6
Exposure Time Measurements
The Model 815 measures the Exposure Time each time a kVp measurement is
made. To display the Exposure Time after an exposure, momentarily push the
white Mode pushbutton switch. The exposure time will be displayed in
milliseconds (ms). In normal or High Speed mode, exposure times less than 40
ms cannot be measured. The x-ray exposure time must be at least 40 ms. If it is
necessary to measure exposure times less than 40 ms, please contact ECC to
purchase our Model 8700 Digital X-ray Time Exposure Meter.
When the instrument is in High Resolution Mode, exposure times less than 80
ms cannot be measured. The x-ray exposure time must be at least 80 ms. We
recommend using normal mode for all exposure time measurements.
More accurate reading of exposure times are obtained at lower kVp / mA
settings. Many high-energy x-ray exposures have a noticeable decay at the
trailing edge (end) that will be added to the exposure time. We recommend
making exposure time readings at 50 to 80 kV and 25 to 50 mA. Also, the
accuracy specifications apply for measurements greater than 150 milliseconds.
High Resolution
The Model 815 was specifically designed to accurately make measurements with
a short exposure time in order to minimize wear on the x-ray head and exposure
to service personnel. However, there are three conditions where more accurate
readings can be obtained by sampling the x-ray waveform for a longer period of
time.
Slow X-rays - Some x-rays, particularly older models take a relatively long time to
reach peak exposure. In the normal or High Speed mode the instrument waits
22 ms before starting the x-ray measurement. The wait time is doubled in the
High Resolution mode, and the actual measurement time is also doubled.
Some DC and multiphase units come on quickly but have a large overshoot. If
the overshoot lasts more than 22 ms, High Resolution Mode will give a more
accurate reading of the true maximum kVp.
Appendix A has more information about making accurate kVp measurements.
