Specifications

16
General Discussion of Calibraon
In order to have condence in the measurements taken
with a radiaon detecon instrument, a calibraon must
be performed. The calibraon compares the response
of the instrument to known values and the instrument
is adjusted (if needed) to bring all indicaons within
the range of the instrument to within ±10% of the true
values. Generally, radiaon detecon instruments should
be calibrated annually.
CAUTION: Errors can occur when using low level sources
or background for calibrang. In the Calibraon mode,
the smallest increment which can be adjusted is .010.
Pulse Calibraon
Since GM-based instruments are essenally pulse
counters, it must be veried that the number of counts
reported equals the number of counts input. This should
be demonstrated for both count rates and total counts.
The issue is confused somewhat with the addion of
dead-me compensaon where the number of counts
in any given second is adjusted upward to account
for pulses “lost” during the me the detector is busy
recharging from a pulse.
Requirements
You will need a PC running Observer USB and a Pulse generator
Procedure
1. With the instrument powered o, remove the back of the unit and aach a pulse generator to
the unlabeled test point immediately to the le of U2. Pulses must be posive with a duraon of
approximately +3.3 to 5V peak to peak square wave. No high voltage is connected to this test point.
2. Power on the instrument.
3. Connect the instrument to the PC running Observer USB using the USB cable. An applicaon window for
the aached instrument should open within a few seconds.
4. Go to View > Cal Panel menu of The Observer USB Soware to access the Calibraon Screen.
5. Record the currently displayed dead me. (Note the current deadme)
6. Change the dead me to 0 and click the Update Sengs buon. Aer a second or two the instrument
should beep to acknowledge that the new seng has been received, stored, and applied. Seng the
dead me to 0 seconds causes the device to not apply any dead-me correcon at all, so the display
should report the true input pulse rate.
7. Run the pulse generator at several dierent rates to span the range of the instrument. For the Inspector
EXP, the maximum working input pulse rate should be about 367,000 CPM (6.123 Hz). Verify that the
value reported by the instrument is within ±10% of the input value in all cases.
8. Change the dead me back to the seng recorded in Step 5.
Chapter 8: Observer USB Calibraon Soware
All calibraon factors must be entered using the
Observer USB soware.