Instrumentation Basics -01- Pressure Measurement

Basic Instrumentation Course

Prepared by

Eng\ Ahmed Mohamed Abdel-Halim

EMC, MIDOR Site

2008

Introduction Instrumentation is the art of measuring the value of some

field parameter, pressure, flow, level or temperature to produce a dial indication or a standard (4 - 20 mA) electronic signal - (3 – 15 psi) pneumatic signal - that represents the value of field parameter in a process.

This signal can be used to directly control other instruments, or sent to a computerized controller system where it can be interpreted into readable values, or used to control other devices and processes in the system.

Instrumentation engineering is the engineering specialization focused on the principle and operation of measuring instruments which are used in design and configuration of automated systems.

1. Pressure Measurements

1- Pressure Measurements General theory (Pascal's law)

Pressure units are pounds per square inch (PSI), (bar) or millimeter water (mmH2O).

Atmospheric pressure = 14.6 psi

= 1.01325 bar

= 10000 mmH2O

Pressure Scales

Gauge and Absolute pressure

Pa = Pg + Patm (when Pg > Patm)

Pa = Pg – Patm (when Pg < Patm)

Pressure In A Fluid Liquids are uncompressible. For an opened vessel,

Pa = Patm + ρgh Pascal's law

Pressure In A Fluid Pascal's law indicates that the shape of the open tank

does not affect the pressure and that the pressure is influenced only by the depth (h) and the density (ρ).

Application: Hydraulic press. The pressure at every point on the same line is equal:

P1 = P2

So that F2 = (A2 / A1) F1

The hydraulic press is a force amplifier where the gain

is (A2 / A1)

The Hydraulic Press

Pressure Exerted by Gases

General law for gases

PV = KT

At const. temperature (Boyle’s law)

P = (K / V) P α (1 / V)

At const. volume (Charle’s law)

P = K T P α T

Manometers Can provide a very accurate measurement of pressure Used as calibration standards for other pressure

measurement devices.

P = ρgh

Manometers

Pressure Gauges Pressure gauges are used for local indication Pressure gauges consist of a dial or indicator and a

pressure element. A pressure element converts pressure into a mechanical

motion. Most mechanical pressure elements rely on the fact that

pressure acts on a surface area inside the element to produce a force that causes a mechanical deflection.

Bourdon Tube Bourdon tubes are circular-shaped tubes with oval cross

sections. The outward pressure on the

oval cross section forces it to

become rounded. This movement provides a

displacement that is

proportional to the applied

pressure. The tube is mechanically linked to

a pointer on a pressure dial to give

a reading.

Bourdon Tube

Bourdon Tube

Diaphragm

A diaphragm is a flexible membrane that expands when pressure applied .

In pressure-measuring instruments, the diaphragms are normally metallic.

When two are fastened together they form a container called a capsule.

Pressure applied inside the diaphragm capsule causes it to expand and produce motion along its axis.

Diaphragm

Bellows The bellows pressure element converts a pressure into a

physical displacement. It is very similar to a

diaphragm-type gauge

the difference is that

typically the movement

in a bellows is much

more of a straight-line

expansion.

Differential Pressure Gauges Measures the difference between two pressures. The measuring element is formed by two diaphragms, acting

on the same movement. In this way the pointer

senses only the difference

between the two pressures

Smart Differential Pressure Transmitter

Capable of measuring differential pressure (that is, the difference between a high pressure input and a low pressure input) and therefore called DP transmitters or DP cells.

The DP transmitter consists of: Body containing display, electronic module & power

module. Manifold with isolation, bypass & vent valves. The transducer (DP cell) inserted in a pressure capsule .

A pressure capsule has to be used to obtain maximum sensitivity.

A pressure capsule has a sensitivity range that closely matches the anticipated pressure of the measured fluid.

Differential Pressure Transmitter

Differential Pressure Transmitter

Transmitter Functional Block Diagram

Three Valve Manifold If the process pressure is accidentally applied to only

one side of the DP capsule during installation or removal of the DP cell from service, over ranging of the capsule would occur and the capsule could be damaged causing erroneous indications.

A three-valve manifold is a device that is used to ensure that the capsule will not be over-ranged during bringing the transmitter in/out of the service.

Allows isolation of the transmitter from the process loop. Consists of two block valves - high pressure and low

pressure block valve - and an equalizing valve. During normal operation, the equalizing valve is closed

and the two block valves are open.

Three Valve Manifold

Three Valve Manifold

Electronic Pressure Sensors Pressure can be converted to some intermediate form,

such as displacement, the sensor then converts this displacement into an electrical output such as voltage, frequency or current.

Most electronic pressure sensors employ capacitive, differential transformer, force balance, photoelectric, piezoelectric, potentiometric, resistive, strain gauge, or thermoelectric means of measurement.

