Figure 1: Radiation Pyrometer.

A total radiation pyrometer is used to measure the temperature by evaluating the heat radiation emitted by a body. All the radiations emitted by a hot body or furnace flames are measured and calibrated for black-body conditions.

Working Principle of Radiation Pyrometer

Every object radiates thermal energy at temperatures above absolute zero and the radiation emitted is a function of its temperature. The energy radiated is proportional to the emissivity at a particular temperature and wavelength. To measure the accurate temperature of a given surface from which radiation is receiving, an operator has to know the emissivity of that material. The radiation is focussed using a lens onto sensor which is a photo sensitive device and generates a voltage proportional to the radiation falling on it (Thermal detector).

Construction of Radiation Pyrometer

A total radiation pyrometer (see Fig. 1) consists of an optical system which includes a lens, a minor, and an adjustable eyepiece. The radiation heat energy emitted from the hot body is focused by an optical system onto a thermocouple or a thermopile and converted to its analogous electrical signal and can be read on a temperature display unit.

The pyrometer should be aligned properly with hot body and should be placed as close to it as possible to minimize the absorption of radiation by the atmosphere. Radiation pyrometers are useful for the measurement of temperature in corrosive environments and in applications where physical contact is impossible.

Advantages of Radiation Pyrometer

1. It is a non-contact-type device.
2. Very quick response is possible.
3. Suitable for high-temperature measurement.

Disadvantages of Radiation Pyrometer

1. Due to emission of radiations to the atmosphere. errors in temperature measurement are possible.
2. Errors due to emissivity affect measurements.

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Figure 1: Capacitance Level Sensor.

Working Principle of Capacitance Level Sensor

The capacitance method of liquid level measurement or Capacitance Level Sensor operates on the principle of parallel plate capacitor, which can be stated as the capacitance of the parallel plate capacitor varies or changes if the area or dielectric constant of it changes.

Construction and Working of Capacitance Level Sensor

The capacitance probe or element is placed inside the tank (generally near to its wall) whose level of liquid is to be measured. The liquid (whose level is to be measured) placed inside the tank can be of two types one is conductive type and the other is nonconductive type. If the liquid is conductive in nature then the capacitance probe acts as one plate of the capacitor whereas the liquid acts as another plate of the capacitor. The dielectric material between these two plates is nothing but the insulation provided for the capacitance probe. In this case, the capacitance varies as the height of the liquid changes i.e., the principle of change in area of plates is used. Therefore when the height of the liquid increases the area between the plates decreases and output capacitance increases. Similarly when the height decreases the capacitance also decreases. In case the liquid whose level is to be measured is nonconductive in nature then the probe acts as one plate of the capacitor whereas the wall of the metal tank acts as another plate of the capacitor. In this case, the dielectric material is liquid. Here the capacitance varies as the dielectric material change (i.e., the principle of change in dielectric constant is used).

As shown in figure 1, the capacitance meter is connected to the capacitance probe and to the wall of the tank. The capacitance meter is calibrated interns of liquid level. If the level of the liquid inside the tank is low (or decreased) the capacitance of the capacitor decreases. The decreased capacitance value is displayed on the capacitance meter.

Similarly the capacitance increase with increase of liquid level and is indicated by the meter which intern indicates the level of the liquid inside the tank.

Advantages of Capacitance Level Sensor

1. This method of level measurement is very sensitive.
2. This method can be used for small systems.
3. No problem of wear-tear since it does not contain any movable pans.
4. It can be used with slurry fluids.

Disadvantages of Capacitance Level Sensor

1. The performance will be affected by the change in temperature.
2. The connection and mounting of metal tank with the meter should be proper, otherwise some errors may occur.

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Working & Construction of Eddy Current Tachometer

The eddy current tachometer consists of a permanent magnet. Here two pole or multiple-pole magnets can be used. This magnet is arranged to a shaft. The shaft is used to make contact with the rotor whose angular velocity is to be measured. An eddy current cup which is nothing but a conducting material is placed near to the magnet. This is carried by a spindle arrangement and restrained by using torsion spring. Mostly aluminium is used to make eddy current cup. When the shaft is in contact with a rotor (which is rotating), the shaft rotates along with magnet.

