Sensors

Measurement of Process Value:
i) Temperature
The most commonly used temperature sensors in oceanography are the Resistance
Temperature Detectors (RTDs) thermistors and thermocouples.
RTD:The principle behind this sensor is that of that pure metals change their resistivity with
temperature in a predictable way. RTDs are constructed with metals whose resistivity increases
linearly with temperature. A Wheatstone bridge circuit employing electrical circuit is used in
measuring the temperature by this sensor. Metals used should a have high boiling point, be easily
available in its pure form, be resistant to corrosion (chemical stability) and have electrical
properties that are highly reproducible. The most commonly used metal is platinum but copper
and nickel are also used.
Basic connection diagram of RTD in Wheatstone bridge circuit

Thermocouples :
Thermocouples are based on the Seebeck effect: two dissimilar metals bearing different
temperatures, joint together at at least two different points (a reference and a measuring point)
will produce electromotive force proportional to the temperature. Different metal-pairs produce
different outputs (i.e., temperature ranges). Thermocouples are not as accurate as thermistors or
RTDs but can take readings of very high temperatures ranges over 2000°C). They are used, for
instance, in High Temperature Probes attached to ROVs when exploring hydrothermalvents.
Thermopile:
A thermopile is defined as several thermocouple connected in series. As T2 output
voltage increases significantly. The advantage of a thermopile is increased sensitivity. Here,
voltage output is thrice the value of one thermocouple with enough sensing junctions a useful
voltage can be generated for example thermopiles are often use to control shut-off valves in
furnaces.
Thermopile

Thermistors :
Thermistors are temperature sensitive resistors. The principle behind the functioning of
this sensor is much like the one from the RTD, however thermistors differ from RTDs in that the
material used in a thermistorsis generally a ceramic or polymer, while RTDs use pure metals and,
also unlike RTDs, the resistance of a thermistors decreases with increasing temperature. The
temperature response is different as well; RTDs are useful over broader temperature ranges,
while thermistors typically achieve a higher precision within a limited temperature range, usually
−90 °C to 130 °C.
Thermistor Circuit
Infrared thermometers:
During infrared measurement (IR measurement), surface temperatures are measured
without contact and also at a distance. IR measurement is ideal example for particularly
monitoring the temperature of small, moving, difficult to access, or extremely hot objects. as
well as for the temperature measurement of entire pallets of goods in the food processing sector.

ii) Pressure:
A pressure sensor is a device that senses pressure and converts it into an electric signal
where the amount depends upon the pressure applied.
Pressure sensors can also be used to indirectly measure othervariables such as fluid/gas
flow, speed, water level, and altitude. Pressure sensors can alternatively be called pressure
transducers, pressure transmitters, pressure senders, pressure indicators, piezometers and
manometers, among othernames.
Pressure Sensors:
 U-Tube Manometer
 Diaphragm
 PiezoElectric
 Air BubblerType
 Differential Pressure (DP Type)
 Inductive or capacitive type
 Opticaltype
 Bourdon tube
 Linear Variable Differential Transformers
U-Tube Manometers:
These are for local indication only. They are made up by glass tube and added with
scales at the tubeside.
Diaphragm Type:
This type is an improvised version of a Bourdon tube type pressure sensor. The
elongation of the element compresses or expands a diaphragm similar to that of a bellow on one
side and the other side connected to a fixed spring. The movement from diaphragm passes
through a leverage mechanism to operate a pointer on a graduated dial or a variable terminal of a
resistance of a Wheatstone bridge circuit.
Diaphragms of Pressure Sensor

Bellow Type:
In this type instead of diaphragm a bellow is install which collapses and expanse against the spring
housed inside which simulates moment of leverage of as in diaphragm type.
Bellow Type Pressure Transducer
Piezoelectric Type:
These sensors use stacks of piezoelectric crystal or ceramic elements to convert the
motion of the force-summing device to an electrical output. Quartz, tourmaline and several other
natural crystals like Rochelle salt, generate an electric charged when strained. Specially
formulated ceramics can be artificially polarized to be piezoelectric and they have higher
sensitivity than natural crystals.
Piezoelectric Pressure Sensor

