Certain gases have excellent dielectric and electric arc interruption characteristics which make their use in electrical installations very desirable.
Water content, as the test parameter, is of great importance in determining the dielectric effectiveness of the gas. Under certain conditions, water may condense and become a conducting liquid resulting in a catastrophic dielectric breakdown of the insulation. The water content of these insulating gases as expressed by dew point is listed in Specifications D 1933
Once the dew point is determined, a conversion to moisture content may be performed using Table 1. Once moisture content is known, the lowest temperature at which gas insulated equipment can be safely operated can usually be determined by reviewing manufacturers'' specifications for the equipment.
The dew point of the test gas is independent of the gas temperature but does depend on its pressure. Many moisture measurement test instruments are sensitive to pressure, and display moisture values at the instrument inlet pressure and not necessarily at the pressure of the system being sampled. It is therefore important to account for this condition to avoid serious measurement errors.
TABLE 1 Relationship Between Dew Point and Moisture Content of Gases
Note8212;With a known dew point which is indicated by the dew point indicator or recorder, the moisture content can be read directly from the table. The table shows the amount of water in air or other gas at various dew points at a pressure of 1 atm (14.7) psi.
| Dew Point | Moisture Content | Dew Point | Moisture Content | ||||||
|---|---|---|---|---|---|---|---|---|---|
| °C | °F | lb/1000 ft3 | mg/L | volumeA percent | °C | °F | lb/1000 ft3 | mg/L | volumeA percent |
| 50 | 122.0 | 5.16 | 82.7 | 12.2 | −16 | 3.2 | 0.079 | 1.27 | 0.149 |
| 49 | 120.2 | 4.92 | 78.9 | 11.6 | −17 | 1.4 | 0.072 | 1.16 | 0.136 |