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INTRODUCTION
A basic instrument system consists of three elements:
Most modern analogue equipment works on the following standard signal ranges.
1. Electric 4 to 20 mA
2. Pneumatic 0.2 to 1.0 bar
Older electrical equipment use 0 to 10 V. Increasingly the instruments are digital with a binary digital encoder built in to give a binary digital output. Pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion.
The advantage of having a standard range or using digital signals is that all equipment may be purchased ready calibrated. For analogue systems the minimum signal (Temperature, speed, force, pressure and so on ) is represented by 4 mA or 0.2 bar and the maximum signal is represented by 20 mA or 1.0 bar.
This tutorial is an attempt to familiarise you with the many types of input sensors on the market today. Usually such sensors are called PRIMARY TRANSDUCERS.
Things that we commonly measure are:
Sensors may operate simple on/off switches to detect the following:
The block diagram of a sensor is shown below.
TEMPERATURE TRANSDUCERS :
THERMOCOUPLES
When two wires with dissimilar electrical properties are joined at both ends and one junction is made hot and the other cold, a small electric current is produced proportional to the difference in the temperature. Seebeck discovered this effect. It is true no matter how the ends are joined so the cold end may be joined at a sensitive
millivolt meter. The hot junction forms the sensor end.
The picture shows a typical industrial probe with a flexible extension and standard plug.
Peltier showed that heat is absorbed at the hot end and rejected at the cold end. Thompson showed that part of the e .m.f. is due to the temperature gradient in the wire as well as the temperature difference between the junctions. Most thermocouple metals produce a relationship between t he two temperatures and the e.m.f as follows.
$e=\alpha(\theta_1-\theta_2)+\beta({\theta_1}^2-{\theta_2}^2)$
$\alpha$ and $\beta$ are constants for the type of thermocouple. The relationship is nearly linear over the operating range. The actual characteristic and suitable operating temperatures depends upon the metals used in the wires. The various types are designated in international and national standards. Typical linear operating ranges are shown for
standard types.
It is important that thermocouples are standard so that the same e.m.f will always represent the same temperature.
Thermocouples come in several forms. They may be wires insulated from each other with plastic or glass fibre materials. For high temperature work, the wire pairs are put inside a tube with mineral insulation. For industrial uses the sensor comes in a metal enclosure such as stainless steel.
RESISTANCE TYPE SENSORS :
These work on the principle that the electrical resistance of a conductor change with temperature. If a constant voltage is applied to the conductor then the current flowing through it will change with temperature. The resistivity of the conductor change with temperature. This usually means the resistance gets bigger as the conductor gets
hotter. The following law relates the resistance and temperature.
$R = R_0(1+\alpha \theta)$
$\alpha$ is the temperature coefficient of resistance.$R_0$ is the resistance at 00C. Sometimes the equation is given as
$R= R_0(1 \alpha \theta - \beta \theta^2)$
A basic temperature sensor is made by winding a thin resistance wire into a small sensor head. The resistance of the wire then represents the temperature. This has an advantage over a thermocouple in that it is unaffected by the temperature of the gauge end. The main type of wire used is PLATINUM. The sensors are usually manufactured to have a resistance of 100 $\Omega $ at 00C and the value of $\alpha$ is 0.00385 to 0.00390. A typical operating range is -200 to 4000C.
A special type of resistance sensor is called a THERMISTOR. They are made from a small piece of semi-conductor material. The material is special because the resistance changes a lot for a small change in temperature and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited. They are only used for a typical range of -20 to 1200C and are commonly used in small hand held thermometers for every day use. The relationship between resistance and temperature is of the form $R=Ae^{\frac{B}{\theta}}$.
LIQUID EXPANSION and VAPOUR PRESSURE SENSORS
These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full. Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurised. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.
Ways and means exist to convert the pressure into an electrical signal. The movement may also directly operate a thermostat. These instruments are robust and used over a wide range. They can be fitted with electric switches to set off alarms.
BIMETALLIC TYPES
It is a well-known principle that if two metals a re rigidly joined together as a two-layer strip and heated, the difference in the expansion rate causes the strip to bend.
In the industrial type, the strip is twisted into a long thin coil inside a tube. One end is fixed at the bottom of the tube and the other turns and moves a pointer on a dial. The outward appearance is very similar to the pressure type. They can be made to operate limit switches and set off alarms or act as a thermostat. (e.g. on a boiler).
