Measurement of a Deep Vacuum without Mercury

Above 30 or 40 mm Hg of vacuum, an ordinary digital battery powered vacuum gauge is sufficient. However, only a vacuum mercury manometer is accurate in the 5 to 15 mm Hg range. As long as the mercury was dry, such a gauge gave excellent results. But few of my clients now permit the use of mercury in the field. 

I have developed an alternate method that works almost as well as the vacuum mercury manometer. It s really the same idea (see Fig. 19.8). Fill the closed end of the tubing with baby oil and mineral oil. Make sure you get the last bubble of air out of the closed end of the tubing. Always use fresh oil to prevent moisture contamination of the oil. Fill the entire glass U tube with oil. 

Connect the open end of the glass tubing to the vacuum pressure point being measured and open the connecting valve very slowly. 

The specific gravity of baby oil is about 0.9 s.g. The sp ecific gravity of mercury is 13.6. Mercury is 15 times denser than baby oil. Referring to Fig. 19.8, divide the 150 mm ofelevation difference inthe manometer legs by 15 to determine that the vacuum is 10 mm Hg. For vacuums above 40 mm Hg, the oil-filled mercury manometer is not practical, as it would be about 30 in long. 

Do not use lighter fluids than baby oil, as they will create a vapor pressure at the closed end of the glass U tube. This will result in a measurement of a better vacuum than really exists. Water is also too volatile at moderate ambient conditions for this purpose. 

---