Apart from the current signals described in the article “Analogue output signals of pressure sensors 1″, the voltage signals 0 … 10 V, 1 … 5 V and 1 … 10 V are used in industrial automation for the transmission of measured values. As with the current signals, their advantage is the ease of use and the possibility to identify problems witha simple multimeter.

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.

In industrial automation and in pressure sensors in particular, the current signal of 4 … 20 mA is the most frequently used signal for analogue transmission of values. The wide use of this signal is due to its ease of handling and especially its interference resistance. A current signal has a higher EMC interference immunity than a voltage signal, due to the fact that electromagnetic interferences are fed into the signal line as voltage signals and result in only very small changes in current at input resistance of the receiver.

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.

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.

A frequently recurring question is about the definition of the so-called IP protection types or sometimes referred to as “IP rating”.

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.

How does an active or passive temperature compensation of pressure transmitters  actually work? High-quality pressure transmitters, especially those used in precision critical applications, are almost always provided with an individual temperature compensation. But what is actually the difference between  an  active and a passive temperature compensation of these sensors?

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.

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Although, for most applications, the choice of sensor measuring principle or functional principle of the pressure sensor is not important, we are still often asked about how the sensor elements integrated in our pressure sensors and pressure transmitters work.

First of all, we would like to give a general definition:

Pressure sensors or pressure sensor elements are measuring elements which convert the physical quantity of pressure into an electrical quantity that is proportional to the pressure. Different physical effects and different sensor materials such as silicon, ceramic or metal are used.

WIKA uses industrial measurement’s 3 most common pressure-measuring principles, the instruments are developed in our own laboratories and also manufactured by ourselves:

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In practice, RoHS-compliant is often equated with “lead-free”. However, this is a widespread mistake. The RoHS directive 2002/95/EC aims to minimise the use of hazardous substances (including, among others, lead), but it does not completely exclude a small percentage.

One of the great obstacles in implementing the directive was the switchover to lead-free solder, because the limit values (0.1 per cent by weight of the inseparable components) do not admit a lead solder. This is probably the most important reason for the mix-up/equating that comes up again and again.

Durable, robust and easy-to-read are the requirements for the display of industrial electronic pressure switches. State-of-the-art is typically a 4-digit 7-segment LED display in red. However, in particular the display of letters on the seven segments is only possible to a very limited extent and therefore these displays are often very difficult to read. However, letters are badly needed in order to make the parameters in the menu self-explanatory. The goal is setting the fundamental instrument functions, such as the units of pressure, as intuitively as possible, without having to read the operating instructions.

Since ten segments offer no real improvement for letters, a 14-segment display is the appropriate solution – with significantly better legibility of the parameters during setup. Make the comparison yourself …

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Manufacturers of electronic pressure switches often offer both PNP and NPN switching outputs. Here is a brief explanation how the two different outputs should be connected. In principle, both are bipolar transistors in which only the internal arrangement of the pn transitions differs. This is why the load must be connected differently to the transistor outputs.

PNP switching output:
The load is connected to the switching output with GND as the reference point. When a change in signal takes place by reaching a pressure value, the supply voltage (+Ub) is “switched through”, allowing current to flow from +Ub through the transistor and through the load to GND.

Equivalent circuit diagram and wiring of pnp transistor switching output

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In connection with the electronic pressure switch PSD-30, I often get asked what is IO-Link and how it is defined. IO-Link is a new communication interface in the area of factory automation/ machine building. Leading providers of automation technology have joined together to form the “IOLink consortium” and specified this new interface. This interface can be used to exchange information on the already existing signal line all the way down to the field level, using a serial protocol. The advantage is that, for example in the case of a pressure switch, bidirectional communication is possible via the already existing 3-wire connection. The following data can be exchanged using the standardised protocol:

• Measurement values: analogue or digital
• Configuration parameters: centrally from the control system to the sensor/actuator
• Diagnostic information: from the instrument to the control system

For further details on IO-Link, please refer to the IO-Link homepage or the product page for the WIKA PSD-30 electronic pressure switch.