Measurement and Control Basics 3rd Edition (complete book)

Source: Ref 1

Source: Ref 1

BIBLIOGRAPHY

1.Thermocouple Reference Tables. 1979. NBS Monogrpah 125, Washington, DC: National Bureau of Standards.

358Measurement and Control Basics

1.8The main reason for using integral action with proportional control is to automatically correct for “off-set” present in proportional only controllers.

1.9Reset windup exists in control loops using proportional plus integral control because under certain conditions the controller will continue to integrate and attempt to change the output even outside the operating range of the controller.

1.10The controller used on the heat exchanger shown in Figure 1-17 is a temperature indicating controller and it is located on the front of a main control panel.

1.11Processes that are slow to respond to disturbances and have long response time benefit the most from the use of PID control.

Chapter 2

2.1In the feedback control loop shown in Figure 2-1, a measurement is made of the variable to be manipulated. This measured process value (PV) is then compared with a set point (SP) to generate an error signal (e = PV-SP). If a difference or error exists between the actual value and the desired value of the process, the process controller takes the necessary corrective action to return the process to the desired value.

2.2The term lag in process control means any relationship in which some result happens after some cause.

2.3The sensor output one second after the input changes rapidly from 20°C to 22°C is 21.77°C.

2.6The system time constant (τ ) is 32 seconds.

2.7Dead time is the time period during which a system does not respond to a change to the input of the system.

2.8The dead time is one second.

2.9KC = 0.36 psi/ft, Ti = 1 min, and Td = 0.25 min

2.10KC = 0.08 psi/°C, ti = 2 min, and td = 0.5 min.

Chapter 3

3.1The current flow is 15 amps.

3.2The current is 4.8 amps.

3.3The area is 9 cmil.

3.4The resistance of 500 feet of copper wire having a cross-sectional area of 4110 cmil is 1.29Ω .

3.5The resistance of 500 feet of silver wire with a diameter of 0.001 inch is 4900Ω .

3.6The distance to the point where the wire is shorted to ground is 1238.2 ft.

12 volts.

3.8I1= 1 A, I2 = 0.5 A, and It = 1.5 A

3.9I1 = 1 mA, I2 = 5 mA, Rx = 400 W, V1 = 2 volts, and V2 = 10 volts.

3.10Electric relays operate as follows: when a changing electric current is applied, a strong magnetic field is produced and the resulting magnetic force moves the iron core that is connected to a set of contacts. These contacts in the relay are used to make or break electrical connections in control circuits.

3.11The three common types of solenoid valves are direct acting, internal pilot-operated, and manual reset.

Chapter 4

4.110102= 1010.

4.2100112= 1910.

4.3101010112= 2538.

4.41011111010002 = 57508.

4.53310 = 418.

4.645110 =7038.

4.74716 = 7110.

4.815716 = 34310.

4.95610 = 3816.

360 Measurement and Control Basics

5.8The pressure is 1.95 psi.

5.9The common sensing elements used in pressure gages are: Bourdon tubes, diaphragms, and bellows.

5.10In the first method, the capacitance change is detected by measuring the magnitude of an AC voltage across the plates when excited. In the second method, the sensing capacitor forms part of an oscillator, and the electronic circuit changes the frequency to tune the oscillator.

5.11(a) Error = (±0.02)(100 inH2O) = ±2.0 inH2O. So, the actual pressure reading is in the range of 48 to 52 inH2O.

(b)Error = (±0.01)(0-100) inH2O = ±1 inH2O. Thus, the actual pressure range is 49-51 inH2O.

(c)Error = (±0.005)(50 inH2O) = ±0.25 inH2O. Thus, the pressure is in the range of 49.75 to 50.25 inH2O.

5.12∆ R = 0.24Ω .

5.13The instrument span is 11 to 66 inH2O.

Chapter 6

6.1The three common sight-type level sensors are: glass gages, displacers, and tape.

6.2The pressure is 3.43 psig.

6.3If an object displaces 3 ft3 of water at 20°C, the buoyancy force on the object is 186.9 lb.

6.4In an air bubbler level-measurement system, a dip tube is installed in a tank with its open end a few inches from the bottom. Air is forced through the bubbler tube; when the fluid bubbles just escape from the open end, the pressure in the tube equals the hydrostatic head of the liquid. As liquid level varies, the pressure in the dip tube changes correspondingly. A pressure-sensitive device measures the change in pressure.

6.5The first disadvantage of bubbler systems is limited accuracy. Another disadvantage is that bubblers might introduce foreign matter into the process. Also, liquid purges can upset the material balance of the process, and gas purges can overload the vent system on vacuum processes. If the purge medium fails, not only is the level indication on the tank lost, but the system also is exposed to process material that can cause plugging, corrosion, freezing, or safety hazards.

362Measurement and Control Basics

6.6In a capacitance probe, variations in fluid level cause changes in capacitance that can be measured with an electronic circuit in the level instrument. In a typical capacitance instrument, a metal probe that is installed in a process tank is insulated from the vessel and forms one plate of the capacitor; the metal vessel forms the other plate. The material between the two plates forms the dielectric of a capacitor. As the liquid level rises, vapors in the tank with a low dielectric constant are displaced by liquid with a high dielectric value. Capacitance changes are detected with an electronic instrument calibrated in units of level.

