" for use in the radiation measurement T&P correction. Lastly, while you're looking at this T&P correction concern, to use the above [(273.15 + T)/295.15] temperature correction, you should validate the normalization done at the time of chamber calibration. That is, ensure it is to "normal" temperature and not "standard" conditions (that is, 0 degrees C). If chamber correction factors are calculated to 0 degrees C, this would require you use a [(273.15 + T)/273.15] temperature correction. David J. Allard, CHP