TEMPERATURE MEASUREMENTBy M. Khurram Saeed

Temperature Measurement Methods Mechanical or Non-Electric Methods Liquid in glass thermometer change in pressure Constant volume gas thermometer Bimetallic thermometer Electric Method Resistance temperature detector Thermistors Thermocouple

Radiation methods Total radiation pyrometerSelective radiation pyrometer Infrared pyrometer

What is thermometer A device to measure temperature

A thermometer has two important elements

a temperature sensor (e.g. bulb on the mercury thermometer) in which some physical change occurs with temperature.

Some mean of converting change into a value.

Glass thermometer Mercury filed in a glass tube and a glass bulb at a bottom.

As the temperature increases the mercury rises in the glass tube.

The glass tube is calibrated with Celsius, Fahrenheit or Both.

The range of the mercury thermometer is 390C to 340oC.

Application like open liquid tank, air ducts, stream line, cooking kettle etc.

Galileo Thermometer

Pressure spring thermometer The volume of liquid increases when heated. This expansion is

proportional to temperature.

It consists of mercury filed bulb, flexible capillary tube and the bourdon tube.

When the steel bulb is heated the mercury level in the bulb expands and hence temperature on the bourdon tube is increases.

Due to this the free end of the bourdon tube moves on the calibrated scale and indicates the temperature.

Advantages Simple and inexpensive design Remote indication upto 100 m is possible Fairly good response, accuracy and sensitive. Easily attached to pen recorder.

Bimetallic thermometer Bimetallic strips consists of two pieces of different metal welded.

Each piece of metal have different coefficient of expansion.

In industry bimetallic strip is wounded in helical form.

The bimetallic strip is made of brass and invar metals.

Typical range of bimetallic thermometer is -74oC to 540oC.

Advantages and application

Simple and inexpensive design. Quite rugged construction ,minimum chances

of damage It is used for automatic control in home

appliances. For the measurement of temperature of

engine , reaction chambers and furnaces

Resistance temperature detectors (RTD)

How it works? It utilizes the fact that resistance of metal change with temperature.

Made Up . Traditionally made up of platinum, nickel, iron or copper wound around a insulator.

Temperature Range from -196oC to 660oC.

RTD geometry

Sheathing and outer covering : stainless steel or Inconel, glass, alumina quartz.

Metal sheath can cause contamination at high temperature and best below 250oC.

At very high temperature, quartz and high-purity alumina are best to prevent contamination.

Types of RTD

RTD Advantages And Disadvantages

Advantages Stable

Very accurate

Change in resistance is linear

Wide temperature operating range -196oC to 660oC.

Good stability at high temperature.

Disadvantages Expensive

Current source required

Small change in resistance

Self heating

Less rugged than thermocouples

Affected by shock and vibration

Applications Air conditioning and refrigeration serving Food processing Stoves and grills Textile production Plastic processing Micro electronics Air ,gas and liquid measurement Exhaust gas temperature measurement

Thermistors How it works? Like RTD

use the fact that resistance of the metal change with temperature.

Make up? Generally made up of semiconductor metals.

Temperature range -60oC to 200oC.

THERMISTOR Advantages and Disadvantages

Advantages Very sensitive (has very

largest output change from input temperature).

Quick response More accurate than RTD

and Thermocouples No 4-wire bridge is

requires as in RTD

Disadvantages Output is a non linear

function. Limited temperature

range. Require a current source Self heating fragile

Thermocouples How it works? It made

up of two different metals joined at one end produce small voltage at given temperature.

Made up of two different metals.

temperature range -9oC to 750oC.

Thermocouple advantages and disadvantages

Advantages

Self power (does not require a current voltage source)

Rugged Simple inexpensive

disadvantages

Extremely low voltage output mV

Not very stable

Needs a reference point

Choice between RTD, thermistors and thermocouples

Cost-thermocouples are cheapest by far, followed by RTD’s

Accuracy-RTD or thermistor

Sensitivity-thermistor

Speed-thermistor

Stability at high temperature-not thermistor

Size-thermocouples and thermistors made quit small

Temperature range-thermocouples has highest range followed by RTD’s

Ruggedness-thermocouples are best

Pyrometer Pyrometer derived from greek root pyro mean fire.

A pyrometer is a non contact instrument that detects an object’s surface temperature by measuring temperature of electromagnetic radiation infrared or visible) emitted from object.

Idea: every object whose temperature is above absolute zero emits radiation.

Disappearing filament pyrometer

Advantages of pyrometer

High temperature measurement Fast response No adverse effect on temperature and

material Measuring moving objects Measuring objects which are difficult to

access.

Infrared thermometer They work by focusing infrared heat

into a sensor that can convert infrared energy into temperature units

Detect IR radiation (ʎ=0.7 -1000µm)

Non contact Wide range (30oC to 4500oC) Accuracy: 1% of reading

Benefits

Can be used for Moving objects Non contact

application where sensor would affect results or difficult to insert or condition are hazards.

Large distances Very high range.

references Industrial instrumentation and control by S.K Singh

www.authorstream.com/temperaturemeasurements