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Theory of Operation
Basic Operating Equations
2
2-7
Q
1
represents the next sample of charge at time T
1
following the activation of the timer. Q
2
corresponds to
the interpolated charge at time T
2
, where T
2
is an integral number of seconds that is restricted to times
within the range: T
1
< T
2
≤ (T
1
+ Timer Interval Setting).
Predicted Timed Charge and Dose
This measurement extrapolates the timed charge (Q
Timed
) at the elapsed time (T
Elapsed
) that should be
accumulated at completion of the timer interval (i.e., TimerIntervalSetting) based on the rate of charge
delivery.
[s]
[s]
[C]
valSettingTimerInter
T
Q
Q
Elapsed
Timed
ictedTimed_Pred
×=
ADCFICCFQDose ictedTimed_PredictedTimed_Pred
×
×= [Units/C] [C]
At the conclusion of the timer interval (i.e., T
Elapsed
equals the TimerIntervalSetting), the predicted timed
charge and dose are equal to the predicted timed charge and dose, respectively.
Accumulated Charge and Dose
This measurement accumulates charge across multiple exposures. The displayed value is updated
during the exposure and held between exposures. It is reset to zero when the RESET/MEASURE button
is pressed while viewing a measurement screen.
[C] [C] _ ExposurepreviousAccumAccum QQQ +=
ADCFICCFQDoseAccum Accum
×
×
= [Units/C] [C]
Exposure Current and Rate
The current (3-1/2 digit) and dose rate displayed when the exposure is in progress are updated once per
second. The current and dose rate return to zero after the end of the exposure. The dose rate
measurement is displayed using the selected time base (seconds, minutes, or hours). If the start and
stop thresholds are set to zero, I
Leakage
is zero.
[A] [A] LeakageExposure III −=
[s] [Units/C] [A] = lierRateMultipADCFICCFIDoseRate Exposure
×
×
×
Average Current and Rate
The average current is calculated using the charge accumulated (
Q
/
U
t
) during a portion of the exposure to
eliminate noise effects. The displayed value (4-/1/2 digit) is held following the exposure and reset at the
start of a subsequent exposure.
Q
1
and Q
2
are measured within the exposure at times T
1
and T
2
respectively. T
1
is 1.33 seconds after the
start of exposure. T
2
is 1.33 seconds before the end of exposure (see Figure 2-3). The exposure must be
more than 3 seconds long. I
Avg
is 0 if the exposure is less than 3 seconds.
2.3.1 Setting Thresholds
The instrument has capability to automatically detect the start and end of an exposure when the current
crosses given limits. Using automatic detection eliminates the sag in readings that occur due to leakage
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
−
=
[s][s]
[C] [C]
12
12
TT
QQ
I
Avg
[s] [Units/C] [A] = lierRateMultipADCFICCFIgDoseRateAv Avg
×
×
×