Fluke Biomedical 8000 Victoreen - Manual (Page 66)

Victoreen 8000
Operators Manual
4-4
data acquisition by 10 milliseconds or select a lower %kV. For more information see Section 4.5--
Waveforms - Overshoot.
Short measured exposure times may also result when the NERO mAx only “sees” only part of an x-ray
exposure. This can happen when exposures are made near the NERO mAx’s minimum detectability and
only a portion of the exposure is above the instrument’s threshold. This also results in low exposure
measurements. To remedy this situation, switch to high sensitivity, switch to the next lower kV filter range
(if the kV is still within range) or decrease the distance between the detector and the x-ray tube. See
Section 4.6--Waveforms - Partial kV Waveforms.
Short exposure times will also be calculated if the x-ray exposure stops for more than 20 milliseconds.
When this occurs, the NERO mAx assumes that the exposure is complete after 20 milliseconds and
proceeds with data analysis.
Errors may occur in the measured exposure time when making measurements on self-rectified dental
machines with the NERO mAx %kV set to 90%, 80% or 75%. On self-rectified generators whose kV
peaks vary by more than the selected %kV, kV peaks that fall below the selected %kV are ignored. If the
peaks that fall below the selected %kV occur at the beginning or end of the kV waveform, they are not
included in the exposure time calculation. 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.
Exposure time errors can 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 measures the exposure time of the first few preheat pulses that is
usually much shorter than the set exposure time. 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.
Generator loading may also effect the exposure time measured by the NERO mAx. The NERO mAx may
indicate longer exposure times for exposures made under light generator loading (low mA, e.g. 25 mA)
than for exposures made under heavy generator loading at the same time and kV settings. This is
because more time is required at the end of the exposure for the tube potential (kV) to discharge when a
generator is lightly loaded.
Measured exposure is not what is expected
The measured exposure may appear to be low when the NERO mAx only “sees” only part of an x-ray
exposure. This happens when exposures are made near the NERO mAx’s minimum detectability and
only a portion of the exposure is above the instrument’s threshold. This also results in short measured
exposure times. To remedy this situation, switch to high sensitivity, switch to the next lower kV filter
range (if the kV is still within range) or decrease the distance between the detector and the x-ray tube.
See Section 4.6--Waveforms - Partial kV Waveforms.
The measured exposure will also be low if the x-ray exposure stops for more than 20 milliseconds. When
this occurs, the NERO mAx assumes that the exposure is complete after 20 milliseconds and proceeds
with data analysis.
The measured exposure is also affected by the user entered ion chamber correction factor and air density
correction factors. Make sure that the internal ion chamber’s correction factor has not been changed by
accident (default is 1.00, see Cal Mode) and make sure that temperature and atmospheric pressure have
been correctly entered (defaults are 22°C, 760 mmHg., see Setup Mode). For more information
regarding NERO mAx calibration see Section 3--Calibration.