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MAMCE/EIE/EI2303 – INDUSTRIAL INSTRUMENTATION-II/QUESTION BANK

UNIT IV- LEVEL MEASUREMENT

1. List the direct level measuring methods.i. Float type level indicator

ii. Displacer type detector,Sight glass type.

2. List the indirect level measuring methods.i. Hydrostatic measurement

ii. Air purge systemiii. Boiler drum system.

3. What are the advantages of sight glass level instrument?

i. Direct reading is possible.ii. Special designs are available.

iii. Glass less devices are available in numerous material for corrosionresistance.

4. What are the advantages of displacer level instrument?

i. High accuracyii. Reliable to clean liquids

iii. Mounted internally or externallyiv. Adaptable to liquid interface measurement.

5. Write short notes on displacer level system.A DISPLACER LIQUID LEVEL MEASUREMENT SYSTEM is a liquid level

measuring system consisting of a buoyant cylindrical object, heavier than the liquid,immersed in the liquid and connected to a spring or torsion device that measures the

buoyancy of the cylinder

6. How CONDUCTIVITY PROBES provide a point level measurement of liquid level.

CONDUCTIVITY PROBES provide a point level measurement of liquid level

through the electrical conductivity of a liquid. The liquid has to be conductive to be able

to provide this level measurement. CONDUCTIVITY PROBES consist of an electrical

circuit of two or more probes (electrodes) inserted in a metal conductive tank where the

metal in the vessel completes the circuit as the liquid level rises to immerse the

electrode(s)

7. Write short notes on ultrasonic methods level measurementUltrasonic transmitters work on the principle of sending a sound wave from a

peizo electric transducer to the contents of the vessel. The device measures the length of time it takes for the reflected sound wave to return to the transducer. A successful

measurement depends on reflection from the process material in a straight line back to thetransducer.

8. Discuss about continuous level measurement of liquids.

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CONTINUOUS LEVEL MEASRURMENT is a method to track the changes of a level over a range of values to monitor inventory or for determining when to add or

remove material from containers.Examples of CONTINUOUS LEVEL MEASRURMENT could include

maintaining a level at a safe level when transferring material, or the water level of a

boiler must be known at all times to prevent a low-water condition that could result in boiler damage or an explosion. These are examples of maintaining levels at a safe limit atall times and if these levels cannot be maintained a system shut down must be initiated to

prevent equipment damage, injury to personnel, or unsafe releases to the environment.

9. How liquid level is measured using float and displacer sensor?A FLOAT is a point level measuring instrument consisting of a hollow ball that

floats on top of a liquid in a tank. Floats are attached to the instrument by a lever to an

On/Off Switch activated by the movement of the float.

A DISPLACER LIQUID LEVEL MEASUREMENT SYSTEM is a liquid

level measuring system consisting of a buoyant cylindrical object, heavier than the liquid,

immersed in the liquid and connected to a spring or torsion device that measures the

buoyancy of the cylinder

10. Write short notes on bubbler system.A LEVEL BUBBLER SYSTEM for a non pressurized vessel consists of a tube

extending to the bottom of a vessel, a pressure gauge, single-leg manometer, transmitter, or recorder; a flow meter to adjust the flow rate of air or nitrogen through the tube; and a pressure regulator to limit the inlet pressure

Part-B

1. DESCRIBE how GAUGE GLASSES are used to measure liquid level.

Ans:

GAUGE GLASSES are devices used to provide a visual indication of a liquid level thatconsist of a glass tube connected above and below the liquid level in a tank that allows the

liquid level to be observed visually. GUAGE GLASSES are used as a visual indication rightat the tank location. As the level of the tank increases or decreases, the liquid level is

observed inside the glass tube. The liquid level is the same as the level inside the tank.The gauge glass occupies the vertical space between the gauge cocks. The gauge cocks

include ball check valves to prevent the loss of process fluid if the gauge glass should break.The gauge class is a thick-walled glass tube fastened to the gauge cocks with a compression

fitting. The gauge glass assembly is attached to the vessel using upper and lower flanges or fittings.

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A guard rod is attached above and below the gauge glass tube to help protect the tube. Insome cases, a thicker plastic tube encloses the glass tube for added protection against

breakage.Because of a limited choice of materials for gauge cocks, gauge glasses are usually used for

non corrosive solutions that can discolor the glass tube or damage the gauge cock materials.

