Industrial Instrumentation

Prepared By

P.Vidhyalakshmi

Assistant Professor(SLG)

FORCE MEASUREMENT

Introduction

• Force: It is defined as the reaction between the two bodies or components.
• The reaction can be either tensile force (Pull) or it can be Compressive force (Push).
• Measurement of force can be done by any two methods:
• Direct Method: This involves a direct comparison with a known gravitational force on a standard mass. Example: Physical Balance.
• Indirect Method: This involves the measurement of effect of force on a body. E.g. Force is calculated from acceleration due to gravity and the mass of the component.

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FORCE MEASUREMENT

Strain Gauge Load cell

Strain Gauge Load cell

Strain Gauge Load cell

Strain Gauge Load cell

Strain Gauge Load cell

Hydraulic Load cell

Hydraulic Load cell

Hydraulic Load cell

Hydraulic Load cell

Pneumatic Load cell

Pneumatic Load cell

Pneumatic Load cell

Pneumatic Load cell

PRESSURE MEASUREMENT

Importance of Pressure Measurement

• Pressure influences boiling and condensation temperatures of some separation operations like distillation and therefore their costs

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• Pressure measurement is necessary for measurement of flow and level

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Units of pressure

• Pressure is the normal force exerted on unit area of a surface

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• According to definition,
• SI unit of pressure is

Pa or k Pa (Pa = N/m²) for low pressures,

kg_f / cm² for high pressures

• Atmospheric pressure is the pressure exerted by the atmosphere at sea level
• P_atm = 101.3 kPa = 14.7 psi

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Vacuum sensors

Mechanical Vacuum Gauges:

1. McLeod gauge

Electrical Vacuum Gauges:

1. Thermal Vacuum Gauges
2. Knudsen gauge
3. Pirani gauge
4. Thermocouple vacuum gauge
5. Ionization Vacuum Gauges

Testing and Calibration of Pressure Detectors: Dead weight tester Pressure Switches.

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McLeod gauge

McLeod gauge construction

• The McLeod Gauge is used to measure vacuum pressure. 
• It also serves as a reference standard to calibrate other low pressure gauges.
• Construction:
• The components of McLeod gauge include a reference column with reference capillary tube.
• The reference capillary tube has a point called zero reference point.
• This reference column is connected to a bulb and measuring capillary and the place of connection of the bulb with reference column is called as cut off point.
• It is called so because if the mercury level is raised above this point, it will cut off the entry of the applied pressure to the bulb and measuring capillary.
• Below the reference column and the bulb, there is a mercury reservoir operated by a piston.

McLeod gauge working

Working:

• The pressure to be measured (P₁) is applied to the top of the reference column of the McLeod Gauge as shown in Fig.
• The mercury level in the gauge is raised by operating the piston to fill the volume as shown by the dark shade in the diagram.
• When the applied pressure fills the bulb and the capillary, again the piston is operated so that the mercury level in the gauge increases.
• When the mercury level reaches the cut-off point, a known volume of gas (V₁) is trapped in the bulb and measuring capillary tube.
• The mercury level is further raised by operating the piston so the trapped gas in the bulb and measuring capillary tube is compressed.
• This is done until the mercury level reaches the Zero reference Point marked on the reference capillary.
• In this condition, the volume of the gas in the measuring capillary tube is read directly by a scale besides it.

McLeod gauge

Working:

• That is, the difference in height “h” of the measuring capillary and the reference capillary becomes a measure of the volume (V₂) and pressure (P₂) of the trapped gas. Now as V₁, V₂, and P₂ are known, the applied pressure P₁ can be calculated using Boyle’s Law given by:

P₁V₁ = P₂ V₂

• The working of McLeod Gauge is independent of the gas composition.
• A linear relationship exists between the applied pressure and height and there is no need to apply corrections to the readings.
• The limitations are that the gas whose pressure is to be measured should obey the Boyle’s law and the presence of vapours in the gauge affects the performance.

