Thermistor is a means of a term "Thermal Resisters".Thermistors are generally made from Semi-Conductor Materials.Most Thermistors have Negative Coefficient of Temperature Resistance i.e.Their Resistance decrease with increase in Temperature.
The Negative Temperature Coefficient of Resistance can be large as several percent per Degree Celsius.This allows the Thermistor Circuits to detect very small changes in Temperature which could not be observed with a Thermocouple.
Thermistors are widely used in applications which involves Measurements in the range of minus 60 degree Centigrade to 15 degree Centigrade.The Resistance of Thermistors ranges from 0.5 ohms to 0.75 mega ohms.
Thermistor is highly sensitive device.the Thermistor exhibits a highly Non Linear characteristics of Resistance versus Temperature.
Construction of Thermistors:-
Thermistors are composed of sintered mixture of Metallic oxides such as Manganese,Nickel,Cobalt,Copper,Iron and Urenium.They are available in variety of sizes and shapes.The Thermistors may be in the form of beads,rods and discs.
A Thermistor in the form of beads is smallest in size and bead may have a Diameter of 0.015 mm to 1.25 mm.(Beads may be sealed in the tips of Solid Glass Rod to form Probes which may be easier to mount than the beads).
Glass Probe have a Diameter of about 2.5 mm and a length which varies from 6 mm to 50 mm.Discs are made by pressing material under high pressure into Cylindrical Flat Shape with Diameters ranging from 2.5 mm to 25 mm.
The three important characteristics of Thermistor make them extremely useful in Measurement and Control Applications,these are as follows.
01)The Resistance:-Temperature characteristics
02)The Voltage:-Current characteristics
03)The Current:-Time characteristics
Applications of Thermistors.
Major Applications of Thermistors are Measurement and Control of Temperature they may be used for number of other applications.
01)Measurement of Temperature.
02)Control of Temperature.
03)Temperature Compensation.
The other Applications of Thermistors includes,
01)Measurement of Power at High Frequencies.
02)Measurement of Thermal Conductivity.
03)Measurement of Level,Flow and Pressure of Liquids.
04)Measurement of Composition of Gases.
05)Vacuum Measurement.
06)Providing Time Delay.
Monday, June 30, 2008
Saturday, June 21, 2008
What is mean by Hygrometer?
A Hygrometer measures the Value of Humidity Directly.Generally,the output of Hygrometer is used to indicate Relative Humidity.
Several Material exhibit changes in Electric Properties that are caused by Humidity.These are frequently used in Transducers that are designed and Calibrated to read Relative Humidity Directly.
01)Resistive Hygrometer:-
Some Hygroscopic Salts exhibit a change in Resistivity with Humidity.The most common is Lithium Chloride.This,with a Binder may be coated on a wire or on Electrodes.
Resulting Resistance changes cover a wide range for example 10 raise to four to 10 raise to nine ohm,as the Humidity changes from 100 to 0 percent.
This makes it impractical to design a single element to operate from 1 to 100 percent Relative Humidity.Instead several elements are used,each in a narrow range,with provision for switching elements.
Resistance is measured either with a Wheatstone Bridge or by a combination of Current and Voltage Measurements.
Most of these must not be exposed to conditions of 100 percent Humidity as the Resulting Condensation may damage the Device.Either they must be operated in a constant Temperature Environment or Temperature corrections must be made.
These are Accurate to within plus or minus 2.5 percent or plus or minus 1.5 percent in some cases.Response times are typically of the order of a few seconds.These are currently the most common Electronics Hygrometers.
A typical Resistive Hygrometer having a mixture of Lithium Chloride and Carbon which acts as Conducting Film.This is put on an Insulating Substrate between Metal Electrodes.
The Resistance of the element changes when it is exposed to variations in Humidity.The higher the Relative Humidity the more Moisture the LithiumChloride will absorb,and the lower will be it's Resistance.
The Resistance of the Sensing unit is a Measure of the Relative Humidity.Resistance should be Measured by applying Alternative Current (A.C) to the Wheatstone Bridge.
D.C. Voltage is not applied because it tends to Breakdown the Lithium Chloride it it's lithium and Chlorine Atoms.The Current flow is a Measure of the Resistance and hence of the Relative Humidity.
