CENTRE OF EXCELLENCE (CHEMICAL
SECTOR)
CHBT-05
Instrument Mechanic Chemical Plant
Pressure
Temperature
Flow
Ex:- Pressure Cooker
Pressure
Flow
Temperature
Level
Level
Week No
Lesson No
1
1
Introduction of instrumentation
2
Pressure
3
Pressure Measuring Instruments
4
Temperature Measuring Instruments
5
Bi – Metallic Thermometer
3
6
Glass & Gas Filled Thermometer
4
7
RTD & Thermocouple
5
8
Flow meter
6
9
Level Measuring Instruments
7
10
Hydrometer
11
pH Meter
12
Anemometer
13
Hygrometer
2
8
Name of Lesson
PRESSURE
Definition of pressure:-Pressure defined as force per unit of area is usually expressed
In terms of familiar units of weight force & area or the height of
a column of liquid that produce like a pressure at its base.
-Pressure measuring devices may be divided in three groups.
(1)Those that are based on the measurement of the height of a
Liquid column.
(2)Those that are based on the measurement of the distortion
of an elastic pressure chamber.
Liquid column methods: Means the pressure being measured is balanced against the
pressure exerted by a column of liquid.
 The density of the liquid is known, the height of the liquid is measure
of the pressure.
P = ƍgh
 Device of this is called Manometer.
 When the height of the liquid is observed visually, the liquid columns
are contained in transparent tubes.
 Liquids used in manometer are generally water and mercury.
ƍ
varies
T
Elastic – Element Method
Pressure measuring devices are those in which the measure pressure
deforms some elastic material (usually metallic) within its elastic limits, the
magnitude of the deformation being approx. proportioned to the applied
pressure.
These devices may be classified into three types.
(i) Bourden Tubes Elements
(ii) Bellows Elements
(iii)Diaphragm Elements
Most frequently used process pressure indicating device is C-Spring
Bourden Tube Pressure Gauge.
These are available in various ranges and different MOC.
Material are selected
Gauges calibrated with
Based on
Pressure
Pressure range
Resistance to corrosion
by the process
Effect of temp. on
calibration
Vacuum
Temperature
 Measurement of the hotness and coldness of a body or fluid is
common place in the process industries.
 Temperature measuring devices utilize systems with properties that
vary with temperature in a simple reproducible manner and known
as reference or secondary thermometers.
 Temperature measuring devices are:(1)Liquid Filled System Thermometers
(2)Bi-metallic Thermometers
(3)Thermo Couple And RTD
(4)Pyrometers
Liquid filled system thermometers:Designed to provide an indication of temp. some distance from the
point of measurement.
The measuring element (bulb) contains a gas or liquid that changes
in volume, pressure or vapour pressure with temp.
These change is communicated through a capillary tube to a
Bourden tube or other pressure or volume sensitive devices.
(1)The Bourden tube responds  to provide a motion related to the
bulb temperature.
(2)Systems responds  to volume changes are completely filled with
liquid.
(3)System responds  to pressure changes either are filled with gas or
are partially filled with volatile liquid.
Bi-metal Thermometers
Thermostatic bi-metal can be defined as a composite material made
up of strips of two or more metals fastened together.
This composite, because of the differential expansion rates of its
components, tends to change curvature when subject to a change in
temperature.
With one end of a straight strip fixed, the other end deflects in
proportion to the temperature change, the square of the length and
inversely as the thickness.
Bi-metal thermometers are used at temperature ranging from 5800C
and down to -1800C and lower.
Flow Measurement:Flow defined as volume per unit of time at specified temperature and
pressure conditions is generally measured by rate meters or positive
displacement.
Positive Displacement:- In which the flow is divided into isolated
measured volumes when the number of fillings of these volumes is
counted in some manner.
Rate Meters:- Applies to all types of flow meters through which the
material passes without being divided into isolated quantities.
Orifice Meter
Venturimeter
Flow Measuring Instruments
Rotameter
Pitot Tube
Turbine Flowmeter
Orifice Meter
The most widely used flow-meter involves placing a fixed area flow
restriction (an orifice) in the pipe carrying the fluid.
This flow restriction causes a pressure drop that can be related to flow
rate.
The sharp edge orifice is popular because its simplicity, low cost and
the large amount of research data on its behaviour.
Flow
Orifice Plate
Venturi - Meter
It operated on exactly the same principle as the orifice.
A venturi gives a definite improvement in power loses over an
orifice and is often indicated for measuring very large flow rates.
The initial higher cost of venturi over an orifice.
Venturi-meter
Rotameter
It consists of a vertical tube with a tappered bore in which a float
changes with the flow rate through the tube.
For a given flow rate the float remains stationary since the vertical
forms of differential pressure, gravity, viscosity and buoyancy are
balanced.
Level Measurement:It can be defined as the determination of the location of the interface
between two fluids separable by gravity with respect to a fixed
detum plane.
The most common liquid measurement is that of the interface
between a liquid and a gas and other are the interface between two
liquids, between a granular or fluidized solid and a gas, and
between a liquid and its vapour.
Direct
Method
Level Measuring Devices
Indirect
Method
Hook Type Level
Indicator
Float Type Level
Indicator
Electric Method
Electric Methods
Two electrical characteristics of fluids are conductivity and electric
constant.
An application of electrical conductivity is the fixed point level detection
of a conductive liquid such as high and low water levels.
A voltage is applicable between two electrodes inserted into the vessel
at different levels.
Both electrodes immersed in the liquid in a current flows.
Capacitance type level measurements are based on the fact that the
electrical capacitance between two electrodes varies with the di-electric
constant of the material between them.
Hydrometer
Hydrometer is direct reading instrument. It is used to measure density.
It works on the principle of buoyancy. It is of fixed weight and made up
of good quality glass.
It immerses objects in the liquid according to its density and reading is
taking according to its stem level which indicates density of fluid.
Hygrometer
Hygrometer is used to measure moisture from atmosphere. It consists
of hygroscopic material which depends on contraction or expansion
giving result of moisture increase or decrease in atmosphere.
Hygroscopic materials include human hair, animal fat, wood or paper.
Hot Wire Anemometer
It can measure flow rate of gas or liquid with the help of electrical
equipments.
PH Meter
It is used to measure PH of liquids.
It consists of electrode. The electrode is made up of Ag / AgCl
which senses the PH of liquids.