Summary
Highlights
This lecture will lay down basic concepts, terminologies, and categories necessary for understanding measurement systems. These introductory concepts are crucial for the later part of the course.
Instruments are categorized as either passive or active. Passive instruments require external energy to perform their task, often consuming the energy they are trying to measure (e.g., a mercury-in-glass thermometer). This can lead to 'loading of the system.' Active instruments, conversely, have their own energy source and require only a stimulus from the outside (e.g., a digital IR thermometer).
Instruments can also be categorized by their output display. Deflection-type instruments show a measurement through a direct deflection (e.g., a standard thermometer or pressure gauge). Null-type instruments, however, achieve a measurement by returning to a zero or null reading, like a weight balance or a potentiometer. Null-type instruments are generally more accurate but more cumbersome to use than deflection-type instruments.
Analog instruments produce a continuous output as the input changes, while digital instruments produce a discretized (step-wise) output. Examples include analog vs. digital bathroom scales and clocks. While analog instruments provide continuous information, converting to digital can lead to a loss of information, as illustrated by the progressive loss of detail when sampling temperature at sparser intervals.
Indicating instruments present their output as a visual or audio indication for human observers (e.g., a physical weight scale or factory floor gauges). Instruments with signal output generate an electrical signal proportional to the measured quantity, which is typically fed to a controller for automatic system maintenance (e.g., a temperature sensor in an AC unit).
Smart instruments are microprocessor-driven, have communication capabilities to transmit signals to control or monitoring systems, and include onboard diagnostics to reduce maintenance costs and increase accuracy. A sensor that merely sends data remotely is not considered a smart sensor based on these criteria.