(Catatan) Pengantar Instrumentasi UTS

Summary of electronics instrumentation chapter 1 untill 8.

 

Chapter 1 : Introduction to Process Control

1.1 Introduction

The technology of controlling a series of events to transform a material into a desired end productis called process control. For instance, the making of fire could be considered a primitive form of process control

1.2 Process Control

Process control can take two forms: (1) sequential control, which is an event-based process in which one event follows another until a process sequence is complete; or (2) continuous control, which requires continuous monitoring and adjustment of the process variables.

1.2.1 Sequential Process Control Control systems can be sequential in nature, or can use continuous measurement; both systems normally use a form of feedback for control. Sequential control is an event-basedprocess,inwhichthecompletionofoneeventfollowsthecompletionof another, until a process is complete, as by the sensing devices.

1.2.2 Continuous Process Control Continuous process control falls into two categories: (1) elementary On/Off action, and (2) continuous control action. On/Off action is used in applications where the system has high inertia, which prevents the system from rapid cycling. This type of control only has only two states, On and Off; hence, its name.

Continuous process action is used to continuously control a physical output parameter of a material. The parameter is measured with the instrumentation or sensor,and compared to a set value.

1.4 Instrumentation and Sensors

The operator’s control function has been replaced by instruments and sensors that give very accurate measurements and indications, making the control function totally operator-independent. Accuracy of an instrument or device is the error or the difference between the indicated value and the actual value.(error calculation page 6)

1.8 Summary

This chapter introduced the concept of process control, and the differences between sequential, continuous control and the use of feedback loops in processcontrol. The building blocks in a process control system, the elements in the building blocks,and the terminology used, were defined. The use of instrumentation and sensors in process parameter measurements was discussed, together with instrument characteristics, and the problems encountered, suchas nonlinearity, hysteresis, repeatability, and stability. The quality of a process control loop was introduced, together with the types of problems encountered, such as stability, transient response, and accuracy. The various methods of data transmission used are analog data, digital data, and pneumatic data; and the concept of the smart sensor as a plug-and-play device was given. Considerations of the basic requirements in a process facility, such as the need for an uninterruptible power supply, a clean supply of pressurized air, clean and pure water, and the need to meet safety regulations, were covered.

 

Chapter 2

2.1 Introduction

The measurement and control of physical properties require the use of well-defined units

2.1.1 Units and Standards As with all disciplines’ sets of units and standards have evolved over the years to ensureconsistencyandavoidconfusion.Theunitsofmeasurementfallintotwodistinct systems: the English system and the SI system [2].

(Basic Units Table di halaman 32/348)

2.5 Standards

Therearetwotypesofstandards:theacceptedphysicalconstants,andthestandards developed by various institutions for uniformity of measurement and conformity between systems.

2.5.1 Physical Constants A number of commonly encountered physical constants are given in Table 2.11.

2.5.2 Standards Institutions Instrumentation and process control use the disciplines from several technical fields, and therefore, use the industrial and technical standards that have evolved in these various disciplines.

2.6 Summary

This chapter discussed the need for well-defined units for physical measurements. The English system originally was the most widely used, but is being replaced by the more scientifically acceptable SI system. SI units are based on centigrade-gramsecond units from the metric system. Measurement units were given in both systems, along with their relation to the base units, and conversion factors between the two systems. Other commonly used metric units not required because of duplication were given as they may been countered. Standard prefixes are given to cover the  wide range of measurements that require the use of multiple and submultiple units.

The digital domain also requires the use of prefixes that have been defined for the base 2, to distinguish between binary and digital numbers. Some of the more common physical constants were given, and the Web addresses of institutions that set industrial standards were given, so that the reader can obtain more specific information.

C H A P T E R 3 Basic Electrical Components

3.1 Introduction

Resistors,capacitors,andinductors—thesearethethreebasicpassiveelementsused in electrical circuits, either as individual devices or in combination.

3.2 Circuits with R, L, and C

Passivecomponentsareextensively usedinaccircuitsforfrequencyselection,noise suppression, and so forth, and are always present as parasitic components, limiting signal response and introducing unwanted delays.

3.2.1 Voltage Step Input When a dc voltage is applied to the series resistor-capacitor circuit shown in Figure 3.1(a) a current flows in the elements, charging the capacitor. Figure 3.1(b) shows the input voltage step, the resulting current flowing, and the voltages across theresistorandthecapacitor.

