Fluke Biomedical 8000 Victoreen - Manual (Page 65)

Troubleshooting
Modes of Operation
4
4-3
Inaccurate kVp measurements may occur when using high sensitivity with a filter card kV setting that is
too high for the kVp being used. Under these conditions, the NERO mAx may detect enough scatter to
induce it to take an exposure and calculate kVp incorrectly. To remedy this situation, switch to the next
lower filter card setting.
If the NERO mAx reports “Low” kV or if the measured kV is much lower than the actual kV, one of its
solid-state detectors may have failed. To easily diagnose this condition, perform the calibration check as
described in Section 3.2 - Calibration Check. The ratios normally displayed after an exposure are
approximately 2 and if the ratios displayed are near 1, one of the NERO mAx solid-state detectors has
failed and must be replaced. Please call Fluke Biomedical at 440.248.9300 for assistance.
Inaccurate kV measurements may result when making measurements on self-rectified dental machines
whose kV peaks vary by more than the selected %kV. On these generators, when the NERO mAx %kV
set to 90%, 80% or 75%, kV peaks that fall below the selected %kV are ignored and are not included in
the kV calculations. Setting the %kV to ZERO (exposure time measured at zero crossing) or 1ØPULSE
(pulse counting mode) can eliminate this effect. See Section 4.7--Waveforms - Self Rectified--for more
information.
Inaccurate kV measurements may also occur when making measurements on self-rectified dental
machines that utilize a filament preheat. When making measurements on these generators, the NERO
mAx typically sees the first few preheat pulses, then the intensity of the remaining preheat pulses falls
below the detectability limit of the NERO mAx, causing the NERO mAx to terminate the measurement
prematurely. When this occurs, the NERO mAx calculates kV based upon the first few preheat pulses
instead of the pulses that occur after the filament preheat. To eliminate this effect, a measurement delay
sufficient to skip over the filament preheat should be used. Additionally, the %kV should be set to ZERO
or 1ØPULSE. For more information, see Section 4.8--Waveforms - Dental with Preheat.
Measured exposure time is not what is expected
The most direct method of determining the cause of a possible exposure time measurement error is to
look at the radiation and/or kV waveform for the exposure in question. Two methods may be used to
examine these waveforms. The first method is to use the NERO mAx Excel Add-In to extract the
waveforms from the NERO mAx. Information on the installation and use of the NERO mAx Excel Add-In
can be found in the NERO mAx Toolkit for Excel Instruction Manual. The second method is to capture
the radiation waveform using a storage oscilloscope connected to the scope output on the back of the
NERO mAx readout. Information regarding use of the NERO mAx scope output may be found in Section
1.6--Scope Output--of the NERO mAx Instruction Manual.
When using the NERO mAx with single phase x-ray machines, the measured exposure time may be
shorter than the actual exposure time when an inappropriate %kV has been selected, such as 75%, 80%,
or 90%. This is because the definition of exposure time for single-phase x-ray machines is not the same
as that for three phase, medium and high frequency machines. For single-phase generators, exposure
time is defined as the number of radiation pulses that occur during the exposure multiplied by the pulse
period. This is equivalent to the time between the first and last passage through 0% of kVp average (zero
crossing). When making measurements with the NERO mAx on single-phase x-ray machines, %kV
should be set to ZERO (for time between zero crossings) or 1ØPULSE (pulse counting mode) to
accurately assess timer performance. Additionally, when measuring exposure time between zero
crossings, the NERO mAx measures the time from when x-ray are detected until they are no longer
detected. This measured time will usually be slightly shorter than the time calculated by multiplying the
number of pulses by the pulse period. This is because the NERO mAx does not "see" the rising and
falling edges of the kV waveform that fall below the bottom of the selected filter range.
If the measured exposure time appears to be short and the %kV is set at 90%, there may be an
overshoot at the beginning of the exposure. If the peak kV of an overshoot is more than 10% higher than
the kV of the rest of the exposure, the NERO mAx will detect the overshoot as a single peak and will find
no other peaks. When this happens, the exposure time will be very short (a few milliseconds) and the
kVp average and kV peak will be the same. This happens when the %kV is set at 90% because the
NERO mAx measures exposure time between the first and last passage through 90% of the peak kV and
will only calculate the time of the overshoot. To eliminate the effect of the overshoot, delay the start of