Manometers - Basics, types and function of pressure gauges

Structure

Pressure gauges are used in a variety of applications, including measuring pressure in car tyres, monitoring atmospheric pressure conditions in weather stations, and measuring pressure in gas and water pipes. Depending on the area of application and design, manometers can measure the relative pressure against the respective static pressure (e.g. atmospheric air pressure) or the absolute pressure against the vacuum (e.g. barometer).

How manometers work

With regard to the elements that transmit the pressure to the display mechanism in a system, the industrial application essentially distinguishes between tube spring manometers, diaphragm manometers and capsule manometers. An analogue scale with a needle or a digital display, in which the physically measured pressure is converted into an electrical signal using pressure sensors and shown on the display, serves as a display device.

Manometers are designed for a specific pressure range that must be taken into account for the selection.

Bourdon tube pressure gauge

A tube spring manometer is a pressure gauge that works according to the principle of elastic deformation. In the encompassing housing there is a tube spring (also called Bourdon spring), which forms the connection between the substance to be measured and the display scale. The tube spring inside the manometer is filled with the measuring material (e.g. water).

If the pressure in the coiled tube spring increases, it deforms elastically and attempts to unwind. The reason for this is physical effects in connection with the spiral or circular design. This movement is mechanically transferred from the tip of the tube spring to the needle via a pull rod, thus indicating the applied pressure on the scale. Tube spring manometers are versatile and suitable for a wide range of applications.

However, since the liquids or gases to be measured penetrate directly into the pressure gauge, they are generally not suitable for measuring the pressure of aggressive media. In these cases, pressure detectors made of special materials must be used to achieve a separation between the manometer and the measuring material or to switch to coated diaphragm manometers.

Diaphragm manometers

Diaphragm manometers transmit the pressure in a system to the display device using a diaphragm. Adjacent to the display device there is a diaphragm, which is usually clamped between two flanges and is connected to the needle via a push rod and physically separates the interior of the pressure gauge from the substance to be measured.

If a force acts on the diaphragm from the outside, it deforms along the acting force and translates the movement via the push rod into a value that can be read on the scale. Since the diaphragm is supported by the flange on the manometer side at maximum load, this form of the manometer is comparatively resistant to overloading.

Diaphragm manometers can display precise results even at very low pressures of a few millibars and can also be used for pressure measurements in systems with aggressive media with appropriate coating of the diaphragm (e.g. with PTFE or gold).

Capsule manometers

Capsule manometers are designed to resemble the structure of diaphragm manometers, but have two diaphragms connected to each other at the edges on the inside.

An opening is provided in the diaphragm on the measuring material side, by means of which the measuring material (usually dry gases) can flow directly into the capsule element. There, the exerted overpressure or vacuum pressure will under negative pressure conditions cause the capsule to deform, which as a result expands or contracts. The deformation is transferred to the display system via a mechanical connection. Capsule manometers are suitable for precise pressure measurements in the mbar range due to their high measuring accuracy.

Properties of manometers

The following list provides an overview of the most important properties of manometers.

Operating principle and construction

Manometers record the pressure in a system using different functional principles. In addition to the principles described above, which are most widespread, there are many other special forms of pressure gauges designed for special applications. For example, barometers with a closed capsule as a special form of capsule spring barometers, pressure scales or liquid manometers.

Display range

The display range indicates in which pressure range the manometer is suitable for the measurement. The pressure is preferably specified in bar.

Accuracy class

The accuracy class of a manometer indicates how large the deviation of the value displayed on the manometer from the actual applied value can be.

According to EN 837-1, the accuracy classes are between 0.1 and 4.0 at a reference temperature of 20°C. The accuracy classes are stated as a percentage of the display range. With an accuracy class of 1.0 and a display range of 100 bar, the error limit of a manometer would therefore be ± 1 bar.

Overload protection

Overload protection refers to the ability of a manometer to withstand pressures beyond the display range. If the pressure increases further and also exceeds this safety range, the elastic deformation of the pressure-absorbing elements transforms into a plastic deformation. The measuring system is permanently deformed.

Nominal size

The size of the manometer display is given as the nominal size in millimetres.

Scale graduation

The scale graduation provides information about the reading accuracy of the display device and denotes the intervals of the display scale.

Installation properties

In addition to the basic properties of manometers mentioned above, other installation-specific properties such as thread size and position of the fastening screw must be considered when selecting the correct manometer for the planned application. Manometers can also be filled with a liquid (e.g. glycerine) to dampen strong vibrations or rapidly fluctuating pressures.

How do differential pressure gauges work

The differential pressure measurement is of great importance for industrial use and is used, for example, to measure the filling level of tanks filled with liquid or to measure flow speeds and associated flow rates.

The differential pressure measurement is of great importance for industrial use and is used, for example, to measure the filling level of tanks filled with liquid or to measure flow speeds and associated flow rates.

The pressure difference is evaluated based on the extent and direction of the deformation of this membrane and transferred to the display system. Various measuring sensors can be used to measure flow speeds, such as dynamic pressure probes or venturi meters/nozzles, depending on the cross-section of the pipeline and operating conditions.

Pressure sensors and flow sensors

Pressure sensors and flow sensors are special forms of pressure gauges.

Pressure sensors are used, for example, to measure the filling level of tanks or containers. They are also used in hydraulic systems to control the pressure of the oil in the system. There are various types of pressure sensors such as piezo-resistive sensors or capacitive sensors.

Flow sensors, on the other hand, are used to measure the amount of liquid or gas through a pipeline system. This is important for precise dosing of media in the chemical or pharmaceutical industry as well as heating systems to check energy consumption.

Selecting the appropriate pressure gauge

When selecting pressure gauges, it is important to consider the specifications of the components and materials.

This can be based on the DIN specifications that are widespread in Europe or the JIS specifications that originate from Japan. However, it is important to note that not all components and materials are compatible between JIS and DIN.

It is therefore recommended to agree on the specifications of a region when selecting combined precision components.