Pressure measurement for absolute, gauge and differential pressure

Pressure measurement is the measurement of the force exerted by a fluid (liquid or gas) on a surface.Pressure is usually measured in units of force per unit surface area.Many techniques have been developed to measure pressure and vacuum.Instruments used to measure and display pressure mechanically are called manometers, vacuum gauges or combined gauges (vacuum and pressure).The widely used Bourdon gauge is a mechanical device that both measures and indicates and is probably the best known type of gauge.Vacuum gauges are used to measure pressures below ambient atmospheric pressure,which is set to zero, negative values (for example, -1 bar or -760 mmHg equals total vacuum).Most gauges measure pressure relative to atmospheric pressure as the zero point, so this form of reading is simply called "gauge pressure".However, anything larger than a perfect vacuum is technically a form of pressure.For very low pressures, a gauge that uses total vacuum as a zero point reference must be used, giving the pressure reading as absolute pressure.Other pressure measurement methods involve sensors that can transmit pressure readings to remote indicators or control systems (telemetry).

Absolute, gauge and differential pressure - zero reference 

Everyday pressure measurements, such as vehicle tire pressure,are usually made relative to ambient air pressure.In other cases,measurements are made relative to vacuum or some other specific reference. When distinguishing between these zero references, the following terms are used:

• Absolute pressure is zero referenced to absolute vacuum and uses an absolute scale so it is equal to gauge pressure plus atmospheric pressure.

• Gauge pressure is zero referenced to ambient air pressure and is therefore equal to absolute pressure minus atmospheric pressure.

• Differential pressure is the pressure difference between two points.

Zero references in use are usually implied by context,and the words are added only when clarification is required.Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressure, deep vacuum pressure, and altimeter pressure must be absolute pressures.For most working fluids where the fluid is present in a closed system, gauge pressure measurement dominates.A pressure gauge connected to the system will indicate the pressure relative to the current barometric pressure.Things change when extreme vacuum pressure is measured, then absolute pressure is often used instead, and the measuring instruments used vary.Differential pressure is commonly used in industrial process systems.A differential pressure gauge has two inlet ports, each connected to one of the volumes whose pressure is being monitored.In effect, such gauges perform the mathematical operation of subtraction mechanically, avoiding the need for an operator or control system to look at two separate gauges and determine the difference in readings.Medium vacuum pressure readings can be ambiguous without the proper context, as they may represent absolute or gauge pressure without a negative sign.Therefore, a vacuum of 26 inches of mercury gauge is equivalent to an absolute pressure of 4 inches of mercury, calculated as 30 inches of mercury (typical atmospheric pressure) - 26 inches of mercury (gauge pressure).Atmospheric pressure is typically around 100 kPa at sea level,but varies with altitude and weather.If the absolute pressure of a fluid remains constant, the gauge pressure of the same fluid will change as atmospheric pressure changes.For example, when a car is driven up a hill, the (gauge) tire pressure rises due to the drop in atmospheric pressure.The absolute pressure inside the tire is basically constant.Using atmospheric pressure as a reference is usually indicated by a "g" after the pressure unit, such as 70 psig, which means the measured pressure is the total pressure minus the atmospheric pressure.There are two types of gauge reference pressure: vented gauge (vg) and sealed gauge (sg).

For example, vented pressure transmitters allow external air pressure to be exposed on the negative side of the pressure sensing diaphragm through a vent cable or a hole in the side of the device so that the pressure always measured refers to ambient air pressure.Therefore, a vented reference pressure sensor should always read zero pressure when the process pressure connection is left open to air.Sealed gauge references are very similar except that atmospheric pressure is sealed on the negative side of the diaphragm.This is typically used in high pressure ranges, such as hydraulic systems, where changes in atmospheric pressure have negligible effect on reading accuracy, so venting is not required. This also allows some manufacturers to provide secondary pressure vessels as an extra precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded.There is another way to create a sealed gauge reference, which is to seal the high vacuum on the back of the sensing diaphragm. The output signal is then shifted so that the pressure sensor reads close to zero when measuring barometric pressure.A sealed gauge reference pressure sensor will never read exactly zero because the barometric pressure is always changing, in this case the reference is fixed at 1 bar.To produce absolute pressure sensors, manufacturers seal a high vacuum behind the sensing diaphragm. If the process pressure connection of the absolute pressure transmitter is open to air, it will read the actual air pressure.

