Metrology Even within the discipline of Electronics, Metrology is an unfamiliar term unless paired with the expression "Traceable to the National Bureau of Standards." Even this term has fallen into bureaucratic disuse due to name change of this office to the National Institutes of Science and Technology. The term above is a legalism which confers absolute authority to the accuracy of a measurement of a physical or electrical phenomena. In simple terms and in day to day applications, such measurements involve the calibration of test equipment, scales, meters, gauges and every method of measure that must be guaranteed to be absolutely accurate. Metrology is the science of measurement (and not the science of weather!). As practiced, this often leads to comparisons of test items to known absolute standards to the highest precision available to science and engineering. For example: a voltmeter is connected to an absolute standard of voltage and the voltmeter's accuracy is expressed in how close it matches. This degree of accuracy can stretch out to more than 6 decimal places and the resolution of measurement often exceeds this. In this case, if one were to submit an electronic meter exhibiting a digital scale extending to 8 places it might measure the absolute standard's Volt as 1.00000115 V. In this case there is an error of 1.15 parts per million (or 0.000115%). The previous example is typical, but is not at the extremes of either accuracy or resolutions available for other types of measurement. In the case of frequency and time, resolutions and accuracies are measured in parts per billion or trillion. With regard to physical items, one could reach for a ruler and accept its 1 inch markings as relatively accurate… for casual purposes. For toolmakers and avionics technicians, they often require their "rulers" to be accurate to 100 millionths of an inch or better. When they reach for their "ruler" they are guaranteed this accuracy after having submitted this "ruler" for calibration against standards that are easily 10 to 100 times more accurate. Such devices are known as Gauge Blocks. The one inch Gauge Block looks like a stainless steel cube and it is often guaranteed to be accurate to within 1 millionth of an inch when it is measured against another standard that is measured to within 1/10 millionths of an inch. The calibration of these Gauge Blocks illustrates the scope of precision but not the difficulty of making their calibration. The methodology employed in the calibration of a Gauge Block demands that all items be placed in a draft free enclosure and allowed to temperature soak in the test environment before measurements are made. This solid stainless steel block literally grows and shrinks as people walk past and drafts warm or cool the area. If the Metrologist were to measure a Gauge Block at 1.0000000 inch, and then step away 6 feet, that block would shrink to 0.9999982 inch. Why? The simple infra-red absorption of the Metrologist's body heat in close proximity by the Gauge Block caused it to expand, and when the Metrologist steps back, less heat is transferred and the block cools and shrinks. The Metrologist reaching for the Gauge Block to reposition it slightly for measurement is guaranteed to introduce a significant error for a number of minutes. This will occur even when the adjustment is performed without actual contact of the hand but through the manipulation with forceps or a simple prod. Metrology is a very demanding discipline that is both formulaic and intuitive. It places constraints upon the Metrologist to be able to perform calibrations through a sequence of interrelated measurements without degrading accuracy or precision. A Metrologist is often in the position of measuring phenomena that are perturbed by subtle mechanisms that most Engineers have dismissed as irrelevant acts of nature (i.e. the heat example above rarely enters into design considerations that only extend to 3 or 4 places of accuracy).