Module 2. Classification and selection of instruments

Lesson 3


3.1  Introduction

In the physical scssssssiences, process engineering and product quality assurance, 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. Measurement generally involves using an instrument as a physical means of determining a quantity or variable. The instrument serves as an extension of human faculties and enables the man to determine the value of an unknown quantity which unaided human faculties cannot measure. An instrument may be defined as a device for determining the value or magnitude of a quantity or variable. Measuring instruments, and formal test methods which define the instrument's use, are the means by which the variables and the relations between variables are obtained

The instruments may be classified as follows:

i)          Mechanical, electrical and electronic instruments

ii)        Absolute and secondary instruments

iii)       Manual and automatic instruments

iv)       Analogue and digital instruments

v)         Self operated and power operated instruments

vi)       Self contained and remote indicating instruments

3.2  Mechanical, Electric and Electronic Instruments

3.2.1  Mechanical instruments

The first instruments were mechanical in nature and the principles on which these instruments worked are even in vogue today. The earliest scientific instruments used the same three essential elements as our modern instruments do. These elements are a detector, an intermediate transfer device and an indicator, recorder or a storage device.

These instruments are very reliable for static and stable conditions. There is a large number of possibilities of mechanical instruments.  It could be calipers, micrometers, scales, measuring tapes, lasers, etc. for measuring distances, a pressure gauge for measuring pressure, strain gauges for measure how much a part is stretched or compressed when a load is applied, tachometer for measuring the rotational speed, multimeter for measuring electrical voltages and currents.

However, the mechanical instruments suffer from a disadvantage that they are unable to respond rapidly to measurements of dynamic and transient conditions. These instruments have several moving parts that are rigid, heavy and bulky and consequently have a large mass. The mass presents inertia problems and hence these instruments cannot follow the rapid changes which are involved in dynamic measurements. Another disadvantage of mechanical instruments is that most of them are a potential source of noise and cause pollution of silence.

Mechanical instruments are simple in design and application. They are more durable and relatively cheaper. No external power source is required for the operation of mechanical instruments. They are quite reliable and accurate for measurements under stable conditions. 

3.2.2  Electrical instruments

Electrical methods of indicating and transmitting the output are faster than the respective mechanical methods. However, an electrical system normally depends upon a mechanical pointer movement as an indicating device. Thus owing to the inertial of mechanical movements these instruments have a limited time and frequency response. For example, some electrical recorders can give full scale response in 0.2 seconds; while the majority of industrial recorders have response time of 0.5 to 24 seconds. Some of the galvanometers can follow 50 Hz variations, but as per present day requirements of fast measurements these are also considered to be slow.

Electrical instruments are light and compact. Amplification produced is greater than that produced by mechanical means. They provide greater flexibility and are lighter in construction. These instruments consume less power and hence cause lesser load on the system.

3.2.3  Electronic instruments

Majority of the modern scientific and industrial measurements require very rapid responses. The mechanical and electrical instruments and systems cannot fulfil these requirements. There is a requirement of decreasing the response time and also the detection of dynamic changes in certain parameters. The monitoring time could be of the order of milli seconds (ms) and many a times, micro seconds (µs). This has led to the design of today’s electronic instruments and their associated circuitry. These instruments involved vacuum tubes or semi-conductor devices. The present day practice is to use semi-conductor devices owing to their many advantages over their vacuum tube counterparts. Since in electronic devices the only movement involved is that of electrons and the inertia of electrons being very small, the response time of these devices is extremely small. For example, a C.R.O. is capable of following dynamic and transient changes of the order of a few nano seconds (10-9 s).

Electronically controlled power supplies are used to provide stable voltages for studies in the field of chemical reactions and nuclear instrumentation. Electronic instruments are steadily becoming more reliable on account of improvements in design and manufacturing processes of semi-conductor devices. Another advantage of using electronic devices is that very weak signals can be detected by using pre-amplifiers and amplifiers. The foremost importance of the electronic instruments is the power amplification provided by the electronic amplifiers. Additional power may be fed into the system to provide an increased power output beyond that of the input. This has been only possible through the use of electronic amplifiers, which have no important mechanical counterpart. This is particularly important where the data presentation devices use stylus type recorders, galvanometers, cathode ray oscilloscopes and magnetic tape recorders.

It is a fact that hydraulic and pneumatic systems may be used for power amplification of signals. However, their use is limited to slow acting control applications like servo-systems, chemical processes and power systems. Electronic instruments find extensive use in detection of electro-magnetically produced signals such as radio, video, and microwave. Electrical and electronic instruments are particularly useful in the intermediate signal modifying stage. Electronic instruments are light compact and have a high degree of reliability. Their power consumption is very low.