Static and dynamic pressure

Static pressure is uniform in all directions, so pressure measurements are independent of direction in a stationary (static) fluid.However, the flow exerts additional pressure on surfaces perpendicular to the direction of flow and has little effect on surfaces parallel to the direction of flow.This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure.Instruments facing the direction of flow measure the sum of static and dynamic pressure; this measurement is called total pressure or stagnation pressure. Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure.While static gauge pressure is critical to determining the net load on the pipe wall, dynamic pressure is used to measure flow and space velocity.Dynamic pressure can be measured by taking the pressure difference between the instruments parallel and perpendicular to the flow. For example, pitot tubes perform this measurement in airplanes to determine airspeed.The presence of measuring instruments inevitably induces shunts and creates turbulence, so their shape is critical to accuracy and calibration curves are often non-linear.

Applications

1. Altimeter

2. Barometer

3. Depth gauge

4. MAP sensor

5. Pitot tube

6. Sphygmomanometer

Instruments

Many instruments for measuring pressure have been invented, each with its advantages and disadvantages.Pressure range, sensitivity, dynamic response, and cost all vary by orders of magnitude from one instrument design to another. The oldest type is the liquid column (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. The U-tube was invented by Christiaan Huygens in 1661.

Hydrostatic pressure

A hydrostatic gauge, such as a mercury column manometer, compares pressure to the hydrostatic force per unit area at the base of the fluid column. Static manometer measurements are independent of the type of gas being measured and can be designed to have a very linear calibration.Their dynamic response is poor.

Piston

0.1 Piston gauges use springs (such as relatively inaccurate tire gauges) or solid weights to balance the pressure of the fluid, in which case it is called a deadweight tester and can be used to calibrate other gauges.

0.2  McLeod Regulation

0.3  The Macleod manometer separates a gas sample and compresses it in a modified mercury manometer to a pressure of a few millimeters of mercury. This technique is very slow and not suitable for continuous monitoring, but has good accuracy.Unlike other manometers, McLeod manometer readings depend on the composition of the gas, since interpretation relies on the compression of the sample to an ideal gas. Due to the compression process, McLeod gauges completely ignore partial pressures from condensation of non-ideal vapors, such as pump oil, mercury, or even water if the compression is sufficient.

0.4  Useful range: Vacuums from about 10−4 Torr[10] (approximately 10−2 Pa) up to 10−6 Torr (0.1 mPa), 0.5  0.1 mPa is the lowest pressure that can be measured directly with current technology.Other vacuum gauges can measure lower pressures, but only indirectly by measuring other pressure-related properties. These indirect measurements must be calibrated to SI units by direct measurement, most commonly the McLeod gauge.

Aneroid

Aneroid manometers are based on a metallic pressure sensing element that flexes elastically under the action of a pressure differential."Aneroid" means "without fluid", a term that originally distinguished these gauges from the static pressure gauges mentioned above. However, aneroid pressure gauges can be used to measure the pressure of liquids and gases, and they are not the only type of pressure gauge that can work in the absence of liquid.Therefore, they are often called mechanical gauges in modern languages. Unlike thermal and ionization gauges, aneroid gauges are not dependent on the type of gas being measured and are less likely to contaminate the system than static pressure gauges. The pressure sensing element can be a Bourdon tube, a diaphragm, a capsule, or a set of bellows that change shape depending on the pressure in the area of interest.The deflection of the pressure sensing element can be read by a linkage connected to the needle, or it can be read by an auxiliary sensor. The most common secondary sensor in modern vacuum gauges measures the change in capacitance due to mechanical deflection.Instruments that rely on capacitance changes are often called capacitance manometers.