McLeod gauge

Advantages of the McLeod Gauge

• Advantages of the McLeod Gauge:
• It is independent of the gas composition.
• It serves as a reference standard to calibrate other low pressure gauges.
• A linear relationship exists between the applied pressure and h
• There is no need to apply corrections to the McLeod Gauge readings.

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Applications of McLeod gauge

• Applications of McLeod gauge
• McLeod gauge is used mainly for calibrating other inferential type of gauges.
• The shortcomings of the McLeod gauge are its fragility and the inability to measure continuously.
• The vapor pressure of Mercury sets the lower limit of measurement range of the gauge.

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Limitations of McLeod Gauge

• Limitations of McLeod Gauge:
• The gas whose pressure is to be measured should obey the Boyle’s law
• Moisture traps must be provided to avoid any considerable vapor into the gauge.
• It measure only on a sampling basis.
• It cannot give a continuous output.

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Thermal Vacuum Gauges

• A. Knudsen Gauge
• B. Thermal conductivity Gauge
• 1. Pirani Gauge
• 2. Thermistor Vacuum Gauge
• 3. Thermocouple Vacuum Gauge

A. Knudsen gauge

• A knudsen gauge is a device which is used to measure absolute pressure in the range of 10^(-8)-10^(-3) torr.

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• Named after it’s inventor Martin Knudsen who was a Danish physicist.

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• Independent of composition of gas ,hence

suitable for laboratory operation.

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• Based on momentum transfer operations.

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• Pressure is determined by interaction of

particles with it surface its kinetic energy

and is temperature dependent.

A. Knudsen gauge Construction

A. Knudsen gauge construction

The parts of a knudsen gauge :

• Vanes : The two vanes are joint to make a rectangular frame suspended by a thin filament fibre .
• Mirror : The mirror is fitted in the thin filament fibre to note the deviations of the pressure.

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• Thin plates : Two fixed heated plates are located opposite to these vanes. These plates are maintained at temperature T. The separation between vanes and plates is less than mean free path of gas.
• Air Inlet : It allow the passage of low chamber gas to the gas chamber .
• Heater : The heater keeps the temperature of the fixed plates higher than surrounding gas pressure

A. Knudsen gauge Working

• Two vanes V along with mirror M are mounted on thin filament

suspension.

• The two heated plates P are maintained at a temperature T.
• The separation distance between two gases is less than mean free path of the surrounding gas.
• The vanes are maintained at gas temperature Tg.
• Gas molecules through the air inlet striking the vanes from the

hot plates have a higher velocity than those leaving the vanes

because of of the difference in temperature.

• Thus there is a net momentum imparted to the vanes which may be measured by observing the angular dispacement of mirror
• Similar o technique use in light beam galvanometer

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A. Knudsen gauge Working

A. Knudsen gauge

A. Knudsen gauge

Applications

• Used for pressure measurement of non condensable gases such oxygen and nitrogen.
• Earlier used for measuring gas pressure in ion discharge experiments.
• Commonly used in petroleum industries to measure gaseous hydrocarbon’s pressure

Thermal Vacuum Gauges

• A. Knudsen Gauge
• B. Thermal conductivity Gauge
• 1. Pirani Gauge
• 2. Thermistor Vacuum Gauge
• 3. Thermocouple Vacuum Gauge

Cool Medium: High Pressure or High Density or high conductivity

Hot Medium: Low pressure or Low Density or Low conductivity

1. Pirani gauge

• The traditional Pirani vacuum gauge, originally invented in 1906 by Marcello Stefano Pirani, is based on a hot metal wire suspended in a tube that is exposed to gas pressure media.
• The Pirani gauge measures the vacuum pressure dependent thermal conductivity from the heated wire to the surrounding gas.
• The heated Pirani sensor filament is typically made of a thin (<25 µm) Tungsten, Nickel or Platinum wire.

Principle:

As gas molecules collide with the filament wire, heat is transported from the hot wire.

