1. Sensors Transducers Presentation

Sensors-transducers

Presented by: I&C Technician

Introduction to process measurement • The purpose of measurement • Kinds of signals • Remote vs local display • Error in measurement system • Calibration • Calibration laboratory • Field calibration

Function of sensor 1. An instrument must sense the process to be measured. part of the instrument must stretch bend, or change position or characteristics when affected by process. 2. The instrument must convert this response into an electrical or pneumatic signals.

CONTROL LOOP

transmitter

Function of transducer • The process sensing element is usually a bellow, a diaphragm or a bourden tube. • The primary conversion element converts the mechanical action of a process sensing element to an electrical signal, usually an electrical resistance or a voltage. • The secondary conversion element is usually an electronic circuit that produces a standard signal according to the need of the control system.

Sensors transducers • • • • • • • •

Pressure sensors. Flow sensors. Thermometers. Liquid level sensors. Vibration and speed sensors. Proximity and displacement sensors. Fire, Flame and smoke detectors. Analyzers.

Transducer principle 1. 2. 3. 4. 5. 6. 7. 8.

Capacitive. Inductive. Piezoelectric. Resistive. Potentiometer. Strain gage. Photoconductive. Thermoelectric.

PRESSURE

BOURDEN TUBE PRESSURE SENSOR

BELLOWS DIAPHRAGM STRAIN GAUGE CAPACITVE INDUCTIVE

PRESSURE TRANSDUCER

POTENTIOMETRIC MAGNETIC PIEZOELECTRIC PHOTOELECTRIC

Bourdon tubes

bellows

diaphragm

Strain gauge

Capacitive

potentiometer

piezoelectric

inductive

Working on transmitter • A measurement device is made up of two parts • Primary element • Transmitter • Converts signal (Transducer) • Transmits signal

flow meters • Flow is the motion of fluid. • Flow rate is the time rate of motion fluid. • Total flow is the rate integrated over a specific time interval. • Flow meter is a flow rate transducer. • Laminar flow. • Turbulent flow. • Reynolds no.

Units of measurement • Cubic foot/minuet. • Gallon/minuet. • Liter/sec.

FLOW

DIFFERENCIAL PRESSURE

PRIMARY ELEMENT

MECHANICAL FLOWMETER

POSITIVE DISPLACEMENT

ELECTRONIC FLOWMETER

TURBINE FLOWMETER MEGNATIC FLOWMETER

ROTARY FLOWMETER

VORTEX FLOWMETER

ULTRASONIC FLOWMETER

Common Devices To Measure Flow • Orifice

plate

• Veturi tube • Flow nozzle

Types and use of orifice plate 1. Concentric opening in center. It is for normal use.

2. Eccentric Opening slightly off center. Used for the fluid or gas which has solid particles.

Types and use of orifice plate 3 . Segmental semi circular in shape. It is for used for slurries.

Venturi Tube

Flow Nozzle

Variable area instruments

Rota meter

Positive displacement flow meters • Trap a known volume of fluid and allow it to pass from meter inlet to outlet. • Total flow measurement. • They have accurate,precise,and have a wide flow range. • They are expensive and object to corrosion. • They are used for water meters and gasoline pumps.

PD flow meters types • Piston flow meter • Notating-disc flow meter • Lobed impeller and oval flow meter • Rotary vane meter • Turbine flow meter

Nutating and rotary-vane flow meter

Piston type flow meter

Lobed impeller and oval flow meter

Turbine flow meter

Magnetic flow meter

Vortex flow meter

Ultrasonic flow meter

temperature Bimetallic and liquid filled system

Resistance temperature detector

thermister

thermocouple

Temperature 

Hotness or Coldness of a Substance



Temperature Units – Celsius

°C

– Fahrenheit °F – Kelvin

°K

Temperature Measuring Instruments

Instrument Inserted or Attached Located at Distance

Pyromete r

Thermomete rs

Basic Types Of Thermometers 

Bimetallic – From –75 °C to 540 °C



Liquid In Glass – From -120 °C to 320 °C



Filled System – From -273 °C to -185 °C – From 650 °C to 760 °C



Metals expand on heating and contract on cooling.



Each metal has a coefficient of thermal expansion



Bimetallic strip bends toward metal of lower rate of thermal expansion, on heating.

