Glossary of Terms

Absolute Pressure (psia): The total force per unit area exerted by a fluid. The sum of atmospheric and gage pressures.
Accuracy: The degree to which an observed value matches the actual value of a measurement over a specified range.
Alternating Current (AC): Current that reverses polarity at a uniform frequency.
ANSI: The American National Standards Institute is a private nonprofit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States.
ASTM: (Formerly known as The American Society for Testing and Materials) An international standards developing organization that develops and publishes voluntary technical standards for a wide range of materials, products, systems, and services.
Atmospheric Pressure: The force exerted per unit area by the weight of the atmosphere.
British Thermal Unit (BTU): The amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Melting a pound of ice at 32°F requires 143 BTU.
BSPT: British Standard Pipe Thread.
Capacitance: A measure of the amount of electric charge stored (or separated) for a given electric potential. The most common form of charge storage device is a two-plate capacitor.
Cavitation: The process where vapor bubbles in a flowing liquid collapse inside a control valve as the pressure begins to increase.
Closed Loop: A control system that provides feedback to a controller on the state of the process variable.
Cold Junction: The end of thermocouple that is kept at a constant temperature in order to provide a reference point.
Contacts: Elements used to mechanically make or break an electric circuit.
Critical Pressure: The ratio of upstream to downstream pressure where the gas velocity out of the valve is sonic and further decreases in downstream pressure no longer increase the flow.
Cv or Valve Flow Coefficient: The number of US gallons per minute of water at 60°F that will pass through the valve with a pressure drop of 1 psi.
Deadband: The value of measurement between when a switch actuates and deactuates.
Density: The mass of a given substance per unit volume.
Derivative Control: A method of changing the output of a controller in proportion to the rate of change of the process variable.
Dewpoint: The temperature to which air must be cooled for the air to be saturated with water.
Dielectric: The insulating material between the conductors of a capacitor.
Dielectric Constant: The ratio of the insulating ability of a material to the insulating ability of vacuum.
DIN: Deutsches Institut für Normung e.V., The German national organization for standardization and is that country’s ISO member body. DIN and mini-DIN connectors, as well as DIN rails are several examples of older DIN standards that are today used around the world.
Direct Current (DC): A current with a constant polarity.
Double Pole Double Throw (DPDT) Switch: Two separate switches that operate simultaneously, each with a normally open and a normally closed contact and a common connection.
Drift: A gradual change in an element over time when the process conditions are constant.
Dry Bulb Temperature: The ambient air temperature measured by a thermometer that is freely exposed to the air but shielded from other heating or cooling effects.
Emissivity: The ratio of energy radiated by the material to energy radiated by a black body at the same temperature. It is a measure of a material's ability to absorb and radiate energy.
Form-C Contact: A contact that has both normally open and normally closed contacts.
Fuzzy Logic: A system that uses mathematical or computational reasoning based on fuzzy sets derived from analog inputs.
Gage Pressure (psig): The measure of force per area exerted by a fluid using atmospheric pressure as the zero reference.
Gain: The ratio of change in output to the change in input of a process.
Hot Junction: The joined end of the thermocouple that is exposed to the process where the temperature measurement is desired.
Humidity: The amount of water vapor in a given volume of air or gas.
Hydrostatic Pressure: The pressure due to the head of a liquid column.
Hysteresis: A property of a device or instrument whereby it gives different output values in relation to its input values, depending on the directional sequence in which the input values have been applied. [IEC 61298-2]
Impedance: The opposition in an electric circuit to the flow of an alternating current consisting of inductive reactance, ohmic resistance, and capacitive reactance.
Inaccuracy: Maximum positive and negative deviation from the specified characteristic curve observed in testing a device under specified conditions and by a specified procedure. [IEC 61298-2]
Note 1: Accuracy is defined in IEC 60050-300, definition 311-06-08.
Note 2: The term inaccuracy is sometimes referred to as measured accuracy. This term should not be used.
Inductive Load: Current passing through wound or coiled wire creates a magnetic field that, in turn, produces mechanical work.
Integral Control: A method of changing the output of a controller by an amount proportional to the error and the duration of that error.
Laminar Flow: Smooth fluid flow that has a parabolic flow profile with no mixing between streamlines.
Linearity: Ability of a measuring instrument to provide an indication having a linear relationship with a defined quantity other than an influence quantity. [IEC 60050-300]
Note: The method of expression of lack of linearity is different for different kinds of instruments and is established in each particular instance.
Long-Term Span Drift: The amount of change of a measured reading with 90% of full-scale range pressure applied and constant ambient conditions over a given period of time, which is typically quoted as an annual figure. [IEC 61298-2]
Long-Term Zero Drift: The amount of change of a measured reading with zero pressure applied and constant ambient conditions over a given period of time, which is typically quoted as an annual figure.
Low Pressure Steam: As defined by ASME, steam under 15 psi pressure.
Manual Reset: A control that must have human input before it will return to its normal state from an alarm state.
Maximum Surge Pressure: Safe pressure for the switch housing but which may damage the mechanism by continuous or repetitive application.