The heat loss is a function of the gas pressure and at low pressure the low gas density and long mean free path between gas molecules provides a low thermal conductivity.

At high pressure the high gas density and short mean free path between molecules will result in high thermal conductivity. 

1. Pirani gauge Construction

1. Pirani gauge working

• The Pirani wire filament is typically operated in a balanced Wheatstone bridge circuit where one leg of the bridge is the Pirani filament and the other three elements of the bridge circuit balance and temperature compensate the circuit. 
• The filament wire is maintained at a constant temperature and when the gas density changes and thereby thermal conductivity changes the energy required to maintain the wire changes accordingly. Consequently, the voltage supply to the Wheatstone bridge becomes vacuum pressure dependent and the measured bridge voltage can be converted to a pressure value.
• The Pirani gauge measures the thermal conductivity of the gas and therefore the measurement is dependent of the gas properties.

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1. Pirani gauge working

• The Pirani gauge consists of a metal wire open to the pressure being measured.
• The wire is heated by a current flowing through it and cooled by the gas surrounding it. If the gas pressure is reduced, the cooling effect will decrease; hence the equilibrium temperature of the wire will increase.
• The resistance of the wire is a function of its temperature and by measuring the voltage across the wire and the current flowing through it, the resistance can be determined and so the gas pressure is evaluated.

1. Pirani gauge working

1. Pirani gauge working

• Fig. shows a Pirani gauge with two platinum alloy filaments which act as resistances in two arms of a Wheatstone bridge.
• One filament is the reference filament and the other is the measurement filament.
• The reference filament is immersed in a fixed-gas pressure, while the measurement filament is exposed to the system gas.
• A current through the bridge heats both filaments. Gas molecules hit the heated filaments and conduct away some of the heat.
• If the gas pressure around the measurement filament is not identical to that around the reference filament, the bridge is unbalanced and the degree of unbalance is a measure of the pressure.
• The unbalance is adjusted and the current needed to bring about balance is used as a measure of the pressure.

2. Thermistor Vacuum working

• It is the same principle of pirani gauge.
• Here Thermistor (semiconductor) is used instead of Resistance wire in the circuit.

3. Thermocouple vacuum gauge

3. Thermocouple vacuum gauge

Construction:

• The thermal conductivity vacuum gauge works on the principle that at low pressure the thermal conductivity of a gas is a function of pressure.
• The Fig. 20.5 shows the basic elements of a thermocouple vacuum gauge. It consists of a linear element which is heated by a known current source and is contact with a thermocouple attached to its centre.
• The heater element together with the thermocouple is enclosed in a glass enclosure.
• The vacuum system to be evaluated is connected to this enclosure.
• The heater element is supplied with a constant electrical energy.
• Working Principle:
• The temperature of the heating element is a function of heat loss to the surrounding gas, which in turn is a function of thermal conductivity of gas that is dependent on the pressure of the gas.
• The temperature is measured by the thermocouple and is calibrated to read the pressure of the gas.
• This gauge is inexpensive and rugged in construction. It provides a convenient and continuous reading with a possibility of remote display.
• It however needs an individual and frequent calibration for different gases.

Ionization Vacuum Gauges

• Principle:
• The ion gauge works by ionising the gas molecules within the gauge volume. The ions are then collected on a thin wire, called the collector. The current formed is proportional to pressure. BA gauges are capable of measuring pressures in the range of around 3x10^-11 mBar to 1x10^-3 mBar.

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The ionization gauges are of two types,

1.The hot cathode ionization gauges

2.The cold cathode ionization gauges

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Ionization Vacuum Gauges: Construction

• The ionization gauge consists of A Triode Vacuum Tube. It has three terminals:
• A heated filament (cathode) to furnish electrons,
• A grid, and
• An anode plate.
• These elements are housed in an envelope, which is connected to the vacuum system under test (where pressure is to be measured) as shown in figure.
• The grid is maintained at a positive potential of 100 - 350 V, while, the anode plate is maintained at a negative potential of about 2 - 50 V with respect to cathode.
• Grid acts as Electron collector and Anode acts as Positive Ion Collector.

