Fluke Biomedical 4000M+ - Manual (Page 21)

3 Theory of Operation
Basic Definitions
3-3
Exposure and Exposure Rate
The Model 4000M+ detector features an ion chamber directly over the diodes. Before an exposure is
made, the Model 4000M+ measures and stores the offsets and drift rate associated with the ion chamber
measurement. Every 300 ms during the radiographic exposure, the ion chamber integrator is read in
order to test for full scale. If the integrator is greater than 40% of full scale, the reading is stored and the
integrator is re-initialized without significant signal loss.
The sample time during the Fluoroscopic Mode is 1 second. After the exposure, the offset and drift rates
are subtracted from the final ion chamber charge measurement, yielding the net charge accumulated
during the exposure. This net measurement is then multiplied by an energy-compensated factor. The
result is an exposure rate value.
Air Kerma and Air Kerma Rate
‘Kerma’ is an acronym for Kinetic Energy Released in the Medium. Air Kerma, then, is a measurement of
the amount of photon energy transferred to atomic electrons in a unit mass of air. The displayed units are
joules per kilogram of air, also referred to as Grays. Grays are computed by multiplying the exposure
measurement (in Roentgens) by 0.00873 Gy/R.
NOTE
The Model 4000M+ is shipped with either an
Exposure/Exposure Rate function or an Air
Kerma/Air Kerma Rate function as specified on the
purchase order. Display units may be changed as
described in Section 2 – Operation.
3.2 Measurement Modes
The Model 4000M+ has four Measurement Modes:
• Radiographic
• Fluoroscopic
• Exposure Only
• Exposure Rate Only
In the Radiographic Measurement Mode, a single exposure is read. At the end of the exposure,
measured quantities are displayed.
In the Fluoroscopic Measurement Mode, a continuous beam is analyzed and measured quantities are
continuously displayed.
In the Exposure Dose Measurement Mode, the accumulated dose is displayed. It may be measured
using the integral ion chamber or an optional external chamber.
In the Exposure Rate Measurement Mode, the exposure rate is displayed. It may be measured using the
integral ion chamber or an optional external chamber.
3.3 Filter Selection
The filter wheel position must correspond to the kVp setting on the x-ray machine being tested. Select
the kVp range by rotating the filter wheel on the rear panel (see Figure 2-2).