6.7An Ultrasonic level measurement system measures the time required for sound waves to travel through material. The velocity of a sound wave is a function of the type of wave being transmitted and the density of the medium in which it travels. When a sound wave strikes a solid medium such as a liquid surface, only a small amount of the sound energy penetrates the surface; a large percentage of the sound wave is reflected. The reflected sound wave is called an echo.

6.8A nuclear level-detection system uses a low-level gamma-ray source or sources on one side of a process vessel and a radiation detector on the other side of the tank. The material in the tank has transmissibility different from that of air, so that the instrument determines the level of the material in the container based on the amount of gamma rays reaching the radiation detector.

6.9The basic principle of a guided wave level probe is Time Domain Reflectometer (TDR) technology. TDR uses pulses of electromagnetic energy, which are transmitted down the probe tube. When a radar pulse reaches a liquid surface that has a higher dielectric constant than the air in the process tank, the pulse is reflected back to the electronic unit. A high-speed electronic timing circuit precisely measures the transit time and calculates an accurate measurement of the liquid level in the tank.

6.10The common level switches used are: inductive, thermal, float, rotating paddle, and ultrasonic.

6.11In a magnetic float-type level switch, a reed switch is positioned inside a sealed and nonmagnetic guide tube at a point where rising or falling liquid level should activate the switch. The float, which contains an annular magnet, rises or falls with liquid level and is guided by the tube.

Chapter 7

7.1T = 271.4°F and T = 406.15 Kelvin.

7.2T = 204.44°C and T = 477.59 Kelvin.

7.3T = 39.2°C.

7.4The expansion in the rod is 0.0166 meters.

7.5The normal operating ranges are: 1) Type J range: –190 to 760°C, 2) Type K range: –190 to 1260°C, and 3) Type S range: 0 to 1482°C

7.6The Seebeck voltage is 1.8 mV.

7.7The temperature is 252°C.

7.8The temperature is 357.4°C.

7.9The isothermal block is a good heat conductor, and this holds measurement junctions at the same temperature. The absolute block temperature is unimportant because the two junctions act in opposition and cancel out the voltage produced.

7.10The resistance ratio is 1.315.

7.11The temperature is 56.75°C.

Chapter 8

8.1The conductance of each solution is:

a) C = 20 mhos; b) C = 5 mhos; and c) C = 4 mhos

8.2[H+] = 10-3, pH = 3, and the solution is acidic.

8.3The no-load voltage of the battery is 12.48 volts.

8.4The Span is 12.5 inches of pressure.

8.5The maximum moisture content of the air is 150 grain/lb.

8.6The frequency of an electromagnetic radiation source that has a wavelength of 100 meters is 3 x 106 Hz.

8.7The photon energy is 6.63 x 10 –25 joule and the number of photons is 1.5 x 10 24 photons.

8.8The intensity of a 1000-w point light source at 10 meters is 0.796 w/m2 and at 20 meters is 0.199 w/ m2..

8.9The maximum wavelength for a resistance change by photon absorption for a CdS semiconductor is λ max = 0.514µ m .

364 Measurement and Control Basics

8.10A photovoltaic cell generates 0.3 volts open-circuit when exposed to 10 w/m2 of radiation intensity, what is the open-circuit voltage of the cell at 20 w/m2 is 0.389 volts.

Chapter 9

9.1The fluid velocity is 25.3 ft/sec.

9.2The volumetric flow is 5.23 cfm and the mass flow is 325.3lb/min.

9.3The fluid velocity is 10.34 ft/s and volumetric flow rate is 0.9 cfs.

9.4The Reynolds number is 82,036 and the flow is turbulent.

9.5The Reynolds number is 3667.15 and the flow is transitional.

9.6The volumetric flow, if the pressure drop across the restriction increases to 5-inH2 O is 10 cfs.

9.7The fluid velocity is 6.6 cfs.

9.8Eccentric and segmental orifices are preferable to concentric orifices for measurement of slurries or dirty liquids as well as for measurement of gas or vapor where liquids may be present, especially large slugs of liquid. Where the stream contains particulate matter, the segmental orifice, which provides an open path at the bottom of the pipe, may be preferable. However, when conditions permit, the eccentric orifice is preferred because of its greater ease of manufacture to precise tolerances and generally more accurate and repeatable performance.

9.9Differential pressure.

9.10The air velocity is 8.02 ft/s.

9.11The meter coefficient is 2pulses/gallon and the scaling factor is 20.

9.12The fluid velocity is 17.6 ft/s.

Chapter 10

10.1The rangeability is 16.

10.2The diaphragm area required to fully open the control valve is 2x10-3 m2.

10.3The valve-sizing coefficient Cv is experimentally determined for each different size and style of valve using water in a test line under carefully controlled standard conditions. The standard test piping arrangement established by the Fluid Controls Institute (FCI) to provide uniform measurement of Cv data is shown in