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Theory of Operation
Basic Operating Equations
2
2-9
Figure 2-3. Average Current and Exposure Time
2.3.3 Air Density Correction Factor
The air density correction factor is used to correct dose and dose rate measurements for a vented ion
chamber.
where the pressure “P” is expressed in mmHg, the temperature “T” is in Kelvin, and the calibration
temperature “T
c
” is in Kelvin. The calibration temperature is either 20°C (293.15 K) or 22°C (295.15 K),
depending on that used by the calibration agency when calibrating the ion chamber. The calibration
temperature must be specified during customization. The ADCF is displayed on the bottom line of the Air
Density screen.
When the user has customized the Model 35040 Therapy Dosimeter to use other temperature and
pressure units, the corresponding values entered by the user at the instrument’s front panel are
automatically converted to mmHg and K before use in calculating the ADCF. The equations used for
these conversions are listed below. The abbreviation FSL stands for “from sea level.”
mm Hg = inch Hg x 25.4
mm Hg = 760 x (54,041 - feet FSL) / (54,041 + feetFSL)
mm Hg = 760 x (16,472 - metersFSL) / (16,472 + metersFSL)
mmHg = mbar x 0.75006
mmHg = hPa x 0.75006
K = (5 / 9) x (F - 32) + 273.15
K = C + 273.15
Time
0
- Sampled charge data points in average current.
T2
- Charge data points for leakage.
T1
Charge
Start of Exposure Detected
End of Exposure Detected
1.33s
1.33s
TOffset
T
Q
X
X
X
X
X
X
X
X
X X
X
c
⎟
⎠
⎞
⎜
⎝
⎛
×
⎟
⎠
⎞
⎜
⎝
⎛
=
T
T
P
mmHg
ADCF
760