3.2.2 Time Constants In an RC network when a step voltage is applied ,   as showning Figure3.1(a),the voltage across the capacitor is given by the equation [1]: Ee= E(1-e^-t/RC)

3.2.3 Sine Wave Inputs

The impedance (Z) of the ac circuit, as seen by the input is given by:

3.3 RC Filters

• High-pass. Allows high frequencies to pass but blocks low frequencies;
• Low-pass. Allows low frequencies to pass but blocks high frequencies;
• Band-pass. Allows a specific range of frequencies to pass;
• Band reject. Blocks a specific range of frequencies;
• Twin–T.Form of band reject filter,but with a sharper response characteristic.

(Halaman 48/348)

3.4.1 Voltage Dividers(Pembagi tegangan, hal 50)

3.4.2 dc Bridge Circuits The simplest and most common bridge network is the DC Wheatstone bridge

 

in the form of a diamond with the supply and measuring instruments connected across the bridge as shown. When all the resistors are equal the bridge is balanced; that is, the bridge voltage at A and C are equal (E/2), and the voltmeter reads zero. Making one of the resistors a variable resistor the bridge can be balanced.

The voltage at point C referenced to D = E × R4/(R3 + R4) The voltage at point A referenced to D = E × R2/(R1 + R2) The voltage (V) between A and C = E R4/(R3 + R4) − E R2/(R1 + R2)

When the bridge is balanced V = 0, and R3R2 = R1R4

3.5 Summary

The effects of applying step voltages to passive components were discussed. When applying a step waveform to a capacitor or inductor, it is easily seen that the currents and voltages are 90° out of phase.Unlike in a resistor, these phase changes give rise to time delays that can be measured in terms of time constants. The phase changes also apply to circuits driven from ac voltages, and give rise to impedances, which can be combined with circuit resistance to calculate circuit characteristics and frequency dependency. Their frequency dependence makes them suitable for frequency selection and filtering. It is required to measure small changes in resistance in many resistive sensors, such as strain gauges. The percentage change is small, making accurate measurement difficult.To overcome thisproblem,the Wheatstone Bridge circuit isused. By

comparing the resolution in a voltage divider and a bridge circuit, a large improvement in resolution of the bridge circuit can be seen. Bridge circuits also can be used for temperature compensation, compensation of temperature effects on leads in remote sensing, and with feedback for automatic measurement of resistive changes. The use of bridge networks also can be extended to measure changes in reactive components, as would be required in a capacitive sensor by the use of ac bridge configurations.

C H A P T E R 4 Analog Electronics

4.1 Introduction

Processcontrolelectronic systems usebothanalog anddigital circuits. Thestudyof electronic circuits, where the input and output current and voltage amplitudes are continually varying, is known as analog electronics. In digital electronics, the voltage amplitudes are fixed at defined levels, such as 0V or 5V, which represent highandlowlevels,or“1”sand“0”s.Thischapterdealswiththeanalogportionof electronics.

4.2 Analog Circuits

The basic building block for analog signal amplification and conditioning in most industrial control systems is the operational amplifier (op-amp).

4.2.1 Operational Amplifier Introduction Op-amps, because of their versatility and ease of use, are extensively used in industrial analog control applications. Their use can be divided into the following categories.

1. Instrumentation amplifiers are typically used to amplify low-level dc and low frequency ac signals (in the millivolt range) from transducers. These signals can have several volts of unwanted noise.
2. Comparators are used to compare low-level dc or low frequency ac signals, such as from a bridge circuit, or transducer signal and reference signal, to produce an error signal.
3. Summing amplifiers are used to combine two varying dc or ac signals.
4. Signal conditioning amplifiers are used to linearize transducer signals with theuseoflogarithmicamplifiersandtoreferencesignalstoaspecificvoltage or current level.
5. Impedance matching amplifiers are used to match the high output impedance of many transducers to the signal amplifier impedance, or amplifier output impedance to that of a transmission line.
6. Integrating and differentiating are used as waveform shaping circuits in low frequency ac circuits to modify signals in control applications.

4.2.3 Op-Amp Characteristics

Theintegratedcircuit(IC)madeitpossibletointerconnectmultipleactivedeviceson a single chip to make an op-amp, such as the LM741/107 general-purpose op-amp. These amplifier circuits are small—one, two, or four can be encapsulated in a single plasticdualinlinepackage(DIP)orsimilarpackage.

