How to choose a gauge

The user should observe some recommendations before selecting a pressure gauge. Among the advices gathered in the EN837-2 Norms we would like to highlight the following ones:

• Compatibility of materials: it is convenient to check out the endurance of the materials on a compatibility table. Most of the common service fluids such as water, air, hydraulic oil, etc are fully compatible with the wetted parts of the standard pressure gauges (usually brass and Cu-Be alloys). For aggressive fluids we suggest the all stainless steel made pressure gauges; For viscous, thick, hot, crystallizing fluids and/or with solids in suspension a diaphragm seal must be installed between the manometer and the fluid process, isolating the wetted parts of the gauge and therefore protecting it. (see Diphragm seals).

Another important caution to take in account is the oxygen service. It that case the wetted parts of the manometer should be oil free, in order to avoid risks of explosion.

• Temperature service: the pressure gauges with internal parts in Cu-Be alloys (ordinary and oil filled types) are suitable for temperature service between -20º and 65ºC. For temperature service up to 150ºC we suggest to install a stainless steel manometer (MIN type). From that temperature onwards it is necessary to isolate the manometer of the fluid process, by means of either a siphon tube, a cooling tower, a capillary tube or a diaphragm seal (see Accessories). The termic drift is around ±0,4% per each 10°C of deviation related to the temperature of reference (20°C).

• Environmental conditions: another particular to take in account. When installing outdoors a sealed manometer is recommended (st. st. cases), whereas for indoor applications a protection degree of IP32 will be enough. For outdoor installations in cold regions or applications below zero we suggest to install non oil filled manometers. For corrosive or aggressive media the stainless steel cased gauges are the most suitable of all (oil filled and all st.st. types).

• Working conditions: main service conditions to avoid:
  • Vibrations: in that case we recommend to install a oil filled gauge (MBA type), where the glycerine absorves the vibrations and makes the lecture easier, besides of reducing the worn of the moving parts.
  • Pulsation dampning caused by work of pumps or other elements, rapid pressure fluctuations may occur. To combine a liquid filled pressure gauge and a pulsation dampener should be the best solution in order to have a good reading and avoid damages and fatigue on the pressure gauge transmission and sensible parts. All MBA type pressure gauges are equipped with a fluid restrictor device. In case the above mentioned measures are not enough, we recommend to install an absorbing shock movement inside the pressure gauge, and even a capillary tube. After installation and before operation please make sure sudden pressure increases will be avoided. The installation of a needle valve will ensure a safety and progressive performance of the device. Not installing the valve is the most common failure on brand-new pressure gauges.
  • Overloads: when the pressure may exceed 1,3 times the pressure gauge range, the installation of a overpressure protector is required (see AC), whose ranges are adjustable depending on the range of the instrument. Some manometers can be equipped with an special movement (called Heav work), which enable the gauges to endure overloads up to 4 times its final scale value).
  • Working range: the pressure service should be situated within the central third area of the scale, it is, between the 35% and 75% of its final scale value.
  • Required accuracy: we will choose the instrument according to the importance of the measures of it in our application. To define the accuracy classes related to the manometers we will apply the European Norm EN 472. The accuracy classes are referred to the maximum error permitted, expressed in % of the internval of measurement. The accuracy classes are the next ones: 0.1, 0.25, 0.6, 1.0, 1.6, 2.5 and 4.0. (see table below). For manometers with zero top, the accuracy class will cover from 10 to 100% of scale. For manometers without zero top the accuracy will cover from 0 to 100% of its scale, and the zero will be used as a test control point.

Example:

Pressure gauge range 0-10 bar, accuracy class 1.0. The maximum error permitted of histeresy is 1% final scale. The difference between increasing and decreasing measurements cannot exceed at any point of its scale 0,1 bar ( =1% de 10 bar).