The Basics Of Pressure Measurement

Sep 2

6 min read

Pressure is a critical factor in manufacturing and process industries. It provides the force that pushes or pulls fluids and gases through a facility and is a key metric used to control processes and ensure safety.

Industrial pressure transmitter used for pressure measurement and process control

In order to control pressure, it’s essential to be able to measure it. That is the role performed by pressure transmitters, mechanical devices that measure the expansive force of liquids and gases. Pressure measurements are expressed as the amount of force per unit of surface area required to stop a liquid, gas, or vapor from expanding, as in pounds/square inch and pascal (one newton per square meter). This document provides a primer on pressure transmitters, including the most common types/technologies, a review of some key components, and an overview of common applications.

Pressure transmitter features and types

Some people use the term transducer and transmitter interchangeably. There are similarities in that they both rely on an internal sensor that converts the applied force into an electrical signal used to derive the pressure measurement. But there are significant differences that make one or the other a clearly better choice for a particular application.

Signal Level: Transmitters typically produce higher signal levels, enabling them to communicate over longer distances, and are able to relay information via fieldbus protocols such as FOUNDATION Fieldbus, Profibus, or HART.

Durability: Transmitters are generally more suitable for harsh and/or industrial environments. Transducers are more often used in labs or niche applications.

Size: Transducers are more compact, but may be too small to be easily be fitted to standard industrial piping.

Environment: Transducers must be protected from contaminants like water or dust. Transmitters are better able to tolerate harsh environments and often carry hazardous-area approvals not available on transducers.

Process variation: Transmitters are more tolerant of wide swings in process conditions and better able to withstand overpressure occurances.

Adjustability: Transmitters provide higher levels of adjustability to accommodate process or measurement requirements across a wider span. This calibration performance range is usually referred to by such terms as “scale down”, “span reset” or “turn down”. For instance, a transmitter with a measuring range of 0 to 400 psi and a range reset 1/10 can be calibrate to a measuring range of 0 - 40 psi while still giving a full output signal (4 - 20 mA, for example).

Transducers are often fixed-range devices where only a fraction of the output signal is relevant to the desired measurement span.

Accuracy: Transmitters provide higher levels of accuracy.

In addition, the secondary electronics package incorporated in many transmitters are highly sophisticated and perform many functions that transducers cannot. For example, variations in process or ambient conditions measured by the primary sensor can be compensated.

Transmitter types

There are basically three types, but multivariable is a fourth, less common type that will also be discussed.

Gauge pressure transmitters

These transmitters compare the gas or liquid pressure relative to atmospheric measure (total pressure - atmospheric pressure = indicated pressure). these are the most-commonly used pressure-measurement devices. Gauge pressure transmitters expose one side of a diaphragm toambient pressure and the other side to the liquid or gas being measured. the flexing or distortion of the diaphragm is converted into an electrical signal. When the gauge pressure is higher than atmospheric pressure, that is referred to as positive pressure. When gauge pressure is lower than atmospheric pressure, it is negative pressure or vacuum.

Absolute transmitters

Absolute pressure transmitters compare a liquid orgas pressure relative to absolute pressure (a perfectvacuum or zero pressure, 0 psia). Absolute pressure is, therefore, always a positive value. These transmitters are designed for applications that require readings not influenced by changes in altitude or weather. They are frequently used in applications using highperformance vacuum pumps, turbines, or where ideal gas properties are critical.

Differential transmitters

These devices indicate the difference or differential between two pressure points. They typically consist of a dual-sided diaphragm, sensing pressure on both sides simultaneously and then calculating the difference between them. These transmitters are commonly used to measure level of liquids and liquefied gases in pressurized tanks, and to measure flow in combination with a differential pressure flow element like an orifice plate or venturi. They can detect differential pressures of only a few millibars.

Multivariable transmitter

A multivariable transmitter captures multiple parameters in a single device. They simultaneously measure differential pressure, static pressure and external temperature all in one device. They are commonly used when flow measurement must be adjusted for temperature and/or pressure. The main advantage of these transmitters is that one instrument does what would otherwise require several transmitters.

Transmitter technologies

A variety of sensing technologies are at work in pressure transmitters. In many applications, and

one of several different technologies will provide satisfactory results. Some technologies, though, are especially well- or unsuited to certain applications.

Piezoelectric crystal: The force (pressure) on the face of a piezoelectric crystal sensor creates an electric charge that is proportional to the pressure. They have excellent dynamic sensitivity, able to detect and measure even changes in pressure over a range of pressure levels.

Inductive: This technology relies on electromagnetic induction, the creation of an electric current by the movement of a conductor (wire coil) in the diaphragm relative to a magnet/magnetic field. While there are various designs, these devices typically consist of a coil, a movable magnetic core, and the pressure-sensing element attached to the core. As pressure varies, the element moves the core inside the coil. There is an AC voltage applied to the coil so that, as the core moves, the inductance changes. Inductive transmitters are particularly well-suited to harsh environments, and are able to withstand shocks and vibration.

Pressure transmitter technology for measuring pressure in industrial process applications

Bridge (Wheatstone bridge or strain gage): In these sensors, the electrical signal is created by the deformation of strain gages (resistors) that are bonded to the diaphragm. “Wheatstone bridge” refers to the design or configuration of the electrical circuit that generates the signal. This technology is usually used for high-pressure applications.

Pneumatic: These transmitters incorporate a bellows capsule that is part of a force-balance measurement system. As the bellows compresses under pressure, an attached bar moves, changing the gap or clearance between a flapper and a nozzle. The changes the output pressure from the pneumatic relay to a feedback bellows. The output pressure continues to adjust until the force on the feedback bellows matches the bellows capsule. This output pressure is proportional to the force applied to the bellows capsule, providing the measured pressure value.

Field termination

Transmitters are available that provide a range of field terminations. Device choice depends on a number of application requirements, including whether the process or operation being measured requires simple pressure measurement indication or digital process data for incorporation in an automation or SCADA scheme.

Typical termination choices include:

4-20mA + HART
1-5V + HART – Low power consumption for remote battery or solar power
WirelessHART– Enables adding devices to existing systems without new wiring
Modbus
FOUNDATION Fieldbus
Profibus PA

Transmitter types

Some pressure-measurement applications require devices beyond standard transmitters. This could include processes with fluid or gasses that are:

Highly corrosive
Dirty, viscous or laden with solids that can block or foul the impulse lines
Likely to solidify in the impulse lines or the transmitter body
Subject to extremes of temperature

In this situations, remote seals provide an alternative solution, isolating the transmitters from conditions that will either shorten their operational life or dramatically affect their performance. They consist of a transmitter body, a capillary and a seal element incorporating a liquid-filled diaphragm. Special seal coatings or metals offer further protection from process hazards. Only the parts actually in contact with the process need to be made from such materials, greatly reducing device costs.Remote seal transmitters also provide an ideal solution for hygienic processes, such as in the pharmaceutical and food and beverage industries, where standardized process connections and food safe materials are in direct contact with the product.

Applications

The range of applications for pressure transmitters in virtual limitless, but each industry has a short list of the most-common uses.

Oil & Gas