• Voltage gain, 200,000;
• Output impedance, 75Ω;
• Input impedance bipolar, 2 MΩ;
• Input impedance MOS, 1012 Ω;
• Input offset voltage, 5 mV;
• Input offset current, 200 nA.

Inputoffsetisduetotheinputstageofop-ampsnotbeingideallybalanced.The offset is due to leakage current, biasing current, and transistor mismatch, and is defined as follows.

1. Input offset voltage. The voltage that must be applied between the inputsto drive the output voltage to zero.
2. Input offset current. The input current required to drive the output voltage to zero.
3. Input bias current. Average of the two input currents required to drive the output voltage to zero.

Slew Rate (SR) is a measure of the op-amp’s ability to follow transient signals(SR = ∆Vo / ∆t), Unity gain frequency of an amplifier is the frequency at which the small signal open loop voltage gain is 1, d or a gain of  0 dB, which in Figure 4.2 is 1 MHz. The bandwidth of the amplifier is defined as the point at which the small signal gain falls 3 dB. Gain bandwidth product (GBP) is similar to the slew rate, but is the relation between small signal open loop voltage gain and frequency. (GBP = BW × Av)

(Moreover see page 60)

4.3 Types of Amplifiers

4.3.1 Voltage Amplifiers

Inverting Amplifier

 

Noninverting Amplifier

 

Differencing Amplifier

 

4.3.2 Converters The circuits shown above were for voltage amplifiers. Op-amps also can be used as current amplifiers, voltage to current converters, current to voltage converters, and special purpose amplifiers.(Examples read at page 66)

4.3.3 Current Amplifiers Devices that amplify currents are referred to as current amplifiers. Figure 4.12 shows a basic current amplifier. The gain is given by:

where the resistors are related by the equation:

R2(R4 + R6) = R3R

 

4.3.4 Integrating and Differentiating Amplifiers Two important functions that are used in process control (see Section 16.6.5) are integration and differentiation [6]. An integrating amplifier is shown in Figure 4.13(a). In this configuration, the feedback resistor is replaced with an integrating capacitor. Using the ideal case, the currents at the input to the amplifier can be summed as follows:

(Page 69)

A differentiating circuit is shown in Figure 4.13(b). In this configuration, the input resistor is replaced with a capacitor. The currents can be summed in the ideal case as followsL(page 69)

4.3.5 Nonlinear Amplifiers Many sensors have a logarithmic or nonlinear transfer characteristic. Such devices require signal linearization. This can be implemented using amplifiers with nonlinear characteristics. These are achieved by the use of nonlinear elements, such as diodes or transistors in the feedback loop [7]. Two examples of logarithmic amplifiers are shown in Figure 4.14. Figure 4.14(a)showsalogarithmicamplifierusingadiodeinthefeedbackloop,andFigure  4.14(b)shows a logarithmic amplifier using a bipolar junction transistor in the feedback loop. The summation of the currents in an ideal case gives:

 

More at page 71

4.3.6 Instrument Amplifiers

4.3.7 Input Protection

Amplifiers, like all ICs, are susceptible to damage from excessive input voltages, such as from input voltages that are larger than the supply voltages, electrostatic discharge (ESD), or EMI pickup [9]. TheinputsofICsareinternallyprotected.However,theprotectioncangiverise toleakagecurrents,sothattheprotectionislimited.Typically, theovervoltageprotection is limited to approximately ±8V greater than the supply voltages. That is, witha±15Vsupply,theprotectionis±23V.Theprotectioncanbeimprovedbythe use of external resistors and clamps, if this is practicable.

4.4 Amplifier Applications

• Capacitance Multiplier;
• Gyrator;
• Sine Wave Oscillator;
• Power Supply Regulator;
• Level Detection;
• Sample and Hold;
• Voltage Reference;
• Current Mirror;
• Voltage to Frequency Converter;
• Voltage to Digital Converter;
• Pulse Amplitude Modulation.