An armored gauge glass assemble is also available for high pressure systems like boilers.These assemblies use a thick flat gauge glass inside an armored enclosure to provide high pressure protection and safety protection against breakage in high pressure vessels or boilers.

If a very high tank uses armored gauges, they will use several of them at overlappinglocations so all levels can be visible.

2. How liquid level is measured using float and displacer sensor?

Ans: Level devices using FLOATS are dependent of the buoyancy of an object to measure

level. A floating object determines the surface of a liquid, whereas a solid object lowered

down to top of material in a silo determines the level of a that product.

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A FLOAT is a point level measuring instrument consisting of a hollow ball that floats ontop of a liquid in a tank. Floats are attached to the instrument by a lever to an On/Off Switch

activated by the movement of the float as depicted below:

Floats are used to indicate a specific tank level, actuate alarms or shutdown switches, or even mechanically control valves. Switches can start a pump when the float is at one position

and stop the pump at another position.

Floats can be located inside of a tank or enclosed in an attached cage or in a stilling wellto minimize turbulence and could also include alarm contacts.

Tape Flow Level Instruments using cables, pulleys, and a float with the float located

inside of a vessel, as the level raises or lowers, the float attached to a cable will cause anexternal indication on the outside of a tank to indicate the level of the solution in that vessel.

Tape Floats are typically used as indication devices only, but they can be used with a

transmitter for continuous level measuring. Both examples are depicted below:

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TAPE FLOATS are continuous level measuring instruments consisting of a floating

object connected by a chain or cable or tape to a counterweight which is the level pointer.The float rides up and down on two guide wires that keep the float in a specific position. A

scale fastened to the outside of the tank shows the reversed tank level with 100% being at the bottom and 0% being at the top. When Float is at the top, the tank if full and when it is at the

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bottom the tank is empty. The counterweight keeps tension on the tape and the pointer movesup or down to indicate the level.

Tape Floats are subject to mechanical problems due to corrosion and buildup of solutions on the tape causing the device to hang up and give a false indication. Sometimes

the float actually develops a leak and falls to the bottom of the tank providing a false

indication.Float and Dial Level Instruments are used with horizontal tanks. A float attached to along arm, long enough for the float to reach the top and bottom of the tank, and is coupled

through a seal to a dial level indicator as depicted below:

These devices are used for measuring clean non corrosive liquids stored under pressure

such as ammonia or methyl chloride, and must be ordered for each specific tank application.The major problem with all float devices is that they are subject to mechanical problems due to

moving parts that become worn and are subject to breakage or defects over time.

3. Describe what a displacer liquid level measurement system is and how it measures level.

Ans:

A DISPLACER LIQUID LEVEL MEASUREMENT SYSTEM is a liquid levelmeasuring system consisting of a buoyant cylindrical object, heavier than the liquid,

immersed in the liquid and connected to a spring or torsion device that measures the

buoyancy of the cylinder as depicted below:

The advantage of using the Displacer Level Instrument is that the movement from the

torque tube assembly can easily be transmitted via a pneumatic 3-15 psig or a 4-20 mA signalfor remote level indication and control of the liquid level.

A DISPLACER LIQUID LEVEL MEASUREMENT SYSTEM is a liquid level

measuring system consisting of a buoyant cylindrical object, heavier than the liquid,

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immersed in the liquid and connected to a spring or torsion device that measures the buoyancy of the cylinder as level increases or decreases.

TheDisplacer Level system is a Continuous Level Measuring Instrument. The Displacer Level Instrument Using The Torque Tube Assembly can easily be

transmitted via a pneumatic 3-15 psig or a 4-20 mA signal for remote level indication and

control of the liquid level.

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4. DESCRIBE how ULTRASONIC SENSORS provide a continuous level measurement.

Ans:

An ULTRASONIC SENSOR is a continuous level measurement device consisting of two electrically energized crystals mounted above the maximum level of the material in the

vessel, with one crystal used as a transmitter and the other used as a receiver.

The transmitter crystal generates a high-frequency sound directed at the surface of thematerial in the vessel or tank. Transit T ime is the time it takes for a transmitted ultrasonicsignal to travel from the ultrasonic level transmitter to the surface of the material to be

measured back to the receiver. The electronic circuitry in the receiver measures the Transit

Time and calculates the distance as depicted below:

This type of sensor is primarily used for granular solids, but is also used with non corrosiveliquids and slurries. Industrial noise and dust can create false signals with Ultrasonic devices.

Ultrasonic Sensors can also be used to provide a point level measurement.

