Digital and analog pressure gauges

Digital and analog pressure gauges are instruments for measuring the pressure of fluids (gases, liquids, steam) in industrial, hydraulic, pneumatic, HVAC, oil-hydraulic, chemical, food and pharmaceutical process plants. They cover pressures from vacuum (mbar) up to hundreds of bar for high-pressure applications, with typical accuracies from 0.025% F.S. for primary references down to 0.5% for standard industrial instruments. Analog models (mechanical dial gauges with a Bourdon tube or diaphragm) and digital models (with piezoresistive or capacitive sensors and an LCD display) are largely complementary in industrial applications.

Analog pressure gauges

Analog pressure gauges use mechanical sensing elements: the Bourdon tube (a curved metal tube that deforms under pressure, with a linkage connected to the pointer), the diaphragm (for low and differential pressures) and the capsule (for low to medium pressures). They are rugged, self-contained (no electrical power required) and inexpensive, and are widely used throughout industry. Typical accuracy classes are 0.5%–1.6% F.S. for industrial models and 0.25% F.S. for precision models. Typical dial diameters are 50, 63, 80, 100 and 150 mm, depending on the legibility required and the space available.

Digital pressure gauges

Digital pressure gauges combine electronic sensors (piezoresistive with a silicon diaphragm, capacitive with a ceramic diaphragm, resonant for high accuracy), amplification and linearization electronics, and a numeric or graphic LCD display. The accuracy of professional models reaches 0.025%–0.1% F.S., with extended ranges (a single instrument can cover a wide span of pressures). Advanced functions include: max/min/avg recording, data logging, USB/Bluetooth/WiFi output, GO/NOGO thresholds, analog output (4–20 mA, 0–10 V) and digital output (RS-485 Modbus, HART, Profibus) for integration into industrial systems, and unit conversion (bar, psi, kPa, mmHg, mmH2O, atm).

Types of pressure

Pressure gauges differ according to the type of pressure measured: gauge pressure (relative to atmospheric pressure, the most widespread in industry); absolute pressure (relative to vacuum, for laboratory and vacuum system applications); differential pressure (between two points, for filters, ventilation and level). Gauges for special applications are certified for: ATEX intrinsic safety in explosive atmospheres; hygienic use in pharmaceutical and food processing (tri-clamp, EHEDG, FDA – with polished surfaces and crevice-free geometries); chemical resistance with PTFE/Hastelloy/titanium diaphragms; high temperature, with glycerin or silicone filling to damp vibration.

Applications

Industrial applications are all but universal: hydraulic and oil-hydraulic systems (monitoring the working pressure of pumps, valves, actuators and cylinders); pneumatics (plant compressed-air lines); chemical and petrochemical processing (pressure control of reactors, distillation columns and pipelines); HVAC (air conditioning, ventilation and heating systems); industrial refrigeration (manifold gauge sets for refrigerants); the food and pharmaceutical sector (with sanitary gauges for CIP/SIP cleaning); automotive and heavy vehicles (gauges for tires, oils and fluids); laboratory testing.

Calibration

Pressure gauges are calibrated with: deadweight testers (the primary reference – calibrated masses on a piston in a cylinder, with the pressure corresponding to weight divided by area); portable pressure calibrators (with a manual or electric pump and a high-class reference sensor); automatic pressure generators (with PID control, for the laboratory). Typical calibration intervals are annual for industrial gauges, six-monthly for precision gauges and on request for primary references. ISO 17025 certificates are required for applications demanding certified accuracy.

Reference standards

The main standards are: EN 837-1, 2, 3 (Bourdon tube, diaphragm and capsule gauges); ASME B40.100 (USA – industrial gauges); OIML R109 (legally controlled pressure gauges); IEC 61326 (EMC for electronic instruments); EN 60079 for ATEX applications; ASME PCC-1 for gauges on pressurized plant. Periodic inspection of safety gauges is required by pressure equipment safety legislation (in Italy, Ministerial Decree 329/2004 on pressure equipment).