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The key bits of data required are:
- Nominal SF6 gas fill pressure at 20
- Minimum (T2 - lock-out) SF6 gas pressure at 20
- What level the low gas warning (T1) is set as
- Total weight of SF6 gas
Threshold values
Gas pressure level at 20
The values for the different alarm thresholds (T1, T2, T3) are readily available from the CB
user's manual or from the CB's name plate.
T3 is usually set a bit below the pressure at which the "burst discs" will rupture. This is to
avoid a dangerous overpressure that would trigger the safety valve. Please refer to the CB
manual.
If a key concern is to ensure that the CB does not run out of gas and remains operational,
then the important thresholds could be:
- T2: The pressure at which the CB lock-out any operation and prevent any operation due
to low SF6 gas pressure
- T1: A warning level a bit before that, to provide a warning that the system is getting
close to lock-out pressure
If a key concern is to ensure that any gas leak is detected as early as possible so that it can
be fixed before too much gas is lost, then the warning level threshold T1 above could be set
to show:
- T1: A small decrease below nominal pressure to alert that there is a leak, closer to
nominal and further away from the warning level of the density meter
One should always use the maximum gas value at which the alarm should be triggered (i.e.
include the tolerance) so that the CBW3 will always alarm if the other gauge does alarm
(since the precision of the CBW3 is usually better than the usual gauge.
In terms of tolerances (min/max values) for each threshold, we would suggest using +/-0.1
Bar or +/-1 psig around the nominal value from the nameplate. If the sensor is exposed to
direct sunlight, larger tolerances should be set to avoid false alarms due to swings in sensor
temperature rather than gas temperature.
Gas liquefaction risk
The gas liquefaction pressure (for the gas or gas mixture selected) is stored in memory so no
need to enter any threshold.
Gas moisture level
CB manufacturers usually recommend a maximum level of moisture in ppmv. If no value is
specified, then you can fall back on IEC 60376 standard recommendation: 120 ppmv.
MA-037 - CB Watch 3 Operating Manual
o
C
o
C
v7.0 - May 2020
75 psig in this example
o
C
62 psig in this example
69 psig in this example
690 lb in this example
Page 58 of 118
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