Generalised measuring system

1 . DEFINITION OF MEASUREMENT

The measuring process is the one in which the property of an object or a system under consideration is compared to an acceptable standard unit.

For a measurement to be meaningful, the following three basic things are required:

i. The standard used for comparison purposes must be accurately defined and should be commonly accepted.

ii. The apparatus used and method adopted must be provable.

iii. The numerical measure is meaningless unless followed by the unit used.

1.1 Significance of Measurements

The basic purpose of measurement is to obtain requisite information pertaining to the fruitful completion of a process. The applications of measurement are monitoring of processes, control of processes and experimental engineering analysis. The significance of measurements are the following 

• As science and technology move ahead, new phenomena and relationships are discovered and these advances make new types of measurements imperative.

• New discoveries are not of any practical utility unless results are backed by actual measurements. 

• The measurements not only confirm the validity of hypothesis but also add to its understanding.

• This results in an unending chain, which leads to new discoveries that require more, new and sophisticated measurement techniques.

• Science and technology are associated with sophisticated methods of measurement.

• Measurement plays a significant role in achieving goals and objectives of engineering because of feedback information supplied by them.

1.2 METHODS OF MEASUREMENT

The methods of measurement may be broadly classified into two categories:

i. Direct Methods

ii. Indirect Methods.

i. Direct Methods 

The information that may be available sometimes indicates the progress the process in a very simple way involving a direct relation. In direct measurement, the meaning of the measurement and the purpose of the processing operation are identical. Such direct measurements are generally accomplished by simple mechanical means.

In direct methods of measurement, the unknown quantity is directly compared against a standard and the result is expressed as a numerical number and a unit (for example, consider an example of collecting 1 litre of water from a tank. In this example, the meaning of the measurement of volume and the purpose of the collecting operation, both are same, i.c., collecting 1 litre of water). Direct methods are quite common for the measurement of quantities like length, mass and time. As human factor is involved in direct measurement, it may not necessarily be very accurate. The sensitivity obtained is less. Direct methods are not preferred and are rarely used.

ii.Indirect Methods 

As direct measurement is not always possible, an indirect measurement technique, involving a derived relationship between the measured quantity and the desired result is adopted. In indirect measurement, the meaning of the measurement and the purpose of the processing operation are not same but they are related to each other. The modem trend in the indirect methods of measurement is to go for electrical methods which offer possibilities of high speed of operation, simpler processing of the measurand and adaptation of computer processing as well. The important aspects of indirect methods are that these methods are comparatively more accurate and have high sensitivity. Equivalent output is obtained indirectly against a standard, and therefore, these methods are common and are preferred for measurement of quantities like temperature, level, flow, etc.

Consider an example of pasteurizing milk. This operation is monitored by noting the temperature of the milk. Here, the temperature measurement is indirect because the purpose of operation is to pas teurize the milk, i.e., to remove the bacteria that may damage the milk, and the meaning of measurement here is to measure the milk temperature. But note that the extent of pasteurization depends upon the temperature of the milk. In this example, direct measurement would be the bacteria count.

1.3 CLASSIFICATION OF MEASURING INSTRUMENTS

A measuring instrument is simply a device for determining or ascertaining the value of some particular quantity or condition. The value determined by the instrument is generally, but not necessarily, quan titative. A measuring instrument may be required to indicate, record, register, signal or perform some operations on the value it has determined. Measuring instruments are classified based upon the mode by which they indicate any change in the quantity to be measured or based on the source of power or by their function or by construction.

a. Classification Based on Standards (scale) Used for Measurements

1. Absolute Instruments 

These instruments give the magnitude of the quantity under measurement in terms of physical constants of the instrument, e.g., Tangent galvanometer and Rayleigh's current balance. Working with absolute instruments for routine work is time consuming, and it takes a lot of time to compute the magnitude of the quantity under measurement. Absolute instruments are seldom used except in standard institutions.

2. Secondary Instruments 

These instruments are so constructed that the quantity being measured can only be measured by observing the output indicated by the instrument, e.g., voltmeter, thermometer, pressure gauge, etc. These instruments are calibrated by comparison against absolute instruments. These instruments are commonly used, as they give direct readings. Usages of these in struments are almost in the sphere of measurement.

b. Classification Based on Working

1. Automatic Instruments 

These instruments do not require manual assistance for their functioning, e.g., mercury thermometer and float-operated level sensors.

2. Manual Instruments 

These instruments require manual assistance for their functioning, e.g., a resistance thermometer with Wheatstone's bridge indicator requires manual adjustment of the null point to get the corresponding temperature reading.

c. Classification Based on Source of Power 

1. Self-operated 

These instruments themselves generate the power required for their operation,e.g., mercury thermometer.

