There are considerable international differences. They can be explained in the following example:

• Europe: 0 … 10 bar / 0 … 10 bar abs
• America: 0 … 150 psig / 150 psia

In Europe only the absolute measuring ranges are identified (by the abbreviation “abs” behind the unit). In America, the pressure type is always indicated but using another naming system (g = gauge = relative pressure).

This is especially confusing if in America the pressure unit “bar” is required for an absolute measuring range or if in Europe a relative pressure device with the unit “psi” is required. How is then the correct nomenclature?

The WIKA group agreed upon the following procedure: if the unit psi is used, a “g” or an “a” is added according to the American system. For all other units, we observe the European standard and only the absolute measuring ranges are marked with a separate “abs”.

The answer to this question is obvious and simple: yes.

Both electronic pressure sensors and mechanical pressure gauges have properties that must be weighed against each other in each application. The table below summarises the individual benefits.

The biggest benefits of mechanical pressure gauges are the easy installation and good readability of the measured value. An electronic pressure transmitter can also be equipped with an on-site display, which is also easy to read even in darkness, but a pressure gauge offers better readability from a distance with a large scale and in daylight. Due to their robust mechanics mechanical pressure gaugesare mandatory in certain applications (e.g. hot-water tanks).

A pressure gauge is quite often combined with a separate electronic pressure sensor. This combines the benefits of both systems: pressure measurement with local display without power supply and availability of a measured value in the PLC. For users who have only one pressure connection available, pressure gauges with integrated electronic sensor are also available.

With voltage signals, electromagnetic interferences can easily lead to wrong readingsof the measured value or of the control signal, which is why shielded lines should be used for such signals. Very often, the voltage signals 0 … 10 V, 1 … 5 V and 1 … 10 V are used for setpoint signalsof motors, although temperature and pressure sensors are also available with these electrical outputs.

Similarly to the current signals, the actual pressure in the sensor is converted into a voltage value and transmitted via two (1 … 5 V, 1 … 10 V) or three (0 … 10 V) wires. The signals 1 … 5 V and 1 … 10 V have the advantage that by setting an active zero value of 1 V, short-circuits in the line can also be detected.

Very widely used is the 4 … 20 mA signal in the transmission of values such as temperature and pressure. For example, the 0 … 10 bar pressure range of a pressure transmitter in a production process is converted by the electronics in the device into a 4..20 mA current signal. As two-wire signal, 4 … 20 mA has in the meantime been given preference over the three-wire version, due to its savings in wiring and its easier error detection. In this version, a cable break is detected by the current value falling below 3.8 mA and a short-circuit by the current value exceeding 20.5 mA (according to NAMUR NE 43). 4 … 20 mA in the three-conductor version is still being used, but only for devices with high supply power requirements.

At WIKA, the term: “pressure transmitter” is commonly used to refer to a pressure sensor equipped with standardised electrical and mechanical interfaces and a standardised output signal.

The working principle of a pressure transmitter is as follows:

The pressure of the medium to be measured is guided via a standard process connection thus affecting the internal pressure sensor element. The internal electronics converts the raw sensor signal into a filtered, amplified, temperature-compensated and standardised signal, such as the 4 … 20 mA signal. This output signal is delivered via a standardised connector or a cable to the subsequent unit for signal processing.

Would you like to have more information about the pressure transmitter A-10 for general industrial application? Then click here.

Source: Bundesverband Wärmepumpe e. V.

The working principle of a heat pump is basically identical to that of an everyday appliance known to all of us: the refrigerator. However, while the refrigerator removes heat from its interior and gives it off to the environment, the heat pump removes heat from the exterior (air, soil, etc.) and gives it off to the house as heating energy. Thus, the system works exactly in reverse.

In order to do that the heat pump performs a compression of a gas (as in the refrigerator) and thus liberates the heat previously removed from the environment by evaporation. This process also consumes energy and must therefore be carefully controlled.

If the compression or evaporation temperature is controlled very accurately in a heat pump system via the pressure, this allows to save energy costs and thus also protect the environment.

WIKA offers products especially for application in heat pumps, the pressure transmitters R-1 and AC-1.

The IP ingress protection types provide a system that describes how the housing of electrical equipment is protected against the ingress of particles/ dust and water.

