The saturation vapor pressure is the level at which air is saturated with water vapor and condensation occurs for further increases in water vapor content. This is a function of the ambient temperature [1]. Absolute humidity in the air is the amount of water (by mass) present in the volume divided by the air volume. All results presented in this section have been corrected for air density and temperature to STP (20°C) as per the most recent IEEE standard [10, 11]. The gap length was first set to an approximate value using the movement mechanism at the bottom of the chamber. Further adjustment was done using Grade 1 (±5.00×10-5 mm accuracy) gauge blocks. Before any testing, the parallelism and gap spacing of the plates was confirmed using the gauge blocks at multiple locations across the gap. The humidity was then adjusted to the required level. Humidity readings before and after each set of breakdown measurements were obtained using an RH meter through one of the metering ports on the side of the chamber. The accuracy of the humidity meter used was ±1% RH. Testing was always commenced at the driest level (between 5-10% RH) and continued in 10% RH steps until a maximum of 90% RH (nearly wet condition) was reached. Water vapor for the chamber was produced using an ultrasonic fog generator and de-ionized water. The water vapor was piped through a set of tubes to the electrode chamber by a high pressure fan on the humidifier enclosure as illustrated in Figure 2a. Valves on the water vapor inlets for the chamber allowed for fine adjustment of the RH in the chamber. Uniformity of the humid air within the chamber was achieved by a circulation fan in the chamber. At each humidity level, electrodes were conditioned with up to 20 breakdown shots. The conditioning was continued until the variation of the breakdown voltage was within 10% of the previous value.
VLF high voltage was generated using a resonant circuit as shown schematically in Figure 3. An electronic signal synthesizer, indicated by (A), was used to supply a sine wave to the amplifier (B). The amplified signal was then fed into the resonant tank circuit composed of the tuned inductor (C) and the capacitor stack (D). A movable tap on the inductor allowed for impedance matching with the amplifier. The output voltage was measured using a capacitive divider (E). The maximum output voltage of this resonant circuit was 18 kVrms at ASU and 300 kVrms at the EBHVTF. At the EBHVTF additional frequencies of 18, 40 and 50 kHz were obtained by changing the connections on the capacitor stacks.