Bimetallic Thermometers

– Spiral – Helix, Multiple Helix

Bimetallic Thermometers (Cont.) Office

and Home Use

Bimetallic Thermometers (Cont.) Industrial

Use

Bimetallic Thermometers (Cont.) Advantages : Inexpensive,

Read

Reasonably

Carefully Best

Rugged and Easy to

Accurate, Handle

for Local Use

RTDs Some Metal’s Resistance Increase on Increase of Temperature Metals Used in RTD Must Be 

Pure



Uniform Quality



Stable With Given Temperature Rage



Able to Reproduce Resistance Temp. Reading



RTDs

Temperature, °C 0 200 400

Metal

Resistance

Platinum Copper Nickel Platinum Copper Nickel Platinum Copper Nickel

100 100 100 175 186 250 250 275 485

RTDs       

RTD act just like a variable resistor Used in resistance bridge circuit Output voltage  resistance of RTD Heating or cooling cause bridge unbalance Lead wire error 250 ft copper wire has 1 ohm resistance. If resistance 139 ohm temp. will be 100°C exclude 2 ohm then R=137 which show 94.4 °C error 5.1 %

RTDs 

Compensating Lead Error – Use of Third Wire, RTD Wire Should Be Opposite Arm – Error With 2 Lead Wires 4°C, With Three Lead 0.1 °C and by Fourth Wire Error 0.03°C

RTDs

–Construction of RTD

RTDs –Measuring Surface Temperature RTDS –Protecting Well for RTDs

RTDs

Advantages of RTD – Extremely accurate – Measure temperature within 0.15 °C – Fast in response and small in size Disadvantages of RTD – Expansive and complex – Require bridge circuit, power supply, meter

Thermistors 

Special Type Of Resistors



High temp. resistance coefficient



Negative as well as positive Temperature Coefficient



Resistance Increases Temp. Decreases

Advantages – Three & four wire bridge circuit not required – Two lead wire provide sufficient accuracy – Less expansive and less complicated Disadvantages – Lack RTD accuracy.

Thermocouple 

Two Different Metals or Alloy – Two Junction • Reference Junction ( to be at constant temperature) – Measuring Junction • Heating measuring junction produce voltage across MJ & reference junction

Thermocouple     

Extension Wires Thermocouple wires Same as thermocouple ( iron constantan, copper constantan Other material for platinum thermocouple Copper or copper alloy

Thermocouple • Construction of Thermocouple

Thermocouple



Advantages – In Expansive, Rugged, Accurate – Do not use bridge circuit – Wide temperature range –273 to 2800 oC



Disadvantages – Low voltage. – Reference junction kept to be constant

Level

Sight glass Pressure head instruments

displacer Differential pressure

Float switches

Electrical instruments

vibration ultrasonic

Definition of Some Points Datum point:

A fixed reference point from which level is measured

Buoyancy:

Upward force exerted by liquid on an object submerged & floating in it

Buoyant force:

Net upward force exerted by liquid on submerged or floating object equal the weight of the displaced liquid. A nearly stationary object whose purpose is to displace liquid, used for liquid measurement.

Displacer:

Level Measurement by Float 

A float, cable, sliding pointer arrangement.



A fixed scale is mounted on the exterior of the tank



A cable connects the float with the pointer through two pullies.



Pointer move up and down and also serve as counter weight.



Float type level indicators are installed on the open tanks and the tank of the atmospheric pressure.

Float-based level switch

Level Measurement by Displacer Displacer do not ride on the surface of the liquid but they are design to submerge as the level rises. Displacer mechanism transfer the slight mechanical motions due to loss of weight which can be used in instruments to indicate level.

Torque Tube Displacers    

The most common type of displacer mechanism is a torque tube displacer. Torque tubes are useful when level measurement is needed on the liquid under pressure. As the liquid level changes, the buoyant force moves the displacer slightly and twist the torque tube. The motion of the torque tube makes the pointer move to indicate the liquid level on the scale.

Displacement level

Level Measurement Using Differential Pressure Instruments 

Differential pressure measurement is suited to level measurement in closed vessels

 

When liquid level is measured in an open tank, piping from the lower end of the tank is connected to the high pressure side of the transmitter and the low pressure side is vented.



When liquid level is measured in a closed tank the effect of tank pressure is compensated by connecting a line from the top of the tank to the low pressure side of the differential.

 

The pressure due to the height of the liquid is a measure of its level.