Electronic instruments make it possible to build analogue and digital computers without which the modern developments in science and technological are virtually impossible. Computers require a very fast time response and it is only possible with use of electronic instruments. The mathematical processing of signal, such as, summation, differentiating and integrating is possible with electronic measurements. With these instruments non contact or remote measurements are also possible.

3.3  Absolute/primary and Secondary Instruments

Electrical measurements of different parameters like current, voltage, power, energy, etc. are most essential in any industry. These are among the oldest of all measurements. The various electrical instruments may be broadly divided into two categories:

1)      Absolute instruments

2)      Secondary instruments

3.3.1  Absolute/primary instruments

Absolute/primary instruments are those which give the value of electrical quantity to be measured in terms of the constants of the instruments and their deflection only e.g. tangent galvanometer. These instruments are rarely used except in standard laboratories, especially for calibration of secondary instruments.

3.3.2  Secondary instruments

Secondary instruments are those in which the values of electrical quantity to be measured can be determined from the deflection of the instruments only when they have been pre-calibrated by comparison with an absolute instrument. Without calibration, the deflection of such instruments is meaningless.

Working with absolute instruments for routine work is time consuming since every time a measurement is made, it takes a lot of time to compute the magnitude of the quantity under measurement. It is the secondary instruments which are most generally used in everyday work, the use of the absolute instruments being merely confined within laboratories as standardizing instruments. A voltmeter, a glass thermometer and a pressure gauge are typical examples of secondary instruments.

Secondary type of measuring instruments has been classified in the following categories:  Indicating instruments

Indicating instruments are those which indicate the instantaneous value of the variables being measured, at the time at which it is being measured. Their indications are given by pointers moving over calibrated dials or scales, e.g., ammeter, voltmeter and wattmeter. This movement of pointer or the deflection is not constant but depends on the quantity it measures. As the needle deflects and indicates the amount of current, voltage or any quantity, these are called deflection type of instruments.  Recording instruments

Recording instruments are those which give a continuous record of variations of the measured variable over a selected period of time. The moving system of the instrument carries an inked pen which rests tightly on a graph chart. These instruments will go on recording on a graph sheet fixed on the instrument all the variations of the quantity in the time it is connected in the circuit. Normally these recordings will be for one day and the recorded sheets are kept as a record of variation of the quantity with time.  Integrating instruments

These are the instruments which will add up the quantity as the time passes or in other words will give a total account of quantity spent in a given time for which it is connected in a circuit. For example, an electric meter measure and register, by a set of dials and pointers, either the total quantity of electricity (in ampere-hours) or the total amount of electrical energy (in watt-hours or kilowatt-hours) supplied to a circuit over a period of time and are known as  ampere-hour meters, watt hour meters, energy meters, etc. Another example is house hold water meter. Deflecting type instruments are again classified as follows:

a)      Depending upon working principle, such as, moving coil, moving iron, dynamometer, electrostatic type, induction type

b)      Depending upon the quantity it measures, such as, voltmeter, ammeter, ohm meter, power factor meter, energy meter etc.

c)      Depending upon the shape of the instruments, such as, portable, panel board type with flush mounting or surface mounting.

Deflection is normally with in 90o, but circular scale instruments are also available which give about 250o deflection. All the deflecting instruments are marked on scale to indicate its working principle by symbols.

3.4  Manual and Automatic Instruments

Manual require the services of an operator, where as in automatic instruments the operator is not required. For example, measurement of rotational speed by a hand operated tachometer an operator is required to make the contact of the instrument with the rotating shaft. For measurement of temperature by a resistance thermometer by Wheat stone bridge in its circuit an operator is required to indicate the temperature being measured. Where as, in measurement of temperature by mercury-in-glass thermometer, no operator is required.

3.5  Self Operated and Power Operated Instruments

A self operated instrument does not require any external power source for its operation. In such instruments the output energy is supplied by the input signal e.g. a dial indicator or mercury-in-glass type thermometer.

In power operated instruments some auxiliary power source is required for its operation. This external power source could be electricity, compressed air etc. In such cases the input signal supplies only the insignificant portion of the output power e.g. an electro-mechanical measurement system.

3.6  Self Contained and Remote Indicating Instruments

A self contained instrument has all the physical elements in one assembly e.g. an analog ammeter or a mercury-in-glass thermometer etc. Whereas, in a remote indicating instrument has primary sensory element and the secondary indicating element are located at two different locations linked by transmitting element. These locations could be long distance apart. In modern instrumentation technology such type of arrangement is quite necessary and vogue. 

3.7  Contact Type and Non-Contact Type Instruments

In contact type instruments the sensing element of the instrument contacts the control medium for the measurements, for example mercury-in- glass thermometer. Where as in non-contact type instruments the sensor does not contact the control medium. The non-contact type measurement includes optical, radioactive or radiation measurements. Such as, radiation or optical pyrometer, non-touch tachometer etc. The Secondary instruments working on analog and digital mode of operation are discussed in Lesson 4.