Measurement and Control Basics 3rd Edition (complete book)

Chapter 6 – Level Measurement and Control

165

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.

The power output of the guided-wave radar probe is only 10 percent that of a conventional radar probe. This is possible because the waveguide offers a highly efficient path for the signal to travel down to reach the surface of the liquid and then bounce back to the receiver. Degradation of the signal is minimized. This means that you can accurately measure the level of liquids that have low dielectric constants of less than 1.7. Variations in media dielectric constant have no significant effect on performance.

Guided-wave radar level probes, like conventional radar probes, use transit time to measure the liquid level. The transit time for a signal reflected from a surface of hydrocarbon liquids with a dielectric constant of 2 to 3 is the same as the transit time for water with a dielectric constant of 80. Only the strength of the return signal is changed. Guided-wave radar level probes only measure transit time.

Since the speed of light is constant, no level movement is necessary to calibrate a guided wave device. In fact, you do not have to calibrate the instrument. You perform field configuration by entering process data into the instrument for the specific level application. You can configure several instruments in very quickly. All that is required is a 24 Vdc power source and the specifications for each process tank.

Level Switches

A wide variety of level-switches available are available for use in measurement and control applications. The most common types are inductive, thermal, float, rotating paddle, and ultrasonic level switches. These switches are used to indicate high and low levels in process tanks, storage bins, or silos. They are also used to control valves or pumps in order to maintain fluid level at a set value or to prevent tanks from being overfilled.

Inductive Switch

Inductive level switches are used on conductive liquids and solids. They are also used on the interface between conductive and nonconductive liquids. The inductive transducers are excited by an electrical source, causing them to radiate an alternating magnetic field. A built-in electronic circuit detects a change in inductive reactance when the magnetic field interacts with the conductive liquid. These switches are generally used in harsh

166 Measurement and Control Basics

environments because the probe is completely sealed and has no moving parts.

Thermal Switch

In a thermal-based level switch, a heated thermal resistor (thermistor) is used to detect the surface of a liquid. The level measurement a thermistor provides is based on the difference in thermal conductivity of air and liquid. Because the thermistor reacts to heat dispersion, it can be used with wateror oil-based liquids.

The thermistor has a negative temperature coefficient of resistance, which means that its resistance decreases as temperature rises. The internal temperature of any device depends upon the heat dispersion of its surrounding environment. Since heat dispersion is greater in a liquid than in a gas or air, the resistance of a thermistor level probe will change sharply whenever the probe enters or leaves a liquid. The operating range of a thermistor assures accurate and dependable operation in liquids up to 1000C.

Float Switch

Float switches are an inexpensive way to detect liquid level at a specific point. When you use float switches on process tanks, you must generally create a seal between the process and the switch. In most cases, magnetic coupling transfers the float motion to the switch or indicator mechanism. Figure 6-16 shows a typical magnetically activated level switch. In this configuration, 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.

In the example in Figure 6-16, the switch is normally closed and will open when the float and magnet are at the same level as the reed switch. You can use the switch opening, for example, to sound an alarm or to stop a pump. The switch can also be designed to close when activated by the magnet.

Rotating Paddle Switches

The rotating paddle level switches shown in Figure 6-17 are used to detect the presence or absence of solids in a process tank. A low-power synchronous motor keeps the paddle in motion at very low speed when no solids are present. Under such conditions, there is very low torque on the motor drive. When the level in the tank rises to the paddle, torque is applied to the motor drive and the paddle stops. The level instrument detects the

Chapter 6 – Level Measurement and Control

167

	Mercury Switch	Mercury Switch
	Closed	Opened
	Magnetic	Permanent
	Piston
		Magnet
	Floats
	Non-magnetic
	Tubes

Figure 6-16. Magnetically activated level switch

High Level

Switch Turns

Freely

Low Level

Switch is

Stopped

Figure 6-17. Rotating paddle level switches

168 Measurement and Control Basics

torque and actuates a switch or set of switches. These switches can then be used to sound an alarm or to control the filling or emptying of the process tank or silo.

Ultrasonic Level Switches

You can use ultrasonic level instruments for both continuous and point measurement. The point detectors or level switches can be grouped into damped sensors or on/off transmitter types.

A damped sensor-type level switch vibrates at its resonant frequency when no process fluid is present. When process material is present, vibration is dampened out. Most units use a piezoelectric crystal to obtain the vibration in the tip of the device. These instruments contain electronic circuits that detect the change in vibration and convert it into a dry-contact switch closure. These level switches are normally limited to liquid service because the damping effect of solids is insufficient in most cases.