4.5 Summary

This chapter introduced and discussed integrated op-amps, and how their low drift characteristics make them a suitable building block for both low frequency ac and dcsmallsignalamplification. However,op-ampsarenotidealamplifiersbecauseof the mismatch at the inputs, input impedance, and different gain at the inputs. The highopenloopgain characteristics oftheop-ampmake itnecessary tousefeedback for stabilization, and the use of a set zero is required for adjustment of the input mismatch.Theop-ampisaveryversatiledevice,andcanbeusedinmanyconfigurations for amplifying low-level voltages or currents, summing voltages, and converting between voltage and current. They also may be used as nonlinear amplifiers, as comparators,andforwaveformshaping.Theuseofop-ampsisnotlimitedtosignal amplification in process control; they have many other applications. Op-amps are susceptible to excessive supply voltage and ESD, so that care and protection is needed in handling.\

C H A P T E R 5 Digital Electronics

5.1 Introduction

Digital electronics has given us the power to accurately control extremely complex processes that were beyond our wildest dreams a few years ago [1]. It would take many volumes to cover the subject of digital technology, so in this text we can only scratch the surface.(read page 75)

5.2 Digital Building Blocks

The basic building blocks used in digital circuits are called gates. The types of gates are Buffer, Inverter, AND, NAND, OR, NOR, XOR, and XNOR [2]. These basic blocks are interconnected to build functional blocks, such as encoders, decoders, adders,counters,registers,multiplexers,demultiplexers,memory,andsoforth.The functional blocks are then interconnected to make systems, such as calculators, computers, microprocessors, clocks, function generators, transmitters, receivers, digital instruments, telephone systems, ADCs, and Digital to Analog Converters (DAC), to name a few.

5.3 Converters(76)

Example of Analog to Digital Converter.

5.3.1 Comparators

The simplest form of information transfer between an analog signal and digital signal is a comparator. This device is simply a high gain amplifier that is used to compare two analog voltages, and depending on which voltage is larger, will give a digital “0” or “1” signal.

5.3.2 Digital to Analog Converters There are two basic methods of converting digital signals to analog signals: DACs, which are normally used to convert a digital word into a low power voltage reference level or waveform generation; and pulse width modulation (PWM), which is usedtoconvertadigitalwordintoahighpowervoltagelevelforactuatorandmotor control [4]. DACs change digital information into analog voltages using a resistor network or a current mirror method (81)

Figure 5.11 shows the generation of the 1 kHz sine wave. In this example, the p-p voltage of the sine wave is generated by the 16 steps of the digital signal, thus giving peakvoltages of0Vand1.5V(1.6× 15/16).Witha20kHzconversionrate, an output voltage is obtained every 50 s or 18°, so that using sin , the voltage of the sine wave can be calculated every 18°. This is given in column 2. The closest DACvoltage isthenselected, andisgiven incolumn3.These stepvoltages are then plottedasshownwiththeresultingsinewave.Inpractice,theconversionratecould behigher,givingabetterapproximationtoacompletesinewave,and/ortheresolutionoftheDACcouldbeincreased.ShownalsoisthebinarycodefromtheDAC(4 bits only). A simple RC filter can smooth the step waveform to get the sine wave. The example is only to give the basic conversion idea.

5.3.3 Analog to Digital Converters

Sensors are devices that measure analog quantities, and normally give an analog output, although techniques are available to convert some sensor outputs directly intoadigital format.Theoutputfrommostsensorsisconvertedintoadigital signal using an ADC. A digital number can represent the amplitude ofan analog signal, as previously stated. For instance, an 8-bit word can represent numbers up to 256, so that it can represent an analog voltage or current with an accuracy of 1 in 255 (one number being zero). This assumes the conversion is accurate to 1 bit, which is normally the case, or 0.4 % accuracy. Similarly, 10-bit and 12-bit words can represent analog signals to accuracies of 0.1% and 0.025%, respectively.

5.3.5 Voltage to Frequency Converters

An alternative to the ADC is the voltage to frequency converter. After the analog voltageisconvertedtoafrequency,itisthencountedforafixedintervaloftime,givingacountthatisproportionaltothefrequencyandtheanalogsignal.Commercial units, such as the LM331 shown in Figure 5.18, are available for this conversion. These devices have a linear relation between voltage and frequency. The operating characteristics of the devices are given in the manufacturer’s data sheets.(page 89)

5.4 Data Acquisition Devices

The central processor is required to interface with a large number of sensors and to drive a number of actuators

5.4.1 Analog Multiplexers

A 4-bit analog multiplexer is shown in Figure 5.19. The analog input signals can be alternately switched by CMOS analog switches to the output buffer, similar to a rotary switch. A decoder controls the switches.