GLASS THERMOMETER
The ordinary glass thermometer is also a complete system. Again the bulb is the sensor but the column of liquid and the scale on the glass is the processor and indicator. Mercury is used for hot temperatures and coloured alcohol for cold temperatures.
The problems with glass thermometers are that they are
1. Brittle.
2. Mercury solidifies at -400C.
3. Alcohol boils at around 1200C.
4. Accurate manufacture is needed and this makes accurate ones expensive.
5. It is easy for people to make mistakes reading them.
Glass thermometers are not used much now in industry but if they are, they are usually protected by a shield from accidental breakage. In order to measure the temperature of something inside a pipe they are placed in thermometer pockets.
PRESSURE TRANSDUCERS Pressure sensors either convert the pressure into mechanical movement or into an electrical output. Complete gauges not only sense the pressure but indicate them on a dial or scale.
Mechanical movement is produced with the following elements.
Bourdon Tube.
Spring and Piston.
Bellows and capsules.
Diaphragm.
1. BOURDON TUBE:
The Bourdon tube is a hollow tube with an elliptical cross section. When a pressure difference exists between the inside and outside, the tube tends to straighten out and the end moves. The movement is usually coupled to a needle on a dial to make a complete gauge. It can also be connected to a secondary device such as an air nozzle to control air pressure or to a suitable transducer to convert it into an electric signal. This type can be used for measuring pressure difference.
2. PISTON TYPE:
The pressure acts directly on t he piston and compresses the spring. The position of the piston is directly related to the pressure. A window in the outer case allows the pressure to be indicated. This type is usually used in hydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement may be connected to a secondary device to convert movement into an electrical signal.
3. CAPSULES AND BELLOWS:
A bellows is made of several capsules. These are hollow flattened structures made from thin metal plate. When pressurised the bellows expand and produce mechanical movement. If the bellows is encapsulated inside an outer container, then the movement is proportional to the difference between the pressure on the inside and outside. Bellows and single capsules are used in many instruments. They are very useful for measuring small pressures.
4. DIAPHRAGMS:
These are similar in principle to the capsule but the diaphragm isusually very thin and p erhaps made of rubber. The diaphragm expands when very small pressures are applied. The movement is transmitted to a pointer on a dial through a fine mechanical linkage.
5. ELECTRICAL PRESSURE TRANSDUCERS: There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal .
The principles of electric strain gauges are covered later. Strain gauges are small elements that are fixed to a surface that is strained. The change in length of the element produces changes in the electrical resistance. This is processed and converted into a voltage. A typical pressure transducer would contain a metal diaphragm which bends under pressure.
PIEZO ELECTRIC TYPES The element used here is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge may be converted into voltage. This principle is used in the pick up crystal of a record player, in microphones and even to g enerate a spark in a gas ignitor. When placed inside a pressure transducer, the pressure is converted into an electric signal.
OTHER ELECTRIC EFFECTS Other electric effects commonly used in transducers are CAPACITIVE and INDUCTIVE. In these cases, the pressure produces a change in the capacitance or inductance of an electronic component in the transducer. Both these effects are commonly used in an electronic oscillator and one way they may be used is to change the frequency of the oscillation. The frequency may be converted into a voltage representing the pressure.
SPEED TRANSDUCERS Speed transducers are widely used for measuring the output speed of a rotating object. There are many types using different principles and most of them produce an electrical output.
POSITIVE DISPLACEMENT TYPES
These types have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid. The quantity of fluid hence depends on the number of revolutions of the meter shaft and the flow rate depends upon the speed of rotation. Both the revolutions and speed may be measured with mechanical or electronic devices. Some of the most common listed below.
1. Rotary piston type.
2. Vane type.
3. Lobe type or meshing rotor.
4. Reciprocating piston type.
5. Fluted spiral gear.
MESHING ROTOR
The MESHING ROTOR type consists of two rotors with lobes. When f luid is forced in, the rotors turn and operate the indicating system.
INFERENTIAL TYPE METERS The flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically. The main types are :
Industrial flow meters:
The turbine type shown has an axial rotor which is made to spin by the fluid and the speed represents the flow rate. This may be sensed electrically by coupling the shaft to a small electric tachometer. Often this consists of a magnetic slug on the rotor which generates a pulse of electricity each time it passes the sensor.
