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 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 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. Can be measured by the difference between total and static pressure.
• Viscosity: The resistance of a fluid to flow when subjected to shear stress.
• Wet Bulb Temperature: The lowest temperature that can be obtained through the cooling effect of water evaporating into the atmosphere.
• 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 percentage of full scale per °C, °F or K e.g. ±0.02%FS/°C.