Glossary of Terms

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 in particular is 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: 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.