Pressure Measurement

Understanding Industrial Pressure Measurement: Key Concepts and Applications

1. Pressure Concept

Pressure is a basic variable in the design, construction and maintenance of industrial processes.

Pressure measurement is the primary variable for a wide range of process measurements, for example:

Flow rate (measuring pressure loss through a restriction).
Liquid level (measuring the pressure created by a vertical column of liquid).
Density of the liquid (measuring the pressure difference across a defined liquid height).

Pressure is defined as the ratio of the normal component of the force on a surface to the area of that surface.

Pressure Formula

The knife will cut better the sharper it is, because the force exerted is concentrated on a smaller area.

The skier does not sink into the snow because the force is spread over a larger area.

1.1 Pressure - Unit Conversion Table

Pressure - Unit Conversion Table

2. Fluid Concept

In general terms matter can be classified into solids and fluids. The word fluid describes something that can flow (this includes liquids and gases).

Ice Cube

Fluid

The distinction between liquids and gases is not well defined, by varying the environmental conditions it is possible to transform a liquid into a gas and vice versa.

Fluid deformation

A fluid can be defined as a substance that does not resist, in a permanent way, the deformation caused by a force, therefore it changes its shape (be water my friend).

2.1 Compressibility

We will distinguish 2 types of fluids:

Incompressible: They are little affected by pressure changes. Their density is constant.
- Most liquids are incompressible.
- Gases can be considered incompressible when the pressure variation is small with respect to the absolute pressure.
Compressible: They are affected by pressure by varying their density. Most of the gases are compressible.

Compressibility

2.1.1 Incompressible Fluids

According to Bernoulli, we can decompose the pressure of an incompressible fluid into 2 components:

Static pressure:
- All points at the same depth will be subjected to the same pressure, regardless of the shape of the vessel in which the liquid is contained.

Static pressure

Dynamic pressure:

Dynamic pressure

According to Euler's continuity equation (mass balance):

Euler's continuity equation

2.1.2 Compressible Fluids

As we know gases are highly compressible fluids.

Compressed gas

From a mechanical point of view, the fundamental difference between liquids and gases is that the latter can be compressed. Their volume, therefore, is not constant and consequently neither is their density.

Considering the fundamental role of this physical quantity in fluid statics, it is understandable that the equilibrium of gases must be considered separately from that of liquids.

Boyle's Law

“The volume of the gas contained in a container is reduced if the pressure is increased.”

Boyle's Law

The theoretical study of the flow of a gaseous fluid is beyond the scope of this presentation, as a note we indicate that the propagation of pressure within a compressible fluid is performed at speeds close to the speed of sound.

3. Pressure Types

Pressure Types

You can find more detailed information about Pressure Types in ./whats-the-difference-between-absolute-gauge-and-differential-pressure.html.

3.1 Atmospheric Pressure

It is caused by the weight of the atmosphere. It depends on climatic changes, the reference value is taken at sea level which is equal to 1013 mbar or 760 mmHg.

Atmospheric Pressure

Static pressure

3.2 Differential Pressure

It is defined as the difference between two pressures, also known as dP, pressure drop, etc....

Differential Pressure

APPLICATIONS:

Level measurement in closed tanks
Density Measurement
Flow Measurement
Interface Level Measurement
Distillation Col. monitoring Flooding
Filter monitoring
Pump and valve monitoring
Fire System Monitoring - Sprinklers
Process Feed Pressure Monitoring

Differential Pressure applications

3.3 Absolute Pressure

It is defined as the pressure referred to absolute zero pressure. It is distinguished by the subscript abs. Another way of defining it is by adding the atmospheric pressure to the relative / gauge pressure indicated by the measurement.

Absolute Pressure

APPLICATIONS:

Applications where high accuracy in vacuum measurement is required
Low pressure or vacuum applications where it is necessary for pressure control to measure the influence of atmospheric pressure.
Low pressure measurement in vacuum distillation columns
Vacuum reactors
Control of leaks in tanks and circuits

Absolute Pressure applications

3.4 Relative Pressure

It is determined by an element that measures the difference between absolute and atmospheric pressure.

It is distinguished by the subscript reg or g.

They make up 95% of the types of meters installed in the chemical industry. It can be positive (P1>Patm) or negative also known as empty (P1<Patm).

Relative Pressure

Relative Pressure Applications

APPLICATIONS:

Level measurement in atmospheric tanks.
Pressure measurement in circuits and pressure equipment where the working pressure suffers variations greater than those caused by variations due to the atmosphere.
Air conditioning and control of air and corrosive gases - Clean rooms.

