True Value

Statements about the accuracy of numbers imply that an accurate, or true value, is known. Measurements that are based essentially on counts, where the items counted are people, money, pulses in digital systems, and so on, can be considered accurate. A statement that a new computer costs $1499 means exactly 1499 dollars.

On the other hand, for measurements of analog variables, and this includes nearly all process measurements, such as temperature and pressure, the exact value is known only to the Great Creator. The value ultimately obtained will only be the best approximation that we humans can make. Even if we are able to get the first 6 digits correct, it is still an approximation. A good quality temperature sensor might output a reading of 68.5 C, but this does not mean that the temperature is exactly 68.500 000 000 It simply means that the true temperature lies somewhere between 68.45 ^o and 68.55 . Still, up until about 1960, engineering calculations were done on sliderules, which were capable of producing only 3 significant figures, but this was good enough to design oil refineries that would produce products and airplanes that would fly.

In industry, the true value of any process measurement is usually taken to be the value indicated by some standard measurement device. Unfortunately, the majority of measurement standards are not rugged enough to tolerate the plant environment and have to be kept in a laboratory. The sensors that are used in the process operations...