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The design for point level measurement uses two similar crystals, one being the transmitter and the other being the receiver. Both crystals are enclosed in a probe, but are separated by a

small intergral air gap. This ultrasonic sensor is called a Gap Switch as depicted below:

A Gap Switch measures the strength of an ultrasonic signal across a small gap todetermine when material in the tank has reached the switch.

When the gap is exposed to air or vapor, the ultrasonic signal is not able to pass throughin sufficient strength to be received; however, when the liquid rises and fills the gap, the

ultrasonic signal from the transmitter is received. This is due to the fact that liquids carry soundwaves more efficiently than air or vapor.

If liquid is in a slurry state or is sticky, a wider gap permits it to drain more readily fromthe gap.

The disadvantage to Gap Switches is that the material used for these devices are not suitable

for corrosive liquids.

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5. DESCRIBE how NUCLEAR LEVEL INSTRUMENTS provide point and continuouslevel measurement.

Ans:A NUCLEAR LEVEL INSTRUMENT is a level measuring system consisting of a

radioactive source that directs radiation through a vessel to a detector, such as a GEIGER

COUNTER on the other side of a vessel. Nuclear level sensors are used for process materials that are extremely hot, corrosive,toxic, or under very high pressure and so are not suitable for intrusive level detectors.

Radioactive elements such as cesium 137 or cobalt 60 provide the radioactive source in theform of gamma rays. The amount of radioactive energy required is calculated based upon a

vessel or tanks wall thickness and distance between source and detector. Nuclear level sensors are relatively expensive to purchase, install, and operate. However,

they are sometimes the only way to measure level under extreme conditions.POINT LEVEL measurement is achieved with a radioactive source mounted externally

on one side of a vessel at the selected level. The source must be enclosed in a protectivehousing with a window allowing the radiation to be directed toward the detector on the

opposite side of the vessel. The nuclear energy source produces a beam of radiation whosefrequency is proportional to the strength of the radiation.

When the material level blocks the Radiation Beam Path, the detected radioactive energyis reduced enough to cause an electrical relay to change its state and provide a level

indication or alarm. This relay can start or stop a feeder, light a lamp, or sound an alarm asdepicted below for Point Level Measurement:

Nuclear Continuous Level Measurement differs from Point Level Measurement in that

it will have several receiving elements instead of just one as depicted below:

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Notice in the picture above, the indicator receiving its signal from the receiver is an indication of

a continuous level as all times and the device receiving its signal from the receiver is providing a

level reading of 0 to 100 % level and it is not an on or off device.For continuous level measurement, the difference from a point level device is that the receiver is

a scintillation counter that detects and measures nuclear radiation as it strikes a sensitivematerial, know as a phosphor, producing tiny flashes of visible light. Phosphors include Zinc,

Sulfide, Sodium Iodide and some liquids and organic substances. The attenuation of the source isused to determine the level.

UNIT V- MEASUREMENT OF VISCOSITY, HUMIDITY AND MOISTURE

1. Define Viscosity.It is a measure of fluidity of the system. Many fluids undergo continuous deformation

with the application of shearing stress

2. Define Relative Humidity.

This is the ratio of moisture content of gas to maximum moisture content of the gas atthat temperature.

3. Define dew point.

This is the saturation temperature of the mixture at the corresponding vapour pressure.

4. Define Hygrometer.Used to measure the moisture content in air. It also used to measure humidity.

5. What is the basic principle of Hygrometer.

It consist of mechanical device measuring the dimension change of humidity sensitivematerials like animal hair, animal membrane , paper etc.

6. Define Moisture.

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Defined as the amount of water absorbed by solids or liquids.

7. What are the various methods of measurement of moisture.

Based on the weight of the particle

Based on the resistance, capacitance,

8. What is Psychrometer?

Psychrometer is a device that uses the bulb thermometers to measure humidity. It is alsoused in air conditioning systems for maintaining humidity.

9. What is Psychrometer?

Psychrometer is a device that uses the bulb thermometers to measure humidity. It is alsoused in air conditioning systems for maintaining humidity.

10. What are the different types of hygrometer?

Hair hygrometers

Wire electrode hygrometersElectrolysis type hygrometers

Resistive type

Capacitive type

Microwave reflector

Part-B

1. Explain the construction and principle of operation of dry bulb Psychrometer with neat .

Principle of measurementWhen water or ice covers the bulb of a thermometer (wet-bulb), latent heat is removed

from the surface of the bulb as the water evaporates, and the wet-bulb temperature becomeslower than the air (dry-bulb) temperature.

