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.
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