There are different types of Hygrometer are as follows.
01)Resistive Hygrometer.
02)Capacitive Hygrometer.
03)Microwave Hygrometer.
04)Aluminium Oxide Hygrometer.
05)Crystal Hygrometer.
Several Material exhibit changes in Electric Properties that are caused by Humidity.These are frequently used in Transducers that are designed and Calibrated to read Relative Humidity Directly.
01)Resistive Hygrometer:-
Some Hygroscopic Salts exhibit a change in Resistivity with Humidity.The most common is Lithium Chloride.This,with a Binder may be coated on a wire or on Electrodes.
Resulting Resistance changes cover a wide range for example 10 raise to four to 10 raise to nine ohm,as the Humidity changes from 100 to 0 percent.
This makes it impractical to design a single element to operate from 1 to 100 percent Relative Humidity.Instead several elements are used,each in a narrow range,with provision for switching elements.
Resistance is measured either with a Wheatstone Bridge or by a combination of Current and Voltage Measurements.
Most of these must not be exposed to conditions of 100 percent Humidity as the Resulting Condensation may damage the Device.Either they must be operated in a constant Temperature Environment or Temperature corrections must be made.
These are Accurate to within plus or minus 2.5 percent or plus or minus 1.5 percent in some cases.Response times are typically of the order of a few seconds.These are currently the most common Electronics Hygrometers.
A typical Resistive Hygrometer having a mixture of Lithium Chloride and Carbon which acts as Conducting Film.This is put on an Insulating Substrate between Metal Electrodes.
The Resistance of the element changes when it is exposed to variations in Humidity.The higher the Relative Humidity the more Moisture the LithiumChloride will absorb,and the lower will be it's Resistance.
The Resistance of the Sensing unit is a Measure of the Relative Humidity.Resistance should be Measured by applying Alternative Current (A.C) to the Wheatstone Bridge.
D.C. Voltage is not applied because it tends to Breakdown the Lithium Chloride it it's lithium and Chlorine Atoms.The Current flow is a Measure of the Resistance and hence of the Relative Humidity.
There are different types of Hygrometer are as follows.
01)Resistive Hygrometer.
02)Capacitive Hygrometer.
03)Microwave Hygrometer.
04)Aluminium Oxide Hygrometer.
05)Crystal Hygrometer.
Thursday, June 19, 2008
How to Measure the Liquid Level by Different Methods.
The Liquid Levels can be measure by different methods but following two are more important from the subject point of view.
01)Electrical Methods:-
The Direct Conversion to Liquid Level position to Electrical Signal is used in many instances.The measurements is generally done by two conversions,so that the Liquid Level is determined indirectly.
The first conversion usually is Liquid Level to a Displacement through a Float in a Liquid or a Spring Loaded Plate in contact with the surface in the case of Granular Solids.
This Displacement is then converted into an Electrical Signals by a Secondary Transducer connected to Float or Plate.These are however,in many applications where this is not possible and hence other methods like Optical or Acoustic means or Gamma rays are used.
The Electric Transducers used for Level Measurements are
01)Resistive
02)Inductive
03)Capacitive
Now we will see about Resistive Method first.
01)Resistive Method:-
This method uses Mercury as a Conductor.A number of Contact Rods are placed at various Liquid Levels.
As Head "h" increases, the rising Level of Mercury above the Datum,shorts successive Resistors "R" and increases the value of "h" directly.
Advantages of Resistive Method:-
01)The System uses Low Voltage to eliminate danger to the operators and to prevent Arcing at the contact points.
02)The Signal can be transmitted to any desired point.
03)The Transducers can be used in Pressured Containers without Packing Glands or Shafts.
04)The unit is simple to calibrate since the distance between the levels of Contact Rods can be accurately measured and the indicated value may be checked for each measured value.
Disadvantages of Resistive Method:-
01)Due to Arcing at the contact points,it is not safe to use this Transducer in Explosive Atmosphere.
02)In order to have a step less indication of the Liquid Level,an extremely large number of Contact Rods are needed.
03)The Contact Rods are corroded by Corrosive Liquids.In addition,the Electric Charges promote Corrosion.
