Planmeca ProMax 3D Max with ProTouch Technical Manual page 101

Exposure time measurement
4.2.3.2 Invasive testing (directly from unit's own feedback signals)
Technical manual
Promax, the first permissible HVL must be at least 2.5mm Al at 84 kV) is to
measure the air kerma rate first without any additional material in the
radiation field, then add 2.5mm Al to the radiation field, measure the air
kerma rate again and check that the air kerma rate with additional 2.5mm Al
is more than one half of the one measured without the added material. That
is,
(Dose rate with added 3 mm Al eq filtration) / (Dose rate without added
filtration) > 0.5
This is sufficient to ensure that the HVL is at least 2.5mm Al. Depending on
the type of the radiation meter used, it is possible that a correction factor
needs to be applied to the result measured with added material in the
radiation field.
The exposure time is controlled by the microprocessor, so the exposure time
accuracy is exactly the same, no matter if the set exposure time is 200 ms
(in film-based cephalostatic mode) or 16 s (in panoramic / tomographic
mode). The exposure time is defined automatically based on the selected
program and is displayed in the upper right corner of the graphical user
interface. In panoramic mode, attach the non-invasive sensor to the front
panel of the cassette holder (or Dimax3 sensor) and make sure its whole
area is in the radiation field. Select 70 kV / 8mA and press ready. Take an
exposure and record the measured exposure time. The measured exposure
time must be within ±10% of the exposure time displayed in the user
interface.
In film-based cephalostatic mode, the exposure time is selected by the user.
Select the shortest possible exposure time, attach the non-invasive sensor
to the cephalostatic cassette holder in the middle of the radiation field, take
an exposure and record the measured exposure time. When placing the
sensor, note that if the unit is equipped with auto-ceph, the cassette size is
automatically detected when the cassette is on its place and the area of
radiation field is adjusted accordingly.
NOTE
The manufacturer does not require the invasive testing. The invasive test
must only be performed if the local authorities require it.
An invasive method should be used for checking the tube current (mA), and
can be used for checking the kVp and exposure time. This method requires
that the covers around the tube head assembly are removed, and a special
measurement adapter cable, Planmeca order code 10008320, is connected
to the connector J2 in the FBK PCB. The FBK PCB is permanently fastened
to the front side of the tube head assembly. The analog feedback voltage
signals can be measured with a calibrated multimeter from the adapter cable
connectors (labelled kVpos, kVneg, mApos and mAneg). An oscilloscope is
required if kV and mA waveforms need to be observed, for an example when
determining the exposure time.
NOTE
The feedback signals are differential, so measuring only one polarity signal
(e.g. kVpos with respect to the X-ray units ground potential) will give false
results. The feedback signals must always be measured differentially, kV
feedback voltage = (kVpos – kVneg) and mA feedback voltage = (mApos –
mAneg).
4 Preventive maintenance
Planmeca ProMax 3D Max 95