Pressure Gauges

A pressure gauge is an instrument used to measure the pressure of gases or liquids within a system. It provides a visual indication of pressure levels, typically in units such as pounds per square inch (psi), bar, or Pascal (Pa). Pressure gauges are essential for monitoring and maintaining safe and efficient operation in various applications, including hydraulic systems, pneumatic controls, boilers, and industrial processes.

The most common type is the Bourdon tube gauge, which uses a curved, flexible tube that straightens under pressure. This mechanical motion is translated into a dial reading. Other types include digital, diaphragm, and capsule gauges, each suited to specific pressure ranges and media.

Selecting the right gauge involves considering factors such as pressure range, fluid type, and temperature. Regular calibration and maintenance are important to ensure accuracy and reliability, especially in critical systems where pressure deviations can lead to safety issues or equipment damage.

A diaphragm seal is used on a pressure gauge to protect it from harsh or corrosive process fluids, extreme temperatures, or clogging by viscous or solid-containing media. The diaphragm acts as a barrier between the process medium and the pressure-sensing element, isolating the gauge from direct contact while still allowing accurate pressure transmission through a fill fluid behind the diaphragm.

Diaphragm seals are especially useful in industries such as chemical processing, pharmaceuticals, food and beverage, and wastewater treatment, where cleanliness, corrosion resistance, and durability are essential. They help prevent contamination, damage to the pressure gauge, and inaccurate readings caused by buildup or crystallisation inside the sensing element.

They are also ideal for sanitary applications, where easy cleaning and sterilization are required. By using a diaphragm seal, you can extend the life of the pressure gauge, reduce maintenance, and ensure reliable operation in challenging or sensitive environments.

Pressure can be measured in various units depending on the system and region. The Pascal (Pa) is the SI unit of pressure, defined as one newton per square meter (N/m²). Since the Pascal is relatively small, kilopascals (kPa) and megapascals (MPa) are commonly used in practical applications.

Another widely used unit is pounds per square inch (psi), especially in the United States. It measures the force in pounds applied over one square inch. Bar is another metric unit, where 1 bar equals 100,000 Pascals, commonly used in industrial and automotive applications.

Other units include atmospheres (atm), where 1 atm equals 101,325 Pa, which is the average atmospheric pressure at sea level. Millimeters of mercury (mmHg) and inches of mercury (inHg) are traditional units used in medical and meteorological fields. Choosing the correct unit depends on the application, industry standards, and required precision for pressure measurement.

Absolute pressure and gauge pressure are two ways of measuring pressure, but they reference different baselines. Absolute pressure is measured relative to a perfect vacuum (zero pressure). It includes atmospheric pressure in its measurement. This means absolute pressure is always positive and is commonly used in scientific calculations and high-precision applications such as vacuum systems or barometric measurements.

Gauge pressure, on the other hand, is measured relative to the surrounding atmospheric pressure. Most everyday pressure gauges, like those used in tires or water systems, measure gauge pressure. When a gauge reads zero, it actually means the pressure is equal to atmospheric pressure, not zero absolute pressure.

The relationship between them is: Absolute Pressure = Gauge Pressure + Atmospheric Pressure

Understanding this difference is important when selecting sensors or interpreting measurements, especially in processes where vacuum or pressurised systems are involved, to avoid errors or equipment damage due to incorrect pressure references.

When using a pressure gauge, several important considerations ensure accurate and safe operation. First, select a gauge with an appropriate pressure range - ideally, the normal operating pressure should fall within 25% to 75% of the full-scale range. This avoids overloading and improves accuracy.

Compatibility with the process media is also critical. The gauge materials, especially the wetted parts, must resist corrosion or damage from the fluid or gas being measured. In harsh or sanitary environments, consider using a diaphragm seal to protect the gauge.

Temperature can affect accuracy and damage internal components, so ensure the gauge is rated for the process and ambient temperatures. Vibration and pulsation can also impact readings or wear out the gauge; using liquid-filled gauges or snubbers can help dampen these effects. Regular calibration and maintenance are essential for long-term accuracy. Finally, proper installation orientation and ensuring no blockages in the pressure port are key to reliable readings.

Metric, NPT, BSPP, and BSPT are different types of screw threads used for pipes and fittings, each with distinct characteristics. Metric threads are standardized by the International Organization for Standardization (ISO). They have a specific pitch and diameter measured in millimetres and are used globally for general mechanical fastening.

NPT (National Pipe Thread) is a U.S. standard with a tapered thread design, meaning the thread diameter decreases along the length. This taper creates a tight, leak-resistant seal when threaded into a fitting, commonly used in plumbing and industrial applications.

BSPP (British Standard Pipe Parallel) threads are parallel (straight) and used mainly in the UK and Commonwealth countries. BSPP fittings seal using an O-ring or gasket rather than the threads themselves.

BSPT (British Standard Pipe Tapered) threads are tapered like NPT but follow British standards. BSPT threads form a seal by wedging the threads together, similar to NPT, but are not interchangeable with NPT due to differences in thread angle and pitch. Understanding these differences is vital for proper fitting and leak prevention.