Module 1. Instruments and measurement system
Lesson 1
Functions of instruments and measurement system
1.1 Introduction
Dairy processing unit operations mainly involve heating, cooling, separating, drying or freezing of the products. These unit operations are carried out under varying conditions of temperatures, pressures, flows and physical compositions. The measurement and control of these variable factors at the various stages of processing call for the accurate and efficient instruments, in addition to the dependence upon human skills. With the advent of large scale milk handling plants the automatic operation and control through efficient instrumentation and automation has become even more necessary. Utilities such as steam, water, electricity air, fuel etc. have to be measured and controlled at appropriate points in the plant. Automatic control instruments are employed to measure and control the temperature, pressure, flow and level of these utilities. The overall aim of the instrumentation/ automation is to improve the product quality and enhance the plant efficiency for better economic returns.
1.2 Variable
A characteristic number or quantity that increases or decreases over time, or takes different values in different situations is known as Variable. It is a factor that can be assigned a measurable dimension of some kind that varies, e.g., length, diameter, area, flow, weight, cost or life-span etc. A dependent variable is any measurable factor whose behavior is controlled by another variable. An independent variable is any measurable factor that produces change or reaction in another variable. A variable is something that is changed or altered in an experiment. In processing of food products the variables involved could be temperature and pressure of steam, processing time, flow rate of various streams etc. For example, to determine the effect of temperature and humidity on storage of a food product will provide evidence on the shelf life of product in different storage conditions. Variable is liable to change, may have a range of possible values and is liable to deviate from an established extension type.
1.3 Measurement
When we decide to study a variable we need to devise some way to measure it. Some variables are easy to measure and others are very difficult. The values of variables are made meaningful by quantifying them into specific units. For example, instead of saying that a particular fluid is hot, we can specify a measurement and specify that the fluid is having a temperature of 80°C. Measurement is collection of quantitative data. A measurement is made by comparing a quantity with a standard unit. An example of measurement means the use of a ruler to determine the length of a piece of paper.
Measurement is thus essentially an act or the result of comparison between the quantity (whose magnitude is unknown) and a predefined standard. Since both the quantities are compared, the result is expressed in numerical values. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement.
There are two essential requirements of the measurements, in order to make the results meaningful;
(i) The standard used for comparison purposes must be accurately defined and should be commonly accepted.
(ii) The apparatus used and the method adopted must be provable.
1.4 Unit of Measurement
A unit of measurement is a definite magnitude of a physical quantity, defined and adopted by convention and/or by law, that is used as a standard for measurement of the same physical quantity. Any other value of the physical quantity can be expressed as a simple multiple of the unit of measurement. For example, length is a physical quantity. The metre is a unit of length that represents a definite predetermined length. When we say 10 metres (or 10 m), we actually mean 10 times the definite predetermined length called "metre".
The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to this day. Different systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system.
The International System of Units (abbreviated as SI from the French language name Système International d'Unités) is the modern revision of the metric system. It is the world's most widely used system of units, both in everyday commerce and in science. The SI was developed in 1960 from the metre-kilogram-second (MKS) system, rather than the centimetre-gram-second (CGS) system, which, in turn, had many variants. During its development the SI also introduced several newly named units that were previously not a part of the metric system. The original SI units for the six basic physical quantities were:
· metre (m) : SI unit of length
· second (s) : SI unit of time
· kilogram (kg) : SI unit of mass
· ampere (A) : SI unit of electric current
· degree kelvin (K) : SI unit of thermodynamic temperature
· candela (cd) : SI unit of luminous intensity
The mole was subsequently added to this list and the degree Kelvin renamed the kelvin.
There are two types of SI units, base units and derived units. Base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units, for example, the watt, i.e. the unit for power, is defined from the base units as m2·kg·s−3. Other physical properties may be measured in compound units, such as material density, measured in kg/m3.
1.5 Significance of Measurements
Science is based on objective observation of the changes in variables. The greater our precision of measurement the greater can be our confidence in our observations. Also, measurements are always less than perfect, i.e., there are errors in them. The more we know about the sources of errors in our measurements the less likely we will be to draw erroneous conclusions. With the progress in science and technology, new phenomena and relationships are constantly being discovered and these advancements require newer developments in measurement systems. Any invention is not of any practical utility unless it is backed by actual measurements. The measurements thus confirm the validity of a given hypothesis and also add to its understanding. This is a continuous chain that leads to new discoveries with new and more sophisticated measurement techniques. While elementary measurements require only ordinary methods of measurement, the advanced measurements are associated with sophisticated methods of measurement. The advancement of Science and Technology is therefore dependent upon a parallel progress in measurement techniques. It can be safely be said that, the progress in Science and Technology of any country could be assessed by the way in which the data is acquired by measurements and is processed.