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Ionization Vacuum Gauges

• Diagram may be followed slide number 40 and 41 for hot and cold cathode.

1 Ip

Pressure= ----X------

K Ig

Ip – Anode Plate Current

Ig- Grid plate current

K - Constant

Ionization Vacuum Gauges: working

• Let us consider that, pressure of gas below the value of atmospheric pressure (vacuum pressure) is to be measured. The Negatively charged electrons emitted by the heated cathode are attracted towards the positively charged grid. The electrons are accelerated due to the high positive charge present on the grid and therefore, electrons rapidly move towards grid (i.e. away from cathode). Some of the electrons are captured by the grid, producing grid current Ig. Electrons having high kinetic energy are not captured by grid and they are passed through the grid and collide with gas molecules, thereby causing ionization of gas atoms.
• The rate of ion production is proportional to the number of electrons available to ionize the gas and the amount of gas present. The process of knocking off an electron from an atom and thus producing a free electron and a positively charged ion is known as Ionization.
• The positive ions so produced are attracted towards anode plate (which is at negative potential) and anode plate current Ip, is produced in the plate circuit. The negative ions (electrons) so produced are collected by the high positive charge present on the grid.
• Thus, the grid current Ig produced is due to,

(a) The collected negative ¡ons on the grid and

(b) The captured electrons by the grid.

The ratio of the anode current Ip to the grid current Ig is a measure of the gas pressure ‘P’. The pressure of gas can be given as,

P=1/k X Ip/Ig

Where, K = Sensitivity of gauge.

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Ionization Vacuum Gauges

• In hot cathode (Fig. version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge.
• The working of cold cathode gauge is also same with the only difference in the production of electrons which are produced in the discharge of a high voltage.

Hot Cathode Ionization Vacuum Gauges

• The hot filament off the hot-cathode gauge emits electrons into the vacuum, where they collide with gas molecules to create ions. These positively charged ions are accelerated toward a collector where they create a current in a conventional ion gauge detector circuit.
• The amount of current formed is proportional to the gas density or pressure.
• Most hot-cathode sensors measure vacuum in the range of 10-2 to 10-10 torr.

Cold Ionization Vacuum Gauges

• Cold Cathode: The major difference between hot and cold cathode sensors is in their methods of electron production. In a cold cathode device, electrons are drawn from the electrode surface by a high potential field. In the Phillips design (Figure 4-8), a magnetic field around the tube deflects the electrons, causing them to spiral as they move across the magnetic field to the anode. This spiraling increases the opportunity for them to encounter and ionize the molecules.

Typical measuring range is from 10^-10 to 10^-2 torr. The main advantages of cold cathode devices are that there are no filaments to burn out, they are unaffected by the inrush of air, and they are relatively insensitive to vibration.

Dead weight tester Pressure Switches

• The dead weight tester is basically a pressure producing and pressure measuring device.
• It is used to calibrate pressure gauges.
• Dead weight testers can measure pressures of up to 10,000 bar.

Construction:

It consists of a piston and cylinder combination fitted above the chamber as shown in Fig.

• The chamber below the cylinder is filled with oil.  The top portion of the piston is attached with a platform to carry weights.
• A plunger with a handle is provided to vary the pressure of oil in the chamber.
• The pressure gauge to be tested is fitted at an appropriate place as shown in the Figure.

Dead weight tester Pressure Switches

Dead weight tester Pressure Switches

Working:

• To calibrate a pressure gauge, an accurately known sample of pressure is introduced to the gauge under test and then the response of the gauge is observed.
• In order to create this accurately known pressure, the valve of the apparatus is closed and a known weight is placed on the platform above the piston.
• By operating the plunger, fluid pressure is applied to the other side of the piston until the force developed is enough to lift the piston-weight combination.
• When this happens, the piston weight combination floats freely within the cylinder between limit stops.
• In this condition of equilibrium, the pressure force of fluid is balanced against the gravitational force of the weights plus the friction drag on the piston.

