Module 3.pptx

7/27/2019 Module 3.pptx

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is a measure of average KINETIC ENERGY of the

particles that makes up a body.

is the ability of one body to transfer thermal energy

to another body.

It is the Degree of HOTNESS of a body.



TEMPERATURE - Measurement Scales

Relative Scales

Fahrenheit (°F)

Celsius (°C)

Absolute Scales

Rankine (°R)

Kelvin (K)



100:C

373:K

273:K

255:K

0:C

-18:C

-273:C 0:K

-460:F 0:R

0:F

32:F

460:R

492:R

672:R 212:F Fahrenheit[°F] = [°C] · 9/5 + 32

Celsius[°C] = ([°F] − 32) x 5/9

Kelvin[K] = [°C] + 273.15

Rankine[°R] = [°F] + 459.67

TEMPERATURE - Measurement Scales



Mechanical Methods Filled – in – System thermometers

Bimetallic Strips/ Expansion of materials

Bulb & Capillary Sensor

Electrical Methods

RTD Thermocouple

Thermistor

Pyrometers etc.

TEMPERATURE - Measurement Methods



Bimetallic Strip Thermometer

Suppose metal A has a smaller coefficient of thermal expansion than

does metal B. As temperature increases, metal B expands more than

does metal A, causing the bimetallic strip to curl upwards as sketched.

Two dissimilar metals are bonded together into

what is called a bimetallic strip.



Bimetallic Thermometer (Expansion of solids)

Effect of unequal expansion of a bimetallic strip

Different metals have difference coefficient.

Configured as spiral or helix for compactness

Can be used with a pointer to make an inexpensive

compact rugged thermometer.




Effect of unequal expansion of a bimetallic strip



One common application of bimetallic strips is in air-conditioningthermostats, where a bimetallic strip is used as the arm of a switch

between electrical contacts. As the room temperature changes, the

bimetallic strip bends as discussed above. When the bimetallic strip

bends far enough, it makes contact with electrical leads which turn the

heat or air conditioning on or off.

Bimetallic Strip as Thermostat



Liquid-in-Glass Thermometer

The most common and well-known thermometer

is the liquid-in-glass thermometer.

As the temperature rises, the liquid expands,

moving up the tube. The scale is calibrated to

read temperature directly. Usually, mercury or

some kind of alcohol is used for the liquid.

Mercury thermometers can range from -38F to 1110 F

Alcohol thermometers range from -328 F to 1110 F

Other thermometer fill fluids include Benzene & ether



Filled-in-System Thermometers

Similar operation as the

liquid in glass system.

Filled system thermometers,

also called pressure

thermometers.



Sensing element is a capillary tube

filled with a liquid or gas which

expands with an increase in

temperature.

This sensing element delivers a

motion of physical change that is

applied to the control element

which either indicates, records, or

by comparing the signal to a setpoint can be used to control the

temperature of a process.

How it works?



Elements

Bulb

Capillary tube

Pressure element

Scale



Class I (A,B) – Liquid filled (Excluding Mercury)

Class II (A,B,C,D) – Vapour filled

Class III (A,B) – Gas filled

Class IV - Deleted

Class V (A,B) – Mercury Filled

Filled-in-System Thermometers - TYPES



Filled-in-System Thermometers - TYPES

Temperature Range Response

Class I: -125 F to + 600 F Slowest

Class II: -40 to 32 or 32 to 600 F Fastest Class III: -450 F to +1400 Fast

Class V: -40 F to +1200 F Fast



The big advantage of the filled system is that

it places the sensor in one place and the

signal processing equipment on another



Class I (Liquid Filled) Systems

Liquid Range (0C)

Ethyl Alcohol -45 to 150

Xylene -40 to 400

Ether (Ethyl) 20 to 90

Toulene -80 to 250



Sensing bulb partially filled with volatile

fluid.

Common fluids include: methyl chloride,

ether, butane, hexane, propane,toluene, sulfur dioxide, water etc.

Based upon the principle that in a

system containing only a liquid and its

vapor, at a given temperature, a given

pressure will exist in the system,

regardless of system volume.

Actual temperature measurement

occurs at interface between liquid and

vapor.

Class II (Vapour Filled) Systems



May exhibit erratic operation when temperature

being measured swings above and below ambient

temperature.

Offers good reliability, inherently accurate, non-uniform scales (non-linear).

Class II (Vapour Filled) Systems …



Utilizes perfect gas law PV = nRT

Constant volume thermometers.