5.4.2 Digital Multiplexers Figure 5.20showstheblockdiagram ofa 4-bitdigital multiplexer. The operationis similar toan analog multiplexer, exceptthattheinputsare digital. Whentheenable is 0, the outputs from the decoder are all 0, inhibiting data from going through the inputNANDgates,andthedataoutputis0.

(See page 90)

5.4.3 Programmable Logic Arrays

Many systems have large blocks of gates to perform custom logic and sequential logic functions. These functions were constructed using the SN 74 family of logic gates. The logic gates can nowbe replaced with a programmable logic array (PLA). One of these devices replaces many gate devices, requires less power, and can be configured (programmed) by the end user to perform all of the required system functions.

5.5 Basic Processor

5.6 Summary

Digital electronics were introduced in this  chapter not only as a refresher,but also to extend digital concepts to their applications in process control. Physical variables are analog and the central processor is digital; therefore, various methods of converting analog to digital and digital to analog were discussed. Analog data can be converted to digital using successive approximation for low-speed applications, or by using flash conversion for high speed,when converting digital to analog weighted resistor techniques for an analog voltage output, or pulse width modulation for power control. Data acquisition devices are used to feed data to the central processor. The use of analog and digital multiplexers in data acquisition systems is shown with demultiplexers, and in the basic processor block diagram. The use of comparators in analog to digital and digital to analog converter swas discussed. The various methods of conversion and their relative merits were given, along with a discussion on analog to frequency converters.

C H A P T E R 6 Microelectromechanical Devices and Smart Sensors

6.1 Introduction

The development of new devices in the microelectronics industry has over the past 50 years been responsible for producing major changes in all industries. The technologydevelopedhasgivencost-effectivesolutionsandmajorimprovementsin all areas. The microprocessor is now a household word, and is embedded in every appliance,entertainmentequipment,mosttoys,andeverycomputerinthehome.In the process control industry, processes and process control have been refined to a level only dreamed of a few years ago.

Micromechanical sensing devices can be produced as an extension of the standard silicon device process, enabling the following types of sensors: • Pressure; • Force and strain; • Acceleration; • Vibration; • Flow; • Angular rate sensing; • Frequency filters

(page95)

6.2 Basic Sensors

6.2.1 Temperature Sensing A number of semiconductor parameters vary linearly with temperature, and can be used for temperature sensing

6.2.2 Light Intensity Semiconductor devices are in common use as photointensity sensors. Photodiodes, phototransistors, and integrated photosensors are commercially available

6.2.3 Strain Gauges(Pengukur regangan) In a resistive type of strain gauge, the gauge factor (GF) is the fractional change in resistance divided by the fractional change in length,

6.2.4 Magnetic Field Sensors Magnetic fields can be sensed using the Hall Effect, magnetoresistive elements (MRE), or magnetotransistors [6]. Some applications for magnetic field sensors are given in Section 11.2.1. The Hall Effect occurs when a current flowing in a carrier (semiconductor) experiences a magnetic field perpendicular to the direction of the current flow. (page 100)

6.3 Piezoelectric Devices(adalah suatu kemampuan dapat menghasilkan suatu arus listrik jika mendapatkan perlakuan tekanan.)

The piezoelectric effect is the coupling between the electrical and mechanical properties of certain materials.If a potential is applied a cross piezoelectricmaterial.

6.3.1 Time Measurements(page 103)

Jika menghitung yang diketahui, pada Squaring Circuit dibagi dengan 2×10^6

6.3.3 PZT Actuators When a voltage is applied across a PZT element in the longitudinal direction(axisof polarization), it will expand in the transverse direction (perpendicular to the direction of polarization). When the fields are reversed, the motion is reversed.

6.4 Microelectromechanical Devices

The techniques of chemical etching have been extended to make semiconductor micromachined devices a reality, and make miniature mechanical devices possible. Micromachining silicon can be divided into bulk or surface micromachining.