4. Pressure Gauges

4.1 PI - Indicators

4.1.1 Pressure Gauges

Process control in modern industry

Pressure gauges are primary standards of most national standards bureaus.

The main advantages of these instruments are their high accuracy, low cost and simplicity, minimum span with inclined ranges from 0 to 2.5mm H2O; maximum span is 4bar.

The accuracy is 0.2 mmH2O for vertical and 0.05 mmH2O for inclined.

The major limitations are possible rupture and problems related to the contained fluid.

Pressure Gauge

Pressure gauges

4.1.2 Bourdon Tubes

In 1852 E.Bourdon patented a curved tube which, when held and pressurized at its open end, produces a movement at its closed end.

Bourdon

Bourdon Tubes

Bourdon Tube

Bourdon tubes are used to detect high pressures, not good at low pressures or vacuum, span from 1bar to 6900bar.

Their accuracy varies from 0.25 to 5% of span. They are used as PI standard in many process industries.

The main advantages are low cost, easy replacement.

Bourdon Tube

4.2 PT - Transmitters

Transmitters

These instruments are composed of two functional units:

Primary Unit: The pressure transducer (primary unit) consists of the interface to the process, the sensor and the primary electronics.

Transmitters

The primary electronics converts these variations into a digital signal that feeds a microcontroller. This performs a linearization of the primary output, compensating for sensor non-linearity, static pressure and temperature changes.
The measured values and sensor parameters are transferred to the subunit, where the communications card is located. The output data value is converted to a 4-20 mA signal.

Transmitter

Secondary Unit: It consists of the rest of the electronics, the terminal block and the enclosure.

Transmitters

4.2.1 Piezo-Resistive

It is the most popular pressure gauge.

It consists of the diffusion of resistive elements inside a single silicon chip, acting as a diaphragm.

Piezo Resistive Diagram

Piezo Resistive

Piezo Resistive Sensor

4.2.2 Capacitive

A capacitor consists of two parallel conducting plates separated by a small gap. Pressure displaces one of the plates that acts as a diaphragm.

Transmitters

Capacitive

Capacitor

4.3 Selection Criteria

Selection Criteria

4.4 Process Connection

Process Connection

5. Pressure Suppliers List

ABB Instrumentation (www.abb.com/us/instrumentation)
Barton Instruments (www.barton-instruments.com)
Brooks Instrument (www.brooksinstrument.com)
Dresser Instrument (www.dresserinstruments.com)
Endress + Hauser Inc. (www.us.endress.com)
Fisher controls (www.fisher.com)
Foxboro/Invensys (www.foxboro.com)
Honeywell (www.iac.honeywell.com)
Marsh Bellofram (marshbellofram.com)
Moore Industries (www.miinet.com)
Rosemount/Emerson (www.rosemount.com)
Siemens (www.sea.siemens.com)
United Electric (www.ueonline.com)
Youkogawa Corp. of America (www.yca.com)
Yokogawa Electric Corp. (www.yokogawa.com)

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Information and Definitions

Absolute Pressure
Absolute pressure is the total pressure measured from a perfect vacuum reference, meaning it includes both the atmospheric pressure and the pressure generated within a system. It is essential in scientific and industrial applications where precise and true pressure values are required, such as in vacuum systems, space environments, or processes sensitive to atmospheric changes. By referencing zero pressure, absolute pressure provides a complete understanding of the force exerted on a surface, independent of atmospheric variations that can affect gauge readings.

Atmospheric Pressure
Atmospheric pressure is the pressure exerted by the weight of the Earth’s atmosphere at any given point. It varies with altitude and weather conditions, with standard atmospheric pressure at sea level being approximately 1013 millibars or 14.7 psi. This pressure acts on all objects and surfaces exposed to the air and serves as a baseline for many pressure measurements. Atmospheric pressure is particularly important in meteorology, aviation, and in determining whether a measured pressure is above or below ambient air pressure.

Barometric Pressure
Barometric pressure is another term for atmospheric pressure and is commonly used in weather forecasting to assess changes in atmospheric conditions. It is measured using a barometer and helps predict weather patterns such as storms or fair weather. Barometric pressure decreases with altitude and fluctuates with temperature and humidity. Understanding barometric pressure is crucial in aviation and environmental monitoring, and it is also used to calibrate pressure instruments that operate relative to atmospheric conditions.

Differential Pressure
Differential pressure is the difference in pressure between two points within a system and is commonly used to measure flow rates, fluid levels, or pressure drops. It is vital in industrial applications such as monitoring filters, measuring liquid levels in pressurized tanks, and determining flow in pipelines. Differential pressure instruments are designed with two pressure ports and can provide insights into system performance, detect clogging or leaks, and maintain optimal operating conditions by highlighting inconsistencies or pressure changes.