At a lower humidity, water evaporates more actively, so that the wet -bulb temperaturelowers sharply. The aspirated psychrometer measures humidity by measuring the difference

between the dry-bulb temperature and wet-bulb temperature.

Structure and composition

The psychrometer consists of two thermometers of the same specifications, which aresuspended side by side in the air. One of them measures the actual air (dry-bulb) temperature

while the other, whose bulb is covered with a wet-bulb temperature.

Psychrometers are classified into the non-aspirated type psychrometer (portableaspirated psychrometers and sling psychrometers) and the aspirated type psychrometers(Assuman type aspirated psychrometer and JAM type aspirated psychrometers). Aspirated

psychrometers are designed to keep the constant flow of air over around the bulbs. Thestructure and composition of the Assuman type aspirated psychrometer, which is commonly

used in Japan, and described below.Figure 3.1 shows the structure of the Assuman type aspirated psychrometer. The

psychrometer

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consists of two enclosed scale type mercury thermometers, which can read temperature inincrements of 0.1.

One of them is called a dry-bulb thermometer, which measures actual air temperatureand the other is called a wet-bulb thermometer, which is measures the temperature of wet-

bulb which covered with a wet sleeve. The wet sleeve is a white thin cotton cloth soaked withwater.

The external and internal cylinders of a metal aspiration tube protect the bulbs from precipitation and radiation of direct sunlight.

As shown in figure 3.1, air floe with a velocity of 2.5 m/s enters from the bottom with anelectromotive fan or a spring fan. The time constant of the psychrometer is about 40 seconds.

A squirt is used to feed water to the wet sleeve of the wet-bulb or to suck excess water fromit.

(3) Psychrometric formula and psychrometric tableWhen the air steadily flows around the wet-bulb, the wet-bulb temperature falls below the air

temperature by water evaporation from the surface of the wet-bulb. When the heat flow

moving into the wet-bulb from the ambient air has reached equilibrium with the latent heatflow removed from the wet-bulb by evaporation, the following equation, called the Sprung psychrometric formula, is derived with the Assuman type aspirated psychrometer,

e= ew- (A/755) p (t-tw) ...................................(1)where,

Psychrometer constant, A is 0.50 when the wet-bulb is not frozen and 0.44 when it is frozen.e: Vapor pressure hPa

ew: Saturation vapor pressure ,hPa

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p: Atmospheric pressure hPa

t: Dry-bulb temperature℃

tw: Wet-bulb temperature℃

Vapor pressure is calculated with this equation (1). Table 3.1 and Table 3.2 show thesaturation vapor pressure for water and ice as a function of temperature.

The second term on the right side of the equation (1) is calculated a function of p and(t-tw), which is tabulated as the vapor pressure table in Table 3.3 and Table 3.4 for the

unfrozen and frozen wet-bulb.

Calculations of vapor pressure, dewpoint temperature, and relative humidity

The vapor pressure, dewpoint temperature, and relative humidity are calculated fromthe measurement with the aspirated psychrometer using the tables described above.

Calculation of vapor pressure1) Make correction of the instrumental error of the dry-and wet-bulb thermometer.

2) Using Table 3.1 or 3.2, determine the value of the saturation vapor pressure for water (ew) or ice (ei) as a function of the wet-bulb thermometer temperature (tw).

3)Using Table 3.3 and 3.4 calculate the second term on the right side of the equation

(1) as a function of atmospheric pressure (p) and (t-tw), where t is the dry-bulbthermometer temperature.4) The vapor pressure e is obtained by making a subtraction between the above two

values.

Precautions for using the aspirated psychrometer

1) Supply water to the wet-bulb with distilled water or soft water, using the squirt.

If the air temperature is 0℃ or less, the water of the wet-sleeve may be frozen. In that

case, make the icymembrane around the wet-bulb as thin as possible, using warmed water.

<Notes>a) Do not supply the wet-bulb with too much water. If too much water is supplied to

the bulb suck excess water by squirt or by attaching a brush to the bottom of the bulb. Do not wet

the inside of the aspiration tube.

b) Use water of the air temperature.c) In the case that the air temperature is high and the humidity is low, the wet -bulb

may dry up by the time when the observer reads the temperature. In such a case, supplywater to the wet-bulb repeatedly.

