Differential pressure gauges

Differential pressure gauges are specialized instruments for measuring the pressure difference between two points, a key parameter in many industrial applications: ventilation and air handling systems (for measuring air flow with Pitot tubes, for monitoring filters and cleanrooms), level control in pressurized tanks, monitoring of pumps and fans, monitoring of heat exchangers (pressure drop), the chemical process industry and HVAC applications. They work by measuring the difference between a “+” pressure and a “−” pressure, regardless of the absolute value of either.

Architecture

Differential pressure gauges use two pressure inlets (+ and −) connected to a single bidirectional sensing element: a central diaphragm that deflects to one side or the other according to the difference, or two opposed Bourdon tubes, or two electronic cells with differential signal processing. The differential pressure may be positive (+P > −P), negative (+P < −P) or zero. The typical range extends from 0–25 Pa (for high-sensitivity cleanrooms) up to 0–100 bar (for high-pressure industrial filters), including bidirectional scales (−1000 / +1000 Pa for flow monitoring). Typical accuracy is 0.5–2% F.S.

Cleanrooms and controlled environments

One of the most widespread applications is monitoring the pressurization of cleanrooms and pharmaceutical GMP environments: the internal pressure must remain constantly positive (5–30 Pa above the surrounding environment) to prevent contaminants from entering from outside (pressure cascade). Analog dial differential gauges (Magnehelic type) or digital gauges are installed in every critical room, providing an immediate reading. Digital transmitters with a 4–20 mA output are connected to the plant BMS for alarms and automatic recording. ISO 14644-3 and EU GMP Annex 1 set out the requirements.

Filter monitoring

Another important application is monitoring the clogging of air, gas and liquid filters: by measuring the pressure drop across the filter, it is possible to identify when the filter is approaching saturation and needs replacing. Low-pressure gauges (0–500 Pa or 0–1000 Pa) are used for air filters; high differential pressure gauges (typically 0–1 bar) for industrial gas and hydraulic filters. Integration with an audible alarm or a signal to the control system allows maintenance to be managed automatically.

Flow measurement with a Pitot tube

Air flow measurement in ducts is often carried out with a Pitot tube (which measures the difference between the total pressure and the static pressure of the flow, from which the velocity is derived using Bernoulli’s equation) connected to a low-pressure differential gauge (typical range 0–1000 Pa). Differential gauges for Pitot use are often integrated directly into the probe tubes. Applications include the balancing of HVAC systems, checking the flow rate of extraction hoods, fan testing and monitoring spray booths.

Level applications

In pressurized or sealed tanks the liquid level is measured with a differential pressure gauge: one pressure tapping at the bottom and one on the pressurized headspace; the difference is proportional to the height of liquid according to h = ΔP / (ρ·g). This application is widespread in chemical process tanks, autoclaves and distillation plants, where traditional float or radar level sensors cannot be used. Dedicated transmitters have high-sensitivity cells (1–100 mbar) for accurate level measurement in tanks of modest height.

Standards and calibration

Differential pressure gauges are calibrated with high-class differential pressure generators, simulating the + and − pressures simultaneously with an accuracy better than the class of the gauge under test. For low-pressure scales, water-column or oil calibrators in micromanometers are used. The reference standards are ISO 4006 (terminology and definitions for differential pressure), EN 837 for mechanical gauges, and sector standards (ISO 14644-3 for cleanrooms, ISO 5167 for flow measurement).