Tutorial: Low pressure gauges for very low and vacuum measurements
Mike EdwardsFeatures tutorial WIKA
To accurately measure pressure below 10 psi (690 mbar), mechanical gauges use a capsule element. Capsule gauges are found in applications that operate in a vacuum or pressures as low as 1″ water column.
In most Bourdon tube pressure gauges, the range limitation on the lower end is 10 psi (690 mbar). Measurements below that call for a different sensing element. A low pressure gauge with a capsule design can go down as low as 1″ H2O (0.036 psi, 2.5 mbar), and measures positive and negative gauge pressure. Capsule elements are also used in other product families, such as differential pressure gauges.
How does a low pressure gauge work?
Low pressure gauges have a very sensitive mechanism consisting of two corrugated discs, each made of brass or stainless steel, that are fused together to create a circular capsule. The slightest changes in pressure cause the capsule to expand or contract; a pinion movement mechanism transfers this motion to the indicator.
Capsule gauges are cost-effective instruments that measure low pressures very precisely and with accuracies up to ±1.0% of span. The accuracy varies according to the gauge’s nominal size. This is because the capsule element is typically placed vertically inside the case, parallel with the dial face. The larger the disc surface area, the better it is able to detect low pressure changes.
For low pressure gauges with high overpressure safety, the capsule sits perpendicular below the case. As pressure enters the pressure-sealed measuring chamber, it deflects the capsule measuring element, which transmits the motion to the movement and indicator. The capsule element is supported by the surrounding housing and, therefore, can achieve high overpressure ratings. This design can be found in the WIKA gauge model 632.51 (4″ or 6″), and it comes standard with an overpressure safety of 50 times of full scale.
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