A patient’s temperature chart shows changes taking place over time. Measurement is a process of mapping actually occurring variables into equivalent values. Deviations from perfect measurement mappings are called errors: what we get as the result of measurement is not exactly what is being measured. A certain amount of error is allowable provided it is below the level of uncertainty we can accept in a given situation. As an example, consider two different needs to measure the measurand, time. | time 0 6 12 18 24 30 Patient temperature FIGURE A patient s temperature chart shows changes taking place over time. Measurement is a process of mapping actually occurring variables into equivalent values. Deviations from perfect measurement mappings are called errors what we get as the result of measurement is not exactly what is being measured. A certain amount of error is allowable provided it is below the level of uncertainty we can accept in a given situation. As an example consider two different needs to measure the measurand time. The uncertainty to which we must measure it for daily purposes of attending a meeting is around a 1 min in 24 h. In orbiting satellite control the time uncertainty needed must be as small as milliseconds in years. Instrumentation used for the former case costs a few dollars and is the watch we wear the latter instrumentation costs thousands of dollars and is the size of a suitcase. We often record measurand values as though they are constant entities but they usually change in value as time passes. These dynamic variations will occur either as changes in the measurand itself or where the measuring instrumentation takes time to follow the changes in the measurand in which case it may introduce unacceptable error. For example when a fever thermometer is used to measure a person s body temperature we are looking to see if the person is at the normally expected value and if it is not to then look for changes over time as an indicator of his or her health. Figure shows a chart of a patient s temperature. Obviously if the thermometer gives errors in its use wrong conclusions could be drawn. It could be in error due to incorrect calibration of the thermometer or because no allowance for the dynamic response of the thermometer itself was made. Instrumentation therefore will only give adequately correct information if we understand the static and dynamic characteristics of both the measurand and the instrumentation. This in turn .
