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to verify all pressure has been relieved. The metal-to-metal sealing surface
initially may cause the probe to stick, so carefully free the joint by tapping
the probe on the metal body. Do not strike the zirconia insulator and do not
use a wrench to turn the probe hex nuts.
(3) After the probe becomes free, loosen the probe retaining nut fully and
remove the probe.
(4) The threads on the probe receptacle and probe retaining nut should be re-
lubricated each time the probe is reinserted (refer to section 2.4 for probe
reinsertion).
4.0 Detection Circuitry
A low-voltage ± symmetrical mixed sine wave (net integral zero value) is generated in
the Model 12B System. This signal is buffered and connected through a resistor to the
probe field terminal blocks. (ref: fig 6, loc 4)
Note: If clock 1 fails, line frequency sampling is used for back-up detection but
± symmetry can not be guaranteed and a small offset voltage may develop.
When the probe tip is immersed in water a signal current bleed path to ground is
completed by the conductivity of the water. Current flow through the circuit causes a
voltage drop to appear across a sensitivity resistor. The voltage is compared to a fixed
reference voltage. When the voltage drop exceeds the reference voltage the amplifier
outputs a signal indicating the presence of water. A green LED turns on when the probe
is in contact with water, (ref: fig 6, loc 9). One test point is provided per channel for
troubleshooting. When the probe is submerged, the voltage should be ≈ 0, when in air or
steam the voltage should be +1.4 VDC (±0.3). Probe channel to test point numbers are:
Channel Test Point
12 28
11 37
10 40
9 43
8 31
7 34
6 25
5 19
4 22
3 16
2 13
1 10
Frequency filters separate the two components of the sinewave and also reduce
noise pick-up by the probes and their associated wiring.