MEASUREMENTS AND ITS TYPES Basics of Measurement Theory

MEASUREMENTS AND ITS TYPES

Basics of Measurement Theory Measurement are “ the assignment of numerical to represent (physical properties. )” Measurement are made to fulfill one or more of several different goals: obtain information about a physical phenomenon, record trends, control some process, correlate behavior with other parameters to obtain insight into their relationship. Data Classes: the data that result from measurements can be divided into two major classes, which are eachother divided into two subclasses. The major divisions include qualitative data and quantitative data.

Qualitative Data Qualitative Data is nonnumerical and categorical. It includes information, such as the presence or nonpresence of some factor, good or bad, defective or not defective, gender, and race. Qualitative data does not inherently result in numbers, and thus is sometimes held in less esteem than quantitative data. This attitude is misguided unless there is an inherent need for numbers in a particular case. Qualitative data can be further broken into two sub groups: Nominal Data and Ordinal Data

Classes of Qualitative Data Nominal Data: It is qualitative data that has no inherent order or rank. Examples include lists of names, labels, and groupings. Ordinal Data: It allows ranking, but differences between data points are either nonexistent or meaningless. Example: Many of these data are binary (i. e. , has only two possible results), one can assign the digits 0 and 1 (e. g. , 1 for yes and 0 for no). A five point scale is often used to assign numerals to questions that are largely value judgments.

Quantitative Data Quantitative data are those which naturally result in some number to present a factor. Example includes amount on money, length, temperature, number of defects per unit, number of defectives unit, voltage, pressure, and weight. Quantitative data can be further divided into two sub classes: Interval Data and Ratio Data.

Classes of Quantitative Data Interval Data: It allow for a meaningful comparison of differences but not the relative values of two or more factors. Ratio Data: Ratio data is based on some fixed or natural zero point, such as weights, pressures, and temperatures (e. g. , the Kelvin scale).

Categories of Measurement There are three general categories of measurement 1. Direct Measurement 2. Indirect Measurement 3. Null Measurement

Direct Measurement Direct measurement are made by holding the measurand up to some calibrated standard and comparing two. Example: Meter stick ruler used to cut a piece of coaxial cable to the correct length. You know that the cable must be cut to a length of 24 cm, so hold a meter stick (the standard or reference) up to the piece of the cable. Measuring cable with a meter stick

Indirect Measurement Indirect measurement are made by measuring something other than the actual measurand. Indirect methods are often used when direct measurements are either difficult or dangerous. Example: on might measure the temperature of a point on the wall of a furnace that is melting metal.

Measuring point on a furnace Indirect Measurement of blood pressure

Null Measurement Null measurement are made by comparing a calibrated source to an unknown measurand then adjusting either one or the other until difference between them is zero. Example: An electrical potentiometer is such an instrument; it is an adjustable calibrated source and a comparison meter (galvanometer). The reference voltage from the potentiometer is applied to one side of the zero center galvanometer, and the unknown is applied to the other side of galvanometer. The output of the potentiometer is adjusted until the meter reads zero difference.

Cont…. Example of Null Measurement

Measurement Standards Metrology, the science of measurement, requires a rule to which things are compared: that rule is called a standard. Not all standards are equal, so there is a hierarchy of standards (Figure): International Reference Standard, Primary Standards. Transfer Standards, Working Standards, and Shop-level Standards, Secondary Standards,

Cont…. International Reference Standards: These standards are agreed on by the International Standards Institute (ISI). For years the reference standard for the meter was a platinum bar 1. 000 m long, which was stored in a vault in Paris and maintained by the ISI. Primary standards: These are the principal standards maintained at a national level in the various countries.

Cont…. Transfer Standards: These standards are second level and are periodically compared with the primary standard. They are used to calibrate lower-order standards used in the country so that wear and tear on the primary standard is reduced. Working Standard: Working standards are compared with transfer standards in a nationally certified laboratory.

Cont…. Secondary Standards: These standards are used locally to calibrate instruments and gauges Gauges and Instruments: The lowest order of standards, these are the devices actually used to make measurements and collect data on the objects being measured. Gauges and instruments are compared with either working standards or secondary standards.