The deep vacuum method of dehydration relies on evacuation alone to remove moisture from the system. A deep vacuum is any vacuum o f 500 microns or less; that is, 29.92 in. Hg vac. or greater. When a deep vacuum is established in a closed system, non-condensibles are reduced to a negligible level. As the pressure is reduced, the boiling point of water is also reduced. As long as the ambient temperature surrounding the system is higher than the boiling point of the internal moisture, it will boil off and be expelled. For example, at a vacuum of about 29.72 in. Hg vac. or 5,000 microns, liquid water will boil anywhere in a system where the temperature is 35 ° F or higher.
In order to know the system has been adequately evacuated, the final equilibrium pressure of the entire system must be found after the system is pumped down and before it is charged. See Figure SP-3-2. First, the system is reduced to a pressure of about 500 microns or below, then it is isolated from the vacuum pump. The reading on the vacuum gauge/indicator is then monitored to note any change in the level of vacuum in the system. If the indicator shows a pressure rise and the pressure continues to rise without leveling off, a leak exists in the system or the connecting tubing. Locate the leak (SP-1) and repair it.
If the indicator shows a pressure rise but levels off between 1,000 and 2,000 microns, this indicates that the system is leak-tight, but still too wet. A constant reading on the indicator of between 500 and 1,000 microns indicates a leak-tight, dry system.