Gauge Pressure
Gauge pressure is the pressure measured relative to the ambient atmospheric pressure, meaning it only reflects the pressure that exceeds the surrounding environment. Most everyday pressure instruments, like those used for tires or water systems, measure gauge pressure. If the internal system pressure equals atmospheric pressure, the gauge reads zero. This type of measurement is widely used in process industries and is practical for monitoring equipment that operates in open environments where atmospheric pressure remains constant or predictable.

Hydrostatic Pressure
Hydrostatic pressure is the pressure exerted by a fluid at rest due to the force of gravity acting on the fluid column above a given point. It is directly proportional to the fluid's density and the vertical height of the fluid column. This concept is fundamental in level measurement for tanks and reservoirs, where sensors at the bottom detect the fluid height by measuring pressure. Hydrostatic pressure is also key in hydraulic systems, dam design, and in calculating fluid forces on submerged surfaces.

Line Pressure
Line pressure refers to the internal pressure present within a pipeline, conduit, or fluid system during normal operation. It is essential for controlling the flow of liquids or gases through industrial systems and ensures that processes function efficiently and safely. Monitoring line pressure helps in identifying system malfunctions, managing energy consumption, and protecting equipment from overpressure damage. Line pressure must be carefully regulated to maintain consistent flow, prevent equipment failure, and ensure compliance with design specifications.

Overpressure
Overpressure occurs when a pressure value exceeds the design or safety limits of a sensor or system. It can lead to equipment failure, sensor damage, or safety hazards if not properly controlled. Overpressure protection is a key design consideration in pressure measurement systems and often involves safety valves or alarms. Understanding and preventing overpressure is crucial in maintaining system integrity, especially in high-pressure environments like chemical processing, oil and gas operations, or pressurized tanks.

Pressure Calibration
Pressure calibration is the process of verifying and adjusting the accuracy of a pressure measurement instrument by comparing it to a known reference standard. It ensures that sensors and gauges provide reliable and consistent readings over time, which is essential in quality control, safety, and regulatory compliance. Calibration compensates for sensor drift and environmental influences, maintaining performance in demanding applications. Regular calibration schedules are part of standard maintenance practices in industries that rely on precision measurement, such as pharmaceuticals and aerospace.

Pressure Sensor
A pressure sensor is a device that detects pressure and converts it into an electrical signal for monitoring or control purposes. These sensors come in various types, such as piezoresistive, capacitive, or strain gauge, and are designed for specific environments and pressure ranges. Pressure sensors are used in virtually every industry to measure fluid or gas pressures and are critical components in automation, safety systems, and process control. Their accuracy, range, and durability determine their suitability for different applications.

Pressure Measurement References

1 ASME PTC 19.2 - Pressure Measurement Instruments and Apparatus — Performance and calibration guidance.

2 ISO 2186:2007 - Fluid flow in closed conduits — Connections for pressure signal transmissions between primary and secondary elements.

3 IEC 61298-2:2008 - Process measurement and control devices — General methods and procedures for evaluating performance — Pressure transmitters.

4 ISA-51.1-1979 (R1993) - Process Instrumentation Terminology — Definitions for accuracy, range, and static performance.

5 NIST Special Publication 250 Series - Pressure Metrology (e.g., NIST SP 250-39) — Calibration practices and uncertainties.

6 Rosemount (Emerson) Pressure Handbook (latest edition) — Application and installation of gauge, absolute, and differential transmitters.

7 Yokogawa Pressure Handbook (2016) — Principles of pressure measurement and transmitter selection.

8 Beamex (2012) Calibration Handbook — Procedures for pressure instrument calibration.

9 API MPMS Chapter 21.1 - Flow Measurement Using Electronic Metering Systems — Guidance on pressure sensing in custody transfer systems.

10 ISO/IEC Guide 98-3 (GUM) - Uncertainty of Measurement — Applied to pressure calibration and traceability.

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Frequently Asked Questions

Q1 What is absolute pressure?

A1 Absolute pressure is the pressure measured relative to a perfect vacuum. It represents the total pressure exerted on a system, including atmospheric pressure and the pressure generated within the system itself. This measurement is crucial in applications involving vacuum systems or when very precise pressure control is required, such as in aerospace or scientific experiments. Unlike gauge pressure, which only considers pressure relative to the atmosphere, absolute pressure provides a complete picture of the actual force applied. Instruments measuring absolute pressure are typically sealed and calibrated to zero pressure at a vacuum reference.

