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Theory of Operation
Basic Operating Equations
2
2-5
Charge and Current Measurements
The instrument’s electrometer charge and current measurements are factory calibrated to a United States
National Institute for Standards and Technology (NIST) traceable standard. Governing regulatory
agencies or best of practice protocols may require that the instrument be calibrated by an independent
calibration agency. The calibration agency may install the correction factors (Q
ScaleFactor
, I
ScaleFactor
) to make
the instrument read out directly as calibrated. These factors are viewable by the operator but may not be
changed or turned off. In almost all cases, the scaling factors will be nearly unity.
[C/C] [C] rScaleFactoerElectromet QQQ ×=
() ( )
() ( )
[A/A] [C/s] rScaleFactoI
iTiT
iQiQ
I ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−
−−
=
1
1
[s] conversionerelectrometoftimeT =
Leakage Measurements
Leakage measurements (Q
Offset
, I
Leakage
and T
Offset
) are only applicable when automatic leakage
compensation is enabled.
The leakage current is defined as the current present between exposures. The displayed leakage current
or dose rate value (3-1/2 digit) is updated at a rate of approximately one time per second and held during
an exposure. Extrapolated charge due to leakage current is nulled from the measurement. In order to do
this, a short-term history of the leakage current is necessary.
When an exposure is not in progress and the absolute value of the current is less than the start threshold,
pressing the RESET/MEASURE button sets the leakage current I
Leakage
to zero and starts the acquisition
of current measurements for determining I
Leakage.
To get the “best” determination of I
Leakage
takes time, at
least 10 seconds. The ion chamber, cables, and instrument should not be disturbed while making the
leakage measurement.
()
[C] OffsetOffset TQQ =
I
I
n
Leakage
n
=
∑
1
[A]
DoseRateLeakage
I
I
CC
F
A
DC
F
RateMultiplie
r
Leakage=×
×
×
[A] [Units
/
C] [s]
T
T
Offset = [s]