On/off transmitter-type level switches contain transmitter and receiver units. The transmitter generates pulses in the ultrasonic range, which the receiver detects. You can locate the transmitter and receiver in the same probe or on opposite sides of the tank, as shown in Figure 6-18. In the latter design, the signal is transmitted in air, and the level switch will be actuated when the sound beam is interrupted by the rising process material. This type of switch is effective for both solid and liquid material applications. The sensor design in which the transmitter and receiver are mounted in the same unit is generally used only for liquids.

EXERCISES

6.1List three common sight-type level sensors.

6.2Calculate the pressure in psig that is detected by a tubular sight glass gauge, if the height of the liquid is 100 inches and the specific gravity of the liquid in the tank is 0.95.

6.3An object displaces 3 ft3 of water at 20°C. Calculate the buoyancy force on the object.

6.4Describe how a typical air bubbler level-measurement system operates.

6.5List several disadvantages of bubbler level detection systems.

6.6Explain how a typical capacitance probe operates, and list some factors that can cause the instrument to have calibration problems.

Chapter 6 – Level Measurement and Control

169

Receiver

Dual Component

Ultrasonic

Level Switch

Transmitter

Single Component

Ultrasonic

Level Switch

Transmitter/Receiver

Figure 6-18. Ultrasonic level switches

6.7Describe how a typical ultrasonic level measurement system operates.

6.8Explain how a nuclear level-detection system works.

6.9Describe the basic principle of a guided-wave radar level probe.

6.10List the level switches commonly used in process control.

6.11Describe how a magnetic float-type level switch works.

BIBLIOGRAPHY

1.Cho, C. H. Measurement and Control of Liquid Level, Research Triangle Park, NC: ISA, 1982.

2.Johnson, C. D. Process Control Instrumentation Technology, 2d ed., New York: John Wiley & Sons, 1982.

3.Kirk, F. W., and N. F. Rimboi. Instrumentation, 3d ed., Homewood, IL: American Technical Publishers, 1975.

4.Liptak, B. G., and V. Kriszta (ed.). Process Control – Instrument Engineers' Handbook, rev. ed., Radnor, PA: Chilton Book, 1982.

5.Murrill, P. W. Fundamentals of Process Control Theory, 3rd Ed., Research Triangle Park, NC: ISA, 2000.

6.Weyrick, R. C. Fundamentals of Automatic Control, New York: McGraw-Hill, 1975.

7

Temperature Measurement

Introduction

This chapter explores the more common temperature-measuring techniques and transducers used in process control, including filledsystem thermometers, bimetallic thermometers, thermocouples, resistance temperature detectors (RTDs), thermistors, and integrated-circuit (IC) temperature sensors. We will discuss each transducer type in detail, but we will first consider at the history of temperature measurement, temperature scales, and reference temperatures.

A Brief History of Temperature Measurement

The first known temperature-measuring device was invented by Galileo in about 1592. It consisted of an open container filled with colored alcohol and a long, narrow-throated glass tube with a hollow sphere at the upper end, which was suspended in the alcohol. When it was heated, the air in the sphere expanded and bubbled through the liquid. Cooling the sphere caused the liquid to move up the tube. Changes in the temperature of the sphere and the surrounding area could then be observed by the position of the liquid inside the tube. This “upside-down” thermometer was a poor indicator, however, since the level changed with atmospheric pressure, and the tube had no scale. Temperature measurement gained in accuracy with the development of the Florentine thermometer, which had a sealed construction and a graduated scale.

In the years to come, many thermometric scales were designed, all of which were based on two or more fixed points. However, no scale was universally recognized until the early 1700s, when Gabriel Fahrenheit, a

171

172 Measurement and Control Basics

German instrument maker, designed and made accurate and repeatable mercury thermometers. For the fixed point on the low end of his temperature scale, Fahrenheit used a mixture of ice water and salt. This was the lowest temperature he could reproduce, and he labeled it “zero degrees.” The high end of his scale was more imaginative; he chose the body temperature of a healthy person and called it 96 degrees.

The upper temperature of 96 degrees was selected instead of 100 degrees because at the time it was the custom to divide things into twelve parts. Fahrenheit, apparently to achieve greater resolution, divided the scale into twenty-four, then forty-eight, and eventually ninety-six parts. It was later decided to use the symbol °F for degrees of temperature in the Fahrenheit scale, in honor of the inventor. The Fahrenheit scale gained popularity primarily because of the repeatability and quality of the thermometers that Fahrenheit built.