6.4.1 Bulk Micromachining A unique property of crystalline material is that it can be etched along the crystal planes using a wet anisotropic etch, such as potassium hydroxide. This property is used in the etching of bulk silicon to make pressure sensors, accelerometers, micropumps, and other types of devices. Pressure sensors are made by etching the backside of the wafer, which can contain over 100 dies or sensors. A photolithographic process is used to define the sensor patterns. The backside of the wafer is covered with an oxide and a layer of light-sensitive resist. The resist is selectively exposed to light through a masked patterned and then developed. The oxide can now be wet etched using buffered hydrofluoric acid. The resist will define the pattern in the oxide, which in turn is used as the masking layer for the silicon etch.(page 106)

6.5 Smart Sensors Introduction

The advances in computer technology, devices, and methods have produced vast changes in the methodology of process control systems. These systems are moving away from a central control system, and towards distributed control devices.

6.5.1 Distributed System

The distributed system has a microprocessor integrated with the sensor.Thisallows direct conversion to a digital signal, conditioning of the signal, generation of a signal for actuator control, and diagnostics. The implementation of smart sensors has many advantages over a central control system [10]. These are as follows:

• Thesmartsensortakesovertheconditioningandcontrolofthesensorsignal, reducing the load on the central control system, allowing faster system operation.
• Smartsensorsuseacommonserialbus,eliminatingtheneedfordiscretewires to all sensors, greatly reducing wiring cost, large cable ducts, and confusion over lead destination during maintenance or upgrades (especially if lead markers are missing or incorrectly placed).
• Smart sensorshave powerfulbuilt-indiagnostics, whichreducescommissioning and startup costs and maintenance.
• Directdigitalcontrolprovideshighaccuracy,notachievablewithanalogcontrol systems and central processing.
• Uniformity in programming means that the program only has to be learned once, and new devices can be added to the bus on a plug-and-play basis.
• Individual controllers can monitor and control more than one process variable. • The set points and calibration of a smart sensor are easily changed from the central control computer.
• The costofsmart sensorsystems is presently higher thanthatofconventional systems,butwhenthecostofmaintenance,easeofprogramming,easeofadding new sensors is taken into account, the long-term cost of smart sensor systems is less.

The implementation of smart sensors does have some drawbacks. These are:

• If upgrading to smart sensors, care has to be taken when mixing old devices with new sensors, since they may not be compatible.
• If a buswire fails, the total system is down,which is notthe case with discrete wiring.However,withdiscretewiring,ifonesensorconnectionfails,itmaybe necessary to shut the system down. The problem of bus wire failure can be alleviated by the use of a redundant backup bus.

6.5.2 Smart Sensors

Smart sensor is a name given to the integration of the sensor with an ADC, aproportional integral and derivative (PID) processor, a DAC for actuator control, and so forth. Such a setup is shown in Figure 6.18 for the mixture of two liquids in a fixed ratio, where the flow rates of both liquids are monitored using differential pressure sensors.The temperatures oftheliquidsare also monitoredtocorrecttheflowrates for density changes and any variations in the sensitivity of the DP cells. All of the sensors in this example can be MEM devices. The electronics in the smart sensor contains all the circuits necessary to interface to the sensor, amplify and condition thesignal,andapplyproportional,integral,andderivativeaction(PID)(seeChapter 16). When usage is varying, the signals from the sensors are selected in sequence by the multiplexer (Mux), and are then converted by the ADC into a digital format for the internal processor. After signal evaluation by the processor, the control signals are generated, and the DACs are used to convert the signal back into an analog formatforactuatorcontrol.Communicationbetweenthecentralcontrolcomputerand the distributed devices is via a common serial bus. The serial bus, or field bus, is a single twisted pair of leads used to send the set points to the peripheral units and to monitor the status of the peripheral units. This enables the processor in the smart sensor to receive updated information on factors such as set points, gain, operating mode, and so forth; and to send status and diagnostic information back to the central computer [11]. Smart sensors are available for all of the control functions required in process control, such as flow, temperature, level, pressure, and humidity control. The distributed control has many advantages, as already noted.

6.6 Summary

Integrated sensors and micromechanical devices were introduced in this chapter. These devices are silicon-based, made using chemical-etching techniques.Properties of integrated silicon devices can be used to accurately measure temperature, light, force, and magnetic field strength. The piezoelectric effect in other materials is used for accurate time generation and in microposition actuators. Integrated micromechanical devices are made using either bulk or surface micromachining techniques. Not only are these devices very small, but conditioning and sensitivity adjustment can be made as an integral part of the sensor, since they are silicon-based. This has the advantage of noise reduction, high sensitivity, improved reliability, and the ability to add features that normally would require extensive

external circuits. As electronic devices become more cost effective, conventional systems will bereplaced with distributed systems using smart sensors,which have a number of advantages in process control facilities, such as reduced loading on the controller, minimized wiring to peripheral units, and simplified expansion with the plug-and-play concept.