ROTATING VANE TYPE
The jet of fluid spins around the rotating vane and the speed of the rotor is measured mechanically or electronically.
VARIABLE AREA TYPES There are two main types of this meter
FLOAT TYPE
The float is inside a tapered tube. The fluid flows through the annular gap around the edge of the float. The restriction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is
just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes the float moves up or down to find a new balance position.
When dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of a non-magnetic metal. The float has a magnet on it. As it moves up and down, the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.
TAPERED PLUG TYPE
In this meter, a tapered plug is aligned inside a hole or orifice. A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered the restriction changes and the plug takes up a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside.
DIFFERENTIAL PRESSURE FLOW METERS These are a range of meters that convert flow rate into a differential pressure. The important types conform to BS 1042 and are
The working principle for all these is that something makes the velocity of the fluid change and this produces a change in the pressure so that a difference $\Delta p = p_2-p_1$ is created. It can be shown for all these meters that the volume flow rate Q is related to Dp by the following formula.
$Q= K(\Delta p)^{0.5}$
K is the meter constant. A full explanation of these meters is covered in the tutorials on fluid mechanics. The picture shows an industrial d.p.meter. Extra instrumentation heads can be fitted to produce an electrical output (4 – 20 mA) or a pneumatic output (0.2 – 1 bar).
FORCE SENSORS
The main types of force sensors are
HYDRAULIC TYPES
Hydraulic types are often referred to as hydraulic load cells. The cell is a capsule filled with liquid. When the capsule is squeezed, the liquid becomes pressurised. The pressure represents the force and may be indicated with a calibrated pressure gauge. The capsule is often a short cylinder with a piston and the pressure produced is given by p = F/A where F is the force and A the piston area.
STRAIN GAUGE TYPE A typical load cell consists of a metal cylinder with strain gauges fixed to it. When the cylinder is stretched or compressed, the strain gauges convert the force into a change in resistance and hence voltage. Since the elements require a supply voltage, the cell usually has 4 wires, two for the supply and two for the output.
POSITION SENSORS
Position sensors are essential elements in the control of actuators. The position of both linear and rotary actuators is needed in robotic type mechanisms. There are three principle types.
A potentiometer is a variable electrical resistance. A length of resistance material has a voltage applied over its ends. A slider moves along it (either linear or rotary) and picks off the voltage at its position or angle. The tracks may be made from carbon , resistance wire or piezo resistive material. The latter is the best because it gives a good analogue output. The wire wound type produces small step changes in the output depending on how fine the wire is and how closely it is coiled on the track.
OPTICAL TYPES
Optical types are mainly used for producing digital outputs. A common example is found on machine tools where they measure the position of the work table and display it in digits on the gauge head. Digital micrometers and verniers also use this idea. The basic principle is as follows. Light is emitted through a transparent strip or disc onto a photo electric cell. Often reflected light is used as shown. The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions are counted electronically and this represents the position or angle. This is very much over simplified and you should refer to more advanced text to find out how very accurate measurements are obtained and also the direction of movement.
INDUCTIVE TYPES
The most common of these is the Linear Variable Differential transformer or LVDT. The transformer is made with one primary coil and two secondary coils, one placed above and the other below the primary. The coils are formed into a long narrow hollow tube. A magnetic core slides in the tube and is attached to the mechanism being monitored with a non magnetic stem (e.g. brass). A constant alternating voltage is applied to the primary coil. This induces a voltage in both secondary coils. When the core is exactly in the middle, equal voltages are induced and when connected as shown, they cancel each other out. When the core moves, the voltage in one secondary coil grows but reduces in the other. The result is an output voltage which represents the position of the core and hence the mechanism to which it is attached. The output voltage is usually converted into D.C. With suitable electronic equipment for phase detection, it is possible to detect which direction the core moves and to switch the DC voltage from plus to minus as the core passes the centre position. These can be very accurate and are widely used for gauging the dimensions of machined components.
DEPTH GAUGES Depth gauges measure the depth of liquids and powder in tanks. They use a variety of principles and produce outputs in electrical and pneumatic forms. The type to use depends on the substance in the tank. Here are a few.
