What is a measurement?
Measurement is the assignment of a number to the character of an object or event, which then can be compared with other objects or events. Measurements can be done unaided by human senses, in which case they are termed as estimates or, more commonly they can also be carried out using instruments.
Basic concepts of measurement begin with a description of the quantity that is to be measured, and in most probability it involves a comparison with a known quantity of the same kind. If that particular object or quantity to be measured is not accessible for direct comparison, then it can be suitably converted / transduced into an analogous measurement signal.
Measurement usually involves some interaction between the object and the observer or observing instrument. There could also be an exchange of energy, which would be negligible in everyday applications but can become considerable in some types of measurement and thereby set a limit on the accuracy.
There are some processes that might seem similar to measurements but they are not so. For example, comparing two sets of strings to see which is longer than the other is not really a measurement. Counting is also not normally viewed as a measurement. So also a test is not deemed as a measurement: because tests could lead to a ‘yes/no’ answer or a ‘pass/fail’ result.
Basic Concepts of Measurement:
The process or the act of measurement consists of obtaining a quantitative comparison between a predefined Standard and a Measurand. The measurand or measured quantity is the physical quantity in metrology that is subject to measurement.
A measurand in the general sense refers to a physical quantity that was or will be subject to measurement. The word Measurand is used to designate the particular physical parameter being observed and quantified; that is, the input quantity to the measuring process. A Measurement is an act of assigning a specific value to a physical variable. That physical variable becomes the Measured Variable.
The standard of comparison must be of the same character as the measurand, and usually is prescribed and defined by a legal or recognized agency or organization.
Few Examples of Standard Organisation:
- ISO: The International Organization for Standardization
- IS: Indian Standard
- ANSI: The American National Standards Institute.
- NIST: The National Institute of Standards and Technology.
The engineer is not only interested in the measurement of physical variables but also concerned with their control. The two function are closely related, however because one must be able to measure a variable such as temperature or flow in order to control it. The accuracy of control is necessarily dependent on the accuracy of measurement.
Measurement is also a fundamental element of any control process. Statistical techniques are available for analysing data to determine expected errors and deviations from the true measurements. The engineer must be familiar with these techniques in order to analyse the data effectively.
The Generalized Measurement System:
Basic concepts of measurement or a measurement system may consist of part or all of four general stages:
- A sensor – Transducer Stage.
- An Intermediate Stage or signal – Conditioning Stage.
- A Terminating Stage – Output Stage.
- Feedback – Control Stage.
The primary function of the first stage is to detect or to sense the physical variable (Measurand) and performs either a mechanical or an electrical transformation to convert the signal into a more usable form.
The sensor is a physical element that employs some natural phenomenon by which it senses the variable being measured. The transducer converts this sensed information into a detectable signal form, which might be electrical, mechanical, optical, etc. In most cases, however, the physical variable is transformed into an electric signal because this is the form of signal that is most easily measured
Signal Conditioning Stage:
The purpose of the second stage is to take the transducer signal and modifies by amplification, filtering or other means so that a desirable output is available
Provides an indication of the value of the measurement. The output equipment might be a simple readout display a marked scale or might contain devices that can record the signal for later analysis. Examples of these devices are tape recorders, chart recorders and computer disk drives.
Feedback – Control Stage:
In those measurement systems involved in process control a fourth stage the feedback control stage, contains a controller that interprets the measured signal and makes a decision regarding the control of the process. This decision results in a change in a process parameter that affects the magnitude of the sensed variable.
It is very important to realize that the accuracy of control cannot be any better than the accuracy of the measurement of the control variable. Therefore one must be able to measure a physical variable accurately before one can hope to control the variable.
A schematic diagram of the generalized measurement system is shown below.
The overall schematic of the generalized measurement systems is quite simple and as one might suspect the difficult problems are encountered when suitable devices are sought to fill the requirements of each of the boxes on the schematic diagram.
Effective measurement techniques include these key concepts:
- Distinguishing between error and uncertainty
- Recognizing that all measurements do have some uncertainty
- Identifying types of error, sources of error and how to detect / minimize these errors
- Estimate, describe, and express uncertainty in measurements and calculations
- Using uncertainty to describe the results of any lab work
- Comparing measured values and determining whether values are the same especially within the stated uncertainty.
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