04)These Systems also present difficulties when there is Saturated Vapour above the Liquid Phase.
05)Any changes in the Conductivity of the Liquid causes serious errors.
01)Electrical Methods:-
The Direct Conversion to Liquid Level position to Electrical Signal is used in many instances.The measurements is generally done by two conversions,so that the Liquid Level is determined indirectly.
The first conversion usually is Liquid Level to a Displacement through a Float in a Liquid or a Spring Loaded Plate in contact with the surface in the case of Granular Solids.
This Displacement is then converted into an Electrical Signals by a Secondary Transducer connected to Float or Plate.These are however,in many applications where this is not possible and hence other methods like Optical or Acoustic means or Gamma rays are used.
The Electric Transducers used for Level Measurements are
01)Resistive
02)Inductive
03)Capacitive
Now we will see about Resistive Method first.
01)Resistive Method:-
This method uses Mercury as a Conductor.A number of Contact Rods are placed at various Liquid Levels.
As Head "h" increases, the rising Level of Mercury above the Datum,shorts successive Resistors "R" and increases the value of "h" directly.
Advantages of Resistive Method:-
01)The System uses Low Voltage to eliminate danger to the operators and to prevent Arcing at the contact points.
02)The Signal can be transmitted to any desired point.
03)The Transducers can be used in Pressured Containers without Packing Glands or Shafts.
04)The unit is simple to calibrate since the distance between the levels of Contact Rods can be accurately measured and the indicated value may be checked for each measured value.
Disadvantages of Resistive Method:-
01)Due to Arcing at the contact points,it is not safe to use this Transducer in Explosive Atmosphere.
02)In order to have a step less indication of the Liquid Level,an extremely large number of Contact Rods are needed.
03)The Contact Rods are corroded by Corrosive Liquids.In addition,the Electric Charges promote Corrosion.
04)These Systems also present difficulties when there is Saturated Vapour above the Liquid Phase.
05)Any changes in the Conductivity of the Liquid causes serious errors.
Tuesday, June 17, 2008
What are the limitations of A.C.Tachometer Generators?
In order to overcome some of the diffculties,A.C.Tachometer Generators are used.The Tachometer has Rotating Magnet which may be either Permanent Magnet or an Electromagnet.
The Coil is wound on the Stator and therefore the problems associated with Commutator (as in D.C.Tachometer) are absent.
The rotation of the Magnet causes an E.M.F to be induced in the Stator Coil.The Amplitude and the Frequency of this E.M.F. are both proportional to the speed of Rotation.Thus either Amplitude or Frequency of induced voltage may be used as a measured of Rotational Speed.
The Output Voltage of A.C.Tachometer Generators is rectified and is measured with a Permanent Magnet Moving Coil Instrument.(PMMCI)
LIMITATIONS:-
01)The difficulty with this system is that at low speed the Frequency of output voltage is low and hence it is very difficult to smooth out the ripples in the output voltage wave-shape and hence A.C.Tachometer Generators are designed to have a large number of Poles,so that the Frequency of output voltage is high,even at low speed.
02)High Speed is also present a problem.At high speeds,the Frequency increases and therefore,the Impedance of the Coil of TachoGeneretors increases. If good linearity is to be maintained in the input Impedance of the display device must be considerably larger than the Impedance of the Coil.
Drag Cup Rotor A.C.Tachogenerators:-
The Tachogenerators consists of a Stator and a rotor.The Stator has two windings mounted at 90 degree to each other.The two Stator windings are,
01)Exitation Winding
02)Sensing Winding
Advantages of Drag Cup Rotor A.C.Tachogenerators:-
01)A Linear Relationship between the output voltage and speed is obtained if the carrier frequency is 5 to 10 times larger than the frequency of the speed signal.For this purpose of excitation winding is supplied with a frequency of 400 hertz.
02)The Drag Cup type tachogenerators are rugged and in-expensive.Also,they requires little maintenance.
03)In some situations these Tachogenerators are very useful since they give a ripple free output.
Disadvantages:-
01)These Tachogenerators are hard to calibrate.
02)The output voltage is proportional to the product of speed and input voltage.