NEMA: The National Electrical Manufacturers Association, a trade association in the US of electrical equipment manufacturers that develops many industry technical standards, such as the standard for electrical equipment enclosures.
NIST: The National Institute of Standards and Technology is a non-regulatory agency of the United States Department of Commerce's Technology Administration. The institute provides standard references and calibration services.
Non-Linearity: Deviation from linearity. [IEC 61298-2]
Note 1: Linearity is defined in IEC 60050(300), definition 311-06-05.
Note 2: Non-linearity does not include hysteresis.
Non-Repeatability: Deviation from repeatability. [IEC 61298-2]
Note 3: Repeatability is defined in IEC 60050(300), definition 311-06-06.
Normally Closed Switch: A switch in which the contacts are normally closed. Actuation opens the contact.
Normally Open Switch: A switch in which the contacts are normally open. Actuation closes the contacts.
NPT: National Pipe Thread.
NSF: A not-for-profit, non-governmental organization that develops standards and provides product certification and education in the field of public health and safety.
Null Switch: A floating contact switch with a zone of no contact. Often used to operate reversible motors.
pH: An indication of the acidity or alkalinity of a solution in units ranging from 0 (most acidic) to 7 (neutral) to 14 (most alkaline).
Pressure Drop: The difference in upstream and downstream pressure of the fluid flowing through a valve.
Proportional Control: A method of changing the output of a controller by an amount proportional to the error.
Proportional-Integral Control (PI): Proportional and integral control combined.
Proportional-Integral-Derivative Control (PID): Proportional, integral, and derivative control combined.
Range: The span of rates within which the sensing element of a given switch can be set to actuate an electric switch.
Rated Pressure: The maximum pressure that the actuating components of the switch in contact with the media can withstand continuously and/or repeatedly without risk of permanent damage.
Relative Humidity: The ratio of the quantity of water vapor in the air to the quantity of water vapor required for saturation at the same temperature.
Repeatability: The closeness of agreement between the results of successive measurements of the same measured, carried out under the same conditions of measurement, i.e., by the same measurement procedure; by the same observer; with the same measuring instruments used under the same conditions; at relatively short intervals of time. [IEC 60050-300]
Repetitive Accuracy: The ability of a switch to operate repetitively at its set point under consistent conditions.
Response Time: The time it takes an element to respond to a change in the value of the measured variable or to produce a change in the output signal.
Rotameter: A variable area flowmeter consisting of a tapered tube and a float.
RS-232: (Recommended Standard 232) is a standard for serial binary data signals connecting between a DTE (Data Terminal Equipment) and a DCE (Data Circuit-terminating Equipment).
RS-485: (Now known as EIA-485) is an OSI model physical layer electrical specification of a two-wire, half-duplex, multipoint serial connection.
Saturation Point: The point at which condensation is formed.
Serial Transmission: Sending one bit at a time on a single transmission line.
Set or Actuation Point: The exact rate which will cause the electric switch to actuate.
Single-Pole Single Throw (SPST) Switch: A switch that only has one of either a normally open or a normally closed contact.
Single-Pole Double-Throw (SPDT) Switch: A switch combining both normally open and normally closed switch contacts.
Solid State: Any element that controls current without moving parts, vacuum gaps, or heated filaments.
Span: The difference between the highest and lowest numbers in a range.
Span Temperature Coefficient: The maximum amount the span reading could change at any point within the compensated temperature range. This error is typically expressed as a percentage of full-scale output of reading. It can also be expressed as a percentage of full scale per °C, °F, or K (e.g., ±0.02%FS/°C).
Specific Gravity: The ratio of the density of a fluid to the density of a reference fluid.
Static Pressure: The pressure exerted by a fluid at rest. The outward push of a fluid against the walls of a container.
Temperature Compensation: The correction for the influence of temperature on a measurement.
3-A: 3-A Sanitary Standards Inc., a non-profit association representing equipment manufacturers, processors, regulatory sanitarians, and other public health professionals that creates standards and accepted practices for dairy and food processing equipment and systems.
Total Pressure: The sum of velocity and static pressure.
Transducer: Any device that generates an electrical signal from physical measurements.
Transmitter: A device that translates the low-level output of a sensor or transducer to a higher-level signal suitable for transmission to a site where it can be further processed.
Turbulent Flow: Fluid flow in which the flow profile is a flattened parabola, the streamlines are not present, and the fluid is freely mixing.
Turndown Ratio: The ratio of the maximum to minimum measurable value that can still produce full-scale output.
Velocity Pressure (Dynamic Pressure): The pressure exerted by the velocity of a fluid. It can be measured by the difference between total and static pressure.
Viscosity: The resistance of a fluid to flow.
Voltage Output: Output in voltage form, typically proportional to the applied measurement.
Wet Bulb Temperature: The temperature at which air can no longer evaporate water, measured with a thermometer whose bulb is covered with a wet cloth.
Zero Temperature Coefficient: The maximum amount the output reading at zero pressure might deviate over the compensated temperature range. This error is typically expressed as a percentage of full-scale output of reading. It can also be expressed as a percentage of full scale per °C, °F, or K (e.g., ±0.02%FS/°C).