2) Operate the fan for aspiration to make the air flow around the bulbs3) Before reading a wet-bulb temperature, it is necessary that the indication is stable.

When the wet-bulb temperature is slightly below 0℃, the water of the wet-sleeve may

not freeze but be super cooled. Thus when the temperature is around or less than 0℃, see

carefully the state of the wet-bulb to find whether the wet-bulb is frozen or super cooledand use the appropriate saturation vapor pressure table. To determine whether the wet-

bulb is frozen or super cooled, gently touch the surface of the wet-bulb with somethinglike a needle. Degree of gloss on the surface of the wet-bulb is also useful to check if the

wet-bulb is frozen.

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a) The time for aspiration required to stabilize the reading is typically five minutes if

the temperature is 0℃ or higher. If the temperature is less than 0℃, an aspiration time

longer than five minutes will be needed.

Sources to cause errorsa) The psychrometer constant A in the psychrometric formula varies, depending on

whether the wet-bulb is frozen or not and the incorrect determination of the wet-bulb leads toerrors. So the state of the wet-bulb should be checked especially in cold conditions before the

calculation. b) As the temperature becomes lower, air contains less vapor, and the saturation pressure

becomes lower. So the wet-bulb temperature reading error affects the vapor pressurecalculations more significantly. Because of this, much care is needed with reading the

psychrometer at low temperatures.c) A portable aspirated psychrometer which is not subjected to forced aspiration is

significantly affected by the natural wind. When a portable aspirated psychrometer is used ina thermometer shelter and the natural wind speed ranges from 0.3 to 4.0 m/s, the error in

humidity may become as high as 7% because the aspiration velocity in the shelter is lower

than the wind speed out of the shelter.d) The wet-bulb temperature is affected by oil on the wet sleeve as well as by anyimpurities, such as salt dissolved in the water. A dirty wet sleeve also prevents correct

measurement. Deposits of dirt on the wet-bulb after the prolonged use may cause errors.e) Generally, the dry-bulb and wet-bulb thermometers have the same size and shape.

Because the wet-bulb has higher thermal conductivity, it responds to changes in air temperature a little more quickly than the dry-bulb. Normally, when the air temperature

changes, the wet-bulb firstly responds, causing a temporary change of humidity indication.On the other hand, the wet-bulb responds less quickly when a thick icy membrane is formed

on the bulb.

2. Write short notes on dew point and explain in detail about the commercial dew point meter.

Ans: DEW POINT:

The dew point is the temperature below which the water vapor in a volume of

humid air at a given constant barometric pressure will condense into liquid water at the same

rate at which it evaporates. Condensed water is called dew when it forms on a solid surface.

The dew point is a water-to-air saturation temperature. The dew point is associated with

relative humidity. A high relative humidity indicates that the dew point is closer to the

current air temperature. Relative humidity of 100% indicates the dew point is equal to the

current temperature and that the air is maximally saturated with water. When the dew point

remains constant and temperature increases, relative humidity decreases.[1]

General aviation pilots use dew-point data to calculate the likelihood of carburetor

icing and fog, and to estimate the height of the cloud base.

http://en.wikipedia.org/wiki/Temperature

http://en.wikipedia.org/wiki/Water_vapor

http://en.wikipedia.org/wiki/Air

http://en.wikipedia.org/wiki/Barometric_pressure

http://en.wikipedia.org/wiki/Condensation

http://en.wikipedia.org/wiki/Dew

http://en.wikipedia.org/wiki/Humidity

http://en.wikipedia.org/wiki/Relative_humidity

http://en.wikipedia.org/wiki/Dew_point#cite_note-Horstmeyer-1



http://en.wikipedia.org/wiki/General_aviation

http://en.wikipedia.org/wiki/Carburetor_icing


http://en.wikipedia.org/wiki/Fog

http://en.wikipedia.org/wiki/Cloud_base

http://en.wikipedia.org/wiki/Cloud_base

http://en.wikipedia.org/wiki/Fog




http://en.wikipedia.org/wiki/General_aviation


http://en.wikipedia.org/wiki/Relative_humidity

http://en.wikipedia.org/wiki/Humidity

http://en.wikipedia.org/wiki/Dew

http://en.wikipedia.org/wiki/Condensation


http://en.wikipedia.org/wiki/Air

http://en.wikipedia.org/wiki/Water_vapor

http://en.wikipedia.org/wiki/Temperature

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This graph shows the maximum percentage, by mass, of water vapor that air at sea-level across a

range of temperatures can contain.