Air Bubbler (Dip Tube) Type:
A controlled amount of flow of air from a tube immersed in a liquid will experience a
certain back pressure which will be a function of liquid column pressure of the immersion.
The same system can be used to measure the level of liquid which in turn will have
pressure proportional to the back pressure experienced by the controlled flow of bubbling air out
of the liquid column.
Air Bubbler or Pneumercator Pressure Sensor
Differential Pressure Type:
The resultant pressure of two pressures acting on either side of a diaphragm pressurizes
and deflects the diaphragm to activate the output mechanism and read in terms of pressure.
Differential Pressure Sensor

Inductive or Capacitive Type:
The resultant mechanical output of the sensor due to the presence of pressure can be used
to activate in case of an inductor the armature of an electromagnetic coil whereby the induced
(back) e.m.f. of the coil [i.e. Linear Variable Differential Transformer, simply named LVDT ]or
in case of a capacitor the plate separation distance can be varied whereby the capacitance can be
varied as a function of pressure.
Inductive Type Pressure Transducer
Capacitive Type Pressure Sensor
Schematic Diagram of Capacitive Sensor

Optical Type:
The mechanical output of the sensor changes the optic incident on a photocell whereby
the change in photocell current is a measure of pressure.
Optical Pressure Transducer

Bourdon Tube:
The basic idea behind the device is that, cross-sectional tubing when deformed in any
way will tend to regain its circular form under the action of pressure. The bourdon pressure gauges
used today have a slight elliptical cross-section and the tube is generally bent into a C-shape or arc
length of about 27 degrees. Detailed diagram of the bourdon tube is shown below.
As the fluid pressure enters the bourdon tube, it tries to be reformed and because of a free tip
available, this action causes the tip to travel in free space and the tube unwinds. The simultaneous
actions of bending and tension due to the internal pressure make a non-linear movement of the free tip.
This travel is suitable guided and amplified for the measurement of the internal pressure. But the main
requirement of the device is that whenever the same pressure is applied, the movement of the tip
should be the same and on withdrawal of the pressure the tip should return to the initial point.

Linear Variable Differential Transformer (LVDT):
This measures linear displacements over a limited range. It consists of one primary and
two secondary coils which are connected differentially with an inner moving magnetic core.
High frequency AC is applied to center coil and this generates magnetic field that induces an
EMF in the secondary coils. Depending upon the core position the voltages in the secondary
coils will change and net voltage from the secondary will be a function of core movement
resulting from either diaphragm or bellow type pressure sensor.
Linear Variable Differential Transformer Output Circuit

iii) Level:
Gauge Glass Type:
A transparent tube with a scale is attached to the bottom and the top of the tank, the top
being open to atmosphere. The level in the tube read at the scale is that inside the tank.
Probe Type:
Electrodes fitted at different levels inside the tank act as electric current path medium and
each connected to a relay to energize and operate its contact which in turn actuates an alarm ,a
pump or a control valve as per the configuration.

Conductivity Probe Level Detection System
Dip Tube (AirBubbler) Type:
A dip tube is inserted from the tank top and is fed with an air pressure at a value above
the maximum static pressure resulting from the liquid head in the tank, such that a small air flow
from the dip tube bottom is available at a controlled rate ( normally 2ft3 /hr. or 0.225m3/hr.).
The back pressure in the dip tube is a measure of level of liquid column.
Air Bubbler Type Level Sensor

