4. Measurement Principles & Theory
Display Messages and Active Messages
Barracuda & QABrowser Reference Manual
2012-10/4.3A
102
Negative Signal
The electrometer module detects a negative signal.
· Most common is that the mAs-probe have been
connected in the opposite direction on the HV cable.
Change the polarity of the current probe.
· Also small negative drift created from the detector source,
typically initially after reset can give this message.
Special care should be taken when using ionization
chambers, since it may take a few minutes before the
initial drift of the electrometer and detector has stabilized.
· Tap Reset to clear the message.
· This message does not appear in the Timed and Free run update modes.
Waveforms and Triggers4.5
To get an understanding of how triggers, delays, and windows work, take a look at the
waveform below. This is what happens during a standard exposure:
1. The radiation starts, i.e. it goes over the detector's lowest trig level.
2. The signal reaches 50 % of its maximum. This is the starting point for the irradiation
time calculation. (The level is user adjustable.)
3. The signal reaches its maximum.
4. The Delay time is reached. (User adjustable.) kV integration window starts.
5. The Delay+Window time is reached. (User adjustable.) kV integration window stops.
6. The signal goes below 50 % of its maximum. This is the end point for the Irradiation
time calculation. (The level is user adjustable.)
7. The radiation ends, i.e. it goes under the detector's lowest trig level.
8. If the signal has been below the trig level during all of Post delay, the exposure is
considered finished. All exposure readings are calculated.
Integrated signal (dose, mAs, etc.)
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