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Pressure gauges - Manometers - Precision instruments for safety!

Manometers are required to measure the physical pressure of gases and liquids. These pressure gauges are also used to measure the pressure exerted by solid bodies. The pressure measurement is relative to the ambient pressure (atmospheric pressure). The precision instruments serve both commercial and private purposes. Pressure sensors sense the pressure and react accordingly. Within the measuring range it is then possible to read off. Especially in the field of pressure measurement technology there are many manufacturers who produce almost all types of pressure gauges, from simple flow manometers to digital manometers, as well as special designs.

How do pressure gauges work?

The way they work varies considerably depending on the pressure gauge. The simplest variant is the liquid manometer. Liquid is located in a u-shaped tube. If the liquid is at the same level in both legs, the pressure is the same in both legs. If, on the other hand, pressure is exerted in only one leg, the liquid moves. The values can then be taken from a scale. The height difference (Δh) is then converted into the pressure difference (Δp) (differential pressure).

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A distinction is made between indirect and direct pressure measuring instruments.

An indirect pressure measuring device derives the pressure e.g. by means of the deformation of elements. Chemical, optical and electrical effects are also possible with this type of device. Indirect pressure measuring instruments are capsule pressure gauges, diaphragm pressure gauges and Bourdon tube pressure gauges.

For pressure gauges with spring-elastic measuring element, a distinction is made between bourdon tube pressure gauges, capsule pressure gauges and diaphragm pressure gauges.

Bourdon tube pressure gauges consist of a helically, helically or circularly wound Bourdon tube. The design depends on the pressure range to be measured. If pressure is applied, the spring stops. The change in travel of the Bourdon tube end is transmitted from a draw rod to a segment gearwheel and consequently to the pointer axis.

Diaphragm pressure gauges

The measuring element here consists of a circular diaphragm spring. This is clamped between two flanges. The pressure bends the diaphragm spring. This is converted by a pointer mechanism into a rotary movement of the pointer axis. In diaphragm seals, however, the deflection is transferred to a liquid. Here the transmission to a pointer mechanism takes place. A grooved profile is stamped into the flat spring. The design depends on the diaphragm diameter, the modulus of elasticity of the material and the diaphragm thickness. Stainless steel is usually used as material. Nickel-based alloys such as Hastelloy or Monel are used for higher resistance in measurement technology. Furthermore, refractory metals (tantalum) and PTFE are used.

Capsule pressure gauges are a special form of diaphragm pressure gauges. In this design, two diaphragm springs are arranged one above the other and welded at the edges. This creates a closed pressure chamber (load cell). The measuring medium is conducted through a capillary tube welded to the capsule spring. Both ends of the spring can bend.

Direct pressure gauges are based directly on the description of the physical quantity. The value given is rooted in one of the values of the equation: p = F/A or Δp = Δhρg (pressure p, area A, height h, density ρ, acceleration of gravity g, force F). Direct manometers are liquid and piston manometers.

The piston barometer measures the pressure through a piston which presses against a counterforce. The source of the force is usually springs. This operating principle is used for very simple but also for high-precision piston barometers, e.g. for calibration and gauging. These are rotary piston pressure gauges, whereby the piston is set in rotation. This avoids misalignment measurement errors.

Special forms include the barometer (absolute pressure measuring instrument), which measures air pressure. The reference pressure is a vacuum. The only difference to other measuring instruments is that the differential pressure is measured between any two systems.

Differential pressure gauge (absolute pressure gauge)

In absolute pressure measurement, the atmospheric air pressure acting on the range spring is replaced by a vacuum. The absolute pressure is measured. With the aneroid barometer, the air pressure acting from outside deforms the diaphragms. If the housing of a Bourdon tube pressure gauge is evacuated, an absolute pressure gauge is created.

Flow meters consist of a measuring sensor (flow sensor) and a supply or evaluation unit. The latter is called a transmitter.

What are accuracy classes?

This is the specification of the error limits in percent of the full scale values. The accuracy classes range from 0.1 to 4. Class 4 means that ± 4 % error in pressure measurement is acceptable.