Pressure measurement

A pressure gauge ( manometer also - from Ancient Greek μανός Manos "thin" and μέτρον métron " measure, measure ") is a measuring device for detecting and displaying the physical pressure of a fluid (liquid, gas). In most applications, the relative pressure - the atmospheric pressure that is based - is measured. Absolute pressure measuring instruments use a vacuum as the reference pressure (eg, barometer). Measure differential pressure gauges, like the others, a pressure difference, however, between any two systems.

• 3.1 Pressure gauges with elastic pressure element 3.1.1 Bourdon tube pressure gauge
• 3.1.2 Diaphragm pressure gauges
• 3.1.3 Capsule pressure gauges

• 4.1 Differential Pressure Measurement

• 5.1 Barometer
• 5.2 Pressure Sensors

• 8.1 European standards
• 8.2 U.S. standards

Classification of pressure measuring instruments

Pressure gauges Pressure gauges are classified based on their measurement procedures in immediate ( directly on the definition of the physical quantity based ) and indirect ( the pressure from other physical effects dissipative ). In the metrological practice, there are further divisions, for example:

• Of application in industrial, chemical or standard gauge.
• The pressure range in low pressure, high pressure or vacuum gauges.
• For accuracy in fine measurement or Gebrauchsmanometer.
• According to procedures in electrical, mechanical or mechatronic pressure gauges.

Direct pressure gauges

Pressure gauges whose display value is based directly on one of the following relationships:

The pressure is physically exerted on the result of a surface motor. The immediate pressure measuring method would thus the determination of a force acting on a given area. This is realized at the pressure balance with his weight loaded piston of a defined cross-sectional area. Also liquid manometer, in which the pressure depends only on the height and density of the liquid column can be regarded as direct pressure gauges. is the gravitational acceleration. indicates the use of difference sizes.

Deadweight

When pressure balance, also called pressure compensator, the pressure is indicated by a piston which moves against a force. The force may be applied by springs ( eg, pressure cooker, because combined with the pressure relief valve ) or by weight ( precision manometer). This principle is used for a very simple manometer, on the other, high-precision pressure balance for the calibration or calibrating other pressure gauges are used. These Drehkolbenmanometern the piston in order to avoid tilting of measurement errors will be caused to rotate.

A particular type are so-called " popouts ". In this construction, when a certain pressure is exceeded, only one pin is pressed from the housing. This is used for display of filter clogging.

Liquid manometer

U- tube manometer

Here the pressure is displayed by moving a column of liquid. To this end, a U-shaped glass tube is used, which is filled to about half full with the sealing liquid such as mercury or water. Then, when a pressure difference is applied between the legs of the U, the liquid column moves to the side with the lower pressure. The level difference is a measure for the pressure difference.

→ Main article: U- tube manometer

McLeod gauge

The McLeod gauge is a compression liquid manometer named after its inventor Herbert McLeod. In this case, a quantity of gas with the volume on the volume is compressed. In the same ratio increases the pressure on from, which can be measured according to the principle of the U- tube manometer. From the output pressure can then be calculated using the Boyle's law with knowledge of the volumes.

Ringbalance

In the ring, a rotatably mounted balance hollow ring with a partition wall is partially filled with a barrier liquid. The chambers lying above the liquid are connected to the pressures to be measured, which turn the ring so far, until a force equilibrium adjusts to a counterweight attached below.

→ Main article: Ringbalance

Mercury manometer as

The reasons for his continuing use of mercury in pressure gauges are for one its beneficial high density. The maximum measurable pressure difference on the other hand, for example, limited by the height of the U-tube on the one hand ( which is generally limited by the space or Ablesehöhe ) and the density of the liquid. This allows the mercury in the same tube length measurement significantly higher differential pressures as for example water. Furthermore, it has low capillary action, so that the mercury in glass tube, a relatively planar surface, which allows accurate reading forms. Mercury remains chemically stable against most gases and allows the measurement of differential pressures between other liquids with which it does not mix.

Problems arise partly from the cleaning process necessary after some time. Although mercury, for example, from air takes up only very small amounts of moisture, there must be cleaned before the measurement result is distorted by the reduction of the density. Until the seventies of the twentieth century it was common practice in laboratories to " boil " the mercury from manometers. While this is simple, but unacceptable because of this, despite everything resulting mercury vapors for reasons of occupational safety. The toxicity is also a problem with a possible contamination of the environment, for example, glass breakage of instruments or spillage during filling or emptying due to large differential pressure.

Indirect pressure gauges

Indirect pressure gauge use secondary physical effects of measurement. To this end, almost all areas of physics have contributed. Mechanical pressure gauges usually take advantage of the elastic deformation of the measuring element. Other methods take advantage of the electrical, optical or chemical effects of pressure, as a method to which the theory of pressure as a measure of the particle number density is based on, may be mentioned:

• Ion current measurement, caused by the ionization of the gas; see ionization vacuum gauges
• Compression of a fixed quantity of gas to a defined volume.
• Friction of a body in the gas (due to its viscosity ); see viscosity vacuum gauge
• Heat transfer of a gas; see Pirani meter

Pressure gauges with elastic pressure element

Bourdon tube pressure gauge

→ Main article: Bourdon tube (metrology) Bourdon tube pressure gauges, pressure gauges, the measuring element called depending on the pressure to be measured range from a circular, snails or helically wound Bourdon tube, the Bourdon tube well, there. Much like a blowouts whistle the tube spring when pressure is striving to handle itself. The change in displacement experienced by the Bourdon tube end up doing is transmitted via a tie rod on a segment gear and thus the pointer axis ( see figure).

