Fluke Biomedical 875 Victoreen User Manual
Page 27
Victoreen 875 High Range Containment Monitor
Operator Manual
4-6
en Clamp
The pen clamp circuit is a
CR502. It
prevents pin 9 from goin
en from being jammed
against the baseline.
Electronic Check So
hematic 876A-1-
30)
Due to the range of the
/h), a remotely activated radioactive check source is
practical, since the sou
and the shielding necessary for this source would
ce is provided. During check
acitive element. Since
the rate of change of
th
y current. It is
th
te the low voltage ramp which,
er
of
while the test is in progress, approximately six (6) seconds.
erly
resting at extreme lower
P
clamping circuit consisting of U501, pins 8, 9 and 10 and diode
g negative. The purpose of this is to prevent the recorder p
urce (ECS) Printed Circuit Board (P/N 876A-1-92, Sc
detector (1 to 10E7 R
rce activity would be high
im
effect detector energy response. For this reason, an electronic check sour
source operation, the detector remains connected to the system as a passive cap
the current-voltage relation in a capacitor is such that the current is proportional to
e applied voltage, an applied voltage, in the form of a linear ramp, will produce a stead
is current which is read during the ECS test.
The ECS board (drawing 876A-1-3D) has two main functions: 1. To genera
in the high voltage supply, develops the high voltage ramp responsible for the test current in the detector
during the ECS test. 2. To monitor the current flowing during the ECS test, and recognize whether the test
signifies a passing of failing condition in the circuits of the unit. One of the circuits involved in this
monitoring process is actually on the power supply printed circuit board.
The voltage ramp which develops the detector current during the ECS test rises from 0 to 506 volts ov
three seconds, so that the rate of change of voltage is approximately 170 V/second. The capacitance
the detector is approximately 435 picofarads and therefore, the steady current during the rise of the
voltage ramp is approximately 7 x 10
-8
amperes, which puts the panel meter at about one-third full scale.
The ramp occurs when ECS is initiated for a period of about six (6) seconds, during which time the
system is not acting as a radiation monitor.
The ECS test may be initiated at the will of the operator. For this purpose there is a manually operated
ECS push button. If the ECS test is not initiated by the operator, it will take place automatically every 17.1
minutes. After each manually initiated test, the automatic circuits are reset so that a test will be initiated
17.1 minutes later.
In addition to causing visual alarms and relay de-energizing if circuit failures are found, the ECS test
affects the panel meter as in the three cases described below. It is the processing of the voltage read by
the panel meter during the ECS test that is responsible for the action of the circuits directly involved in
driving the visual (green light) and relay (fail) alarm circuits. Recorder and computer outputs are muted
during the ECS and will indicate zero
Panel Meter Action During ECS Test
The meter action to be expected during the test period with the containment monitor operating prop
can best be explained by considering three initial conditions: 1) Panel meter is
end of scale; 2) Panel meter on scale but below 10
3
R/h; 3) Meter above 10
3
R/h at start of test.
1. Meter at extreme lower end of scale
In this case, the meter needle should remain motionless for about 1 second, rise to 10
3
R/h and
remain there for about three seconds, and then fall to its initial position.
2. Meter on scale, but below 10
3
R/h
In this case, the meter needle should fall to zero at the beginning of the test period. It will then rise
to approximately one-third full scale, and remain in this position for about four seconds. After this,
will resume its original reading, w
it
ith perhaps some slight negative overshoot.
3. Meter above 10
3
R/h at start of test
This case is quite similar to the above except that the meter may not fall to zero at the start of the
test, and at the upswing, it will take a position higher than 10
3
R/h, the excess depending on the
ambient radiation. It will ultimately resume the same position as before the test as in item 2 above.