Tank Radar Type:
In this system a transmitter emits radar waves to the surface of the liquid. The reflected
signal is processed in the electronic control box and finally the usage is calculated. The
frequency of the transmitted signal decreases over a time period. The incoming signal is
compared with the outgoing signal. The difference between these two signals is a low-frequency-
signal. Its frequency is directly proportional to the distance from the Transmitter to surface of the
product.
SAAB Tank Radar uses advanced method to detect the surface echo and measure the
distance to the surface accurately. The signal is filtered in a digitally controlled analog filter.
And removes any echoes smaller than a threshold value. Then a narrow filter is around the
frequency corresponding to the surface echo. The remaining frequency is compared with the one
calculated in the previous sweep, resulting in a very accurate signal with only a few Hertz.
When propagating through a media, radar signals are absorbed and signal strength decreases.
High frequency signals suffer more attenuation than mid and low frequency signals.
Ultrasonic Type:
A specially designed, intrinsically safe, sealed 4 – 20 mA loop powered ultrasonic
level transmitter is used to determine the level of liquid especially in Waste Water and
Effluent Treatment Plants.

iv) Flow:
 Tube
 Differential Pressure Flow switch
 Rota meter:
When no flow through the rotameter the float rests at the bottom of the tube the
as fluid enters the tube the higher density as the float causes the float to remain at the
bottom. The space between float and tube allows for flow past float and as flow increases
the pressure drop increases and the float will rise to indicate the amount of flow. This flow
meter is usually used to measure low flow rates.
Turbine Flowmeter (Mechanical) -Fig from mobile
The vanes on the rotor are pressurized by the flow on process liquid and this
causes the rotor to rotate and via gear train the display is made available.
This type of meter is used for H.F.O/D.O systems in engine room.
Rotameter
 Electromagnetic Flow meter:
The rotor is floating type and the design reduces friction. The AC electrical
output from pickup coil is amplified to operate an electromagnetic counter indicating total
quantity, and/ or calculated when a rate of flow required.

Electromagnetic Flow Meter
 Ultrasonic Flow Equipment :
This meter employs latest technology with clamp on transducer assembly design to
detect flow rate from outside. This uses Doppler Effect which uses the frequency shift of
ultrasonic signals reflected from discontinuities in the fluid stream. The ultrasonic beam
from one piezoelectric crystal is transmitted through the pipe wall into the fluid at an angle
to the stream flow. A second piezoelectric crystal in the same sensor detect signals
reflected of flow disturbances. Transmitted and reflected signal are compare in an
electrical circuit and the corresponding frequency shift is proportional to the velocity. This
meter can measure pure water, wash-water, sewage, process liquids, oils and other light
homogeneous liquids that should be capable of ultrasonic wave propagation and have a
reasonably axis-symmetrical flow.
Ultrasonic Flow Equipment

 Doppler Flow meter :
This works on the Doppler Effect principle that frequencies of received sound waves
depend on the motion of the source relative to the propagating medium. One transducer
transmits ultrasonic beam into the flow stream whose moment of materials alters the
frequency of the beam reflected on to a receiving transducer. The frequency shift is
linearly proportional to the rate of flow of materials in the flow stream.
iv) RPM Detector:
Tacho generator -- a generator with permanent magnet poles, whose output is given to a basic
D.C. voltmeter – tachometer – graduated inRPM
Inductive sensor – a proximity detector, i.e. a coil on a permanent magnet placed near a
ferromagnetic gear wheel gives output voltage as per the wheel teeth proximity.
Conversely an inductive proximity sensor which generates an electromagnetic field detects the
eddy current loss induced when a metal target enters the field.
e.g. : M/E and A/E rpm sensors
Similarly as in the case above instead of the inductive proximity sensor a capacitive proximity
sensor can also be used.
e.g. : OWS oil level sensor, Aux. boiler cascade tank feed water oil detector
sensor consists of four elements: coil, oscillator, trigger circuit, and an output.
The oscillator is an inductive capacitive tuned circuit that creates a radio frequency.
Theelectromagnetic field produced by the oscillator is emitted from the coil away from the face
of
the sensor. The circuit has just enough feedbackfrom the field to keep the oscillator going.

