CB Watch 3 User Manual v6.3 - May 2019 Page 41 of 100
HMI: Measurements/Alarms
Figure 54 – Gas alarms
Gas moisture level
The presence of a high level of moisture in the SF6 gas affects the dielectric withstand
strength of the gas. Therefore, it is always desirable to test the moisture content of any
gas being added to the CB before doing so. Failing that, the resulting gas moisture level
can be measured as an option and displayed in the HMI. A special gas sensor is used that
not only measures pressure, and temperature but also the moisture level in the gas.
The moisture level in the gas is continuously displayed in the HMI:
HMI: Measurements/Gas
Figure 55 – Gas moisture level
The moisture level is indicated in ppmv: parts per million by volume.
This is the units most commonly used and quoted in the IEC standard 62271-1.
This value represents one million times the ratio of the volume of moisture (water vapor)
present in the gas to the total volume of the gas (including water vapor).
An alarm can be set if the moisture level exceeds a maximum threshold:
HMI: Settings/Gas
Figure 56 – Moisture alarm threshold
Note: In order to avoid spurious alarms and because moisture measurements without
averaging are notoriously variable and “spiky” in the short term (due to the influence of
temperature), one can set a minimum time (in minutes) for the value to exceed the
threshold before the alarm is raised.
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