At a given temperature but independent of barometric pressure, the dew point is a consequence

of the absolute humidity, the mass of water per unit volume of air. If both the temperature and

pressure rise, however, the dew point will increase and the relative humidity will decrease

accordingly. Reducing the absolute humidity without changing other variables will bring the dew

point back down to its initial value. In the same way, increasing the absolute humidity after a

temperature drop brings the dew point back down to its initial level. If the temperature rises in

conditions of constant pressure, then the dew point will remain constant but the relative humidity

will drop. For this reason, a constant relative humidity (%) with different temperatures impliesthat when it's hotter, a higher fraction of the air is water vapor than when it's cooler.

At a given barometric pressure but independent of temperature, the dew point indicates the mole

fraction of water vapor in the air, or, put differently, determines the specific humidity of the air.

If the pressure rises without changing this mole fraction, the dew point will rise accordingly;

Reducing the mole fraction, i.e., making the air less humid, would bring the dew point back

down to its initial value. In the same way, increasing the mole fraction after a pressure drop

brings the relative humidity back up to its initial level. Considering New York (33 ft elevation)

and Denver (5,280 ft elevation),[2]

for example, this means that if the dew point and temperature

in both cities are the same, then the mass of water vapor per cubic meter of air will be the same,

but the mole fraction of water vapor in the air will be greater in Denver.


http://en.wikipedia.org/wiki/Humidity#Absolute_humidity_.28Volume_basis.29

http://en.wikipedia.org/wiki/Mole_fraction


http://en.wikipedia.org/wiki/Specific_humidity

http://en.wikipedia.org/wiki/Dew_point#cite_note-denfacts-2




http://en.wikipedia.org/wiki/Specific_humidity




http://en.wikipedia.org/wiki/Humidity#Absolute_humidity_.28Volume_basis.29


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3. Explain briefly about moisture measurement of various substances

Ans:Moisture analysis covers a variety of methods for measuring moisture content in both

high level and trace amounts in solids, liquids, or gases. Moisture in percentage amounts is

monitored as a specification in commercial food production. There are many applications wheretrace moisture measurements are necessary for manufacturing and process quality assurance.Trace moisture in solids must be controlled for plastics, pharmaceuticals and heat treatment

processes. Gas or liquid measurement applications include dry air, hydrocarbon processing, puresemiconductor gases, bulk pure gases, dielectric gases such as those in transformers and power

plants, and natural gas pipeline transport.

Chilled-mirror dewpoint hygrometer

(1) Structure and composition sensor (mirror)The basic structure of the sensor unit for a chilled-mirror dewpoint hygrometer is shown in

Figure 3.7.Sample air is drawn to the metallic mirror surface through piping to determine the dewpoint

temperature. As the mirror cools, condensation forms when its surface temperature falls belowthe dewpoint temperature, but evaporates and disappears at higher temperatures. The temperature

of the metallic mirror when condensation forms is measured using a platinum resistancethermometer, and the result is taken as the dewpoint temperature. Condensation conditions are

monitored using a photo-detector with the reflection of a light-emitting diode (LED) on themirror. Irradiated light is scattered when condensation is present, and the amount of reflected

light changes with the mirror’s surface condition. A peltier element is used to control themirror’s temperature.

(2) Structure

Chilled-mirror dewpoint hygrometers consist of a sensor unit with a mirror, an indicator tooutput the measurement results, and a pump to draw sample air into the sensor unit. The sample

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flow can be adjusted using the pump, and a filter should be installed if the sample air has a highcontaminant content (Figure 3.8).

With models to which a thermometer can be attached to measure the temperature of the sampleair, relative humidity can be calculated based on the sample temperature and the dewpoint

temperature.

(3) Error factorContaminants such as salt, dust and oil mist on the mirror may result in artificially elevated

dewpoint temperature readings or difficulties in stable condensation layer formation due totemperature control malfunction. As absorbent piping will draw vapor from the sample and

create large errors, it is important to use stainless steel or fluoride-based resin pipes and to makethem as short as possible.

(4) Maintenance

As mirror contamination can cause errors, the mirror should be cleaned with a special detergent before measurement. Leaving the unit on high temperature after measurement can also result inthe development of mold or corrosion. After measurement ends, the hygrometer should be dried

completely by blowing dry air through it.

(5) Calibration

If a humidity generator tank is attached, calibration should be conducted by connecting piping in parallel from the tank to both the instrument to be calibrated and the standard instrument,

measuring the dewpoints at the same time and comparing them. Thermometers should also becalibrated when relative humidity is to be determined.

