The measured time will sometimes not be
exactly what was set under settings, but the
measured value is the one used for all
calculations. The dose rate shown for Timed
mode will always be mean dose rate during
the measurement cycle, i.e. the measured
dose divided by the measured time.
The Timed mode can be very useful both for very low dose rate measurements as well
as for long duration measurements.
· For extreme low-level dose measurements you can improve your reading by
subtracting the background level. First do a Timed measurement without exposing
the detector to radiation and then do the same with radiation. The timed mode will
use the same measuring time and the first reading can be subtracted from the first.
Just make sure not to tap Reset between these measurements, as the Reset button
will do an offset adjustment. Note also that low-level readings may give inaccurate
kV readings.
· For long duration measurements, cases with slowly rising and falling output, or
cases with very low pulse rate, timed mode may also be useful. For instance on CT
machines where the rotation cannot be stopped.
4.3.2 Using Free Run Update Mode
Free run update mode works almost exactly as the ordinary Continuous update mode.
There are however two differences:
1. Since there is no trig level, you will be able to measure lower, but there will be no
time reading unless the signal goes over the trig level.
2. You can select a moving average function. This lets you set a time for moving
average, this time acts as a averaging window, moving through time.
Moving average
This function is intended for low level dose rate measurements where increased
sensitivity and stability is needed. The function uses a moving average algorithm
where the number of seconds is selected by the user.
During the reset process the user must make sure that the detector is not exposed to
radiation. After the reset procedure the Barracuda will start to show a value calculated
as the sum of the last X values divided by X (X is the number of seconds chosen by
the user). For each new second that passes the last value in the stack will be
discarded and a new value added. This means of course that it will take X seconds
before the Barracuda starts to show a valid value when the detector attached is
exposed to a steady radiation level. In the same way it will take X seconds for the
Barracuda to show a zero value after the radiation has ended. Great care must be
taken into choosing a time constant fitted to the nature of the signal.
Example
If you set the time to 8 seconds, each reading, will be the mean of the reading of the
last 8 seconds. This means that it will take 8 seconds until the reading reaches a
started set radiation level.
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