Actually, the standard only describes – by means of a code of max. 4 digits - whether the instrument is protected against the penetration of water and foreign particles/dust but does not indicate if the instrument is suitable for special operating conditions.

The abbreviation IP stands for “International Protection” but in the English speaking countries it is often translated by “Ingress Protection” .
This designation is factually correct since only a description is made whether water or particles may “ingress” into the housing.

There are two different standards for the classification of the IP codes:

• DIN EN 60529 à Degrees of protection provided by housings (IP code)
  Often used in the industrial area.
  
• DIN 40 050 part 9 à Road vehicles; IP protection types; protection against foreign bodies, water and contact;
  electrical equipment
  Is used if the requirements exceed the normal immersion of the instruments, e.g. high-pressure cleaning

Both standards are applicable and there are slight differences in some details. It is therefore useful to refer to the respective standard for each individual case.

Additional information regarding the “Ingress of water”:
NB: Only the behaviour of the equipment when coming into contact with pure water is described here. As soon as additives have been added, these protection types are no longer valid or apply only to a limited extent.

Generally it can be said that, depending on the type of medium to be measured (gaseous, liquid, harmful substance), pressure measuring instruments are subject to different requirements, depending on the pressure range and volume. If the medium is not known, it is best to design pressure measuring instruments conservatively, that is, also for harmful gaseous and liquid media, with a pressure channel and an internal volume of < 0.1 l.

Whereas it is sufficient that pressure equipment for up to 200 bar is generally designed and manufactured following the rules of “Good Engineering Practice” (GEP), for pressures from 200 bar, a conformity assessment procedure must be used, which in the simplest case can be an “internal production control”.

For pressures from 200 bar, it is also required to meet the basic safety requirements of the PED Appendix I, which, among other things, requires a pressure test as proof of the pressure strength. An EC Declaration of Conformity must be prepared, and the instrument will be marked with CE.

In any case, details should be looked up in the original text of the PED, while information on the interpretation of this directive can be found in the PED guidelines.

Here are the most important key points:

1. Pressure equipment for more than 0.5 bar is subject to the PED.
2. Between 0.5 and 200 bar, “Good Engineering Practice” must be applied. No CE marking and Declaration of conformity possible according to the PED.
3. At pressures greater than 200 bar, the “Basic Safety Requirements” of the PED Appendix I must be met, CE marking is required, and an EC Declaration of conformity according to the PED must be prepared.

Passive temperature compensation:

Sections of the characteristic accuracy curve of the pressure sensor are measured at different temperatures during the manufacturing process. Then, the previously determined temperature errors are compensated by passive elements (resistors) within the electronics of the sensor or by corrections of specifically  designed resistance structures directly on the sensor  element itself (e.g. by laser-trimming). The (passive) resistor elements used have an almost linear temperature behaviour, it is, however, only possible to compensate 1st order errors. Temperature errors of higher order, i.e. strong bending of the characteristic curve under temperature, can not be compensated.

Active temperature compensation:

Here too, the characteristic curve of the pressure transmitters is measured at different temperatures during the manufacturing process. However, the pressure transmitter has an additional integrated temperature sensor which constantly measures the temperature of the sensor and transfers it to the pressure transmitter’s signal processing. In practice, two methods of active temperature compensation are common: the first method compensates by means of a limited number of samples, , i.e. discrete correction values, between which interpolation takes place. The second method uses the electronics of the transmitter and a higher-order equation resulting from the regression of the acquired measurement values in order to compensate then the expected error.

During operation, this signal processing makes it possible to automatically, i.e. “actively”, compensate the pressure transmitter ‘s temperature error using the calculated correction factors within a specified temperature range (e.g. 10-60°C).

The most commonly used method to minimise temperature errors of pressure sensors is a passive temperature compensation. This is the traditional method which is widely used. However, active temperature compensation is the top class of possible compensation methods. WIKA has constantly improved and refined this technology in recent years.

The pressure transmitters of WIKA using active temperature  compensation therefore feature a temperature error which is almost zero in their specified temperature range.

Process industry users need this information for a SIL evaluation of the complete application.

For the requirements of the machine building industry, MTTF_d (Mean Time To Dangerous Failure) values are provided in order to be able to determine the relevant Performance Level (PL).