(6) RepairSeverely corroded mirrors cannot be repaired and must be replaced. As the procedures for

identifying faults, replacing units and conducting similar work depend on the model, theinstruction manual should be followed.

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4. Explain the construction and principle of operation of Say bolt Viscometer with neatsketch

Ans: Viscosity is a measure of fluidity of the system. Many fluids undergo continuousdeformation with the application of shearing stress.

Fig1 Saybolt viscometer.

Figure 3-2. – Various styles of glass capillary viscometers.3-2

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It is used to express the fluid’s viscosity, in Sayboltuniversal seconds or Saybolt furol seconds.The glass capillary viscometers, shown infigure 3-2, are

examples of the second type ofviscometer used. These viscometers are usedtomeasure kinematic viscosity. Like the Saybolt viscometer, the glass capillary

measures the time in seconds required for the tested fluid to flow through the

capillary. This time is multiplied by the temperature constant of the viscometer inuseto provide the viscosity, expressed in centistrokes. The following formulas may be used to convert centistrokes (cSt units) to approximate Saybolt universal

seconds (SUS units).For SUS values between 32 and 100:For SUS values greater than 100:Although the viscometers discussed above areused in laboratories, there are

other viscometers in the supply system that are available for local use. These viscometerscan be used to test the viscosity of hydraulic fluids either prior to their being added to

a system or periodically after they have been in an operating system for a while.measure kinematic viscosity. Like the Sayboltviscometer, the glass capillary measures

the timein seconds required for the tested fluid to flowthrough the capillary. Thistime is multiplied bythe temperature constant of the viscometer in useto provide the

viscosity, expressed in centistrokes.The following formulas may be usedtoconvert centistrokes (cSt units) to approximateSaybolt universal seconds (SUS

units).For SUS values between 32 and 100:For SUS values greater than 100:Althoughthe viscometers discussed above areused in laboratories, there are other viscometersin

the supply system that are available for localuse. These viscometers can be used totest theviscosity of hydraulic fluids either prior to theirbeing added to a system or

periodically after theyhave been in an operating system for a while.

5. Explain any two types of hygrometers used for Humidity measurement, with neat sketch.

Ans: Hair hygrometer

(1) Principle of measurement and structure

The hair hygrometer uses the characteristic of the hair that its length expands or shrinksresponse to the relative humidity. the dimensions of various organic materials vary with their

moisture content. A humidity change takes an effect on the moisture content in such materials.The length of human hair from which liquid are removed increases by 2 to 2.5% when relative

humidity changes by 0 to 100%. Different types of human hair show different changes in length.However, there is still a relationship between the length of hair and relative humidity.

The hair hygrograph is a hair hygrometer to which a clock-driven drum is installed to

record humidity no a recording chart. When the humidity in the air changes, a hair bundle ⑪

expands or shrinks, so hair joint metal attached to a lever ⑩

moves, making a rotation of a maincan③. The weight of a pen arm attached to the shaft⑥ give a downward moment.

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①Indicator adjusting screw②Weight ③Main cam④Sub cam⑤Rotation axis for main cam⑥Rotation axis for sub cam

⑦Plate attaching sensor part of humidity , ⑧Screw attaching sub cam⑨Connecting spring

⑩lever ⑪Hair bundle.

The plumb ② of the main cam balances with momentand a small change of the hair

bundle⑪ is magnified to the movement of the pen.

Since the length of the hair increases almost logarithmically with the increase of humidity,changes in humidity are not indicated correctly when the elongation of hair is linearly recorded.

The hair hygrometer uses two special cams to put graduations on the hygrometer at equal

intervals. A spring⑨ joints cams③ and④ to prevent them from each other. The movement of

the main cam ③ differ from that of the sub cam ④ depending on the position of the contact point of these two cams. At low humidity, the movement of the sub cam ④ is less than that of

the main cam ③. As humidity increases, the movement of the sub cam ④increases. The hair

hygrometer is designed so that the two special cams cause the movement of the pen arm to be proportional to the change in humidity. The hair hygrometer uses a recording chart with a

humidity scale divided into 100 equal segments. Each segment corresponds to 1%. So, humiditycan be directly read from the recording chart.

