Modern Standards of Measurement

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. For example, Ohm's law is the most widely understood concept in all of electricity usage. Of the three units of measure involved, only current (ampere) is an independent unit. Voltage and resistance units are dependent on current units, per Ohm's law. Two supplemental independent units are also recognized internationally, both dealing with angle measurement.

It is believed that each of independent units of measure will be defined in terms of the other four independent units eventually. Length (meter) and time (second) are already connected this way. If an accurate time base is available, then a length standard can be reproduced without a meter bar artifact. Lesser known is the relationship between the luminance (candela) and current (ampere). The candela is defined in terms of the watt, which in turn is derived from the ampere. This difficult to recreate standard is supplemented by an incandescent bulb design that is used as a secondary and transfer standard. These bulbs recreate the candela when a specific amount of current is applied.

The development of standards follows the needs of technology. As a result, some units of measure have much more resolution than others. The second is reproducible to 1 part in 10 to the 14th power. As this resolution capability increased, what was believed to be a constant proved to be very slightly irregular. Luminance (candela) can only be reproduced to 5% of reading despite having sensors that are capable of 50 parts per million (0.0005%) precision. Reproducibility of the standard is the constraint.

Temperature (kelvin) is defined by accepted fixed points. These points are defined by the state changes of nearly pure materials, generally as they move from liquid to solid. Between these fixed points, Standard Platinum Resistance Thermometers (SPRTs) constructed a very specific way are used to interpolate temperature values. This mosaic of approaches produces uncertainty that is not uniform over the entire range of temperature measurement. Temperature measurement is coordinated by the International Practical Temperature Scale, maintained by the BIPM.

Non-commercial measurement details like these used to be academic curiosities. But as the frontiers of science moved forward, it pulled applied science along. Engineering, manufacturing and ordinary living now routinely challenge the limits of measurement.

For example, most owners of 'atomic clocks,' more correctly known as radio clocks, know that there are no radioactive materials in their clocks. Fewer users know that the clocks are synchronized by internal radio receivers for broadcasted time signals from real atomic clocks. There are too many other measurement devices used by people who don't have adequate comprehension of the basic principles involved. Without this basis, the need for metrology and calibration is difficult to substantiate.

After many centuries of effort, there still are many unanswered questions and a lot of work remaining to be done. There also are plenty of surname-less units of measure waiting for new champions. They would join among others such as Kelvin, Watt, Ampere, Hertz and, in 1971, Siemens in the ranks of those who received the ultimate acknowledgement of their contributions to technology and measurement.

Date: 2016-04-22; view: 1327