Fluke Biomedical 8000 Victoreen - Manual (Page 83)

A
Appendix
Temperature and Pressure
A-1
Appendix A
Temperature and Pressure
A.1 Temperature and Pressure
Any ionization chamber consists of a defined volume of air in which ions produced by radiation passing
through the chamber can be collected and measured. A potential difference (voltage) is placed across
the plates of the ion chamber. When ionizing radiation passes through the chamber, ion pairs are
produced, each pair consists of one positive and one negative ion. Under the influence of the electric
field produced by the potential on the plates, the ions move toward their opposite charged plate. The net
effect of this is to cause a current to flow through the electronics in the NERO mAx connected to the
plates, the magnitude of which is proportional to the rate of the exposure of radiation.
The sensitivity of any ion chamber depends upon the number of air molecules in the chamber. These
quantities are directly proportional. The number of air molecules is a function of volume, temperature,
and pressure. The volume of air in the internal and external ion chambers attached to the NERO mAx
are fixed, although at different quantities. The difference in sensitivity between different size ion
chambers are addressed in the CAL mode. Since all of these chambers communicate with the
atmosphere, temperature and pressure will vary with ambient conditions.
In the NERO mAx the ambient temperature and pressure are manually entered. These values are used
to calculate the correction factor that is automatically applied to the output of both internal and external
ion chambers. These ion chambers provide rate and exposure measurements, and the corrections are
applied in all modes as appropriate. Temperature and pressure does not affect other readings such as
kV, time, or mAs. The correction factor is computed as follows:
273.16 T
273.16 T
P
P
cf
0
0
+
+
× =
Where T is the temperature in degrees Celsius and P is the pressure in mm of Hg. T
0
and P
0
is the
temperature and pressure, respectively, at which the chamber was calibrated.