PRESSURE SENSORS

By

Jean Raynell Bello

Khazel Bustillo

BASIC TERMS

Pressure force applied over a surface area

Fluid any substance that can flow

Pressure sensor devices designed to accurately detect the magnitude of external applications.

Sensitivity measure of the change in output in response to a change in input

Accuracy specifies the maximum error to be expected from a device

PRESSURE SENSORS

They can alternatively be called

pressure transducers

pressure transmitters

pressure senders

pressure indicators

Piezometers

and manometers, among other names.

PRESSURE SENSORS

It is important that the pressure sensor used be able

to give accurate and precise readings as needed for a long period of time without need for maintenance

to endure the conditions of the system.

PRESSURE SENSORS

Several factors influence the suitability of a particular pressure sensor for a given process:

the characteristics of the substances involved in the process

the environmental conditions of the system

the pressure range of the process

the level of precision and sensitivity required in measurements.

THEORY

TYPES OF PRESSURE

Absolute Reference

Pre

ssu

re

0

Gage

Pressure Total

Pressure

Reduced (or Vacuum)

Pressure

Absolute

Pressure

Atmospheric Reference

(Standard Atmospheric Pressure)

Barometric

Pressure

PRESSURE-MEASURING DEVICES

There are two types of measuring devices:

Mechanical Pressure Transducer and elements

Electrical Pressure Transducer and elements

MECHANICAL

Bourdon-tube

As pressure is applied internally, the tube straightens and returns to its original form when the pressure is released

The tip moves with internal pressure and is converted with a pointer onto a scale

MECHANICAL

Bourdon-tube

Advantages:

Inexpensive

Wide operating range

Fast response

Good sensitivity

Direct pressure measurement

Disadvantages:

For indication only

Hysteresis on cycling

Sensitive to temperature variation

Vulnerable to shock and vibration

MECHANICAL

Helix and Spiral tubes

The pressure causes the tube to straighten out

The degree of its uncoiling is linked to a pointer that indicates the pressure reading

MECHANICAL

Helix and Spiral tubes

Advantages:

Increased accuracy and sensitivity

Advantage over the Bourdon-tube as there are no movement losses due to links and levers

Disadvantages:

Very expensive

MECHANICAL

Spring and Bellows Bellows made up of series

of folds which allow expansion. One end is fixed while the other moves in response to the applied pressure

Spring opposes the applied pressure and a linkage connects the end of the bellows to a pointer for indication

This device is used for ON/OFF controls

MECHANICAL

Spring and Bellows

Advantages: Simple construction

Easily maintained

Inexpensive

Disadvantages: Sensitive to temperature

variation

Work hardening of bellows

Hysteresis

Poor over range protection

MECHANICAL

Diaphragm Used as a means of

isolating the process fluids or for high pressure application

Provides pressure measurement with electrical transducers

MECHANICAL

DiaphragmAdvantages:

Simple construction

Easily maintained

Inexpensive

Disadvantages: Sensitive to temperature

variation

Work hardening of bellows

Hysteresis

Poor over range protection

MECHANICAL

Manometer

Pressure measured are applied to the open ends of the tube

Difference in height will be read as the pressure difference

MECHANICAL

Manometer

Advantages:

Simple operation and construction

Inexpensive

Disadvantages:

Low pressure range

High pressure range requires mercury

Readings are localized

MECHANICAL

Single and Double Inverted Bell

It measures the pressure difference in a compartment on each side of a bell-shaped chamber

ELECTRICAL

Strain gauge

Uses semiconductor chip to measure pressure changes

Change in pressure also causes a change in resistance as the metal is deformed

ELECTRICAL

Strain gauge

Advantages: Wide range

Accuracy of 0.1%

Small in size

Stable with fast response

No moving parts

Disadvantages: Temperature sensitive

Thermo-elastic strain causes hysteresis

ELECTRICAL

Vibrating Wire Consists of an electronic

oscillator which causes a wire to vibrate at its natural frequency under tension

As the pressure changes on the diaphragm, so does the tension on the wire which affects the frequency on the wire

Frequency changes then cause pressure changes

ELECTRICAL

Vibrating Wire

Advantages:

Good accuracy and repeatability

Stable

Low hysteresis

High resolution

Absolute, gage, or differential measurement

Disadvantages:

Temperature sensitive

Affected by shock and vibration

Physically large

ELECTRICAL

Piezoelectric

Pressure sensing is applied to crystals that becomes elastically deformed upon application

When a crystal deforms, it generates an electric charge signals

ELECTRICAL

Piezoelectric

Advantages: Accuracy 0.0075%

Very high pressure measurement

Small in size

Fast response

Self-generate signal

Disadvantages: Dynamic sensing only

Temperature sensitive

ELECTRICAL

Capacitance

Pressure measurement involves sensing the change in capacitance that results from the diaphragm movement

ELECTRICAL

Capacitance

Advantages:

Accuracy 0.01 to 0.2%

Wide range

Linearity

Fast response

Disadvantages:

Temperature sensitive

Vibration

Limited overpressure capability

Costly

ELECTRICAL

Optical Used to measure the

movement of the diaphragm due to pressure

An opaque vane is mounted to the diaphragm and moves in front of an infrared light beam

As the light is disturbed, the received light on the measuring diode indicates the position of the diaphragm

ELECTRICAL

Optical

Advantages:

Temperature corrected

Good repeatability

Negligible hysteresis

Disadvantages:

Expensive

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