Fluke Biomedical 10100AT - Manual (Page 32)

10100AT
Operators Manual
2-14
After the exposure is over, which is defined as the waveform having dropped below the filter pack's kVp
range and not having returned to it in 100 ms, the instrument performs an analysis of the stored
waveforms to calculate the exposure time.
2.7.2.1 Advanced Measurement Options
When in the Combined Exposure, kVp, and Time Measurements mode, several measurement options are
available that enhance the usability of the 35050AT. These features are available via the options button.
Measurement Delay – Allows the user to specify a measurement delay to skip over unwanted portions of
an x-ray output waveform. The measurement delay range is from –999 to 999 milliseconds. When a
negative measurement delay is selected, the 35050AT does not include the delay time in the measured
exposure time. When a positive measurement delay is selected, the 35050AT includes the delay time in
its measured exposure time. If the selected delay time is longer than the exposure time, a “-delay”
message will be displayed.
When no measurement delay is used, time measurement starts either when the waveform first attains
75% of its peak value, or when the waveform first enters the kVp range, if 75% of its peak value lies
below the range. Time measurement ends when the waveform last attains this value.
Exposure Time Units – Allows the user to select exposure time units of seconds (milliseconds) or
pulses.
When seconds are selected as exposure time units, exposure time measurement method is based on a
kV threshold crossing. The threshold is set at 75% of the kVp, or the minimum kVp for the filter pack in
use, whichever is greater. Exposure time is measured between the first and last passage through this
threshold.
When Pulses are selected as exposure time units, the 35050AT counts the number of x-ray pulses in a
pulsed or single-phase radiographic exposure. Pulses are counted by detecting each passage through a
threshold on the rising and falling edges each pulse in the output waveform.
Short Exposures <5 ms - To prevent measurement of spurious signals, the 35050AT Dosimeter can be
set to ignore exposures of less than 5ms. Use of this option does not affect exposure time
measurements.
2.7.2.2 Time Corrections
For a pulsed single-phase x-ray unit operating on a power supply frequency of 60 Hz, division of the pulse
count by 120 for full-wave rectified units yields the exposure time in seconds (divide by 60 for half-wave of
self-rectified systems). Because the Model 35050AT Dosimeter measures exposure-time as the time
between the first and last crossings of the 75% of the kVp level, the values obtained from the Model
35050AT Dosimeter will be less than those marked on the generator. The following table gives values
that may be added to the Model 35050AT Dosimeter's results to correct for this effect.
Line Frequency/Machine Type Add to Displayed Time
60 Hz/Half Wave 12.8 ms
60 Hz/Full Wave 4.5 ms
50 Hz/Half Wave 15.4 ms
50 Hz/Full Wave 5.4 ms
It has been assumed that 75% of kVp lays within the filter pack's specified measurement range.
2.7.3 Making kVp and Time Measurements without an Ion Chamber
kVp and time measurements can be made without the use of an ion chamber. Simply follow the
procedures in sections 2.7.1 and 2.7.2. When the ion chamber is not connected, disregard the displayed
value of exposure or exposure rate.