The ultrasonic system reflects sound waves from the surface and determines the depth from the time taken to receive the reflected sound. The electronic version uses a variety of electrical affects including conduction of the fluid and capacitance. The pneumatic version bubbles air through the liquid and the pressure of the air is related to the depth. A simple pressure gauge attached to a tank is also indicates the depth since depth is proportional to pressure.
STRAIN GAUGES Strain gauges are used in many instruments that produce mechanical strain because of the affect being measured. In their own right, they are used to measure the strain in a structure being stretched or compressed.
The strain gauge element is a very thin wire that is formed into the shape shown. This produces a long wire all in one direction but on a small surface area. The element is often formed by etching a thin foil on a plastic backing. The completed element is then glued to the surface of the material or component that will be strained. The axis of the strain gauge is aligned with the direction of the strain. When the component is stretched or compressed, the length of the resistance wire is changed. This produces a corresponding change in the electrical resistance.
Let the length of the gauge be L and the change in length be $\Delta L$.
The mechanical strain $\varepsilon =\frac{\Delta L}{L}$.
Let the resistance of the gauge be R (typically 120 $\Omega $) and the change in resistance be $\Delta R$.
The electrical strain $\xi =\frac{\Delta R}{R}$.
The electrical and mechanical strain are directly proportional and the constant relating them is called the gauge factor (typically 2).
Gauge Factor = Electrical Strain/Mechanical strain = $\frac{\xi}{\varepsilon} =\frac{\Delta R}{R} \Delta L$.
STRAIN GAUGE ARRANGEMENTS A strain gauge is of little use unless we can convert the change in resistance into a voltage. This is best done with a Wheatstone bridge.
If only one active gauge is used, this would be $R_1$ or $R_2$. $R_1$ and $R_2$ must be equal, so must $R_3$ and $R_4$. In this case, the voltage at points 1 and 2 are equal to
$\frac{V_s}{2}$ and so the output Vo is zero. In order to ensure this, the balancing resistor $R_3$ is adjusted to make the output zero with no strain applied to the gauge. Suppose that $R_1$ is the active gauge. If the bridge is balanced then the voltage at points 1 and 2 is half the supply voltage. $V_1 = V_2 = \frac{V_s}{2}$.
When $R_1$ changes its resistance by $\Delta R$ the voltage at point 1 becomes:
The output becomes
$\frac{V_sR}{2R+ \Delta R}$(using ratio of resistances)
$V_0 = V_2-V_1 = \frac{V_s}{2}- \frac{V_sR}{(2R+\Delta R)}$
$V_0= \frac{V_s\Delta R}{4R+2\Delta R}$
Dividing top and bottom by R we get $ V_0 = V_s \frac{\frac{\Delta R}{R}}{\frac{4+2\Delta R}{R}}$
The gauge factor is defined as $G = \frac{electrical strain}{mechanical strain} $
$G = \frac{(\frac{\Delta R}{R})}{\varepsilon}$
So $(\frac{\Delta R}{R})= G\varepsilon$.
Finally we get $V_0 = \frac{V_sG\varepsilon}{4+2G\varepsilon}$.
TEMPERATURE EFFECTS One of the problems with strain gauges is that the resistance also changes with temperature and so it is vital that each pair of resistors is maintained at the same temperature.
If one active gauge is used, say R1, then the other resistor R2
must be placed near to it and this is best done by using a
DUMMY GAUGE fixed close to the active gauge but in a
position where it is unstrained. Better still, make R2 another
active gauge and so double the output from the bridge. For
example, if a beam is used to produce the strain, one gauge is
placed on top and the other on the bottom as shown. Let R1
increase and R2 decrease by DR. The voltage at point 1
becomes
$\frac{V_s(R-\Delta R)}{2R}$ (using ration of resistances)
$V_0 = V_2-V_1 = \frac{V_s}{2}- \frac{V_s(R-\Delta R)}{2R}$
$V_0= \frac{V_s\Delta R}{2(2R+\Delta R)}$
Dividing top and bottom by R we get $V_0 = \frac{V_s\Delta R}{2R}
Finally we get $V_0 = \frac{V_sG\varepsilon}{2}$.
If the load cell only produces tension or compression, the active gauges are R1 and R4 with R2 and R3 being dummy gauges. All 4 gauges are then at the same temperature. This is shown in the diagram.
