Automation Lecture 9

8/13/2019 Automation Lecture 9

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Automation Lecture 9


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Pressure Sensor types

Abso lu tepressure sensorsmeasure pressurewith respect to a zero pressure reference value(vacuum reference).

Differentialpressure sensorsmeasure the

difference between two pressure environments. Negat ivepressure sensorsmeasure vacuum

pressure with respect to atmospheric (ambient)pressure.

Gauge pressure sensorsbehave much thesame way as differential pressure sensors, butmeasure positive pressure in relation to theenvironment's ambient pressure


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Piston Based Pressure Sensor


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Burdon Tubes

A Bourdon tube is a short bent tube, closed at one end.

When the tube is pressurized, it tends to straighten out. This

motion is proportional to the applied pressure.


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Bellows

This sensor uses a small metal bellows to convert pressureinto linear motion. As the pressure inside increases, the

bellows expand against the resistance of a spring (the spring is

often the bellows itself). This motion is detected with a position

sensor such as a pot.


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Piezoresistive Pressure sensor

A high-density diffusion layer of piezoresistive material is formed

to make a resistive layer on a film. Compression of thissemiconductor film element bends the crystalline form of the

piezo material so as to change its resistance value in proportion

to the amount of stress on its surface. The signal changes are

then amplified and interpreted as pressure readings.


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http://www.omega.com/ppt/pptsc_lg.asp?ref=PX760&Nav=preb05http://www.omega.com/ppt/pptsc_lg.asp?ref=PX771A-PMM&Nav=preb05http://www.omega.com/ppt/pptsc.asp?ref=PX750&nav=preb05


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Flow Transducers

Flow sensors measure the volume of material that passes the

sensor in a given time. Such sensors are widely used in

process control industries.

Measured material can be:

-Fluid

-Gas

-Conductive

-Not Conductive


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Mechanical flow Transducers
http://www.omega.com/ppt/pptsc.asp?ref=FTB900&nav=gref02


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http://www.omega.com/ppt/pptsc.asp?ref=FTB8000A&nav=gref02http://www.omega.com/ppt/pptsc.asp?ref=FTB900&nav=gref02http://www.omega.com/ppt/pptsc.asp?ref=FPR200&nav=gref02


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Flow Transducers based on

pressure Measurement

Pressure sensors are often used to measure fluid flow. The faster the fluid is flowing,the more pressure it will create in the open end of the pitot type flow meter.

Pressure upstream from a restricted orifice in a pipe is always higher than pressure

downstream from that restriction. The greater the flow rate, the greater the pressure

difference, so if a differential pressure sensor compares pressures before and after

the restriction, then flow rate can be determined. The restriction orifice required by

such a sensor reduces the flow.


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Magnetic Flow Measurement

The operation of a magnetic flowmeter or mag meter is based upon

Faraday's Law, which states that the voltage induced across any

conductor as it moves at right angles through a magnetic field is

proportional to the velocity of that conductor.

-fluid being measured must be electrically conductive

-No pressure loss

-No moving parts

- ~3000Euro
http://www.omega.com/ppt/pptsc.asp?ref=FMG400&nav=


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Ultrasonic Flow Meter

An ultrasonic flowmeter (non-intrusive Doppler flow meters) is a volumetric flow meter

which requires particulates or bubbles in the flow. Ultrasonic flowmeters are ideal for

wastewater applications or any dirty liquid which is conductive or water based.

Ultrasonics flowmeters will generally not work with distilled water or drinking water.

Aerations would be required in the clean liquid applications. Ultrasonic flowmeters are

also ideal for applications where low pressure drop, chemical compatibility, and lowmaintenance are required.

The basic principle of operation employs

the frequency shift (Doppler Effect) of an

ultrasonic signal when it is reflected bysuspended particles or gas bubbles

(discontinuities) in motion. This metering

technique utilizes the physical

phenomenon of a sound wave that

changes frequency when it is reflected by

moving discontinuities in a flowing liquid.Ultrasonic sound is transmitted into a pipe

with flowing liquids, and the discontinuities

reflect the ultrasonic wave with a slightly

different frequency that is directly

proportional to the rate of flow of the liquid


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http://www.omega.com/ppt/pptsc_lg.asp?ref=FDT100&Nav=gree02


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Rotameter

A rotameter consists of a tapered tube,

typically made of glass, with a float inside that

is pushed up by flow and pulled down by

gravity. At a higher flow rate more area(between the float and the tube) is needed to

accommodate the flow, so the float rises.

Floats are made in many different shapes, with

spheres and spherical ellipses being the most

common.
http://www.kinginstrumentco.com/products/products.html


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Temperature Sensors

posi t ive temperature coeff ic ient

(desirable), which means that the sensor

output goes up as the temperature goes

up

negative temperature coeff ic ient, which

means that the output goes down as the

temperature goes up


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Bimetallic Temperature Sensors

The bimetallic strip is a laminate of two metals with different

coefficients of thermal expansion. As the temperature rises,

the metal on the inside expands more than the metal on the

outside, and the spiral tends to straighten out. These sensors

are typically used for on-off control


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Resistance Temperature Detector

(RTD) The resistance temperature detector (RTD) is a temperature

sensor based on the fact that metals increase in resistance astemperature rises.

Platinum wire has a temperature coefficient of 0.0039 //C,which means that the resistance goes up 0.0039 for each

ohm of wire for each Celsius degree of temperature rise. RTDsare available in different resistances, a common value being100 . Thus, a 100- platinum RTD has a resistance of 100 at 0C, and it has a positive temperature coefficient of 0.39/C. Platinum wire has a temperature coefficient of 0.0039//C, which means that the resistance goes up 0.0039 foreach ohm of wire for each Celsius degree of temperature rise.RTDs are available in different resistances, a common valuebeing 100 . Thus, a 100- platinum RTD has a resistance of100 at 0C, and it has a positive temperature coefficient of0.39 /C.


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http://www.omega.com/ppt/pptsc.asp?ref=NB1-ICIN_INDUST_TC&nav=tema14


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Thermocouples

The thermocouple is based on the Seebeck effect, a phenomenonwhereby a voltage that is proportional to temperature can be produced

from a circuit consisting of two dissimilar metal wires.


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Thermistor

A thermistor is a two-terminal device that changes

resistance with temperature. Thermistors are made of oxide-

based semiconductor materials and come in a variety of

sizes and shapes. Thermistors are nonlinear; therefore, they

are not usually used to get an accurate temperature reading

but to indicate temperature changes, for example,

overheating. Also, most thermistors have a negative

temperature coefficient, which means the resistance

decreases as temperature increases, as illustrated with thesolid line in the graph of Figure 6.50(a). A very desirable

feature of these devices is their high sensitivity. A relatively

small change in temperature can produce a large change in

resistance.


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Discrete Liquid Level Detection


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Continuous Level Detection

Automation Lecture 9

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