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 As soon as additives have been added, these protection types are no longer valid or apply only to a limited extent.

In connection with the pressure equipment directive 97/23/EC (PED), a pressure limit of 200 bar is often mentioned. But what does this limit mean for manufacturers and/or companies that place pressure sensors, pressure transmitters and pressure equipment on the market?

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.

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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For the pressure transmitter IS-20 with the ignition protection type ia (intrinsically safe) and ATEX approval, the safety-relevant data for SIL applications have been determined and summarised in the additional instructions “safety-related data”. These include the Average Probability of Failure on Demand (PFD_a), the Hardware Fault Tolerance (HFT) and the Safe Failure Fraction (SFF).

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).

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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Due to the worldwide rising energy costs, the energy-efficient operation of refrigeration systems becomes a must. According to estimates, more than 90% of the refrigeration systems worldwide are not yet equipped with continuous control. They have been optimised and set for a certain operating state/refrigeration capacity – which means that they operate outside the optimum in all other operating states, whenever less capacity is required. This results in a waste of valuable energy.

In the future, systems without closed-loop control will no longer be acceptable. This is why manufacturers of refrigeration systems are increasingly using electronic pressure sensors, in order to control pressure and thus the resulting temperature of the refrigerant with high precision. The pressure sensors R-1 and AC-1 have been developed specifically for this use and in practice achieve substantial savings in electricity costs for the operators.

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.

Unfortunately, we are asked too rarely regarding the specification of the IP 68 rating for our pressure sensors and submersible pressure transmitters, i.e. how deep such an instrument may be submersed. However, this is not only a very meaningful question but also a mandatory one, because, contrary to almost all other IP ratings, the IP 68 rating is only described and not specified by means of concrete values in the IEC 60529:

1. The first digit, the number “6″, means that the instrument has total dust ingress protection.
2. The second digit, the number “8″, means that the instrument is suitable for permanent submersion in water.

However, the maximum submersion depth of such an instrument is not described. This makes total sense because there is a huge difference between submersing an instrument in a 1 m high water tank or using an instrument for measurements in a depth of 300 m on the ocean ground. According to the standard, the manufacturer and the user must therefore comply with the requirements for IP 68. Anyway, the IP 68 rating must always be better than the IP 67 rating, that means that the instrument may be submersed at least 1 m.

In practice, it means that we indicate different immersion depths for our pressure transmitters and level probes depending on the area of application. The immersion depths range from 3 m to 300 m maximum. I.e. completely different submersion depths are specified depending on the application.

If you are looking for the suitable solution for your application, just ask your contact person for advice.

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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