In R&D applications the design of equipments and processes require the basic engineering design data on the properties of the input raw materials and processed products. The operation and maintenance of equipments for optimal processing variables to achieve best quality product and energy efficient equipment utilization require the monitoring and control of several process variables. Both these functions require measurements. The economical design, operation and maintenance require a feedback of information. This information is supplied by appropriate measurement systems.
1.6 Function of Instrumentals and Measurement Systems
The measurement systems and the instruments may be classified based upon the functions they perform. There are four main functions performed by them: indicating, signal processing, recording and control.
i). Indicating Function: This function includes supplying information concerning the variable quantity under measurement. Several types of methods could be employed in the instruments and systems for this purpose. Most of the time, this information is obtained as the deflection of a pointer of a measuring instrument.
ii). Recording Function: In many cases the instrument makes a written record, usually on paper, of the value of the quantity under measurement against time or against some other variable. This is a recording function performed by the instrument. For example, a temperature indicator / recorder in the HTST pasteurizer gives the instantaneous temperatures on a strip chart recorder.
iii). Signal Processing: This function is performed to process and modify the measured signal to facilitate recording / control.
iv). Controlling Function: This is one of the most important functions, especially in the food processing industries where the processing operations are required to be precisely controlled. In this case, the information is used by the instrument or the systems to control the original measured variable or quantity.
Thus, based on the above functions, there are three main groups of instruments. The largest group has the indicating function. Next in line is the group of instruments which have both indicating and or recording functions. The last group falls into a special category and perform all the three functions, i.e., indicating, recording and controlling.
In this lesson only those instruments would be discussed whose functions are mainly indicating and recording, especially those instruments which are used for engineering analysis purposes. The process control functions and the related instruments are discussed in Lesson 27.
1.7 Basic Requirements of a Measurement System / Instrument
The following are the basic requirements of a good quality measurement system / instrument:
a) Ruggedness
b) Linearity
c) No hysteresis
d) Repeatability
e) High output signal quality
f) High reliability and stability
g) Good dynamic response
1.8 Applications of Measurement Systems
Before discussing the instrument characteristics, construction and working, it is pertinent to understand the various ways in which the measuring instruments are put in use. Different applications of the instruments and measurement systems are:
i). Monitoring a process/operation
ii). Control a process/operation
iii). Experimental engineering analysis
i). Monitoring a Process/Operation
There are several applications of measuring instruments that mainly have a function of monitoring a process parameter. They simply indicate the value or condition of parameter under study and these readings do not provide any control operation. For example, a speedometer in a car indicates the speed of the car at a given moment, an ammeter or a voltmeter indicates the value of current or voltage being monitored at a particular instant. Similarly, water and electric energy meters installed in homes and industries provide the information on the commodity used so that its cost could be computed and realized from the user.
ii). Control a Process/Operation
Another application of instruments is in automatic control systems. Measurement of a variable and its control are closely associated.
To control a process variable, e.g., temperature, pressure or humidity etc., the prerequisite is that it is accurately measured at any given instant and at the desired location. Same is true for all other process parameters such as position, level, velocity and flow, etc. and the servo-systems for these parameters.
A block diagram of a simple process control system is shown in Fig. 1.1.
Fig. 1.1 Process control system
Let us assume that the output variable to be controlled is non-electrical quantity and the control action is through electrical means. Since the output variable is a non-electrical quantity, it is converted into a corresponding electrical form by a transducer connected in the feedback loop. The input to the controller is reference which corresponds to the desired value of the process parameter. The output process variable is compared with the reference or desired value with the help of a comparator. In case the desired value and the process variable differ, there is a resultant error signal. This error signal is amplified and then fed to an actuator, which produces power to drive the controlled circuitry.
The corrective action goes on till the output is at the same level as the input which corresponds to the desired output. At this stage, there is no error signal and hence there is no input to the actuator and the control action stops.
Common examples of this application are the domestic appliances, such as, refrigerator, air conditioner or a hot air oven. All of these employ a thermostatic control. A temperature measuring device (often a bimetallic element) measures the temperature in the room, refrigerated chamber or in the oven and provides the information necessary for appropriate functioning of the control system in these appliances.
iii). Experimental Engineering Analysis
Experimental engineering analysis is carried out to find out solution of the engineering problems. These problems may be theoretical designs or practical analysis. The exact experimental method for engineering analysis will depend upon the nature of the problem. The analysis could be grouped into following categories:
1. Obtaining solutions of mathematical relationships with the help of analogies.
2. Formulating the generalized empirical relationships in the cases where no proper theoretical backing exists.
3. Testing the validity of theoretical predications.
4. Generating the basic engineering design data on the properties of the input raw materials and processed products for R&D application.
5. Design of process equipments for specific applications.
6. Optimization of machine / system parameters, variables and performance indices.