There are four types of pressure measurements:

Measurement of absolute pressure: measurements are made with respect to absolute pressure, for example as with atmospheric pressure.

Measurement of relative or effective pressure: measurements are made with respect to ambient pressure, generally atmospheric pressure.

Measurement of differential pressure: this is the difference between two pressures.

Measurement of vacuum pressure
: this is referred to ambient pressure, usually atmospheric pressure.

One can also distinguish pressure measurements according to their behaviour with time:
Static or quasi-static pressure, which varies only slowly with time.
Examples are atmospheric pressure, level measurements and certain pressures in industrial processes such as in the petrochemical, food and pulp and paper industries.
Fluctuating pressure, which can be random or cyclic. Examples are the pressure inside a gun barrel, in a plastic injection mould, in an internal combustion engine cylinder, etc.


Mechanical apparatus indicating the value of the local pressure

1. Bezeiring
2. Window
3. Window gasket
4. Dial
5. Movement
6. Tube
7. Case
8. Blow out disc
9. Connection
10. Connection gasket
11. Pointer

The tube is displaced in proportion to the applied pressure, which causes the pointer to move by means of a mechanically amplifying linkage.

_________ position of tube at P = 0

- - - - - - - - position of tube at P measured

Components of a metallic pressure gauge:
BOURDON tube* or «C» tube:
The sensing element consists of a metallic tube of various cross sections
shaped in the form of a «C». One end is fixed and connected to the pressure circuit and the other is sealed and free to move.
When pressure is applied to the tube the free end is forced outwards.
(Measuring range from around 400 mbar to 60 bar).
* invented by E. Bourdon in 1849

The sensing element consists of two thin corrugated
sections laser welded together. The capsule thus formed
acts as a cavity that slightly deforms with variations in
pressure. It is suitable for measuring very low pressures.
(Measuring range from 0 to about 600 mbar).

Helicoil tubes (or pig tails) :
The coil is cylindrical. It has the same characteristics as a
spiral tube but with greater resistance to fatigue due to
better stress distribution.
(Measuring range from about 60 to 3000 bar).
The sensing element consists of two flanges connected to
a flexible cylinder. Pressure applied to the interior of the
chamber causes it to deform by an amount proportional to
the pressure. They are mainly used in differential pressure
gauges and pressure switches.
Spiral tubes:
These use the same principle as the «C» tube but with the
deflection of the free end of the tube amplified due to the
spiral shape.
They are mainly used in gas expansion thermometers.

Accuracy is the maximum allowable error expressed as a percentage of the measurement span P.
Accuracy does not apply in a region P/10 1O from the zero point for pressure gauges and vacuum gauges; for compound gauges it does not apply in a region P/20 on either side of the zero point. It applies to a calibration carried out on a new equipment, at ambient temperatures between + 18°C and + 28°C before shipping. It includes repeatability, hysteresis (reversibility).

Accuracy classes
The accuracy class of an instrument is defined conventionally as the greatest value of the maximum allowable error over the specified measurement range.

The difference between a Directive and a Regulation needs to be explained:
A Directive is a group of laws, issued by the European Union, which all products/services sold in the European Union have to respect. A Regulation defines specific requirements that a manufacturer may apply to its products/services. Regulation is voluntary, never compulsory.
According to the Directive 97/23/CE , pressure gauges are classified as pressure accessories and in order to determine whether the CE symbol is required the following criteria must be evaluated:
- Bottom full scale pressure.
- Internal volume.
- Types of fluids for which pressure gauge is projected.
Characteristics of Italmanometri’s pressure gauges are the following:
- Pressure of bottom scale: from -1 bar until 1,000 bars.
- Internal volume: < 0.1 l.
- Types of fluids for which pressure gauge is designed: liquids and gas.
According to the Directive 97/23/CE , our pressure gauges are not subject to obligatory CE marking and have to be designed and manufactured complying with current EU Countries technology rules
aiming to guarantee safety during operation.
Exception: all pressure gauges designed to control pressure of dangerous gasses listed in Directive 67/548/EC are subject to obligatory CE marking if their bottom scale pressure doesn’t exceed 200 bars.
EN 837-1 Bourdon tube pressure gauges. Dimensions, metrology and assay. Aim and application field: this European regulation defines the requirements of pressure, vacuum and compound gauges with Bourdon tube circular,
spiral or helical indicators, of nominal dimensions between 40 and 250, used for the measurement of relative pressures until 1,600 bars.
EN 837-2 Recommendations for choosing and installing a pressure gauge.
EN 837-3 Membrane and capsule pressure gauges. Dimensions, metrology, requirements and assay
UNI EN ISO 5171 Pressure gauges used in soldering, cutting and similar processes.
Object and application field: this European regulation defines requirements of Bourdon type pressure gauges used in soldering, cutting and similar processes for measuring compressed gas pressure up to 300 bars, melted acetylene and liquefied gas pressure.
In all above cases, U.E. Directives or Regulations will be printed on the device, either with the CE symbol (in case of U.E. Directives) or by indicating the Regulation number (for example EN -837 or UNI EN ISO).

(Pressure Equipment. Directive) European Directives regulating pressure measurement instruments became effective as of 30.05.2002. Here is a summary of the instructions for this purpose:
Pressure gauges with bottom scale values exceeding 0.5 bar, being maximum pressure components, must comply with DGRL 97/23/EG (regulations for pressure instruments) and related requirements.
As their use conditions are not completely known, we design and manufacture our pressure gauges according to the most rigorous rules (class I gas). That means that our pressure gauges are marked
with the CE symbol starting from bottom scale value of 200 bars, according to conformity procedures. Pressure gauges with connection flanges exceeding DN 25, starting from bottom scale value of 0.5 bar are also marked with the CE symbol.
The CE symbol is shown on the external part of the package.
A Conformity Certificate is issued on request.
Pressure gauges with a bottom scale value lower than 0.5 bar, as well as possible separators are not subject to DGRL regulations and cannot be marked CE.
Pressure gauges with a bottom scale value between 0.5 and 200 bars must only comply to good engineering practice rules (Art. 3 – Paragraph 3) and do not have to be marked CE .
No CE symbol is required for pressure gauges without the manufacturer’s name and brand.
In order to protect possible overpressure within allowed limits (“common components with safety functions”), pressure gauges applied to safety devices, have to be considered separately.
Our pressure gauges are manufactured and tested according to European Regulation EN 83 7.

We can currently confirm that all our pressure gauges RoHS i.e., spring coils, stainless steel membrane bellows, membrane and differential gauges …comply with European Directives.

The features and measurements of the equipment listed in this catalogue are not binding. For technical reasons, ITALMANOMETRI s.r.l. reserves the right to make any modifications considered opportune.


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