When a metal target enters the field, eddy currents circulate within the target. This causes a load
on the sensor, decreasing the amplitude of the electromagnetic field. As the target approaches the
sensor the eddy currents increase, increasing the load on the oscillator and further decreasing
theamplitude of the field. The trigger circuit monitors the oscillator's amplitude and at a
predetermined level switches the output state of the sensor from its normal condition (on or off).
As thetarget moves away from the sensor, the oscillator's amplitude increases. At predetermined
level the trigger switches the output state of the sensor back toits normal condition (on or off).
Capacitive proximity sensors are similar to inductive proximity sensors. The main difference
between the two types is that capacitive proximity sensors produce an electrostatic field
insteadof an electromagnetic field. Capacitive proximity switches will sense metal as well as
nonmetallic materials such as paper, glass, liquids, and cloth.
The sensing surface of a capacitive sensor is formed by two concentrically shaped metal
electrodes of an unwound capacitor. When an object nears the sensing surface it enters the
electrostatic field of the electrodes and changes the capacitance in an oscillator circuit. As a
result, the oscillator begins oscillating. The trigger circuit reads the oscillator's amplitude and
when it reaches a specific level the output state of the sensor changes. As the target moves away
from the sensor the oscillator's amplitude decreases, switching the sensor output back to its
original state.

 Opto-switch sensor – a proximity detector with a photocell where an incident light
ray is interfered by a non-ferromagneticmaterial
 Encoder – it uses a slotted wheel placed between a LED light source and a
phototransistor. The phototransistor turns ON/OFF with the flashing light,
creating the pulse output which is then processed in a processor and rpm
calculate

vi) Salinometer:
The condensate or product, if of acceptable quality, is delivered to the appropriate
tanks by the distilled water pump. Quality is continuously tested by the salinometer both at
start up and during operation. If the device registers an excess of salinity it will dump the
product and activate the alarm using its solenoid valves. The product is recirculated in
some installations.
The electric salinometer
Pure distilled water may be considered a non-conductor of electricity. The
addition of impurities such as salts in solution increases the conductivity of the water, and
this can be measured. Since the conductivity of the water is, for low concentrations,
related to the impurity content, a conductivity meter can be used to monitor the salinity of
the water. The instrument can be calibrated in units of conductivity (micromhos) or
directly in salinity units (older instruments in grains/gall, newer instruments in pprn or
mg/litre) and it is on this basis that electric salinometers (Figure ‘a’) operate.
The probe type electrode cell (Figure ‘b’) is fitted into the pipeline from the
evaporator, co-axially through a retractable valve which permits it to be withdrawn for
examination and cleaning. The cell cannot be removed while the valve is open and
consists of two stainless steel concentric electrodes having a temperature compensator
located within the hollow inner electrode.

vii) General Measurement of processes:
a. Viscorator:
To obtain a continuous reading of viscosity of a fluid an application theory covering
isothermal laminar flow in circular tubes has to be considered. This involves a constant quantity
of fluid supplied under isothermal conditions and laminar flow to a capillary tube of known
diameter and length. This is accomplished by a precision gear pump driven at 40 rpm by an
electric motor via reduction gear. A differential pressure cell measures the pressure drop across
the tube and this signal obtained is proportional to the viscosity of the oil. This signal is used to
operate a control system adjusting the steam valve to an oil fuel heater.

Viscosity Sensor
Viscotherm Sensor
b. Fire Detectors:
SmokeDetector:
The Alpha particles emitted by the elements like Americium and Iridium collide with air
and produce charged particles called ions. A low level electric voltage across the enclosed
chamber causes a steady small current to flow between two electrodes. When smoke enters this
space the Alpha particles are absorbed by the smoke and rate of ionization of air and hence the
electric current to fall which eventually sets off the fire alarm.
The other type uses photoelectric sensor to detect the change in light level caused by
smoke. This type is more expensive as well to install and less effective.

Ionization Chamber Combustion Detector
Heat Detector:
One type is that operates at a predetermined temperature and the other when the rate of
temperature rise of surrounding air reaches a set limit.

