Thesenon-commercial measurement details used to be academic curiosities. However, engineering,manufacturing and ordinary living now routinely challenge the limits of measurement.

Industry-specific standards

In addition tostandards created by national and international standards organizations, many large andsmall industrial companies also define metrology standards and proceduresto meet their particular needs for technically and economically competitive manufacturing.These standards and procedures, while drawing in part upon the national andinternational standards, also address the issues of what specific instrument technology willbe used to measure each quantity, how often each quantity will be measured, andwhich definition of each quantity will be used as the basis for accomplishingthe process control that their manufacturing and product specifications require. Industrialmetrology standards include dynamic control plans, also known as "dimensional controlplans", or "DCPs", for their products.

In industrialmetrology, several issues beyond accuracy constrain the usability of metrologymethods. These include:

1. The speed withwhich measurements can be accomplished on parts or surfaces in the
process ofmanufacturing, which must match the TAKT Time of the production line.

2. Thecompleteness with which the manufactured part can be measured such as
described inhigh-definition metrology,

3. The abilityof the measurement mechanism to operate reliably in a manufacturing
plantenvironment considering temperature, vibration, dust, and a host of other potential
hostile factors,

4. The abilityof the measurement results, as they are presented, to be assimilated by the
manufacturingoperators or automation in time to effectively control the manufacturing
process variables,and

5. The totalfinancial cost of measuring each part.

National standards

Every countrymaintains its own metrology system. In the United States, the National Institute ofStandards and Technology (NIST) plays the dual role of maintaining and furthering bothcommercial and scientific metrology. NIST does not enforce measurementaccuracy directly.

Modern standards

Currently,only five independent units of measure are internationally recognized: temperature interval, linear distance, electrical current, frequency and mass. All measurements of all types are based on one or more of these independent units. Two supplementalindependent units are also recognized internationally, both dealing with anglemeasurement.

Forexample, Ohm's law is a widely known concept in electrical study. Of the three H units ofmeasure involved, only current (ampere) is an independent unit. Voltage and resistanceunits are dependent on current units, as defined by Ohm's law.

Inthe United States, ASTM Standard Practice E 380,replaced h' IEEE/ASTM SI 10 [1], adaptsindependent unit of measure theory to practical measurement activity.

Itis believed that each of independent units of measure will be defined in terms of the otherfour independent units eventually. Length (meter) and time (second) are already connectedthis way. If an accurate time base is available, then a length standard can be reproduced without a meter bar artifact, using the known constant speed of light. Lesser knownis the relationship between the luminance (candela) and current (ampere). The candelais defined in terms of the watt, which in turn is derived from the ampere. This difficult torecreate standard is supplemented by an incandescent bulb design that is usedas asecondary and transfer standard. These bulbs recreate the candela when a specific amount of current is applied.

Thedevelopment of standards follows the needs of technology. As a result, some units ofmeasure have much more resolution than others. The second is reproducible to 1 part in1014. As it became possible to measure time more precisely, solar time, believed to be aconstant, proved to be very slightly irregular. This resulted in leap second adjustmentsto keep UTC synchronised with solar time.

Temperature (kelvin)is defined by agreed fixed points. These points are defined by

■§ the statechanges of nearly pure materials, generally as they move from liquid to solid.

Betweenthese fixed points, Standard Platinum Resistance Thermometers (SPRTs),

• constructedin a specified manner, are used to interpolate temperature values. This

mosaicof approaches produces measurement uncertainty which is not uniform over the

entire rangeof temperature measurement. Temperature measurement is coordinated by

the InternationalPractical Temperature Scale, maintained by the BIPM.

Date: 2016-04-22; view: 762