Instrumentation Terminology
Change of Measure or Range
The spectrum or set of variable values as they fall within the upper or lower limits of measurement capability or transmission of the instrument is expressed by setting the two extreme values (e.g., 0-100 bar).
Rangeability
The quotient of the top and bottom values of an instrument, also called dynamic or far, is dimensionless.
Scope or Span
The difference between the upper and lower measurement range of the instrument.
Error
The algebraic difference between the value measured or transmitted by the instrument and the actual value of the measured variable.
Uncertainty
The spread of values that can reasonably be attributed to the true value of the measured quantity (each time a measurement is taken with an instrument, it is known not to be exact).
Accuracy
The quality of a measuring instrument that tends to give readings close to the true value of the measured quantity.
Accuracy: Tolerance
Tolerance is the allowable error in measurement or transmission of the instrument and defines the limits of errors made when the instrument is used under normal service conditions. It is expressed as:
- % of range
- Variable units of measure
- % of reading completed
- % of full scale or measurement range
Dead Zone
The field of values of the variable that does not change the indication, i.e., produces no response. It is expressed as a percentage of the range.
Sensitivity
The reason for the increase in reading and the increase in the variable that results after the resting state. Indicated as a percentage of the range, it produces a response.
Repeatability
The ability to reproduce the position of the pen, index, or instrument output signal by repeatedly measuring the same variable values under the same conditions of service and in the same sense of variation, covering the entire field.
Reproducibility
Describes the closeness of the output readings for the same input when there is no change in the method of measurement (the observer, measuring instrument, measurement, conditions of use, and time of measurement). Usually expressed in % of span over a period of time.
Hysteresis
The maximum difference observed at the levels indicated by the pen, index, or instrument for the same value as any other in the field when the variable goes full scale in both directions, either upward or downward. It is expressed as a percentage of the span.
Measurement Range: Zero Elevation
A range in which the zero value of the variable or measured signal is greater than the lowest value of the field.
Field with Suppressed Zero Measure
A field in which the variable or zero measured signal is lower than the lowest value of the field.
Zero Elevation
The amount by which the zero value of the variable exceeds the lower value of the field. It can be expressed in units of the measured variable or as a percentage of the range.
Suppression of Zero-Sum
The value lower than the zero field of the variable. It can be expressed in units or as a percentage of the variable range.
Drifting
The change in output that occurs over a period of time while holding the measured variable and all environmental conditions constant. It is widely believed that zero drift is the change of the variable to zero due to any internal cause, and thermal drift of zero is where the variation is due only to temperature effects. Usually expressed as a percentage of the output signal of full scale at room temperature or temperature change intervals.
Resolution
The magnitude of step changes in the output signal as it changes continuously over the entire field.
Reliability
A measure of the likelihood that an instrument will continue to behave within the error limits specified over a given time and under specific conditions.
Stability
The ability of the instrument to maintain its behavior during its life and specified storage.
Temperature
The temperature field within which the instrument is expected to work within specified error limits.
Noise
Any accidental electrical disturbance or unwanted signal that changes the transmission, display, or recording of the instrument.
Linearity
The approach that a calibration curve has to a specified straight line.
Service Life
The minimum specified time during which the characteristics of a continuous and intermittent instrument apply without submitting changes beyond the specified tolerances.
Blind Instruments
Instruments that have no visible indication of the variable, e.g., pressure switches, alarms, and thermostats. Flow, pressure, level, and temperature transmitters without indication are also considered blind instruments.
Indicator Instruments
Instruments that have an index and a scale where the value of the variable can be read. According to scale, the amplitude of the indicators is divided into concentric and eccentric. Digital tools are also considered indicator instruments.
Recording Instruments
Instruments that record the variable with continuous strokes or dots. They may have a circular, rectangular, or elongated graphic, depending on the shape of the graph.
Primary Elements
Instruments that are in contact with the variable and use or absorb energy from the controlled environment for the measurement system in response to an indication of the variation of the variable. The effect of a primary element may be a change in pressure, force, position, or power measurement, not necessarily related to the variable nature of the prosecution if their variation.
Transmitters
Capture the process variable across the primary element and transmit it remotely through pneumatic (3 to 15 psi) or electronic (4 to 20 mA DC) signals. Other electronic signals are also used (1 to 5 mA DC, 10 to 50 mA DC, 0 to 20 mA DC, 1 to 5 volts, and 10 to -10 volts). The primary element may or may not be part of the transmitter.
Transducers
Receive an input signal based on one or more physical quantities and do not modify or convert the signal. Transducers can be relays, primary elements, transmitters, converters, etc.
Converters
Instruments that receive an input signal from a pneumatic or electronic instrument and, after modification, send out a normalized output signal. For example, a P/I converter receives a pneumatic input signal (P) and outputs an electronic signal (I).
Receiving Elements
Receive the signal from the transmitters and indicate or record it.
Controllers
Compare the controlled variable (pressure, level, temperature) to a desired value and take corrective action according to the deviation. The controlled variable can be received directly, in the case of local controllers, or indirectly via a pneumatic, electronic, or digital signal from a transmitter.
Final Control Element
Receives the signal from the controller and modifies the flow of the fluid or control agent. In a pneumatic control system, this is typically a pneumatic valve with a diaphragm or a servomotor. In an electronic control system, a servo valve is actuated through a pressure-to-current converter. Motorized valves are also often used in electronic control systems.