When temperature raised at constant volume pressureraises to operate Bourden gauge.

Helium approximates perfect gas, but tends to leak and isnot often used

Nitrogen usually is used

Compensation generally not necessary if a large bulb is

used.

Class III (Gas Filled) Systems



1. Ambient Temperature Effect

2. Head or Elevation Effect

3. Barometric Effect

4. Immersion Effect

5. Radiation Effect


ERRORS



The change in ambient temperature causes volume

changes in the capillary and the pressure spring cases

error in measurement.

Its value is given by tE = (Vctc+Vptp)/Vb

where c = capillary; p = pressure spring; b = bulb;

E = error; t=change in temperature; V=Volume

1. Ambient Temperature Effect



If the thermometer bulb is placed at a different height with

respect to the pressure spring, elevation error arises.

For the bulb at higher position, pressure reading is also high.

The error in % span,

2. Head/Elevation Error

h = height difference;

ρ=fluid density

Kb&kf = bulb & fluid stiffnesses

% Es = 100hρ(kb+kf )Vb/∆

= 100hρ(kb+kf )/(Rδ)



Due to atmospheric pressure changes, changes the

effective differential pressure at the pressure spring.

For a pressure change ∆p in the system and a

barometric pressure change of ∆pB, error in percentage

of the span of ts,

tb= 100 ∆pB ts/∆p

3. Barometric Effect



If the bulb is not properly or fully immersed in the

process and the head of the bulb is properly insulated,

heat from the bulb is lost due to conduction throughthe extension neck and thermal well causing error.

the error is roughly proportional to the unexposed area

of the bulb.

4. Immersion Effect



Occurs due to the temperature difference between the

bulb and the other solid bodies around it.

5. Radiation Error



CASE COMPENSATION

FULL COMPENSATION (CASE+CAPILLARY COMPENSATION)

COMPENSATION TECHNIQUES



CASE COMPENSATION

Inserting a bimetal strip between the free end of Bourden Gauge

and the pointer.

The deflection of the bimetal strip due to ambient temperature

is in such a way that it compensates the deflection of the

Bourden Gauge due to ambient temperature effect.



FULL COMPENSATION



FULL COMPENSATION…

A dummy system excluding bulb is arranged by side.

The dummy system provides a rotation exactly equal in

magnitude but opposite in direction compared to the ambient

temperature effect on the measurement system.



Advantages

Self Contained. They do not require auxiliary power source.

They are simple, rugged, lasting and have minimum

possibility of being damaged during shipping andinstallation.

Inexpensive due to simple design.

Sensitivity and Speed of response fairly good.

They can be designed to deliver significant power to drive

indicating or controlling mechanisms, including valves.

The capillary allows considerable separation between the

point of measurement and the point of indication.




Disadvantages

In the case of system failure entire units need to be replaced.

Many applications may not allow large bulb volume within the

measured medium. Performance characteristics vary for different liquids, so care

should be taken for particular application.

The maximum temperature is limited compared to electrical

systems. The separation of the sensing and indicating elements is

limited compared to electrical systems.

Electronic sensors are more preferred due to flexibility,

distant measurement, cost etc.




Electrical MethodsElectrical Temperature Transducers



Thermistor, a word formed by combining thermal with

resistor, is a temperature-sensitive resistor fabricated from

semiconducting materials.

The resistance of thermistors decreases proportionally withincreases in temperature.

The operating range can be -200°C to + 1000°C



The thermistors can be in the shape of a rod, bead or

disc.

Manufactured from oxides of nickel, manganese, iron,

cobalt, magnesium, titanium and other metals.



The word that best describes the thermistors is

“sensitive”



Advantages:

Small sizes and fast response

Low cost

Suitability for narrow spans

Disadvantages:

More susceptible to permanent decalibration at high

temperatures.

Use is limited to a few hundred degrees Celsius.

Respond quickly to temperature changes, thus,

especially susceptible to self-heating errors.

Very fragile



RTD (Resistance Temperature Detector) is a temperature

sensitive resistor.

It is a positive temperature coefficient device, which

means that the resistance increases with temperature.

The resistive property of the metal is called its

resistivity.

The industry standard is the platinum wire RTD

(Pt100) whose base resistance is exactly 100.00

ohms at 0.0 °C.



linear temperature sensors.

Resistance Vs temperature characteristics are stable andreproducible.

linear positive temperature coefficient (-200 to 800°C).

very accurate and suitable for use as a secondarystandard.

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