Dead weight tester Pressure Switches

Applications:

It is used to calibrated all kinds of pressure gauges such as industrial pressure gauges, pressure transmitters etc.

Advantages:

• It is simple in construction and easy to use. It can be used to calibrated a wide range of pressure measuring devices.

• Fluid pressure can be easily varied by adding weights or by changing the piston cylinder combination. Limitations:

• The accuracy of the dead weight tester is affected due to the friction between the piston and cylinder.

TEMPERATURE MEASUREMENT

Introduction

• Temperature: Degree of hotness or coldness of a body
• Bonding
• Solids – Tight – fixed shape
• Liquids – less tightly – take the shape of the container
• Gas – loosely bonded – fill completely the enclosed spaces

Introduction

• Body is heated - the vibrational speed of its molecules increases rapidly
• Dimensional changes in solids
• Pressure change in liquids
• Solid to liquid
• Liquid to gas

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Physical effects utilized to measure temperature

• Expansion of liquid or solid
• Change in pressure
• Change in electrical resistance
• Thermoelectricity
• Radiation
• Special temperature indicating devices

Physical effects utilized to measure temperature

(i) Expansion of liquid or solid

- Materials expand and contract with temperature.

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Bimetallic thermometer – Linear expansion

Expansion of mercury in thermometer

Physical effects utilized to measure temperature

ii) Change in pressure

When a fluid is confined, its pressure

increases when the temperature rises

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Ex : filled system thermometer , gas thermometer

iii) Change in electrical resistance

Ex : RTD , Thermistor

Physical effects utilized to measure temperature

iv) Thermoelectricity

The thermoelectric effect is the direct conversion of temperature is the direct conversion of temperature differences to electric voltage is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple

v) Radiation �Radiations emitted from a hot object is used to measure temperature� Ex: Radiation Pyrometer

Physical effects utilized to measure temperature

iv) Thermoelectricity

The thermoelectric effect is the direct conversion of temperature is the direct conversion of temperature differences to electric voltage is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple

v) Radiation �Radiations emitted from a hot object is used to measure temperature� Ex: Radiation Pyrometer

Physical effects utilized to measure temperature

• Special temperature indicating devices
• Temperature indicating paints , applied by brush or spray (Thermo-colour)
• Temperature indicating Crayons ( Thermochromes) with which the work piece stocked usually show a single colour change.
• Temperature indicating pellets , shapes formed of selected metals or alloys will melt at pre-established temperature.
• Seger Cones – In the ceramic industry , the cones are prepared by the mixture of minerals have some pyrometric value. The point at which the tip of the cone softens and bend over to touch the base gives the desired temperature for which the cone is prepared.

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Three basic fixed point

• Fixed point is a standard degree of hotness or coldness such as the melting point of ice or boiling point of water
• Boiling point

Temperature at which the substances changes from the physical state to gaseous state.

• Freezing point

Temperature at which the substances changes from the physical state to solid state.

• Triple point

At particular temperature and pressure , three different phases of 1 substance can exist in equilibrium is called as an triple point.

Temperature Scales

Class 1 : Liquid filled thermometer.

• Change in volume and the temperature is measured.
• It can be used upto a range of -200 °C to 300 °C
• Height of the liquid column indicates the temperature .

 V_t = V_o(1+αt)

V_t = Volume at temperature ‘t’

V_o= Volume at 0° temperature

α = Coefficient of cubical expansion

• Commonly used liquid alcohol, toluene and pentane

Limitation

• Parallalex error
• Object whose temperature to be measured should be freely accessible
• Upper limit of the liquid should be below the boiling point of the liquid.

