Fluke Biomedical 06-526 - Manual (Page 15)

Operation
Accuracy Considerations
2
2-7
2.9 Accuracy Considerations
2.9.1 Electrometer Drift
Each instrument has a constant drift which can be measured in a zero radiation field. Drift is expressed
as mR/min and has a typical value of 1 mR/min or less. By recording the drift rate, this error can be
corrected. Refer to Table 2-2. Drift of the internal chamber can only be measured in the manual mode.
2.9.2 Air Density Corrections
Temperature and pressure have a definite effect on the response from the ion chamber; they determine
the number and density of molecules present for ionization. Figure 2-2 shows the correction factors used
for changes in temperature and altitude (from 22°C and sea level). To achieve maximum accuracy, use
the following equation:
Corrected Exposure =
(Measured Exposure - Drift x Exposure Time) (CF
T
) (CF
P
)
Where CTT and CFP are the correction factors for temperature and pressure respectively. Note that no
more than 2% errors result over the temperature range of 60° to 82°F (16° to 28°C), so that the
temperature correction can normally be disregarded.
In the above equation, CF
T
and CF
P
can be calculated (rather than approximating using Figure 2-2).
CF
T
= T(°C) + 273
295
CF
P
= 760
P(mm Hg)
Where:
T(°C) = temperature measured in degrees C, and
P (mm Hg) = pressure measured in mm Hg