Diaphragm pressure gauges

Diaphragm as measuring element have a circular diaphragm which is usually clamped between two flanges. Upon pressurization of the diaphragm experiences a deflection, which is implemented by a pointer in a rotational movement of the pointer axis. Diaphragm similar in structure diaphragm seals, except that the deflection of the diaphragm is not transferred to a liquid, but on a pointer. The stroke of the deflection of the diaphragm springs is transmitted to the pointer movement, is not linearly dependent on the pressure. To compensate for this non-linearity, a non-linear scale can be used either or, more commonly in practice, a plate spring in which a grooved profile is pressed. The interpretation of the membrane springs is dependent upon the diameter of the diaphragm, the diaphragm thickness and the modulus of elasticity of the material used. Usually stainless steel is used as a membrane material. If a higher resistance is required, even nickel-based alloys such as Monel or Hastelloy are used. If materials are needed that make the production of a plate spring impossible or very costly, eg of plastics such as PTFE or refractory metals such as tantalum, membrane templates come from the appropriate material to use.

Capsule pressure gauges

Capsule are a special form of Plattenfedermanometers. Capsule springs consist of two superimposed plate springs which are welded together at their edges, so that a closed pressure chamber is created ( " load cell "). The measured medium is passed through a densely welded also with the capsule spring capillary tube into the capsule spring. They shall be stored in the manometer that both sides of the capsule element can bend and so at the same applying pressure to the double suspension travel of the diaphragm can be exploited by measurement. Due to the series connection of several capsule springs, the sensitivity can still be increased. Capsule springs are used for measuring low pressures. The aneroid barometer doses or sets an evacuated capsule element for the determination of atmospheric pressure (see figure).

Absolute and differential pressure gauges

Practically pressure gauges are usually used with elastic measuring element for absolute and differential pressure measurement. For the absolute pressure measurement in this case the bearing on the range spring atmospheric pressure is to be replaced by a vacuum. Thus, in the interior of an aneroid capsule element is evacuated so that the pressure acting from the outside air pressure deforms the membranes. Evacuating the housing of a Rohrfedermanometers, which also gives an absolute pressure gauge. In a Diaphragm to the measurement pressure side facing away from the plate spring has to be evacuated.

Differential pressure measurement

Usually diaphragm measuring systems are used (see figure) for differential pressure measurement. At the same pressure in the positive (3), as in the negative pressure chamber ( 2), which plate springs (1 ) undergo no deflection. Only a pressure difference in the chambers causes a deflection of the disc springs, and the pressure difference is caused by transmission of the spring stroke of a connecting rod (6) to the measuring element for display. The two plate springs are hydraulically coupled via a filling medium (7). The resilient corrugated pipes ( 4) provide for the sealing of the two pressure chambers to the atmosphere. The two sealing elements (5) provide for overload protection: As soon as one side of the pressure-difference is overloaded, close the pressure chamber to the atmosphere from certain.

Special Pressure Gauges

Barometer

A barometer is used for determining the air pressure. There are usually used absolute pressure gauges that measure the pressure relative to a vacuum. This pressure differential causes a force (typically a membrane ) is applied to a surface and can be determined by means of force measurement. Barometer usually have a measuring range of 800 to 1200 mbar absolute pressure ( pressure relative to vacuum).

Pressure sensors

A pressure sensor is a measuring element which converts the physical quantity into a pressure proportional to the pressure electrical output. For determining the contact pressure, the definition of the pressure is used, and due to a force measurement. There are thus suitable all measurement methods, which are also used for force and weight measurement: piezoelectric sensors, strain gauges as well as pressure balances.

Flüssikeitsfüllungen of Zeigermanometern

Are Indicating pressure gauge mounted on machines that vibrate strongly, eg Compressors and hydraulic power units, are frequently used instruments, the housing is filled with a transparent liquid. Most here glycerol ( hence the name Glycerinmanometer ) is used, as well as silicone oil, and in the pharmaceutical and food industry also white oil. The viscosity of the fluid dampens the needle deflection when the pressure to be measured varies dynamically or the housing is exposed to strong vibrations and thus facilitates easy reading. By the vibration damping and lubricating properties of the liquid In addition, the life of the mechanical parts of the plant is increased. The liquid has not directly to do with the acquisition of reading.

Others

• Biology Baroreceptors register the pressure of the flowing blood on the vessel walls
• The eardrum is a sensor of the organ of hearing, which is only sensitive to pressure ( sound pressure )

• Technology Sound pressure receivers, microphones are a special type which produce the sound pressure at the microphone the received electrical signals proportional

Standardization

European standards

• DIN EN 472, pressure gauges - Definitions
• DIN EN 837-1, Pressure gauges with bourdon tubes; Part 1: Dimensions, metrology, requirements and testing
• DIN EN 837-2, pressure gauges; Part 2: Selection and installation recommendations for pressure gauges
• DIN EN 837-3, Pressure gauges with capsule plates and springs; Part 3: Dimensions, metrology, requirements and testing

U.S. standards

• B40.100 -2005: Pressure gauges and gauge attachments.
• PTC 19.2-2010: Performance test code for pressure measurement.