Fluke Biomedical 4000M+ - Manual (Page 20)

Victoreen 4000M+
Operators Manual
3-2
Time
The Model 4000M+ measures exposure time by determining the time between the first and last passage
through 75% of kVp AVG. This is the technical definition used on most x-ray equipment, including 3-
phase, medium-frequency and high-frequency generators.
On single-phase machines, 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 AVG, i.e., timing on “zero crossing”. The difference in time between 0% and 75% is
approximately 2 ms, which occurs before the first and after the last passage, making a total time
difference of 4 ms. In order to resolve this timing definition discrepancy, it is recommended that four (4)
milliseconds be added to the displayed time.
NOTE
The Model 4000M+ indicates single-phase x-ray
machines by displaying a ( ) with the time value
(e.g., 35 msec ).
The Model 4000M+ measures exposure time from the internally stored waveform. The first 300
milliseconds and the last 20 ms of the hardened radiation waveform are stored along with the total
exposure time determined from hardware “on” time (reference is crystal-controlled).
There will be circumstances when the x-ray timer is operating properly but the x-ray output time is
incorrect. Due to this technique, occasional time measurement errors may be misdiagnosed. Consider
the following examples:
1. Time Measurement Errors Due To Light Loading
Exposures which result from a light load (e.g., 25 mA) on the generator may indicate a longer
exposure time (for the same time setting) on the Model 4000M+ than heavily loaded exposures.
The difference is caused by the fact that more time is required at the end of the exposure for kV
tube potential to discharge when low mA has been selected. As long as sufficient kV is on the tube,
radiation is being emitted and, if it is above 75% of kVp AVG, the time will be measured by the
Model 4000M+.
2. Time Measurement Errors On Machines Without Filament Pre-Heat
If an x-ray machine filament pre-heat malfunctions, the output may be very low during the first part
of the exposure and then increase. Model 4000M+ time measurements may be significantly shorter
than the time set on the machine. This is because the radiation, during first part of the exposure,
may be below the level necessary to start the Model 4000M+ measuring time. This constitutes a
machine malfunction discovered by the Model 4000M+ in which a timing error is caused by a pre-
heat problem.
NOTE
Some x-ray machines, such as Dental x-ray
machines, do not pre-heat the filament. If a time
measurement error is seen on other x-ray
machines, it may indicate a filament pre-heat
malfunction.