Due to this, flux passes through the cup. With the change in the rotation of the magnet the direction of induced flux also changes causing the alternate flux density (B) to develop in the cup. So an electric field is produced with the rate of change of flux density B.

\[ \text{Curl E = }\frac{-dB}{dt}\]

This electric field will generate eddy current in the shell of the cup. The generated eddy current density is indicated by J and it will produces a secondary magnetic field of intensity H. The relation between magnetic field of intensity H and eddy current density J is given by,

\[\text{Curl H = J}\]

Due to these two magnetic fields a torque is generated in the eddy current cup, which is proportional to the angular speed. A pointer scale arrangement is provided at the back of the spindle. If the eddy current cup is restrained by torsion spring, an angular deflection of spindle occurs and is indicated over a calibrated scale. If both magnetic torque and spring torque are balanced then it will be indicated by pointer.

Applications of Eddy Current Tachometer

Eddy current tachometers are used,

1. In automobile speedometers
2. To measure speed in aircraft engines
3. To measure locomotive speeds
4. In control systems and industrial applications.

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Working Principle of Ultrasonic Flow Meter

The velocity of propagation of ultrasonic sound waves in fluid changes when the velocity of the flow of fluid changes.

Working & Construction of Ultrasonic Flow Meter

The arrangement of flow rate measurement using ultrasonic transducer contains two piezoelectric crystals placed in the fluid (gas or liquid) whose flow rate is to be measured. Of these two crystals, one acts as a transmitting transducer (transmitter, T) and the other acts as a receiving transducer (receiver, R). The transmitter and thevreceiver are separated by some distance, say d. Generally, the transmitting transducer is placed in the up stream and it transmits ultrasonic pulses when an electronic oscillator energizes it. These ultrasonic pulses are then received by the receiving transducer placed at the downstream flow.

Let the time taken by the ultrasonic pulses to travel from transmitter and received at the receiver is Δt. If the direction of propagation of the signal is same as the direction of flow then the transit time can be given by,

\[\Delta {{t}_{1}}=\frac{d}{{{V}_{s}}+{{V}_{f}}}\]

Where,

d = Distance between two crystals (i.e., transmitter and receiver).

V_s = Velocity of propagation of ultrasonic sound waves in the fluid.

V_f = Linear velocity of flow.

If the direction of the signal is opposite with the direction of flow then the transit time is given by,

\[\Delta {{t}_{2}}=\frac{d}{{{V}_{s}}+{{V}_{f}}}\]

The signal traveling in the same direction of flow is sinusoidal in nature having f Hz frequency has a phase shift of,

\[\Delta {{\phi }_{1}}=\frac{2\pi fd}{{{V}_{s}}+{{V}_{f}}}\text{ rad}\]

Similarly, the sinusoidal signal traveling in the opposite direction of flow has a phase shift of,

\[\Delta {{\phi }_{2}}=\frac{2\pi fd}{{{V}_{s}}-{{V}_{f}}}\]

Now the determination of transit time or phase shift gives the velocity of fluid flow. Using this ultrasonic flow meter the flow rate can be determined in two methods. One method involves the measurement of difference in transit time (Known as transit time difference method) and the other method involves the measurement of difference in frequency (Known as oscillating loop system).

The output of oscillator which is a sinusoidal signal of 100 kHz frequency is supplied to the transmitter which transmits these signals to the receiver. With the help of commutating switch the functions of both transmitter and receiver are reversed.

Therefore, the difference that exists in the transit time can be given by,

\[\Delta t=\Delta {{t}_{2}}-\Delta {{t}_{1}}\]

\[\Delta t=\frac{d}{{{V}_{s}}-{{V}_{f}}}-\frac{d}{{{V}_{s}}+{{V}_{f}}}\]

\[\Delta t=\frac{2dv}{V_{s}^{2}-V_{f}^{2}}\]

This difference in transit time can be determined using a phase sensitive detector, driven in synchronization with the commutator. Generally the flow velocity inside the pipe is very low and can be neglected. Therefore the difference in transit time becomes,

\[\Delta t\approx \frac{2dv}{V_{s}^{2}}\]

Thus, the Δt is linearly proportional to velocity of flow.

