3.1 Pressure Measurement Instruments

Pressure Measurement Instruments

Pressure measurement is one of the most fundamental requirements in process control. Pressure is defined as force per unit area ($P = F / A$). In industrial applications, pressure is typically measured in pounds per square inch (psi), Pascals (Pa), or inches of water column (inH2O). Understanding how these measurements are obtained, the operating principles of various sensors, and their appropriate applications is critical for any CCST Level I technician.

Elastic Element Sensors

Elastic element sensors convert fluid pressure into mechanical motion. They rely on the principle that a metal or elastomeric material will deform predictably when subjected to pressure. This deformation is then linked mechanically to a pointer on a dial or electrically to a transmitter.

Bourdon Tubes

The Bourdon tube is perhaps the most ubiquitous pressure-sensing element in industrial gauges. Invented by Eugene Bourdon in 1849, it consists of a flattened metal tube sealed at one end and open to the process fluid at the other. As pressure increases, the tube attempts to regain its original circular cross-section, causing the tube to uncoil or straighten slightly.

There are three primary configurations of Bourdon tubes:

1. C-Type: The most common form, bent into an arc of roughly 250 degrees. It is widely used for general pressure indication ranging from 15 psi to 10,000 psi.
2. Spiral: The tube is wound in a flat spiral. This provides a larger tip movement for a given pressure change, eliminating the need for complex mechanical amplification (pinion and sector gears) and increasing sensitivity.
3. Helical: The tube is wound like a spring in a helix. Helical Bourdon tubes are commonly used in high-pressure recording instruments because they provide high torque and substantial tip travel.

Diaphragms

A diaphragm is a thin, flexible circular plate that bulges under pressure. Diaphragms are excellent for measuring relatively low pressures (fractions of an inch of water to a few hundred psi) and are often used to isolate sensitive measuring elements from corrosive process fluids (chemical seals). They can be flat or corrugated; corrugated diaphragms offer greater linear displacement.

Bellows

A bellows is an expandable, accordion-like metallic cylinder. As pressure is applied to the inside (or outside) of the bellows, it expands (or contracts) linearly. Bellows are highly sensitive and are typically utilized for low-pressure applications (up to roughly 100 psi). They provide a large amount of mechanical force and motion, making them ideal for driving pneumatic transmitters or electrical switches.

Electronic Pressure Transmitters

While elastic elements are great for local indication, modern process control relies on electronic pressure transmitters to send signals (usually 4-20 mA) back to a controller or DCS. These transmitters employ various sensor technologies to convert mechanical deformation into an electrical signal.

Capacitive Cell Transmitters

In a capacitance-based pressure transmitter, a central sensing diaphragm sits between two rigid capacitor plates. The process pressure (or differential pressure) causes the sensing diaphragm to deflect toward one plate and away from the other. This changes the capacitance on both sides (since capacitance is inversely proportional to the distance between the plates). An electronic oscillator circuit detects this change in capacitance and converts it into a proportional electrical signal. Capacitive transmitters are renowned for their high accuracy, stability, and broad rangeability.

Piezoresistive / Strain Gauge

Strain gauges rely on the piezoresistive effect—the change in electrical resistance of a material when it is mechanically stretched or compressed. In a piezoresistive pressure transmitter, tiny strain gauges are bonded to or diffused into a silicon sensing diaphragm. When pressure deflects the diaphragm, the strain gauges deform, altering their resistance. These gauges are typically arranged in a Wheatstone bridge circuit to maximize sensitivity and compensate for temperature variations.

Resonant Wire Transmitters

A resonant wire transmitter operates on the principle that the resonant frequency of a stretched wire changes with tension (similar to tuning a guitar string). Process pressure is applied to a diaphragm, which changes the tension on a wire oscillating in a magnetic field. An oscillator circuit maintains the wire's vibration and measures its frequency. As pressure increases, wire tension increases, raising the resonant frequency. This frequency shift is converted into a standard 4-20 mA output.

Differential Pressure (DP) Cell Construction

Differential pressure (DP) transmitters measure the difference between two pressures: High (H) and Low (L). The construction typically involves an isolation diaphragm for each port. The process fluid presses against the isolation diaphragms, transferring the pressure via a fill fluid (such as silicone oil) to a single internal sensing diaphragm. This fill fluid protects the delicate sensing element from harsh process media and extreme temperatures. DP cells are incredibly versatile, used not only for differential pressure but also for inferring flow rate (across an orifice plate) and liquid level (in closed tanks).

Manometers

Before electronic transmitters, manometers were the primary standard for pressure measurement, and they remain invaluable for calibration and low-pressure readings. They measure pressure by balancing the process pressure against a column of liquid (like water or mercury).

U-Tube Manometer

The simplest form is a clear U-shaped tube partially filled with liquid. One leg is connected to the process, and the other is open to the atmosphere (for gauge pressure) or another process point (for differential pressure). The pressure difference is directly proportional to the difference in height ($h$) of the liquid levels in the two legs: $P = h \times \text{fluid density}$.

Inclined Manometer

To measure very low pressures accurately, an inclined manometer is used. One leg of the U-tube is tilted at an angle. A small vertical change in pressure forces the liquid to travel a long distance along the inclined tube, greatly amplifying the reading scale.

Well-Type Manometer

The well-type (or reservoir) manometer has one very large cross-sectional area leg (the well) and one narrow transparent tube. When pressure is applied to the well, the liquid level in the well drops very slightly, while the level in the narrow tube rises significantly. This allows the technician to read the pressure from a single scale next to the narrow tube without having to calculate the difference between two moving levels.

Installation Considerations: Impulse Lines and Manifolds

Proper installation is critical for accurate pressure measurement. The tubing connecting the process to the transmitter is called an impulse line. For liquid service, impulse lines should slope downward to the transmitter so any trapped gas can bubble back up into the process. For gas service, they should slope upward so any condensed liquid drains back into the process line.

Manifold valves are used to isolate, equalize, and vent pressure transmitters for maintenance and calibration.

• 3-Valve Manifold: Commonly used for DP transmitters. It consists of a High-pressure block valve, a Low-pressure block valve, and an Equalizing valve. Manifold operation must follow the site procedure and the manifold/transmitter drawing. A common isolation sequence closes the high-side block, opens the equalizer to remove differential pressure, and then closes the low-side block before controlled venting; blindly closing both blocks first can trap differential pressure.
• 5-Valve Manifold: Adds two vent/bleed valves to the 3-valve design, allowing the technician to easily drain the impulse lines or apply calibration test pressures without breaking tubing connections.

Equalization Procedure (3-Valve Manifold)

The equalizing valve connects the high- and low-pressure sides so the sensing cell sees approximately zero differential pressure. Exact block/equalize/vent order depends on the installed manifold, process service, and whether the task is an in-service zero check or full isolation. Review the P&ID, manifold drawing, and site procedure; obtain operations authorization; wear the specified PPE; move valves slowly; verify pressure at the vents; and never loosen tubing until both sides are isolated, depressurized, drained or purged, and proven safe.

Summary Table: Pressure Sensors