TM 5-3610-257-14
d.
When the READ pushbutton is pressed, it causes the measured density to be displayed as a digital
readout. During a portion of each cycle of the power source, the photomultiplier tube dynode network charges to a voltage
greater than that required for measurement of the maximum density within the range of the densitometer. During the
remaining portion of the power source cycle, the photomultiplier tube dynode network voltage decreases with time. This
produces the change in dynode voltage required for a linear density measurement.
e.
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. The oscillator 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.
f.
The counter continues counting until it has accumulated 400 counts. This 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 4 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 cont rol
incorrectly.
4-6