Another method of flow rate measurement using ultrasonic flow meter is shown in figure (2). This arrangement of flow rate measurement is based on frequency. It uses two self excited oscillating system in order to utilize the received pulses to trigger the transmitted pulses in feedback mechanism.

This helps to generate a train of pulses. The pulse repetition frequency for transmitter placed at upstream or in the forward propagating loop is given by,

\[{{f}_{1}}=\frac{1}{\Delta {{t}_{1}}}=\frac{1}{\frac{d}{({{V}_{s}}+{{V}_{f}}\cos \theta )}}=\frac{({{V}_{s}}+{{V}_{f}}\cos \theta )}{d}\]

Similarly, the pulse repetition frequency for transmitter placed at downstream or in the backward propagating loop is given by,

\[{{f}_{2}}=\frac{1}{\Delta {{t}_{2}}}=\frac{1}{\frac{d}{({{V}_{s}}-{{V}_{f}}\cos \theta )}}=\frac{({{V}_{s}}-{{V}_{f}}\cos \theta )}{d}\]

Therefore, the difference in these two frequencies is given by,

\[f={{f}_{1}}-{{f}_{2}}\]

\[f=\frac{({{V}_{s}}+{{V}_{f}}\cos \theta )}{d}-\frac{({{V}_{s}}-{{V}_{f}}\cos \theta )}{d}=\frac{2{{V}_{f}}\cos \theta }{d}\]

This frequency difference gives the flow rate of the fluid.

Advantages of Ultrasonic Flow Meter

1. Not required any obstruction to the flow.
2. It is not affected by changes in density, viscosity, and temperature.
3. The output is linearly proportional to the input.
4. Effectively used in bidirectional flow measurements.
5. High accuracy and also has an excellent dynamic response.

Disadvantages of Ultrasonic Flow Meter

1. The circuit arrangement is difficult.
2. The cost of the arrangement is high.

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Working Principle of Electromagnetic Flow Meter

The measurement of flow rate using an electromagnetic flow meter depends on Faraday’s law of electromagnetic induction. When a pipe or tube carrying electrically conducting fluid is placed in a transverse magnetic field an e.m.f. will be induced across the electrodes connected to it. This voltage gives the measure of the velocity of the fluid or flow rate of the fluid.

Construction of Electromagnetic Flow Meter

An electromagnetic flow meter consists of a nonmagnetic and non-conducting pipe to carry the flow whose velocity or flow rate is to be determined (see Figure). To the opposite sides of this pipe a pair of insulated electrodes which are in contact with the fluid flow inside the pipe are connected. Now this pipe is placed between the two poles of an electromagnet or permanent magnet that produces magnetic field.

Working of Electromagnetic Flow Meter

When the conducting fluid whose flow rate is to be measured is made to flow through the pipe, it cuts the magnetic field causing some e.m.f. to be induced across the electrodes. This induced voltage is given by,

\[e=Blv\]

Where,

B — Flux density

l — Conductor length (this is equal to diameter of the pipe)

v — Velocity of the fluid (conductor)

From the above equation it is clear that the voltage induced across the electrodes is directly proportional to the diameter of pipe, average velocity of fluid and hence gives the volume flow rate of the fluid.

Advantages of Electromagnetic Flow Meter

1. This method does not cause any obstruction to the flow of fluid, hence pressure will not drop.
2. It can be used with pipes of any size.
3. Accuracy is good.
4. The relation between output voltage and flow rate is linear.
5. Effectively measures the flow rates of slurries, conducting fluids, sludge etc.
6. The output is independent of variations in temperature, viscosity, density and so on.

Disadvantages of Electromagnetic Flow Meter

1. Highly expensive.
2. The presence of gas and air bubbles in the fluid leads to errors.
3. The fluid under measurement must be conductive in nature.

Limitations of Electromagnetic Flow Meter

1. Its use is limited to conductive fluids only.
2. Its flow range is limited to 10^-3 g pm to 10⁴ g pm i.e., 0.5 to 4.95 × 10² m³/hr.

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