Potentiometric-type Sensor

Piezoelectric-type Sensor Class of crystals, called piezoelectric, produce an electrical

signal when they are mechanically deformed. The voltage level of the signal is proportional to the amount of

deformation. The crystal is mechanically attached to a metal diaphragm. One side of the diaphragm is connected to the process fluid to

sense pressure, and a mechanical linkage connects the diaphragm to the crystal.

The output voltage signal from the crystal is very small (normally in the microvolt range), so you must use a high-input impedance amplifier.

Piezoelectric-type Sensor

Capacitance-type Sensor In a capacitance-type pressure sensor, the process

pressure is applied to a diaphragm. The diaphragm is exposed to the process pressure on

one side and a reference pressure on the other. The deflection of the diaphragm causes a change in the

distance between the metal plates, so, the capacitance changed.

The change in capacitance is detected by a bridge circuit & converted into either a direct current or a voltage signal.

Capacitance-type Sensor

Strain Gauges Strain gauge sensors are used for high and low pressure

applications, and can measure absolute or differential pressure. The strain gauge is a device that can be affixed to the surface of

an object to detect the force applied to the object. One form of the strain gauge is a metal wire of very small

diameter that is attached to the surface of a device being monitored.

When force is applied the overall length of the wire tends to increase while the cross-sectional area decreases.

For a metal, the electrical resistance will increase as the length of the metal increases or as the cross sectional diameter decreases.

Very small pressure changes can be detected if there are a large number of wire runs.

Strain Gauges

Strain Gauges

Variable Reluctance Sensor

Usually used for low differential pressure measurements. The operating principle of the VRP transducer is based on the

reluctance of the L1 and L2 coils is directly proportional to the length

of the flux path and inversely proportional to its permeability. Each electromagnetic circuit associated with coil L1 and coil L2

contains two reluctance elements, iron and air-gap paths. When a permeable material such as iron is introduced into the flux

field of the coil, the lines of magnetic flux are redirected and concentrated in the permeable material.

As a differential pressure is applied, the diaphragm deflects, one side decreasing and the other increasing, and the air gap reluctances in the electromagnetic circuits change proportionally to the differential pressure applied.

Variable Reluctance Sensor

Pressure Measurement Error Over-Pressure: All of the pressure gauges are designed

to operate over a rated pressure range. If a pressure gauge is over ranged, pressure is applied to

the point where it can no longer return to its original shape, thus the indication would return to some value greater than the original.

Diaphragms and bellows are usually the most sensitive and fast-acting of all pressure gauges.

Bourdon tubes are very robust and can handle extremely high pressures although, when exposed to over-pressure, they become slightly distended and will read high.

Pressure Measurement Error Faulty Sensing Lines : When the pressure lines become

partially blocked, the dynamic response of the sensor is naturally reduced and it will have a slow response to change in pressure.

Depending on the severity of the blockage, the sensor could even retain an incorrect zero or low reading.

Periodic draining and flushing of sensing lines is a must. A cracked sensing line has the characteristic of

consistently low readings.

Pressure Measurement Error Over ranging Damage to the D/P Cell: The valve

manifolds are provided on DP cells to prevent over-pressuring and aid in the removal of cells for maintenance.

Loss of Loop Electrical Power : The output of the DP

transmitters will drop to zero with a loss of power supply.

Pressure Switches The switch is a device that monitor the system & senses any change in

the concerned quantity, opens or closes an electrical circuit when a pre-determined value is reached.

The pressure switch is a device that will trip at the set point, and remain tripped until a fixed re-set point is reached, at which point the switch will return to its original operating position. Pressure switch consists of

Electrical Contacts: the elements in the switch that electrically respond to the media applied to the actuator.

Actuator: The member in the switch which receives the media and ultimately strokes the electrical contacts to open or close at the designated set point.

Pressure Switches Switch configured to be SPST or SPDT. The fluid medium determines the diaphragm material, and in

some cases, the type of switch that can be used. We may need a pressure, vacuum or differential switch. In a pressure switch, positive pressure pushes the diaphragm In a vacuum switch, negative pressure pulls the diaphragm. In a differential switch, both sides of the switch housing are

ported to two pressure sources, and the diaphragm responds to the resulting net force.

The switch healthy status is the status that the switch gives no alarm or trip signal and configured to be N.O. or N.C.

according to the application.

Diaphragm Sensing Element Switch

Many pressure switches are configured simply by coupling a diaphragm to the actuator of a snap switch.

In operation, force against the diaphragm is transferred to a guide disk, which depresses the actuator of a snap switch.

To depress the actuator, the guide disk must also push against the opposing force of a spring.

The compression of the spring can be modified by an adjustment screw, and this permits fine calibration of the switch set point.

Snap Action Switch

Snap Action Switch

Diaphragm Sensing Element Switch

Piston Sensing Element Switch Combines a durable piston sensing element

with a reliable switch mechanism. The switch is capable of extremely high

pressure ratings.