The voltage at point 1 becomes $\frac{V_sR}{(2R+\Delta R)}$
and at point 2 becomes $\frac{V_s(R+\Delta R)}{(2R+\Delta R)}$
The output becomes $V_0 = V_2-V_1 = \frac{V_s\Delta R}{2R+\Delta R} $
Dividing top and bottom by R we get $V_0 = \frac{\frac{\Delta R}{R}}{\frac{2+\Delta R}{R}}$
Finally $V_0 = \frac{V_sG\varepsilon}{2+G\varepsilon}$
This is double the output of a single active gauge and fully temperature stable.
If a beam is used in the load cell, all 4 gauges may be made active as shown. The output at point 1 becomes
$V_1 = \frac{V_s(R-\Delta R)}{2R}$.
and at point 2 becomes
$V_2 = \frac{V_s(R+\Delta R)}{2R}$.
The output becomes $V_0 = V_2-V_1= \frac{V_s\Delta R}{R} $.
$V_0 = V_sG\varepsilon$.
This is 4 times the output of a single active gauge and fully temperature stable.
A basic instrument system consists of three elements:
- Sensor or Input Device
- Signal processor
- Receiver or Output Device
1. Electric 4 to 20 mA
2. Pneumatic 0.2 to 1.0 bar
Older electrical equipment use 0 to 10 V. Increasingly the instruments are digital with a binary digital encoder built in to give a binary digital output. Pneumatic signals are commonly used in process industries for safety especially when there is a risk of fire or explosion.
The advantage of having a standard range or using digital signals is that all equipment may be purchased ready calibrated. For analogue systems the minimum signal (Temperature, speed, force, pressure and so on ) is represented by 4 mA or 0.2 bar and the maximum signal is represented by 20 mA or 1.0 bar.
This tutorial is an attempt to familiarise you with the many types of input sensors on the market today. Usually such sensors are called PRIMARY TRANSDUCERS.
Things that we commonly measure are:
| Temperature | Pressure |
| Speed | Flow rate |
| Force | Movement, Velocity and Acceleration |
| Stress and Strain | Level or Depth |
| Mass or Weight | Density |
| Size or Volume | Acidity/Alkalinity |
| Objects(Proximity switch) | Empty or full (level switch) |
| Hot or cold (thermostat) | Pressure high or low (pressure switch) |
It is important that thermocouples are standard so that the same e.m.f will always represent the same temperature.
| Type J | 0 to 8000C |
| Type K | 0 to 12000C |
| Type T | -199 to 2550C |
| Type E | 0 to 6000C |
| Type R/S | 0 to 16000C |
| Type B | 500 to 18000C |
| Type N | 0 to 12000C |
| Type L | 0 to 8000C |
$\alpha$ is the temperature coefficient of resistance.$R_0$ is the resistance at 00C. Sometimes the equation is given as
A basic temperature sensor is made by winding a thin resistance wire into a small sensor head. The resistance of the wire then represents the temperature. This has an advantage over a thermocouple in that it is unaffected by the temperature of the gauge end. The main type of wire used is PLATINUM. The sensors are usually manufactured to have a resistance of 100 $\Omega $ at 00C and the value of $\alpha$ is 0.00385 to 0.00390. A typical operating range is -200 to 4000C.
A special type of resistance sensor is called a THERMISTOR. They are made from a small piece of semi-conductor material. The material is special because the resistance changes a lot for a small change in temperature and so can be made into a small sensor and it costs less than platinum wire. The temperature range is limited. They are only used for a typical range of -20 to 1200C and are commonly used in small hand held thermometers for every day use. The relationship between resistance and temperature is of the form $R=Ae^{\frac{B}{\theta}}$.
These are thermometers filled with either a liquid such as mercury or an evaporating fluid such as used in refrigerators. In both cases the inside of the sensor head and the connecting tube are completely full. Any rise in temperature produces expansion or evaporation of the liquid so the sensor becomes pressurised. The pressure is related to the temperature and it may be indicated on a simple pressure gauge.
Ways and means exist to convert the pressure into an electrical signal. The movement may also directly operate a thermostat. These instruments are robust and used over a wide range. They can be fitted with electric switches to set off alarms.
1. Brittle.
2. Mercury solidifies at -400C.
3. Alcohol boils at around 1200C.
4. Accurate manufacture is needed and this makes accurate ones expensive.
5. It is easy for people to make mistakes reading them.
Glass thermometers are not used much now in industry but if they are, they are usually protected by a shield from accidental breakage. In order to measure the temperature of something inside a pipe they are placed in thermometer pockets.
