Electricity + Control June 2018

PRESSURE + LEVEL MEASUREMENT + INSTRUMENTATION

Legend P FSV = Full scale value (pressure at the end of the pressure gauge scale) PS = “Maximum permissible pressure” – defined by the manufacturer, the highest pressure for which the pressure gauge has been designed. With non-full-scale loadable pressure gauges: PS = 0.75 * PFSV, otherwise: PS = PFSV (a) Dependent on pressure range (b) The change of the class accuracy is measured after an additional load cycle test (not explained here). A specification about the allowed class ac- curacy change after overload only (without load cycle test) does not exist. The table values must therefore be viewed as the absolute maximum. (c) For non-full-scale loadable instruments, the fol- lowing applies: Overload = 1.43 * PS = 1.43 * 0.75 * PFSV = 1.07 * PFSV Conclusion When selecting a Bourdon tube pressure gauge, overload safety is an important parameter to take

into consideration. Depending on the structural de- sign of the selected instrument, the overload safe- ty will be different. Pressure gauges can cope with slight and short-term overloads. The indication accuracy generally is maintained or only worsens slightly. A high overload with up to 2.5 times FSV manifests itself in pressure gauges through an in- creasing deterioration of the indication accuracy and a clearly visible zero point error. However, the pressure-bearing components do not burst and the instrument remains sealed. Sources: (1) EN 837-1, sect. 9.4.1: Pressure gauges for a maximum static pressure load of 75 % of the full scale value. (2) EN 837-1, sect. 9.4.2: Pressure gauges suita- ble for maximum steady pressure load equal to the full scale value. (3) Directive 2014/68/EU, annex 1, point 7.4 Hydro- static test pressure.

Mark Syguda is a Product Manager at Wika Instrumentation Visit www.wika.com