Hydrostatic tank gauging

Differential pressure cell design

Ultrasonic level

analyzers Hydrazine monitor

Sodium monitor Dissolve oxygen monitor Flue gas oxygen monitor

Silica monitor Conductivity monitor

Ph monitor

hydrazine

Main component

sensor

Sample and solution flow

Calibration sequence

Sodium • Reagent solution • Standard solution • Etch solution

Dissolved Oxygen Sensor • Oxygen molecules diffuse through the membrane based on the partial pressure of oxygen. • ORP Reaction – Oxygen is reduced to hydroxide ions (OH-) at the gold cathode. – Silver is oxidized to silver bromide (M-4) or silver chloride (M-4HD) at the silver anode

• The current flow is directly proportional to the concentration of Dissolved Oxygen in solution

Calibration • Zero calibration • Air calibration

Silica monitoring

Sample and solution flow

Calibration and maintenance • Reagent solution • Standard solution

Flue gas oxygen monitor

installation

Main probe component

Cell and tube assembly

Diffuser and detector

Inner probe assembly

Calibration setup

CONDUCTIVITY ANALYZERS

Measuring principle Conductance is a measure ability of electric circuit to pass current.

G = 1/R(siemense) Na+ Cl-

Variables affecting conductivity • Temperature • Concentration • contamination

Conductivity probes • Electrode type • Inductive type

Na+ Cl-

Typical Conductivity Values 10% NaOH

355,000

Sea Water

50,000

Mains Water Supply

50

Boiler Feed Water

1

Distilled Water

0.5

Ultra Pure Water

0.055

0

1

100

10,000

1,000,000

CONDUCTIVITY IN HUB PLANT

CONDENSER/EXTRACTION PUMPS

STATOR COOLING WATER

BOILER FEED WATER

Calibration Zero standard solution

Span standard solution

Grab sample

Conductivity ranges vs. resistance input

Decade box

PH monitor

pH Theory • pH is a Unit of Measurement – pH = Power of Hydrogen (H+) – Defined as the Negative Logarithm of Hydrogen Ion Activity • pH = log (1/H+)

• Used for Determining the Acidity or Alkalinity of an Aqueous Solution – Practical pH Scale for Industrial Instrumentation • 0 - 14 pH

pH Sensor Components • pH Measuring Electrode pH Measuring Electrode

Reference Cell

– Purpose is to Develop a Millivolt Potential Directly Proportional to the Free Hydrogen Ion Concentration in an Aqueous Solution

• Reference Cell KCl Buffered to 7 pH KCl Gel Ag/AgCl Element

pH Sensitive Glass

Liquid Junction

– Purpose is to Maintain a Constant Reference Potential Regardless of pH Change or Other Ionic Activity in the Solution

• Reference Cell Liquid Junction – Purpose is to Maintain Electrical Contact Between the Reference Electrode and the Measuring Electrode by way of the Solution

pH is a Potentiometric Measurement pH Measuring Electrode

Reference Electrode

• The Measuring System consists of a pH Measuring Electrode and Reference Electrode • The Potential Difference Between the Two Electrodes is a Function of the pH Value of the Measured Solution • The Solution Must Be Conductive and is Part of the Electrical Circuit

Ion Concentration Acid

Neutral

Alkaline

pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Hydrogen Ion (H+) 1 0.1 0.01 0.001 0.0001 0.00001 0.000001 0.0000001 0.00000001 0.000000001 0.0000000001 0.00000000001 0.000000000001 0.0000000000001 0.00000000000001

Hydroxyl Ion (OH-) 0.00000000000001 0.0000000000001 0.000000000001 0.00000000001 0.0000000001 0.000000001 0.00000001 0.0000001 0.000001 0.00001 0.0001 0.001 0.01 0.1 1

Typical pH Values 4.0% NaOH Crackers Blood Pure Water Milk Beer Tomatoes Oranges Lemons 4.9 % H2SO4

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

pH

8.0

9.0 10.0 11.0 12.0 13.0 14.0

Ph measurement in hub plant

Boiler feed water

Waist water treatment

calibration

Standard buffers

Grab sample

Fire and smoke detectors

Ionization sensors

Photo sensors

Infrared and ultraviolet sensors

Thermal sensors

Flam detector heat generated

Ionization

radiation

Fire and smoke detectors

Ionization sensors

Photo sensors

Infrared and ultraviolet sensors

Thermal sensors

Flam detector heat generated

Ionization

radiation

Proximity and displacement sensors Capacitive sensors

Inductive sensors

Magnetic sensors

LVDT sensors

Mechanical limit switches

Optical sensors

Ultrasonic sensors

Digital output and encoders