Fixed Temperature Heat Detector:
Fixed Temperature Heat Detector works when the heat exceeds a pre-determined
temperature, the bi-metal deflects and closes the contact, triggering the fire signal.
Fixed Temperature Heat / Thermal Detectors can respond to:
 Fixed temperature limit
 Rapid rate of change of the temperature in the protected area
 Combination of these types of detection
Typical fixed temperature spot-type detectors contain a bimetallic switch element that
closes at a specified temperature limit. The switch is normally composed of two metals, each
having a different temperature coefficient of expansion.
As this bimetallic element heats the metal with higher coefficient of expansion, it causes
the switch to bend or curve, closing the switch; thus indicating an alarm condition.
Bimetallic spot and coaxial style thermal detectors are self restoring. Fusible link and
melting insulation types of line thermal detectors are not self-restoring.
Rate of Rise Type Temperature Detector:
This type works on the principle that provided the rate of increase in the surrounding
temperature is above a given minimum value, the detector will operate given time limits, the
latter depending upon the rate of increase of temperature. It will operate when the temperature
exceeds 54°C for a rate of rise of less than 1 minute and generally operate before the temperature
exceeds 78°C. The response time of heat sensitive detectors depend up on various factor one of
which is the height between detector and floor level.

Rate Of Rise Type –Fire Detector
Flame Detector:
Detectors of this type can be either infrared or ultra-violet type. They have been designed
to respond only to that particular part of the low frequency flickering produced by flames
(typically 1-15 Hz) which is characteristic of a flame. This is achieved by appropriate light
filters. The circuitry of the system is so arranged that the detector will not activate alarm on
immediately sensing the radiation, but only if flame persists for a pre-determined time.
The drawback of these detectors is that if smoke happens to screen the detector from the fire
before the detector sensing it is unlikely to function. Also the detector`` may react to light
being received from a vibrating source. Hence flame detector are always use along with other
type of detector.

Infra-Red Flame Detector
Ultra-Violet Flame Detector

c. Oxygen Analyzer:
The portable or fixed type oxygen analyzer is used to evaluate the
O2 content in the atmosphere. The most vital part of the instrument is the
sensor which can be of different types:
i) Electrolytic cell
ii) Paramagnetic cell
iii) Chemical absorptioncell
d. Oil Mist Detector:
To ascertain the level of oil mist content inside the crankcase of an
engine and to safeguard the engine from danger of explosion this is used.
This works on the principle of obstruction in the incident light rays on a
photocell by the process medium changing the current value in the
photocellcircuit.
Oil Mist Detector

e. O2 Sensor:
The measurement methods of the oxygen analyzer currently available in the
industry can be classified into the following categories.
1. Zirconia type measurement system
2. Paramagnetic type
3. Optical type
4. Electrochemical type
Since each of the measurement methods has its advantages and
disadvantages, it is important to select an oxygen analyzer of an appropriate
method for your application and usage.
The following describes an overview of each of the measurement methods and
their advantages and disadvantages.
1. Zirconia type measurement system:
Concentration cell type:
A solid electrolyte like zirconia exhibits conductivity of oxygen ions at high
temperature.
When porous platinum electrodes are attached to both sides of the zirconia
element to be heated up and gases of different partial oxygen concentrations are
brought into contact with the respective surfaces of the zirconia, the device acts as
an oxygen concentration cell. This phenomenon causes an electromotive force to
be generated between both electrodes and it is proportional oxygen concentration.
Advantages: Can be directly installed in a combustion process such as a
boiler’s flue and requires no sampling system, and response is
faster.Capable of measuring trace oxygen concentration.
Disadvantages: If the sample gas contains a flammable gas, a measurement error
occurs (combustion exhaust gas causes almost no problem
because it is completely burned)

