Swagelok PGI-63C-PC0-LA02 Deluxe Bourdon Needle Dial Vacuum Pressure Gauge With 1/4 Inch NPT, 0 - 30 inHg

These Swagelok® deluxe Bourdon needle dial vacuum pressure gauges have a 1/4 inch male NPT inlet vacuum connection and can measure vacuum pressure in the 0 - 30 inHg range. These deluxe bourdon gauges are liquid filled inside which dampens the effects of pulsation and pressure spikes, lubricates the gauge’s moving parts so that everyday friction is reduced, and helps prevent internal corrosion among other things. The air bubble is needed in the gauge for dampening the internal pressure for more accurate readings and acts as a cushioning effect.

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.

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