PRESSURE TRANSDUCERS Pressure sensors either convert the pressure into mechanical movement or into an electrical output. Complete gauges not only sense the pressure but indicate them on a dial or scale.
Mechanical movement is produced with the following elements.
2. PISTON TYPE:
3. CAPSULES AND BELLOWS:
4. DIAPHRAGMS:
5. ELECTRICAL PRESSURE TRANSDUCERS: There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal .
- Strain Gauge types.
- Piezo electric types.
- Other electric effects.
PIEZO ELECTRIC TYPES The element used here is a piece of crystalline material that produces an electric charge on its surface when it is mechanically stressed. The electric charge may be converted into voltage. This principle is used in the pick up crystal of a record player, in microphones and even to g enerate a spark in a gas ignitor. When placed inside a pressure transducer, the pressure is converted into an electric signal.
OTHER ELECTRIC EFFECTS Other electric effects commonly used in transducers are CAPACITIVE and INDUCTIVE. In these cases, the pressure produces a change in the capacitance or inductance of an electronic component in the transducer. Both these effects are commonly used in an electronic oscillator and one way they may be used is to change the frequency of the oscillation. The frequency may be converted into a voltage representing the pressure.
SPEED TRANSDUCERS Speed transducers are widely used for measuring the output speed of a rotating object. There are many types using different principles and most of them produce an electrical output.
- OPTICAL TYPES
- MAGNETIC PICK UPS
- TACHOMETERS
- POSITIVE DISPLACEMENT TYPES
- INFERENTIAL TYPES
- VARIABLE AREA TYPES
- DIFFERENTIAL PRESSURE TYPES
1. Rotary piston type.
2. Vane type.
3. Lobe type or meshing rotor.
4. Reciprocating piston type.
5. Fluted spiral gear.
MESHING ROTOR
INFERENTIAL TYPE METERS The flow of the fluid is inferred from some effect produced by the flow. Usually this is a rotor which is made to spin and the speed of the rotor is sensed mechanically or electronically. The main types are :
- Turbine rotor types
- Rotary shunt types
- Rotating vane types
- Helical turbine types
ROTATING VANE TYPE
VARIABLE AREA TYPES There are two main types of this meter
- Float type (Rotameter)
- Tapered plug type.
FLOAT TYPE
When dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of a non-magnetic metal. The float has a magnet on it. As it moves up and down, the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.
TAPERED PLUG TYPE
DIFFERENTIAL PRESSURE FLOW METERS These are a range of meters that convert flow rate into a differential pressure. The important types conform to BS 1042 and are
- ORIFICE METERS.
- VENTURI METERS.
- VENTURI METERS.
- NOZZLE METERS.
- PITOT TUBES.
- Mechanical types.
- Hydraulic types.
- Electrical strain gauge types.
STRAIN GAUGE TYPE A typical load cell consists of a metal cylinder with strain gauges fixed to it. When the cylinder is stretched or compressed, the strain gauges convert the force into a change in resistance and hence voltage. Since the elements require a supply voltage, the cell usually has 4 wires, two for the supply and two for the output.
- RESISTIVE
- OPTICAL
- INDUCTIVE
OPTICAL TYPES
Optical types are mainly used for producing digital outputs. A common example is found on machine tools where they measure the position of the work table and display it in digits on the gauge head. Digital micrometers and verniers also use this idea. The basic principle is as follows. Light is emitted through a transparent strip or disc onto a photo electric cell. Often reflected light is used as shown. The strip or disc has very fine lines engraved on it which interrupt the beam. The number of interruptions are counted electronically and this represents the position or angle. This is very much over simplified and you should refer to more advanced text to find out how very accurate measurements are obtained and also the direction of movement.
INDUCTIVE TYPES
The most common of these is the Linear Variable Differential transformer or LVDT. The transformer is made with one primary coil and two secondary coils, one placed above and the other below the primary. The coils are formed into a long narrow hollow tube. A magnetic core slides in the tube and is attached to the mechanism being monitored with a non magnetic stem (e.g. brass). A constant alternating voltage is applied to the primary coil. This induces a voltage in both secondary coils. When the core is exactly in the middle, equal voltages are induced and when connected as shown, they cancel each other out. When the core moves, the voltage in one secondary coil grows but reduces in the other. The result is an output voltage which represents the position of the core and hence the mechanism to which it is attached. The output voltage is usually converted into D.C. With suitable electronic equipment for phase detection, it is possible to detect which direction the core moves and to switch the DC voltage from plus to minus as the core passes the centre position. These can be very accurate and are widely used for gauging the dimensions of machined components.
