Model 3-97 Survey Meter
June 2020
page 4
5. CALIBRATION
5.1 Detector Operating Point
CAUTION: The detector operating voltage is
connected to both detectors at all times,
regardless of which position the detector
INT/EXT switch is set. Typically an external
GM type detector is used and the maximum
operating voltage is 1000 Vdc; therefore, the
internal detector operating voltage must be kept
below 1000 Vdc.
5.1.1 Internal Detector Calibration
Switch the INT/EXT switch to the INT
position. Expose the unit to a 241Am
source and develop an operating
voltage (HV) versus count-rate plot.
Adjust the HVI potentiometer to vary
the internal detector operating voltage.
The operating voltage can be monitored
through the external connector. Set the
operating voltage at the flattest portion
of this curve.
Note: Measure high voltage with a Model
500 pulser or a high-impedance voltmeter
with a minimum of 1000 megohm input
resistance.
The Model 3-97 internal scintillation
detector radiation response is energy
dependent. An energy response curve is
included in the drawing section at the
back of this manual.
Place the Model 3-97 on a certified
calibration range. Use the indentations
on the sides of the can to position the
internal detector to the center of the
calibration line. Calibrate each scale
for the best fit at two-thirds and one-
third-scale readings (10 and 20 R/hr).
Confirm that the calibration is within
10% of each reading.
If the appropriate radiation fields are not
available, switch the instrument to the lowest
calibrated range position and perform the
following:
Connect a Ludlum Model 500 Pulser to
the external connector and switch the
INT/EXT switch to the EXT position.
With the Model 3-97 switch to the
lowest calibrated range position,
determine the pulse rate versus R/hr
calibration point. Decade the pulser
and Model 3-97 to the next lower range
and calibrate the meter to the
corresponding point determined in the
previous step. Decade down and
calibrate the remaining range positions.
After all the range positions are calibrated,
monitor the background radiation level.
Typical radiation background levels range
from 5-10 R/hr.
5.1.2 External Detector Calibration
For most external GM detector
applications, the detector operating may be
adjusted for 900 Vdc. Set the HV to the
recommended detector operating voltage.
Place the external GM detector on a
certified calibration range. Check each
range position at three-fourths and one-
fourth-scale and confirm the reading is
within 10%. If the reading error is
greater than 10% but less than 20% of
reading, record the field versus the
meter reading at three-fourths and one-
fourth scale. State the correction factor
on the calibration sticker for the
specific range.
