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Many techniques have been developed for the measurement of reduced or increased pressures. Pressure gauges are either direct- or indirect-reading. Those that measure pressure by calculating the force exerted on the surface by incident particle flux are called direct reading gauges. Indirect gauges record the pressure by measuring a gas property that changes in a predictable manner with gas density.
Bourdon Tube Type
In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon.
A pressure or vacuum gauge usually consists of a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the pressure increases the tube will tend to uncoil, while a reduced pressure will cause the tube to coil more tightly. This motion is transferred through a link 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 the following pictures the transparent cover face has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:
- the left side of the face, used for measuring manifold vacuum, is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.
- the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/[square centimeter|cm² on its inner scale and pounds per square inch on its outer scale.
Mechanical details
Stationary parts:- A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.
- B: Chassis Plate. The face card is attached to this. It contains bearing holes for the axles.
- C: Secondary Chassis Plate. It supports the outer ends of the axles.
- D: Posts to join and space the two chassis plates.
Moving Parts:
- Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.
- Moving end of Bourdon tube. This end is sealed.
- Pivot and pivot pin.
- Link joining pivot pin to lever (5) with pins to allow joint rotation.
- Lever. This an extension of the sector gear (7).
- Sector gear axle pin.
- Sector gear.
- Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to the short distance between the lever arm link boss and the pivot pin and the difference between the effective radius of the sector gear and that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle.
- Hair spring to preload the gear train to eliminate gear lash and hysteresis.
Aneroid chamber (bellows) type
In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an aneroid, which means "without liquid". (Early barometers used a column of liquid such as water or the liquid metal mercury suspended by a vacuum.) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), altimeters, altitude recording barographs, and the altitude telemetry instruments used in weather balloon radiosondes. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as air speed indicators and rate of climb indicators (variometers) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber.
Absolute pressure vs. gauge pressure
Pressure gauges can be further classified into those reading absolute pressure and those reading gauge pressure. Absolute gauges measure the pressure of a fluid referenced against a vacuum. Aneroid barometers and altimeters are good examples, and the Manifold Absolute Pressure (MAP) sensor used in the engine control systems of modern fuel-injected automobiles is another. Gauge-pressure gauges measure the pressure of a fluid referenced against ambient air pressure. The Bourdon tube gauge discussed above is an example. The displayed reading of such a gauge will vary as local atmospheric pressure changes. However, if the gauge is designed to read pressures of many atmospheres, the error between gauge pressure and absolute pressure is usually so small as to be negligible.
A differential pressure gauge is a special case of gauge-pressure measuring instrument, designed to display the difference in pressure between two points. A liquid-column manometer is one example of such a gauge. Such instruments have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction through mechanical means, obviating the need for an operator to watch two separate gauges and mentally determine the difference in readings.
"Pressure gauge"