MODEL 9-4
Technical Manual
Section 4
Ludlum Measurements, Inc.
Page 8
June 2018
Range Change
When the instrument is switched to the ×100, ×1K, or ×10K range, RL1
is closed, reducing the feedback resistance to approximately 4.2 x 10
9
ohms. At 500 mR/hr on the x100 range, the chamber current will be
approximately 7 pA (7 x 10
-12
amps) and the voltage out at pin 6 of U1
will be approximately 30 mV (0.03 V). Full-scale voltage outputs relative
to the guard voltage for the x1K and x10K are 300 mV (0.3 V) and 3 V
respectively.
Reset
Relay RL2 allows the chamber to be shorted to guard and the electrometer to
be isolated for input bias current compensation.
Offset Adjustment
The electrometer op amp will have some offset voltage. Internal calibration
control R111 will provide an offset voltage that can be adjusted to null out
the electrometer offset voltage. Place instrument in a field less than 0.1
mR/hr, then select the
×100
scale and adjust R111 for zero meter reading.
Zero Adjust
The primary function of the zero adjust is to control input bias current in the
electrometer. With the
RESET
(SW1) closed, the reset relay (RL2) opens the
electrometer input and shorts the chamber output to guard. With the
RESET
held closed, input bias current is adjusted with the zero control for a
zero meter dial reading.
Allow a five-second recovery period for the meter to settle before
and after using the
RESET
button on the
×1
and
×10
scales.
