Needle Dial Vacuum Pressure Gauge, 0 to 30 inHg, With 1/4 inch NPT male brass fitting

These dial gauges are good for a quick and cheap measurement of the vacuum pressure. They are designed to read in the 0 to 30 in Hg range. Each gauge has a 1/4 male NPT pip thread on the back for making the vacuum connection.

Bourdon Dial Needle Vacuum Pressure Measurement Gauges, by Wikapedia.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 presure directly relative to the surrounding atmospherice pressure. For example, Albuquerque New Mexico at our facility is at 5,000 feet above sea level with an atmospherice pressure of 630 mmHg. A bourdon gauge calibrated at sea level (760 mmHg) would read about 5 inHg of vacuum at our location.