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Stem Correction Factor

Class 2 : Vapour pressure thermometer

• Vapour pressure of liquid is measured as function of temperature.
• It has a high speed of response, low cost, ease of repair
• A bulb is filled with volatile liquid partially.
• Rest of the bulb is filled with same volatile liquid in vapour state.
• Commonly used liquids are methylene chloride , methane , propane , toluene, ethyl alcohol , ether.

Condition of selection of liquid.

• Boiling point of the liquid must be below the lowest temperature to be measured.
• Liquid must be inert with the bulb capillary and pressure element.
• Liquid must be commercially available in pure form.
• Highest temperature to be measured must be below the critical pressure of the liquid.

Class 3 - Gas thermometer

• Principle – Charles Law

At a Constant Volume , absolute temperature is directly proportional to absolute pressure.

• The gas must be chemically inert and should possess good expansion coefficient and possess low specific heat.
• Ex – Nitrogen , Helium
• Range – 35 °C to 600 °C

Class 1 : Liquid Filled thermometer

Class 2 : Vapour pressure thermometer

• Vapour pressure of liquid is measured as function of temperature.
• It has a high speed of response, low cost, ease of repair
• A bulb is filled with volatile liquid partially.
• Rest of the bulb is filled with same volatile liquid in vapour state.
• Commonly used liquids are methylene chloride , methane , propane , toluene, ethyl alcohol , ether.

Dual Filled Vapour pressure thermometer

Condition of selection of liquid.

• Boiling point of the liquid must be below the lowest temperature to be measured.
• Liquid must be inert with the bulb capillary and pressure element.
• Liquid must be commercially available in pure form.
• Highest temperature to be measured must be below the critical pressure of the liquid.

Class 3 - Gas thermometer

• Principle – Charles Law

At a Constant Volume , absolute temperature of a confined gas is directly proportional to absolute pressure.

• Bulb must be larger one
• The gas must be chemically inert and should possess good expansion coefficient and possess low specific heat.
• Ex – Nitrogen , Helium
• Range – 35 °C to 600 °C

Class 4/5 : Mercury filled thermometer

• Principle – Volumetric Expansion.
• Mercury is commercially available in pure form h and it has high expansion coefficient
• It has high boiling point and it can be used for high reasonable range .
• It will not stick on capillary walls
• It is clearly visible with respect to glass capillary tube
• It can be used to a range of -30 °C to 280 °C
• With mercury alloys, upto 750 °C

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Errors Possible

• 1. Ambient Temperature Effect
• 2. Head or Elevation Effect
• 3. Barometric Effect
• 4. Immersion Effect
• 5. Radiation Effect

Errors Possible

• 1. Ambient Temperature Effect
• 2. Head or Elevation Effect
• 3. Barometric Effect
• 4. Immersion Effect
• 5. Radiation Effect

Ambient Temperature Effect

• The change of ambient temperature causes volume changes in the capillary tube and the Bourdon tube thereby causing error in measurement
• As in the vapour-pressure thermometer, the liquid surface temperature is the only determining factor, it does not need correction for the ambient temperature effect

Ambient Temperature Effect

• Reduced by
• Larger bulb size
• Case compensation
• Full Compensation

Head or Elevation effect

• If the thermometer bulb is placed at a different height with respect to the Bourdon tube, elevation errors are produced

Head or Elevation effect

• Reduced by
• The filling of fluid is done at a high pressure compared with the height of the bulb to avoid this error

Barometric Effect

• The effect due to change in the atmospheric pressure is known as the Barometric Effect

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• Reduced by
• This error may be avoided by keeping the filled system at a pressure sufficiently larger than the atmospheric pressure.

Immersion Effect

• If the bulb is not properly immersed or fully immersed and the head of the bulb is not properly insulated, heat from the bulb is lost due to conduction through the extension neck and thermal well. This causes immersion error, and due to this a lower temperature is indicated by the thermometer.

Radiation Effect

• Radiation error occurs due to temperature difference between the bulb and other solid bodies around.
• Reduced by
• A radiation shield is used around the bulb to minimize this error.

Case Compensation

Full Compensation

Full Compensation

Full Compensation

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