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(2) Precautions for using the hair hygrograph1) Before taking a reading of the hair hygrograph, gently tap the hygrometer to remove any

mechanical tension added to the hair bundle.2) At every measurement with the hair hygrograph, the reading should be compared with the

humidity measured with the aspirated psychrometer at the same time. The difference of the

humidity between them is used as a correction value.3) Time marks as well as the degree of clock accuracy should be recorded on the chart.a) When making a time mark on the recording chart by moving the pen, take care to

move the pen arm downward. Moving the pen arm in the pen arm in the oppositedirection(upward)makes the hair

bundle to expand, causing the hygrograph to become defective. b) To determine the humidity from the recording chart, read the indication on the record

then correct it with correction values obtained by the procedure above.(3) Sources to cause errors

a) Hair expands or shrinks due to changes in temperature as well as those in humidity.

The expansion or shrinkage of a hair corresponding to a temperature change of 1℃ is about 1/15

of the expansion or shrinkage of a hair corresponding to a temperature change of 1% in usual air temperatures. Thus no special temperature compensation is made in hair hygrograph. However,if the temperature varies considerably, slight errors will occur. Because the hygroscopicity of

hair begins to decrease at around -15℃ and becomes almost nil at -40℃, the hair hygrometer does not serve at extremely low air temperatures.

b) The response of hair to humidity has hysteresis. The hair length changes more whenhumidity increases than when it decreases.

Electronic hygrometer (capacitive type)

(1) Structure and compositionElectronic hygrometers detect the change in the electrostatic capacity or electric

resistance of a sensor when it absorbs moisture. In this section, the electrical capacitivehygrometer is described.

The electrical capacitive hygrometer uses a dielectric material made of high polymer membrane, as a sensor.

Figure 3.4 Structure of hygrometer sensor with high polymer membrane

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The sensor is fitted with a filter which protects the sensor from contaminants, such as toxicgases, and has pores to tale moisture in it. Figure 3.6 shows an example of such a filter.

(2) Characteristics of the sensorThe measurement range of the electrical capacitive hygrometer is from 0 to 100%, and its

accuracy can be improved by calibration. By calibrating with the standard hygrometer, the

electrical capacitive hygrometer attains the error of 1% or less in the range from 0 to 90% anderror of 2% or less in the range from 90 to 100%.The hysteresis becomes large when the humidity changes from high to low. It is within 1% at

relative humidity of 60-80%. when relative humidity increases from 0 to 90% and the sensor absorbs moisture, the time constant of the sensor is about six seconds. On the other hand, when

relative humidity decreases from 90 to 0% and the sensor releases moisture, the time constant isabout 10 seconds.

For meteorological purposes, the sensor is put in a ventilation shelter to protect the sensor from precipitation and sunlight with the aspiration speed of 2 to 4 m/s around the sensor. The

time constant with the shelter from the saturation to the room humidity is about 20 minutes,which is longer than that without the shelter, because of the shelter’s large thermal capacity.

A high polymer membrane humidity sensor has temperature dependence of about 0.1%/℃

for thetemperature range from 5 to 30℃and 0.2%/℃ for the temperature range from – 30 to 0℃.

Therefore, a temperature sensor is installed together with the humidity sensor to compensate itstemperature dependency.

(3) Sources to cause errorsa) Any difference between the ambient temperature and the sensor temperature causes an

error. For example, at 20℃and 50%RH, a difference of 1℃between the ambient temperature andthe sensor temperature results in an error of about 3%. At 90%RH, the error becomes up to about

6%. When the sensor temperature is lower than the ambient temperature in a low humiditycondition, dew may form on the surface of the sensor. This will make a large measurement error.

The sensor is housed in an ventilation shelter to reduce or eliminate the difference of temperature between the sensor and the ambient air to prevent dew formation.

b) The electronic capacitive hygrometer can be used in any environment where thehuman can live. However, do not use the hygrometer in the atmosphere containing oil mist,

flammable gas, dust,organic solvents, acid, alkaline or ammonia. Using the hygrometer in the atmosphere may cause

itssensor electrodes to corrode, thus the sensor life is shortened. To prevent the sensor electrode

fromcorrosion, a protection filter is used to keep out dust or organic solvents.

(4) Maintenance

Routine maintenance

Routine maintenance is not needed.

Periodic maintenance

a) Compare the electrical capacitive hygrometer with the aspirated psychrometer once threemonths to

observe time-dependent changes. b) Replace the protection filter with a new one twice a year. In rural areas where little soot is

found , the interval between replacements may prolonged to a maximum of once a year.

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