The Process of Measurement:
An Overview
The process of measurement consists of obtaining a quantitative comparison between a predefined standard and a measurand.
Measurand = Physical parameter being measured: length, temperature, fluid flow, acoustics, motion,…..
Standard of comparison: must be of the same characters as the measurand, and is prescribed and defined by a legal of recognized agency, e.g.:
NIST = The National Institute of Standards and Technology.
ISO = The International Organization for Standardization.
ANSI = The American National Standards Institute.
The significance of Mechanical Measurement:
Measurement is the fundamental basis for all research, design, and development and its role is prominent in many …… such as:
-fault diagnosis and trouble shooting (process monitoring).
-Quality control.
-Fits and tolerances.
-CL (Feedback) system.
-Feed-forward control systems.
-New understanding of physical world.
-Test for any theory, design or development.
Note: “One thing you learn in science is that there is no perfect answer, no perfect measure.
Fundamental Methods of Measurement:
-Direct Comparison with either a primary or secondary standard.
-Indirect Comparison through the use of a calibration.
The generalized Measuring System
Most measuring systems fall within the framework of a general arrangement consisting of three phases (stages):
Stage 1 : Detection- transduction, or sensor- transducer, stage.
Stage 2 : Signal conditioning stage.
Stage 3 : Readout- recording stage.
Example:
Consider the familiar tire gauge used for checking automobile tire pressure. It consists of:
-Cylinder and piston.
-Spring resisting the piston movement
-Steam with scale divisions.
Calibration:
Known magnitudes of the measurand (input) is fed into the sensor- transducer, and the systems output behavior is observed, Such a comparison allows the magnitude of the output to be correctly interpreted I terms of the magnitude of the input. The calibration procedure established the correct output scale for the measuring system.
Single- point calibration
Is used when the output is exactly proportional to the input i.e.,
Output = Constant* input
Multipoint- calibration
Is used when a number of different input values are applied. It works when the output is not simply proportional, and, more generally, improves the accuracy of the calibration.
In summary:
Then establish a relationship between the output and the known input.
Uncertainty: Accuracy of Results
Error may be defined as the difference between the measured result and the true value of the quantity being measured.
We do not know the true value; hence we do not know the error.
We can discus as error and can estimate the size of an error, but we can never know its actual magnitude as will be seen in chapter 3.
Reporting Results
Experimental results should be recorded in one of the following forms:
- Executive summary.
- Laboratory note or technical memo.
- Progress report (describes the current status of the project).
- Full technical report.
- Technical paper.
- Executive summary:
Shows only the key features of the work:
-What was done and
-What was conducted, outlined in a few paragraphs.
- Laboratory note or technical memo:
Is written to be read by someone thoroughly familiar with the project, such as an immediate supervisor or the experimental list himself.
In some case a sigle page is sufficient, including a sentence or two stating the problem, a block diagram of the experimental setup and some data presented (table or/ and plot).
- Full Report:
It tells the complete story to one who is interested in the subject but who has been in direct touch with the specified work- perhaps top officials of a large company or a review committee of a sponsoring agency. One format for proper full report includes:
• main body (to be short and to the point),
• stating the supporting materials,
• data,
• detailed descriptions of the equipment,
• review of literature,
• sample calculations,
• plots,
• discussion,
• conclusions,
• references,
• appendixes.
- Technical Report:
Its purpose is to make (to advertise) the work of writer.
It is a brief summary of the project. It must be tailored to fit either a time allocated in a meeting or space in a publication.
It should include:
• problem statement and
• results.
These two items will attract the attention of other workers interested in the field who can then make direct contact with the writer(s) for additional details and discussion.
Dr. Mostafa S. Habib MEG 381 Instrumentation- The Process of Measurement 1/8