C H A P T E R 7 Pressure

7.1 Introduction

Pressureistheforceperunitareathataliquidorgasexertsonitssurroundings,such as the force or pressure ofthe atmosphere onthe surface of the Earth, and the force that liquids exert on the bottom and walls of a container.

7.2 Pressure Measurement

Pressure units are a measure of force acting over unit area. It is most commonly expressedinpoundspersquareinch(psi)orsometimespoundspersquarefoot(psf) in English units; or Pascals (Pa) in metric units, which is the force in Newtons per square meter (N/m2).

Pressure = Force(Gaya) / Area(Luas)

7.2.1 Hydrostatic Pressure The pressureat a specific depthina liquid is termed hydrostatic pressure.The pressureincreasesasthedepthinaliquidincreases.Thisincreaseisduetotheweightof the fluid above the measurement point. The pressure p is given by:

p = (gamma) x h

where (gamma) is the specific weight(lb/ft3 inEnglishunits,orN/m3 inSIunits),andhisthe distance from the surface in compatible units (e.g., ft, in, cm, or m).

7.2.2 Specific Gravity

 

7.2.3 Units of Measurement

1. Atmospheric pressure is measured in pounds per square inch (psi), in the English system.
2. AtmosphericpressureismeasuredinPascals(PaorN/m2),intheSIsystem.
3. Atmospheric pressure can be stated in inches or centimeters of water.
4. Atmospheric pressure can be stated in inches or millimeters of mercury.
5. Atmosphere (atm) is the equivalent pressure in atmospheres.
6. 1 torr = 1 mm mercury, in the metric system. 7. 1 bar (1.013 atm) = 100 kPa, in metric system.

MORE Details at page 117.

7.2.4 Buoyancy (Kemampuan mengaapung)

Buoyancy is the upward force exerted on an objec timmersedorfloatinginaliquid. The weight is less than it is in air, due to the weight of the displaced fluid. The upward force on the object causes the weight loss, called the buoyant force, and is given by:

B = (gamma) V (7.3)

where B is the buoyant force in pounds, (gamma)  is the specific weight in pounds per cubic foot, and V is the volume of the displaced liquid in cubic feet. If working in SI units, then B is in newtons, (gamma)  is in newtons per cubic meter, and V is in cubic meters.

7.3 Measuring Instruments

7.3.1 Manometers

7.3.2 Diaphragms, Capsules, and Bellows

Gauges are a major group of sensors that measure pressure with respect to atmospheric pressure. Gauge sensors are usually devices that change their shape when pressure is applied. These devices include diaphragms, capsules, bellows, and Bourdon tubes. Diaphragms consist of a thin layer or film of a material supported on a rigid fram

7.3.3 Bourdon Tubes

Bourdon tubes are hollow, flattened, or oval cross-sectional beryllium, copper, or steel tubes,

7.3.4 Other Pressure Sensors

Barometers are used for measuring atmospheric pressure. A simple barometer was the mercury in glass barometer, which is now little used due to its fragility and the toxicityofthemercury. Theaneroid(nofluid)barometerisfavoredfordirectreading[e.g.,thebellowsinFigure7.7,orthehelicalBourdontubeinFigure7.9(b)],and the solid state absolute pressure sensor is favored for electrical outputs. A Piezoelectric pressure gauge is shown in Figure 7.10. Piezoelectric crystals produceavoltagebetweentheiroppositefaceswhenaforceorpressureisappliedto the crystal.                             (See more at page 125)

7.3.5 Vacuum Instruments

Vacuum instruments are used to measure pressures less than atmospheric pressure. Bourdon tubes, diaphragms, and bellows can be used as vacuum gauges, but they measure negative pressures with respect to atmospheric pressure. The silicon absolute pressure gauge has a built-in low-pressure reference, so it is calibrated to measure absolute pressures. Conventional devices can be used down to 20 torr (5 kPa), and this range can be extended down to approximately 1 torr with special sensing devices.