Q2 What are common types of pressure sensors?

A2 Common types of pressure sensors include piezoresistive, capacitive, strain gauge, and piezoelectric sensors. Piezoresistive sensors use the change in electrical resistance of a material under stress to measure pressure. Capacitive sensors work by detecting changes in capacitance caused by diaphragm movement. Strain gauge sensors use metal or semiconductor strain elements bonded to a diaphragm that deforms with pressure. Piezoelectric sensors generate a voltage when subjected to mechanical stress. Each type is suited for different applications depending on factors like accuracy, pressure range, environmental conditions, and response time.

Q3 What is barometric pressure?

A3 Barometric pressure, also known as atmospheric pressure, is the pressure exerted by the weight of the atmosphere at a specific location. It changes with altitude and weather conditions. Barometric pressure is an important variable in meteorology, aviation, and environmental science. At sea level, the average barometric pressure is about 1013 millibars or 14.7 psi. Pressure sensors designed for barometric measurements are used in altimeters and weather forecasting equipment. Variations in barometric pressure help predict weather changes, such as incoming storms or fair weather conditions.

Q4 What is differential pressure?

A4 Differential pressure is the difference between two pressure readings taken at two different points within a system. It is used to monitor flow rates, filter conditions, and fluid levels. For example, in a pipeline, measuring the pressure before and after a restriction allows calculation of flow. Differential pressure sensors are designed to measure this difference accurately and often have two ports for connection. This type of pressure measurement is critical in industrial processes, HVAC systems, and fluid dynamics analysis. It ensures efficient system operation and helps detect clogs or leaks.

Q5 What is gauge pressure?

A5 Gauge pressure is the pressure measured relative to the local atmospheric pressure. It does not include the atmospheric pressure in its reading. Most pressure gauges used in everyday applications, like tire pressure or water pressure in plumbing, display gauge pressure. A gauge pressure reading of zero means the measured pressure is equal to atmospheric pressure. If the pressure is below atmospheric, the gauge shows a negative reading. This type of measurement is widely used because it reflects the pressure that actually acts on containers or machinery in a given environment.

Q6 What is hydrostatic pressure?

A6 Hydrostatic pressure is the pressure exerted by a fluid at rest due to the force of gravity. It increases with depth in a fluid column and depends on the fluid density and height of the fluid column. This principle is used in level measurement of tanks and in various hydraulic systems. Hydrostatic pressure sensors are used in applications like water treatment, dams, and reservoirs. Understanding hydrostatic pressure helps engineers design structures that can withstand fluid forces and allows accurate monitoring of fluid levels in containers.

Q7 What is line pressure?

A7 Line pressure refers to the pressure within a pipe, duct, or conduit in a fluid system. It is the operating pressure under normal conditions and is important for maintaining flow and system performance. Line pressure is commonly measured in systems like water distribution, natural gas pipelines, and pneumatic controls. It must be monitored to ensure safety and efficiency, and excessive line pressure can indicate blockages or pump malfunctions. Proper regulation and monitoring of line pressure help maintain consistent operation and protect equipment from damage.

Q8 What is overpressure in pressure measurement?

A8 Overpressure is a condition where the measured pressure exceeds the maximum rated pressure limit of a sensor or system. It can result in damage to the pressure sensor, system components, or even lead to failure. Sensors are often designed with a certain level of overpressure protection, but frequent or extreme overpressure events should be avoided. Engineers must select pressure instruments with appropriate pressure ranges and safety margins to accommodate occasional pressure spikes. Monitoring systems are also used to detect overpressure conditions and trigger safety responses.

Q9 What is the importance of pressure calibration?

A9 Pressure calibration ensures that pressure measuring instruments provide accurate and consistent readings over time. It involves comparing the output of the pressure device against a known reference standard and making adjustments if needed. Calibration is crucial in industries where precision is critical, such as pharmaceuticals, aerospace, and oil and gas. Regular calibration prevents drift, improves safety, and ensures compliance with quality standards. It also helps maintain trust in data used for process control, safety decisions, and product quality assurance. Proper documentation of calibration is part of regulatory compliance.

Q10 What units are used in pressure measurement?

A10 Pressure is measured in various units depending on the application and region. The most common units include pascal (Pa), bar, pounds per square inch (psi), atmospheres (atm), and millimeters of mercury (mmHg). In scientific contexts, the pascal is widely used, with 1 bar equal to 100,000 pascals. Psi is commonly used in North America, especially in automotive and industrial settings. Atmospheric pressure is often referenced in atm or mmHg in weather and medical applications. Selecting the correct unit is essential for interpreting data and ensuring system compatibility.