The Coil is wound on the Stator and therefore the problems associated with Commutator (as in D.C.Tachometer) are absent.
The rotation of the Magnet causes an E.M.F to be induced in the Stator Coil.The Amplitude and the Frequency of this E.M.F. are both proportional to the speed of Rotation.Thus either Amplitude or Frequency of induced voltage may be used as a measured of Rotational Speed.
The Output Voltage of A.C.Tachometer Generators is rectified and is measured with a Permanent Magnet Moving Coil Instrument.(PMMCI)
LIMITATIONS:-
01)The difficulty with this system is that at low speed the Frequency of output voltage is low and hence it is very difficult to smooth out the ripples in the output voltage wave-shape and hence A.C.Tachometer Generators are designed to have a large number of Poles,so that the Frequency of output voltage is high,even at low speed.
02)High Speed is also present a problem.At high speeds,the Frequency increases and therefore,the Impedance of the Coil of TachoGeneretors increases. If good linearity is to be maintained in the input Impedance of the display device must be considerably larger than the Impedance of the Coil.
Drag Cup Rotor A.C.Tachogenerators:-
The Tachogenerators consists of a Stator and a rotor.The Stator has two windings mounted at 90 degree to each other.The two Stator windings are,
01)Exitation Winding
02)Sensing Winding
Advantages of Drag Cup Rotor A.C.Tachogenerators:-
01)A Linear Relationship between the output voltage and speed is obtained if the carrier frequency is 5 to 10 times larger than the frequency of the speed signal.For this purpose of excitation winding is supplied with a frequency of 400 hertz.
02)The Drag Cup type tachogenerators are rugged and in-expensive.Also,they requires little maintenance.
03)In some situations these Tachogenerators are very useful since they give a ripple free output.
Disadvantages:-
01)These Tachogenerators are hard to calibrate.
02)The output voltage is proportional to the product of speed and input voltage.
Tuesday, June 3, 2008
What is Tachometer Generators?
Before going to Tachometer Generators we will see about Angular Velocity in short.
In many cases the only way to measure Linear Velocity is to convert it into Angular Velocity.For example a Speedometer uses the wheels rotational speed as a measure of Linear road speed.
The measurement of Angular Speed may be made with Tachometers which may be either Mechanical or Electrical type.
Electrical Tachometers.
The Electrical Tachometers are professed over Mechanical Tachometers for all applications because these Tachometers offer all the advantages associated with Electrical Transducers.
Electromagnetic Tachometer Generators.
There are two types of Electromagnetic Tachometer Generators called Tachogenerators.
01)D.C.Tachometer Generators.
02)A.C.Tachometer Generators.
The details of D.C.Tachometer Generators is as below.
01)D.C.Tachometer Generators.
D.C.Tachometer Generators consists of a small Armature which is coupled to the Machine whose Speed is to be measured.This Armature revolves in the field of a Permanent magnet.The Electro Magnetic Force (EMF) generated is proportional to the product of Flux and Speed.Since the flux of the Permanent magnet is constant,the voltage generated is proportional to Speed.
The Polarity of Output Voltage indicates the Direction of Rotation.This Electro Magnetic Force is measured with the help of moving Coil Voltmeter having a uniform scale and calibrated directly in terms of Speed.
Advantages are,
01)The direction of rotation is directly indicated by the Polarity of Output Voltage
02)The Output Voltage is typically 10 mV/rpm and can be measured with conventional type D.C.Voltmeters.
Disadvantages are,
01)Brushes on small Tachometer Generators often produce Maintenance problems,as their contact resistance may vary and produce appreciable errors.This is commutator and the brushes requires periodic Maintenance.
02)The input resistance of Meter should be very high as compared with Output Resistance of Generators.This is required to limit the Armature Current to small value.If the Armature Current is large,the field of the Permanent Magnet is distorted giving rise to non-linearity.
In many cases the only way to measure Linear Velocity is to convert it into Angular Velocity.For example a Speedometer uses the wheels rotational speed as a measure of Linear road speed.
The measurement of Angular Speed may be made with Tachometers which may be either Mechanical or Electrical type.
Electrical Tachometers.
The Electrical Tachometers are professed over Mechanical Tachometers for all applications because these Tachometers offer all the advantages associated with Electrical Transducers.