DEPTH GAUGES Depth gauges measure the depth of liquids and powder in tanks. They use a variety of principles and produce outputs in electrical and pneumatic forms. The type to use depends on the substance in the tank. Here are a few.
The ultrasonic system reflects sound waves from the surface and determines the depth from the time taken to receive the reflected sound. The electronic version uses a variety of electrical affects including conduction of the fluid and capacitance. The pneumatic version bubbles air through the liquid and the pressure of the air is related to the depth. A simple pressure gauge attached to a tank is also indicates the depth since depth is proportional to pressure.
STRAIN GAUGES Strain gauges are used in many instruments that produce mechanical strain because of the affect being measured. In their own right, they are used to measure the strain in a structure being stretched or compressed.
The strain gauge element is a very thin wire that is formed into the shape shown. This produces a long wire all in one direction but on a small surface area. The element is often formed by etching a thin foil on a plastic backing. The completed element is then glued to the surface of the material or component that will be strained. The axis of the strain gauge is aligned with the direction of the strain. When the component is stretched or compressed, the length of the resistance wire is changed. This produces a corresponding change in the electrical resistance.
Let the length of the gauge be L and the change in length be $\Delta L$.
The mechanical strain $\varepsilon =\frac{\Delta L}{L}$.
Let the resistance of the gauge be R (typically 120 $\Omega $) and the change in resistance be $\Delta R$.
The electrical strain $\xi =\frac{\Delta R}{R}$.
The electrical and mechanical strain are directly proportional and the constant relating them is called the gauge factor (typically 2).
Gauge Factor = Electrical Strain/Mechanical strain = $\frac{\xi}{\varepsilon} =\frac{\Delta R}{R} \Delta L$.
STRAIN GAUGE ARRANGEMENTS A strain gauge is of little use unless we can convert the change in resistance into a voltage. This is best done with a Wheatstone bridge.
When $R_1$ changes its resistance by $\Delta R$ the voltage at point 1 becomes:
The output becomes
$V_0 = V_2-V_1 = \frac{V_s}{2}- \frac{V_sR}{(2R+\Delta R)}$
$V_0= \frac{V_s\Delta R}{4R+2\Delta R}$
Dividing top and bottom by R we get $ V_0 = V_s \frac{\frac{\Delta R}{R}}{\frac{4+2\Delta R}{R}}$
The gauge factor is defined as $G = \frac{electrical strain}{mechanical strain} $
$G = \frac{(\frac{\Delta R}{R})}{\varepsilon}$
So $(\frac{\Delta R}{R})= G\varepsilon$.
Finally we get $V_0 = \frac{V_sG\varepsilon}{4+2G\varepsilon}$.
TEMPERATURE EFFECTS One of the problems with strain gauges is that the resistance also changes with temperature and so it is vital that each pair of resistors is maintained at the same temperature.
$V_0 = V_2-V_1 = \frac{V_s}{2}- \frac{V_s(R-\Delta R)}{2R}$
$V_0= \frac{V_s\Delta R}{2(2R+\Delta R)}$
Dividing top and bottom by R we get $V_0 = \frac{V_s\Delta R}{2R}
Finally we get $V_0 = \frac{V_sG\varepsilon}{2}$.
and at point 2 becomes $\frac{V_s(R+\Delta R)}{(2R+\Delta R)}$
The output becomes $V_0 = V_2-V_1 = \frac{V_s\Delta R}{2R+\Delta R} $
Dividing top and bottom by R we get $V_0 = \frac{\frac{\Delta R}{R}}{\frac{2+\Delta R}{R}}$
Finally $V_0 = \frac{V_sG\varepsilon}{2+G\varepsilon}$
If a beam is used in the load cell, all 4 gauges may be made active as shown. The output at point 1 becomes
and at point 2 becomes
$V_2 = \frac{V_s(R+\Delta R)}{2R}$.
The output becomes $V_0 = V_2-V_1= \frac{V_s\Delta R}{R} $.
$V_0 = V_sG\varepsilon$.
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