CB Watch 3 User Manual
v6.3 - May 2019
Page 48 of 100
The key bits of data required are:
-
Nominal SF6 gas fill pressure at 20
o
C
75 psig in this example
-
Minimum (T2 - lock-out) SF6 gas pressure at 20
o
C
62 psig in this example
-
What level the low gas warning (T1) is set as
69 psig in this example
Threshold values
Gas pressure level at 20
o
C
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 (as shown in the figures
above).
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
should 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
In terms of tolerances (min/max values) for each threshold, we would suggest using from
+/-3% to +/-5% around the nominal value. 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 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.
Gas extrapolated pressures
The same thresholds 1 and 2 are used as for the gas pressures. If want earlier warning,
then need to extend both the short term and the long-term horizon during setup.