Electromagnetic Tachometer Generators.
There are two types of Electromagnetic Tachometer Generators called Tachogenerators.
01)D.C.Tachometer Generators.
02)A.C.Tachometer Generators.
The details of D.C.Tachometer Generators is as below.
01)D.C.Tachometer Generators.
D.C.Tachometer Generators consists of a small Armature which is coupled to the Machine whose Speed is to be measured.This Armature revolves in the field of a Permanent magnet.The Electro Magnetic Force (EMF) generated is proportional to the product of Flux and Speed.Since the flux of the Permanent magnet is constant,the voltage generated is proportional to Speed.
The Polarity of Output Voltage indicates the Direction of Rotation.This Electro Magnetic Force is measured with the help of moving Coil Voltmeter having a uniform scale and calibrated directly in terms of Speed.
Advantages are,
01)The direction of rotation is directly indicated by the Polarity of Output Voltage
02)The Output Voltage is typically 10 mV/rpm and can be measured with conventional type D.C.Voltmeters.
Disadvantages are,
01)Brushes on small Tachometer Generators often produce Maintenance problems,as their contact resistance may vary and produce appreciable errors.This is commutator and the brushes requires periodic Maintenance.
02)The input resistance of Meter should be very high as compared with Output Resistance of Generators.This is required to limit the Armature Current to small value.If the Armature Current is large,the field of the Permanent Magnet is distorted giving rise to non-linearity.
Monday, June 2, 2008
Application And Advantages of Digital Systems.
In last topic,we have discussed about Digital Instruments and Analog Instruments.In this post we will see about Digital Systems and Microprocessors used for measurement system and control.
The major reasons for Digital and Microprocessor are.
01)Stability and accuracy of control.
02)Flexibility.
03)Lower cost per function.
04)Greater reliability and equipment life.
05)Human factors favouring Digital Interface.
The most general and versatile circuit that can be placed on a single Chip is the Digital Microprocessor.the Microprocessor is versatile because it can be programmed to perform an almost unlimited computing tasks.
Application And Advantages of Digital Systems.
01)The device used in Digital Circuits generally operate in one of the two states,known as,ON & OFF resulting in a very simple operation.
02)There are only a few basic operations in Digital Circuit which are very easy to understand.
03)Digital Technique requires Boolean Algebra which is very simple and easily be learnt.
04)Digital Circuit requires basic concept of Electrical Network Analysis which is easily learnt.
05)A large number of Integrated Circuits ( IC )are available for performing various operations.These are highly reliable,accurate and the speed of operations is very high.
06)The effect of fluctuations in the characteristics of the components,ageing of components,temperature and noise etc. is very small in Digital Circuit.
07)Digital Circuit have capability of memory which makes these circuit highly suitable for Computers,Calculators,Electronic Watches etc.
08)It is very fascinating & challenging field of study because most of the latest Electronics systems are Digital in nature.
The major reasons for Digital and Microprocessor are.
01)Stability and accuracy of control.
02)Flexibility.
03)Lower cost per function.
04)Greater reliability and equipment life.
05)Human factors favouring Digital Interface.
The most general and versatile circuit that can be placed on a single Chip is the Digital Microprocessor.the Microprocessor is versatile because it can be programmed to perform an almost unlimited computing tasks.
Application And Advantages of Digital Systems.
01)The device used in Digital Circuits generally operate in one of the two states,known as,ON & OFF resulting in a very simple operation.
02)There are only a few basic operations in Digital Circuit which are very easy to understand.
03)Digital Technique requires Boolean Algebra which is very simple and easily be learnt.
04)Digital Circuit requires basic concept of Electrical Network Analysis which is easily learnt.
05)A large number of Integrated Circuits ( IC )are available for performing various operations.These are highly reliable,accurate and the speed of operations is very high.
06)The effect of fluctuations in the characteristics of the components,ageing of components,temperature and noise etc. is very small in Digital Circuit.
07)Digital Circuit have capability of memory which makes these circuit highly suitable for Computers,Calculators,Electronic Watches etc.
08)It is very fascinating & challenging field of study because most of the latest Electronics systems are Digital in nature.
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