Limiting Current type:
If the flow of oxygen into the cathode of a zirconia element heated to high
temperature is limited, there appears a region where the current becomes constant
even when the applied voltage is increased.This limited current is proportional to
the oxygen concentration.
Advantages: Capable of measuring trace oxygen concentration.Calibration is
required only on the span side (air).
Disadvantages: If the sample gas contains a flammable gas, a measurement
error occurs. The presence of dust causes clogging of the gas
diffusion holes on the cathode side; a filter must be installed in a
preceding stage.
2. Magnetic Type measurement system:
Paramagnetic Type:
This is one of the methods utilizing the paramagnetic property of oxygen.
When a sample gas contains oxygen, the oxygen is drawn into the magnetic field,
thereby decreasing the flow rate of auxiliary gas in stream B. The difference in
flow rates of the two streams, A and B, which is caused by the effect of flow
restriction in stream B, is proportional to the oxygen concentration of the sample
gas. The flow rates are determined by the thermistors and converted into electrical
signals, the difference of which is computed as an oxygen signal.

Advantages: Capable of measuring flammable gas mixtures that cannot be
measured a zirconia oxygen analyzer. Because there is no sensor
in the detecting section in contact with the sample gas, the
paramagnetic system can also measure corrosive gases.
Among the magnetic types, the paramagnetic system offers a
fasterresponse time than other systems.
Among the magnetic types, the paramagnetic system is more
resistant vibration or shock than other systems.
Disadvantages: Requires a sampling unit corresponding to the sample gas
properties or applications.
Munday Cell Type: (Dumb-Bell System)
The unique paramagnetic (attraction towards a magnetic field) property
compared to other gases is utilized to measure the oxygen content of a process.
Two diamagnetic spheres of glass filled with Nitrogen are mounted at the
ends of a bar to form a dumb-bell. This is suspended horizontally from a platinum
ribbon suspension. It operates in a strong magnetic field. As nitrogen is
diamagnetic (repelled by magnetic field) the spheres are repelled from the strong
magnetic field and rotate twisting the suspension to its zero position when 100%
nitrogen flows in the magnetic field. The deflection of the pointer from zero
position is a proportional function to the force acting on the two spheres which in
turn proportional to oxygen content in that magnetic field.
A mirror is attached to the center of suspension where a light source
incidents and reflection is directed to a photo cell whose output current is
indicated in a milliammeter graduated in ‘O2 %’.

Sensitive element of the Munday cell
Basic Circuit of the Munday Cell

3. Optical type-Tunable Diode Laser measurement system:
Tunable Diode Laser (or TDL) measurements are based on absorption
spectroscopy. The TruePeak Analyzer is a TDL system and operates by
measuring the amount of laser light that is absorbed (lost) as it travels through the
gas being measured. In the simplest form a TDL analyzer consists of a laser that
produces infrared light, optical lenses to focus the laser light through the gas to be
measured and then on to a detector, the detector, and electronics that control the
laser and translate the detector signal into a signal representing the gas
concentration.
Tunable Diode Laser System of ‘O2’ Measurement
Advantages: Capable of measuring a number of near infrared absorbing
gases in difficult process applications.
Capability of measuring at very high temperature, high
pressures and under difficult conditions (corrosive, aggressive,
high particulate service).
Most applications are measured in-situ, reducing installation
and maintenance costs.
Disadvantages: The installation of the flange is necessary for both sides of
the process.

4. Electrochemical type: (Galvanic cell type)
If oxygen is dissolved via the diaphragm in an electrolytic solution in
which an anode (base metal) and cathode (noble metal) are adjacent to each other,
a current proportional to the quantity of dissolved oxygen is generated. The
amount of oxygen passing through the diaphragm is proportional to the partial
oxygen pressure of the sample gas, therefore, the oxygen concentration can be
determined by measuring the current.
Advantages: The detecting system can be made compact,
This measurement system is available in portable or
transportable form.
Relatively inexpensive in comparison with oxygen
analyzers of other measurement systems.
Disadvantages: The cell life is limited. As it is a kind of oxygen cell, the galvanic
cell deteriorates even if not used. In general, it should be
replaced approximately every year.

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