TRANSDUCER

What is TRANSDUCER ?“A device which convert the one form of energyinto another form of the energy”

OR“A device which converts a non-electrical quantity

into an electrical quantity”Functions:1. Detects or senses the presence , magnitude and

changes in physical quantity being measured.2.Provides a proportional electrical signal

CLASSIFICATION OF TRANSDUCERSA] Broadly classified in TWO groups;1] Active Transducer or Self generating typetransducer :- develop their own voltage or current(eg. Thermocouple and thrmopile , piezoelectricpick up , photovoltaic cell etc.)

2] Passive Transducers or Externally poweredtransducers :- derive power from external source(e.g RTD , Thermistor, potentiometric devices etc)

B] Based on type of output

1] Analog transducer:- convert i/p physical quantityinto an analogous o/p which is contineous function oftime. (e.g Strain guage , Thermocouple , Thermistor,LVDT etc)

2] Digital transducer :- convert i/p physical quantityinto an electrical o/p which may be in pulse form

C] Based on Electrical principle involved1.Variable-resistance type: a. Strain and pressuregauges b. Thermistors, resistance thermometers c.Photoconductive cell2.Variable –inductance type- a. LVDT b. Reluctancepick up c. Eddy current gauge3.Variable-capacitance type- a. capacitor microphoneb. Pressure gauge c. Dielectric gauge4.Voltage generating type- a. Thermocouple b.Photovoltaic cell c. Rotational motion tachometer d.Piezzoelectric pick up5. Voltage divider type- a. Potentiometer positionsensor b. Pressure actuated voltage divider

Selection of Transducer:

1. Ranges available 7. Maximum depth

2.Squaring system 8. Linearity and hysteresis

3. Sensitivity 9. Output for zero input

4. Maximum working temp. 10. Temp. coe. Of zero drift

5. Method of cooling 11. Natural frequency

6. Mounting details

Temperature Measurement using RTD(Resistance Temperature Detector)

The Birth of RTD• The classical resistance temperature detector (RTD)

construction using platinum was proposed by C.H. Meyers in1932.

• He wound a helical coil of platinum on a crossed mica web andmounted the assembly inside a glass tube.

• This construction minimized strain on the wire whilemaximizing resistance.

Platinum Sensing Resistors• Platinum, with its wide temperature range and stability, has

become the preferred element material for resistancethermometers.

• Platinum sensing resistors are available with alternative Rovalues, for example 10, 25 and 100 Ohms.

• A working form of resistance thermometer sensor is defined inIEC and DIN specifications.

• This forms the basis of most industrial and laboratoryelectrical thermometers.

• The platinum sensing resistor, Pt100 to IEC 751 is dominant inmany parts of the world.

• Its advantages include chemical stability, relative ease ofmanufacture, the availability of wire in a highly pure form andexcellent reproducibility of its electrical characteristic.

• The result is a truly interchangeable sensing resistor which iswidely commercially available at a reasonable cost.

RTD

Resistance MeasurementSeveral different bridge circuits are used todetermine the resistance. Bridge circuits helpimprove the accuracy of the measurementssignificantly. Bridge output voltage is a function ofthe RTD resistance.

FROM THE EQUATION OF RESISTANCE,

• R = resistance• ρ = specific resistance of the

conductor material• L = the length of the

conductor in meters• A = the area of the conductor

in square meters• When a strain produced by a

force is applied on the wires,L increase and A decrease.

0

50

100

150

200

250

300

350

400

-300 -100 100 300 500 700 900

t, oC

R, O

hm

Platinum RTD

Copper RTD

COMPARISON OF ELEMENT TYPES

Calendar—Van Dusen equation

30 01.0101.001.0101.01 TTTTTRR

• Where, a,b and dare calibration constants, dependenton the purity of the platinum which is countrydependent.•The dominant constant is a which has a value of either0.003921/°C for the so-called U.S. calibration curve, or• 0.003851/°C for the "European" calibration curve.•RTD sensors corresponding to either curve are available.•For the U.S. calibration curve, d = 1.49, b = 0 for T < 0and b = 0.11 for T>0.

