What are the different types of measuring instruments?

 INTRODUCTION

By measurement of a quantity, we mean "an act of comparison" between the quantity to be measured and a predefined standard, e.g., when we say a "5 meter piece", we mean that the piece is 5 times of 1 meter standard "kept in the laboratory". Measurements helps in the proper and economical design of equipment and processes. Various "measuring instruments" are used for measurement of various quantities.

We always measure quantities "through comparison".

MEASUREMENT METHODS

The methods of measurement are broadly classified as :

(1) Direct Comparison Method

(2) Indirect Comparison Method

(1) Direct Comparison Methods : This is the most common method of measurement used by common man. We measure length, mass etc, of an unknown quantity by comparing it with the known standard. This is not very accurate method; as human errors are involved.

(2) Indirect Comparison or Calibration Methods : The direct method of measurement is not always possible. For accurate measurements and to attain high degree of sensitivity. Indirect methods are preferred.

In indirect method, the quantity to be measured is first converted into a suitable analogous signal. This analogous signal is processed and fed to the instrument for measurement. The device which converts the quantity into the suitable form is called a Transducer.

The signal processing involves the following:

(a) Amplification of the weak signal before being given to the instrument or,

(b) Telemetring the data for remote reading e.g., reading temperature of the Sun.

TYPES OF MEASUREMENTS

There are following types of measurements : 

(1) Primary Measurements : The measurement obtained directly by observation and comparison is called primary measurement. It needs no conversion, e.g., measuring length, time, mass of a body etc.

(2) Secondary Measurements : These measurements involve one conversion, e.g. measurement of pressure. In this, pressure will be converted into a suitable form by a transducer and then fed to the instrument.

(3) Tertiary Measurement : The measurements involve two conversions of the quantity under measurement, e.g., temperature measurement by thermo-couple. In this measurement, the temperature is first converted into voltage, which is again converted into the legible (readable) form through a meter.

INSTRUMENT

An instrument may be defined as a device which helps to determine value of an unknown quantity.

CLASSIFICATION OF INSTRUMENTS

(A) The instruments may be of the following types:

(1) Mechanical Instruments: They are mechanical in construction and are suitable for measurement of static and stable quantities like Force, Pressure etc. But they can not measure a dynamic or varying quantity. These are having heavy moving parts and hence also produce noise or pollution. The Pressure gauge, speedometer or wattmeter are the examples of mechanical instruments.

(2) Electrical Instruments: They use electrical methods of measurement and are faster in action than the mechanical instruments. However, the measuring part of these instruments is also mechanical consisting of needle, spindle, springs etc. Their accuracy and sensitivity are no doubt superior than the mechanical instruments, but not so much as to cope with the modern days requirements. They can measure a quantity varying at the rate of 50 Hz per second. The ammeters, voltmeters are the examples of these instruments.

(3) Electronic Instruments: In modern times, Electronic Instruments have replaced the previous two types. These use semi conductor devices (diodes, transistors etc.), which have excellent time response and they work without any noise or pollution. In electronic devices, the only movement involved is that of electrons hence the "response time" is very small due to negligible inertia of electrons. The Cathode Ray Oscilloscope (C.R.O) the much used electronic instrument has a time response of the order of 10-10 seconds.

The "transistorized power supplies" are available which provides constant voltage supply to the electronic instruments; it reduces errors. Moreover, very weak signals can be measured after increasing their value by "amplifiers". The electronic instruments can measure audio, video and microwave signals. Moreover, the instruments are light, compact and consume little power.

The field of space communication today is totally depended on electronic instruments. These instruments make it possible to build computers, which require very fast time response which is only possible through electronic devices.

(B) The Instruments can also be classified as:

(1) Absolute Instruments: These are the instruments, which give magnitude of a quantity in terms of some Constant of the instrument. These are used in laboratories for purpose of calibration and standardisation. e.g., Tangent Galvanometer and the Raleigh's current balance. The later is used to measure absolute value of current.

(2) Secondary Instruments: These instruments give magnitude of the quantity directly on the scale provided with the instrument. These instruments are calibrated with the help of absolute instruments, e.g., Frequency meter, voltmeter, ohmmeter etc.

The secondary instruments may be further of two type :

(a) Analog Instruments: These have a calibrated scale with a pointer which moves on the scale and gives reading of the quantity. In other words, these instruments measure analog signals. The signals that vary in a continuous fashion and can take infinite values in any given range are called Analog signals.

(b) Digital Instruments: These instruments have neither a scale nor the pointer; they give reading directly in the digital form.

The signal, which vary in discrete steps and take up only finite different values in a given range are known as digital signal. These instruments can read digital as well as analog signals. 

The analog instruments suffer from the following errors:

(a) Parallex error: While taking reading the pointer and eye of the person, should be in the same line otherwise an error known as parallax is introduced. For this purpose, a mirror is provided in analog instruments. But this error can not be removed completely.

(b) Human error: While reading an analog instrument, human error is also introduced we can take a reading wrongly or record it wrongly. The "parallax" is also a type of human error.

The digital instruments are free from these errors and hence are very accurate as compared to analog instruments. 

(C) The instruments can also be classified according to the "function they perform, i.c..

(1) Indicating Instruments,

(2) Recording Instruments,

(3) Integrating Instruments,

(4) Controlling Instruments.

(1) Indicating Instruments: These instruments actually indicate i.e. provide "instantaneous" value of the quantity under measurement. At another instant, the value may change and thus the position of the pointer will also change on the calibrated scale. The ac ammeter or voltmeter gives instantaneous/rms value of the current or voltage under measurement. Similarly, deflection of pointer of a speedometer indicates the speed of the automobile at that moment/instant

(2) Recording Instruments: These instruments maintain a written record of the quantity under measurement. For this purpose, they have a pen (replacing the needle) and a paper (replacing the scale), the pen moves on the paper and draws a curve or graph. For example we have an instrument (load curve plotter) which can draw a load curve (load Vs time) of an installation. The paper may be re moved and kept for record and another paper may be fixed on the instrument for another curve. Similarly a potentiometric type of recorder is used for monitoring temperature. Yet another example is ECG (Electro cardio graphs) used by doctors to record heart beat. The graph can be put into the record file of the patient for later use.

(3) Integrating Instruments: These instruments neither provide instantaneous value nor they maintain record of the quantity, but they integrate (sum up) the quantity under measurement, e.g., energy meter goes on integrating the units consumed and reads on its monitor the total sum of the value since beginning.

(4) Controlling Instruments: This is one of the most important instruments especially in the field of industrial controls. In this case the information is used by the instrument to control the original measured quantity. The examples are thermostats for temperature control and floats for liquid level control.

Note: Commercially there are 3 main groups of instruments. (a) The largest group is of the Indicating Instruments (b) The second group is of instruments which have indicating and/or recording functions (c) The last group is of special instruments which can perform all the 3 functions, i.e., indicating, recording & integrating.