Types of pressure and their significance

Along with temperature, pressure is one of the most important physical variables in modern technologies. It is defined as the force acting perpendicular to a surface, and its unit in the SI system is the pascal. Other commonly used units of measurement include pounds per square inch or bar.

In the world of pressure measurement technologies, there are various types of pressure that differ from each other by their reference point (reference pressure). Absolute pressure, relative pressure, sealed pressure, and differential pressure are all types of pressure. KELLER piezoresistive pressure sensors measure pressure changes by deforming the membrane of the sensitive element. This deformation results from the difference in pressure between the front process side of the membrane and the rear reference side. Depending on the type of pressure, the reference side is either open to the environment or isolated from it.

Absolute Pressure

With absolute pressure, the process pressure is measured relative to a vacuum. During manufacturing, the rear reference side of the membrane inside the sensor element is exposed to a vacuum and sealed. If atmospheric pressure acts on the membrane, the sensor measures the barometric atmospheric pressure. All fundamental physical formulas related to pressure are based on absolute pressure data.

Unit of pressure measurement: bar abs.

Reference pressure: 0 bar abs. (vacuum)

KELLER designation: PAA (e.g., PAA-23SX)

Typical application areas:

• Barometers
• Vacuum packaging processes
• Vacuum pump monitoring
• Steam pressure measurement
• Level measurement in sealed containers (with two sensors)

Relative Pressure (Gauge Pressure)

Relative pressure, also known as gauge pressure, is the process pressure measured relative to atmospheric pressure. In other words, it is the difference between the process pressure and the prevailing atmospheric pressure. Atmospheric pressure is influenced by both altitude and weather, so it is subject to constant changes. Therefore, measurements using relative pressure are well suited for processes affected by changes in atmospheric pressure. Relative pressure sensors always have a vent hole on the rear side of the measuring cell so that atmospheric pressure serves as the reference point for the sensitive element inside the housing. Relative pressure is the most common type of pressure for pressure sensors and is used in virtually all fields and industries.

Unit of pressure measurement: bar rel.

Reference pressure: atmospheric pressure

KELLER designation: PR (e.g., PR-33X) 

Typical application areas

• Hydrostatic level measurements
• Pneumatic systems
• Air compressors
• Process control pumps

Sealed Pressure (Absolute Compensated)

A special form of gauge pressure is sealed pressure, which combines the principle of relative pressure measurement with the advantages of a sealed sensor element. This type of pressure is especially useful in harsh operating conditions where ventilation of the measuring cell is not possible. Sealed pressure refers to a predefined reference pressure. For calibration purposes, KELLER usually uses 1 bar abs. as the reference point. Since sealed pressure sensors are isolated from the environment, they cannot compensate for changes in atmospheric pressure. Depending on the application conditions, the measurement error may be negligible, or additional atmospheric pressure measurement may be required for compensation. For such gauge pressure sensors, KELLER uses absolute sensitive elements, as this prevents increased influence of temperature changes on the sensor signal.

Unit of pressure measurement: bar

Reference pressure: 1 bar abs.

KELLER designation: PA (e.g., PA-23SY)

Typical application areas

• Measurement of gauge pressure in harsh conditions where ventilation of the measuring cell is impossible or undesirable.
• Use at high pressure: since the measurement error caused by changes in atmospheric pressure is negligible compared to the accuracy of the manometer.
• Measurements of hydrostatic level when there is a risk of moisture ingress or flooding in the area around the cable end.

Differential Pressure

When measuring differential pressure, the difference between two process pressures is measured. Therefore, pressure gauges for measuring differential pressure always have two connections. Differential pressure measurements can be performed either with a single separate sensor (bidirectional) or with two separate sensors whose signals are subtracted from each other. With only one sensor, the smallest pressure difference can be measured even if the line pressure (system pressure) is very high. Using two sensors, it is possible not only to measure differential pressure but also to determine the absolute pressure in the line.

Pressure unit: diff. bar.

Reference pressure: second process pressure

KELLER designation: PD (e.g., PD-39X)

Typical application areas

• Filter monitoring
• Flow measurement
• Measurement of fill level in sealed tanks
  
  

Overview of pressure types

For more information about the instruments, you can contact IZMERKON by phone at +7 (812) 309 56 05 or via the feedback form.