Before we can begin to develop an understanding of the static and time changing characteristics of measurements, it is necessary to build a framework for understanding the process involved, setting down the main words used to describe concepts as we progress. Measurement is the process by which relevant information about a system of interest is interpreted using the human thinking ability to define what is believed to be the new knowledge gained. This information may be obtained for purposes of

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... controlling the behavior of the system (as in engineering applications) or for learning more about it (as in scientific investigations). The basic entity needed to develop the knowledge is called data , and it is obtained with physical assemblies known as sensors that are used to observe or sense system variables. The terms information and knowledge tend to be used interchangeably to describe the entity resulting after data from one or more sensors have been processed to give more meaningful understanding. The individual variables being sensed are called measurands . The most obvious way to make observations is to use the human senses of seeing, feeling, and hearing. This is often quite adequate or may be the only means possible. In many cases, however, sensors are used that have been devised by man to enhance or replace our natural sensors. The number and variety of sensors is very large indeed. Examples of man-made sensors are those used to measure temperature, pressure, or length. The process of sensing is often called transduction , being made with transducers. These man-made sensor assemblies, when coupled with the means to process the data into knowledge, are generally known as (measuring) instrumentation. The degree of perfection of a measurement can only be determined if the goal of the measurement can be defined without error. Furthermore, instrumentation cannot be made to operate perfectly. Because of these two reasons alone, measuring instrumentation cannot give ideal sensing performance and it must be selected to suit the allowable error in a given situation.