2. Power-operated 

These instruments require external power supply for their functioning. This power may be in the form of electricity or compressed air or hydraulic supply.

d. Classification Based on Construction

1. Self-contained 

These instruments have all of their parts enclosed in one physical assembly,eg., mercury thermometer.

2. External construction 

Some instruments have different elements contained in different physical assemblies connected by data transmission elements, c.g., RTD.

e. Classification Based on Function

1. Indicating Type 

These instruments have some kind of calibrated scale and pointer. Any change in the quantity to be measured is indicated by a change in the pointer position on the scale. The scale has calibrations in terms of values of the measured quantity, e.g., mercury thermometer.

2. Recording Type 

These instruments continuously make a written record of the values of the measured quantity against some other variable like time, e.g., if the furnace is cooled and these cooling temperatures are sensed by a recording-type temperature-measuring instrument then the plot or graph of the furnace temperature against time is produced by the instrument.

1.4 GENERALIZED MEASUREMENT SYSTEM

It is possible and desirable to describe the operation of a measuring instrument or a system in a gener alized manner without resorting to intricate details of the physical aspects of a specific instrument or system. The whole operation can be described in terms of functional elements.Most measurement systems contain the following four functional elements:

1. Primary sensing element

2. Variable conversion and manipulation element(s)

3. Data transmission element (s)

4. Data presentation element

1. Primary Sensing Element 

This element first receives the energy from the measured medium and utilizes it to produce a condition representing the value of the measured variable. The quantity under measurement makes its first contact with the primary sensing element of the mea surement system. This act is then immediately followed by the conversion of the measurand into an analogous electrical signal. This work is done by a device which converts a physical quantity into elec trical quantity termed transducer. The first stage of a measurement system is known as the detector transducer stage.

2. Variable Conversion 

Element This element converts the condition produced by the primary element into the condition useful for functioning of the instrument. The output of the pri mary sensing element may be an electrical signal of any form. It may be voltage, frequency, current, change in resistance or some other electrical parameter. For the instrument to perform the desired function, it may be necessary to convert this output to some suitable form while preserving the information content of the original signal. For example, suppose the output of the primary sensing element is an analog signal and the next stage of the system may accept the signal only in the digital form. Then an analog-to-digital converter is used to convert the signal into the desired form. Many instruments do not need any variable conversion element, while others need more than one variable conversion element.

Variable Manipulation Element 

This element performs certain operations on the condition produced by the secondary element. It manipulates the signal presented to it, preserving the original nature of the signal. Manipulation here means only a change in the numerical value of the signal. For example, an electronic amplifier accepts a small signal as input and produces an output signal which is also a voltage but of greater magnitude. It is not necessary that a variable manipulation element should follow the variable conversion element, as shown in Fig.  It may precede the variable conversion element. In case the voltage is too high, attenuators are used which lower the voltage or power for the subsequent stage of the system. This element represents the parts used for indicating, recording, signaling, registering or transmitting the measured quantity. The process of variable conversion and manipulation is called signal conditioning.

3. Data Transmission Element 

When the elements of an instrument are physically separated, it becomes necessary to transmit data from one element to another, or when the primary element is far away from the secondary element, and an element is essential that transmits the condition of the primary element to the secondary element. The element that performs this function is called a data transmission element.

Example Spacecraft are physically separated from the earth where the control stations guiding their movements are located. Therefore, control signals are sent from these stations to the spacecraft by telemetry system using radio signals.

4. Data Presentation Element 

The information about the quantity under measurement must be displayed in an intelligible form to the personnel or the system for monitoring, control or analysis purpose. This function is done by the data presentation element to monitor data; visual display devices are required. These devices may be analog or digital. In case data is to be recorded, recorders like magnetic tapes, plotters, printers, xy or y-t recorders and digital storage oscilloscopes may be used. Using the functional elements we can measure any physical parameter.

Example Suppose we are measuring weight. The function elements will remain the same as shown . Figure shows block diagram for weight measurement. In this case, the primary sensing element used is the load cell, which is connected to the platform where we will put weights. Weight is the measurand; load cell is the transducer. When the weight is kept on the platform, it will exert force on the load cell. The output of the load cell is in millivolts. So voltage proportional to the weight is produced. Voltage is amplified which is calibrated in terms of weight and given to the conversion ele ment. The conversion element here is an analog-to-digital converter. The converted data is given to the display, which is a digital display. As the display is located in the system, there is no need of the data transmission element.