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Information Sheets
Measurement, Calibration and Quality control
Measurements
For Customer and Suppliers to agree on the characteristics of products or services, there must be a system of measurement to demonstrate those characteristics. Developed by international agreement, a system that defines fundamental physical units, prototype physical standards, instruments for transferring measurements and the assessment of accuracy of instrumentation has been established.
Units of measurements are given by the “basic”, “derived” and “supplementary” metric units of the Systemé International d’Unites (SI).
“Basic” units are a limited number of base units from which all other “derived” units can be determined. Base units are defined in terms of physical phnomena or a physical reference standard.
Basic units are:
- Length (metre, m)
- Mass (kilogram, kg)
- Time (second, s)
- Electric current (ampere, A)
- Thermodynamic temperature (Kelvin, K)
- Amount of substance (mole, mol)
- Luminous intensity (candela, cd)
All these units with the exception of Mass, are defined in terms of a physical phenomena that can be reproduced anywhere (with the correct equipment and conditions). For example, the second as a measure of time is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between two hyperfine lines of the ground state of the caesium-133 atom. These are called “natural” standards.
The on exception is a “material” standard and is the kilogram. This unit is defined by an international prototype made of platinum-iridium being kept by the International Bureau of Weights and Measures in Sèvries, Paris, France. The British copy (No 18) is kept at NPL. (Efforts to produce a natural standard for mass are still ongoing.)
“Derived” units are determined in terms of the basic units. Examples of these include:
- Frequency (hertz, Hz) ..........1/s
- Force (Newton, N) ...............m kg/s²
- Energy (Joule, J) .................m² kg/s²
“Supplementary” units are dimensionless but are clearly needed. These include:
- Plane angle (radian, rad) The radian is the plane angel between two radii of a circle which cut off in the circumference an arc equal in length to the radius.
- Solid angle (steradian, sr) The steradian is the solid angle which, having its vertex in the centre of the sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere.
All SI units include an indicator of the scale of the unit. The scaling factor is shown by a prefix to the unit and indicates the scale in multiples or submultiples of 10 – such as millimetre being one thousandth of a meter or kilometre being one thousand metres.
“Subjective” standards are standards that require subjective judgements by people using their senses and reasoning. Examples include the colour matching of paints, blending whisky or tea. Subjective standards are commonly avoided as achieving consistency is very difficult. However there are times when the use of these standards cannot be avoided or is suitably convenient.
Practicable application of measurements standards
For general use, the complexity of the basic standards is far too great Thus a hierarchy of standards has been established where “working standards” used by working instruments can be calibrated by the use of transfer standards and primary reference standards, all being traceable back to the basic National unit standards with know errors.
Knowledge of the errors is vital when using any measurements system. The make-up of the uncertainty of measurement is generally due to accuracy and the precision of measurement. “Accuracy” is the extent to which the average matches the true value. These errors are a combination of “systematic” error and “random” error. The systematic error can be corrected by adjustment, but the random error will remain. “Precision” is the variation between individual measurements and is the precision of the measuring process.
As knowledge of the errors is vital, calibration is essential to achieve reliable absolute measurements and consistency of measurement. Calibration of measuring equipment needs to be regularly conducted to ensure that the extent of errors is know and to eliminate the effects of damage and drift. Calibration should be made by comparison to superior standards and documented. This is usually done by test facilities that have National accreditation to the United Kingdom Accreditation Service (UKAS). UKAS is a body set up by an initiative of the DTI in 1995 to serve British industry by providing assurance of the Competence of evaluators, i.e. laboratories, certification bodies and inspection bodies, forming an essential element of the National Measurement System through traceable standards.
Quality control
What does is mean?
Quality control is only part of the total Quality story. A widespread view of the objectives of quality control is to produce a quality of product or service that
- Satisfies the Customer
- Is as cheap as possible
- Can be achieved in time to meet the delivery requirements
What each of these statements means is different to different people. Many want to exceed their Customer’s expectations, many believe making a reasonable (or even “excessive”) profit is honourable - it all depends on the Customer’s perception of what he wanted, the phrase “as cheap as possible” and delivery requirements are, of course, negotiable in many cases. So this is not a simple equation to success.
What is under control?
This usually means that is a confidence that what is intended to happen, will happen. The essential elements of a control system are:
- There is a plan
- There is the intent to carry out the plan
- The pan is implemented with feedback comparing achievements with the plan
- Deviations from the plan are wither corrected to made to enable continual improvements to the plan
The essence of any control system is that of constant feedback to optimise or eliminate deviations from the plan. Clearly the sooner faults are detected and corrected; the less “out of limit” work will be produced.
The corrective measures are taken with the object of learning from the mistakes and so avoiding that fault in the future. Quality control systems provide information about quality shortcomings and enable corrections and improvements to be made. The information also indicates what machines, processes, materials and people are capable of making. IF the expectations are higher that this information indicates, excessive scrap or complete failure may result.
What is quality control?
Quality control generally had five stages:
- Set the quality standard or quality of design required by the Customer
- Plan to achieve the required quality involving
- Planning methods
- Planning equipment
- Getting the right material
- Getting and training people
- Planning product/service quality assessments
- Make it right first time
- Correct and quality deficiencies
- Implement for long term quality control and planning
Staying aheadIt is thus important to understand present and future needs and capabilities to stay ahead of the game. Equally important is continual improvement. Just meeting today’s Customer needs is not enough if long term targets are to be met. When needs and capabilities are understood, the system can be improved.
Efforts are needed to ensure it continues to work and provide Customers with the assurance that their business or desires are safe. All this is worthless without management commitment, lack of which is a key reason for failure.