If the moisture content is known in PPMV, to find the dew point in C the water vapour pressure is calculated by multiplying the ppmv measurement by the system absolute pressure to obtain the partial pressure of the water vapour. Table below shows the relationship between temperature and saturated water vapour pressure. If the water vapour pressure at the corresponding temperature shows in table, the water vapour begins to condense.
Table : 1
Pressure of saturated aqueous vapour in mm, Hz for temperature of -59 0C to +109 0C
What is the dew point of an SF6 system having a moisture content of 300 PPMV at guage pressure 152 kPa (22psig)
Pressure = 300 * 10 -4 (22 + 14.7) 51.7 = 0.57mmHg
The 51.7 multiplier is used to convert the pressure from psia to mmHg for use with Table :1
(760 mmHg = 14.7 psi, so 1 psi = 760/14.7 = 51.7 mmHg)
The pressure act which the measurment was taken now enters the converssion calculation. For example : if the guage pressure was 600 kPa (87 psig) when the same moisture reading of 300 PPMV was taken, the dew point is now - 12 C. Thus, when converting and comparing PPMV or PPMW and dwe point reading, the pressure at which the reading is recorded is extreamly important
Many instruments that measure dew pointoperate at the system pressure. Instruments that measure PPMV are designed for use at pressure slightly higher than atmospheric pressure - gauge pressure slightly above 0 kPa (0 psig) to provide gas flow for sampling. Normaly, the system sampling valve is opened slightly to mentain the gas flow and take a PPMV measurment at a pressure of slightly above atmospheric pressure. This difference in instrument operation shall be taken into account when converting and comparing dew point and PPMV levels
SF6 decomposition products in electrical discharges
Table 2 - Recommended moisture limits and specification from various manufacturers.