4320 1001 TCTCTBTARR

Properties of RTD1.Linearity of resistance2. Resist to corrosion and oxidation under the temp. range3. To provide reproducible and consistent results4. Good sensitivity5.High sensitivity i.e unit can be fabricated in compact andconvenient size.6.No change of phase or state within a reasonable temperaturerange

Note:- Industrial thermometers are usually made as Platinum ,Nickel , Copper and basically for precise measurement Platinumis preferred because physically stable , high electrical resistancecharacteristics .*Platinum resistance thermometer to its accuracy, stability,sensitivity temp range from b.p of Oxygen -182.9 deg. C tofreezing point of antimony 630.5 deg. C

Advantages of RTD1.Resist to corrosion and oxidation2. Physically stable3. Measurement is very accurate4. It has a lot flexibility5. It does not require a reference junction6. Temp. sensitive resistance element can be replaced andinstalled easily7. High working signal level8. The limits of error of resistance element are claimed to be +/-0.25 % of the scale range9.The accuracy of measuring circuit can be checked easily bysubstituting a std resistor.10. Self heating effect can be reduced to negligible with properdesign11.The resistance element response time is order of 2 to 10seconds

Dis-advantages of RTD1. It is more costly

2. They suffer from time lag

3. Leakage current is possible between resistance element toground

4. Resistance change due to temperature changes of measuringresistors

Temperature Relationships

(°F) = 9/5*(°C) +32

(°C) = 5/9*[(°F) –32]

(°F) = (°R) – 459.67

(°C) = (K) – 273.15

THERMOCOUPLE

A thermocouple is in simplest form, consist of twodissimilar metals or alloys which develop emf whenthe reference and measuring junction are at differenttemperatures.

The reference or cold junction is maintained atconstant temp. such as 0 deg cen.

Fig shows a simple circuit of thermocouple and tempmeasuring device.

In industrial installations , equipment's are equippedwith automatic compensating devices for temp.changes of the reference junction , thus eliminating thenecessity of maintaining this junction at constanttemp.

• Seebeck effect

1. If two wires of dissimilar metals are joined at bothends and one end is heated, current will flow.

2. If the circuit is broken, there will be an open circuitvoltage across the wires.

3. Voltage is a function of temperature and metal types.

4. For small DT’s, the relationship with temperature islinear

5. For larger DT’s, non-linearities may occur.

V T

Properties of Thermocouple:

1. The temp and e.m.f relation should be linear andreproducible

2. Adequate thermo e.m.f per degree of temp. change tofacilitate detection and measurement

3. It should be strong for withstand high temp.

4. It should maintain its calibration without drift for longperiod of time.

5. Cost should be reasonable

6. It should have long life

TYPES OF THERMOCOUPLE

Thermocouple Types• Type B – very poor below 50ºC; reference junction temperature not

important since voltage output is about the same from 0 to 42 ºC

• Type E – good for low temperatures since dV/dT is high for lowtemperatures

• Type J – cheap because one wire is iron; high sensitivity but also highuncertainty (iron impurities cause inaccuracy)

• Type T – good accuracy but low max temperature (400 ºC); one lead iscopper, making connections easier; watch for heat being conductedalong the copper wire, changing your surface temp

• Type K – popular type since it has decent accuracy and a widetemperature range; some instability (drift) over time

• Type N – most stable over time when exposed to elevatedtemperatures for long periods

Advantages of Thermocouple:

1. Better response

2.Higher range of temp. measurements

3. Sensing element can be easily installed

4. Cheap

5. Very convenient for measuring the temp. atone particular point in a piece of appratus.

Disadvantages of Thermocouple:

1. Low accuracy

2. Circuit is very complex

3. For long life they need to be amplyprotected.

Choice Between RTDs, Thermocouples, Thermisters• Cost – thermocouples are cheapest by far, followed by RTDs

• Accuracy – RTDs or thermistors

• Sensitivity – thermistors

• Speed – thermistors

• Stability at high temperatures – not thermistors

• Size – thermocouples and thermistors can be made quite small

• Temperature range – thermocouples have the highest range, followed by RTDs

• Ruggedness – thermocouples are best if your system will be taking a lot ofabuse

Thank You