TM 5-3610-285-14
g.
Current from the photomultiplier tube is monitored, and a START pulse is generated when the output current
equals a predetermined reference value. When the dynode voltage has decreased to the standard reference value,
approximately 150 volts, a READ pulse is generated. The time between these two pulses (START and READ) increases
directly proportional to increasing density values. The START pulse starts the operation of a 60,000 Hz oscillator, which
supplies clock pulses to the integrated circuit counters, which begin counting upward from zero. Occurrence of the READ
pulse transfers the accumulated clock pulse count into the storage registers, where it is used to produce the
corresponding reading in the digital readouts.
h.
The counter continues counting until it has accumulated 400 counts, which is electrically equivalent to zero
counts. At this time, the counter is stopped until the next measurement cycle. Releasing the probe read pushbutton
allows the last measured value to remain displayed in the digital readout. If the READ pulse occurs before the START
pulse, a minus (-) sign will appear at the left side of the display. This is to prevent the possibility of setting a zero control
incorrectly.
7-7
