These Bourdon needle dial vacuum measure gauges measure both positive pressure from 0 to 30 PSI (pounds per square inch) and vacuum pressure from 0 to 30 inHg (inches of mercury). There is 760 mmHg (29.92 inHg) of atmospheric pressure at sea level. These Bourdon vacuum pressure measurement gauges are low cost being quick and easy to use. However, they are very low accuracy gauges. Each tick mark in the vacuum is around 25.4 Torr. These Bourdon vacuum pressure measurement dial gauges are 3 inches high and have an 1/4 inch NPT vacuum flange fitting. They are made of corrosive resistant stainless steel.

Bourdon Dial Needle Vacuum Pressure Measurement Gauges, by Wikipedia.org
A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube.The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed. The Bourdon pressure gauge measures pressure relative to atmospheric pressure, that is, they measure pressure directly relative to the surrounding atmospheric pressure. For example, Albuquerque New Mexico at our facility is at 5,000 feet above sea level with an atmospheric pressure of 630 mmHg. A bourdon gauge calibrated at sea level (760 mmHg) would read about 5 inHg of vacuum at our location.