How are absolute and relative pressure measuring ranges distinguished?

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”.

For some years now, it has been possible to increasingly automate production processes, and by doing to to achieve substantial cost reductions in some cases. This raises the question: does it still make sense to use mechanical pressure measuring instruments?

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.

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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.

Unfortunately, we are asked too rarely regarding the specification of the IP68 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 IP68 rating is only described and not fully 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 specified. Thismakes 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 agree on the exact specification for IP68. Anyway, the IP68 rating must always be better than the IP67 rating, that means that the instrument may be submersed for at least 1 m.

In practice, it results in totally 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 for the same IP rating but depending on the application.

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

For more information please see our article “Definition of IP protection types“

The response time of pressure sensors is reflected in a large number of varying parameters, such as the response time, settling time or rise time in specifications or data sheets. In general, it can be assumed that the response time is defined as the interval required by the output signal of a pressure sensor  to display a change in the applied pressure. Of greatest practical relevance is the so-called rise time.

The graphic shows a simplified diagram of a steplike change in pressure (shown in blue) with a time-delayed change in signal of the pressure sensor (shown in red).

For the sake of simplicity, the picture only shows an ideal situation. In reality, the response time of pressure sensors contains further influencing factors, such as dead time or overshoot, due to their particular constructive setups.

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The two definitions of MTTF and MTBF differ only slightly.

Written out, the two acronyms read as follows:

Mean Time To Failure  (MTTF) – i.e., the component/instrument will be replaced, following a failure

Mean Time Between Failures (MTBF)–  mean time between (two) failures, i.e., the component/instrument will be repaired, following a failure

Both are purely statistical parameters and are used to represent the reliability of (electronic) components. However, MTTF and MTBF must not be mixed up with the average service life.

For pressure sensors and pressure transmitters, it is customary to use the MTTF values, as these instruments are normally not repaired, following a failure, but completely replaced.

To obtain the MTTF values of WIKA pressure sensors, you can request them from your local WIKA Customer Support.

Since temperature affects the measuring accuracy of a pressure sensor, there always remains a small temperature error in the rated temperature range despite a wide range of compensation measures. This error is often expressed in the data sheets of manufacturers of pressure sensors as temperature coefficient (abbr. TC). This coefficient describes a (linear) error, starting from a reference point, which in most cases is room temperature. Accordingly, the temperature error at room temperature is zero and increases with increasing difference of the temperature from room temperature with the specified coefficient in linear fashion (see figure).

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What exactly is the difference between the 4-20 mA output signal in 2- and 3-wire technology?
This question is answered best from the viewpoint of the user:

An output signal in 2-wire technology means:

• less wiring required
• better EMC protection, since interferences can be filtered more easily
• better protection against wiring errors

The only advantage of 3-wire technology is that higher ohmic loads are possible, i.e., the current loop can also be operated on a measuring instrument of relatively high input impedance.

Conclusion: With the exception of a high ohmic load requirement, 4-20 mA in 2-wire technology offers the user clear advantages- also over other signals, such as 0-10 V.