Around 1742, Swedish astronomer Anders Celsius proposed that the melting point of ice and the boiling point of water be used for the two fixed temperature points. Celsius selected zero degrees for the boiling point of water and 100 degrees for the melting point of water. Later, the end points were reversed, and the centigrade scale was born. In 1948, the name was officially changed to the Celsius scale and the symbol °C was chosen to represent “degrees Celsius or centigrade” of temperature.

Temperature Scales

It has been experimentally determined that the lowest possible temperature is -273.15°C. The Kelvin temperature scale was chosen so that its zero is at -273.15°C, and the size of one Kelvin unit was the same as the Celsius degree. Kelvin temperature is given by the following formula:

T = T(°C) + 273.15

(7-1)

Another scale, the Rankine scale (°R) is defined in the same way—with -273.15°C as its zero—and is simply the Fahrenheit equivalent of the Kelvin scale. It was named after an early pioneer in the field of thermodynamics, W. J. M. Rankine. The conversion equations for the other three modern temperature scales are as follows:

T ( oC ) =	5		(T ( o F ) − 32o )		(7-2)
9
T (°R) = T (°F) + 459.67					(7-3)
T ( o F ) =		9		T ( oC) + 32o	(7-4)

5

Chapter 7 – Temperature Measurement

173

You can use these equations to convert from one temperature scale to another, as illustrated in Examples 7-1 and 7-2.

EXAMPLE 7-1

Problem: Express a temperature of 125°C in (a) degrees °F and (b) Kelvin.

Solution: (a) We convert to degrees Fahrenheit as follows:

T ( o F ) = 9 T ( oC) + 32o 5

T ( o F ) = 9 (125 o ) + 32o 5

T (°F) = (225 + 32) °F

T = 257°F

(b) Convert to Kelvin as follows:

T (K) = T°(C) + 273.15

T (K)= 125°C + 273.15

T = 398.15 K

Reference Temperatures

We cannot build a temperature divider the way we can a voltage divider, nor can we add temperatures as we would add lengths to measure distance. Instead, we must rely on temperatures established by physical phenomena that are easily observed and consistent in nature.

The International Temperature Scale (ITS) is based on such phenomena. Revised in 1990, it establishes the seventeen reference temperatures shown in Table 7-1. The ITS-90, as this new version is called, is designed so that temperature values obtained on it do not deviate from the Kelvin thermodynamic temperature values by more than the uncertainties of the Kelvin values as they existed at the time the ITS-90 was adopted. Thermodynamic temperature is indicated by the symbol T and has the unit known as the Kelvin, symbol K. The size of the Kelvin is defined to be 1/273.16 of the triple point of water. A triple point is the equilibrium temperature at which the solid, liquid, and vapor phases coexist.

174 Measurement and Control Basics

EXAMPLE 7-2

Problem: Express a temperature of 200°F in degrees Celsius and then degrees Kelvin.

Solution: First, we convert 200°F to degrees Celsius as follows:

T ( oC) = 5 (T ( o F ) − 32o ) 9

T= 5 (200 − 32)o C 9

T = 93.33°C

Now, we convert the temperature in degrees Celsius to Kelvin as follows:

T (K) = T°(C) + 273.15

T (K)= 93.33°C + 273.15

T = 366.48 K

Table 7-1. Defining Fixed Points of the ITS-90

Description	K	0C
Vapor pressure (VP) point of helium	3 to 5	-270.15 to -268.15
Equilibrium hydrogen at triple point (TP)	13.8033	259.3467
Equilibrium hydrogen at VP point	≈ 17	≈ -256.15
Equilibrium hydrogen at VP point	≈ 20.3	≈ -252.85
Neon at TP	24.5561	248.5939
Oxygen at TP	54.3584	218.7916
Argon at TP	83.8058	189.3442
Mercury at TP	234.3156	38.8344
Water at TP	273.16	0.01
Gallium at melting point (MP)	302.9146	29.7646
Indium at freezing point (FP)	429.7485	156.5985
Tin at FP	505.078	231.928
Zinc at FP	692.677	419.527
Aluminum at FP	933.473	660.323
Silver at FP	1234.93	961.78
Gold at FP	1337.33	1064.18
Copper at FP	1357.77	1084.62