7.5 Summary

Pressure measurements can be made in either English or SI units. Pressures can be referenced to atmospheric pressures, where they are termed gauge pressures; or referenced to vacuum, where they are referred to as absolute pressures. Hydrostatic pressures are the pressure at a depth in a liquid due to the weight of liquid above, which is determined by its density or specific weight. Similarly, when an object is placed in a liquid, a force is exerted (buoyancy) on the object equal to the weight of liquid displaced. A number of pressure measuring instruments are available, such as the Bourdon tube and bellows, although some of the older types, such as the U-tubemanometer, are being replaced by smaller, easier to use, and less fragile devices, such as the silicon pressure sensor. All of the pressure sensors can be configured to measure absolute, differential, or gauge pressures. However, to measure very low pressures close to true vacuum, special devices, such as the ionization and Pirani gauges, are required. Pressures are often used as an indirect measure of other physical parameters. Care has to be taken not to introduce errors when mounting pressure gauges. The choice of the pressure gauge depends on the needs of the application. A list of characteristics was given,along with the precautions that should be used when installing pressure gauges.

Definitions (at page 130)

C H A P T E R 8 Level

8.1 Introduction

This chapter discusses the measurement of the level of liquids and free-flowing solids. There are many widely varying methods for the measurement of liquid level. Levelmeasurementisanimportantpartofprocesscontrol.Levelsensingcanbesinglepoint,continuous,direct,orindirect.Continuouslevelmonitoringmeasuresthe level of the liquid on an uninterrupted basis. In this case, the level of the material willbeconstantlymonitored,andhencethevolumecanbecontinuouslymonitored, if the cross-sectional area of the container is known.

8.2 Level Measurement

Levelsensingdevicescanbedividedintofourcategories:(1)directsensing,inwhich the actual level is monitored; (2) indirect sensing, in which a property of the liquid, such as pressure, is sensed to determine the liquid level; (3) single point measurement, in which it is only necessary to detect the presence or absence of a liquid at a specific level; and (4) free-flowing solid level sensing.

8.2.1 Direct Level Sensing

A number of techniques are used for direct level sensing, such as direct visual indication using a sight glass or afloat. Ultrasonic distance measuring devices also maybe considered.

8.2.2 Indirect Level Sensing A commonly used method of indirectly measuring a liquid level is to measure the hydrostatic pressure at the bottom of the container. The level can be extrapolated from the pressure and the specific weight of the liquid. The level of liquid can be measured using displacers, capacitive probes, bubblers, resistive tapes, or by weight measurements.

Pressure is often used as an indirect method of measuring liquid levels. Pressure increases as the depth increases in a fluid. The pressure is given by:

p = (GAMMA)h (8.1)

where p is the pressure, (GAMMA) is the specific weight, and h is the depth. Note that the units must be consistent, the specific weight is temperature-dependent, and temperature correction is required.

8.2.3 Single Point Sensing

In On/Off applications, single point sensing can be used with conductive probes, thermal probes, and beam breaking probes. Conductive probes are used for single point measurements in liquids that are conductive and nonvolatile, since a spark can occur. Conductive probes are shown in Figure 8.11. Two or more probescan be used to indicate set levels. If the liquid is in a metal container, thenthe container can be used as the common probe. Whenthe liquid is in contact with two probes, the voltage between the probes causes a current to flow, indicating that a set level has been reached. Thus, probes can be used to indicate when the liquid level is low, and when to operate a pump to fill the container.A third probe can be used to indicate when the tank is full, and to turn off the filling pump. The use of ac voltages is normally preferred to the use of dc voltages, to prevent electrolysis of the probes.

8.2.4 Level Sensing of Free-Flowing Solids (More at page 142)

8.4 Summary

This chapter introduced the concepts of level measurement. Level measurements can be director indirect continuous monitoring, or single point detection. Direct reading of liquid levels using ultrasonic devices is noncontact, and can be used for corrosive and volatile liquids and slurries. Indirect measurements involve the use of pressure sensors, bubblers, capacitance,or load cells,which a reall temperature-sensitive and will require temperature data for level correction. Of these sensors, load cells do not come into contact with the liquid, and are therefore well suited for the measurement of corrosive, volatile, and pressurized liquids and slurries. Single point monitoring can use conductive probes, thermal probes, or ultrasonic or radioactive devices. Of these devices, the ultrasonic and radioactive devices are noncontact, and can be used with corrosive and volatile liquids, and in pressurized containers. Care has to be taken in handling radioactive materials. The measurement of the level of free-flowing solids can be made with capacitive probes, a paddle wheel, or with a vibration-type of device.