A – Z of Calibration
The field of calibration has a huge vocabulary describing the methods, parameters, equipment, and processes used to verify the measurement accuracy of masters, gages, and other measuring instruments. The following glossary of definitions includes the most commonly used terms.
Absolute Pressure: Actual pressure relative to a perfect vacuum.
Absolute Temperature: The temperature measured from absolute zero as in the Kelvin and Rankine scales.
Absolute viscosity: The measure of resistance to the shear, unit is dyne-second per square centimeter, or more commonly known as centipoise (cP).
Accreditation: The process used by a qualified independent agency to verify the quality system and technical capability of a calibration laboratory to a recognized standard such as ISO 17025.
Accuracy: It defines how close a measured value is to the true value of the dimension. The conformity of measurement to an accepted standard value. Accuracy includes traceability to NIST or some appropriate national or international standards organization
Adjustment of an instrument: The operation of bringing a measuring instrument into a state of performance suitable for its use.
Adjustment to Nominal: The maximum allowable difference between the actual value supplied with the standard and the nominal value.
Air Velocity: The velocity of air at a measurement point or plane, measured using an anemometer laser Doppler or pitot tube.
Altimeter: An instrument that measures the height above the ground.
Ambient Temperature: The local temperature of the air in the immediate vicinity.
Atmospheric Pressure: The local air pressure in the immediate vicinity.
Anemometer: A device for measuring wind speed or air velocity. Readings indicated from the device are generally at actual test conditions.
ASTM G93: ASTM Standard Practice for cleaning methods and cleanliness levels for materials and equipment used in oxygen-enriched environments.
Barometer: An instrument that measures atmospheric pressure.
Bell Prover: A gas calibrator that immerses a "bell" of known dimensions into a bath of liquid over a specified time. This is generally used a primary source for gas flow meter calibrations.
Blowby Meter: A flowmeter used in the automotive industry for the measurement of engine blow-by gases, often utilizing vortex or orifice type flowmeters.
Bubble Flowmeter: An instrument that uses a soap film across a known bore and times it passages of movement. Care must be taken with soap film meters due to issues with vapor pressure, the permeability of the soap film, changing the shape of the film, and the build-up of the solution on the walls.
Calibration: The set of operations which establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material and the corresponding values realized by standards.
Calibration Accuracy: The sum of the uncertainties in the calibration procedure, including the uncertainties in the references, test instruments, transfers, etc. Calibration accuracy must be better than the stated accuracy or initial accuracy.
Calibration Certificate or Report: The document that presents calibration results and other information relevant to a calibration.
Calibration Frequency: The time intervals at which instruments, gages, and masters are calibrated. These intervals are determined by their user based on the conditions of their use to ensure their performance or size remains within acceptable limits.
Calibration Limits: The tolerance applied to gages and instruments beyond which they are not considered suitable for use.
Clamp-on Flowmeter: An ultra-sonic type flowmeter that is positioned on the outside of a pipe to measure the flow in the pipe using time of flight or doppler techniques.
Clinometer: An instrument to measure an angle of inclination or elevation.
Comparator: An instrument for comparing some measurements with a fixed standard.
Critical Flow Nozzle: A flow nozzle where the gas in the throat of the nozzle is kept above sonic velocity whilst in use, sometimes known as a sonic nozzle.
Cup Anemometer: A type of wind speed indicator used mainly in weather stations.
Density: The mass per unit volume.
Differential pressure: The difference between the two measurements of pressure in a system.
Electromagnetic Flowmeter: A non-intrusive style flowmeter for use on conductive liquids, mainly water. Often referred to as a Magmeter.
Error of measurement: The result of a measurement minus the true value of the measurand.
Flow Straightener: A device for conditioning flow profile mainly used upstream of flowmeters.
Gauge Pressure (g): A measurement of the force/area exerted by a gas or fluid, relative to atmospheric pressure.
Gas Meter: A generic term for a gas flow meter.
Gear Meter: A flowmeter that uses the meshing of gears to meter small volumes of a liquid. Most meters will be supplied with a Meter Factor calibration constant.
Gravimetric Calibration Rig: A measurement system that utilizes fluid mass and time for the calculation of flow rate.
Hertz: The unit of frequency.
Hot Wire Anemometer: An anemometer type for measuring wind speed or air velocity meter. These meter types often state the velocity reading to standard conditions. These are also referred to as Thermal Anemometers.
Hydrometer: A measuring instrument for determining the specific gravity of a liquid or solid.
Hygrometer: Measuring instrument for measuring the relative humidity of the atmosphere.
Initial Accuracy: Accuracy at the time of shipment.
International (Measurement) Standard: The standard recognized by an international agreement to serve internationally as the basis for fixing the value of all other standard of the quantity concerned.
ISO: International Organisation for Standardisation.
ISO/IEC 17025:2017: International standard for the General requirements for the competence of testing and calibration laboratories.
K factor: A calibration constant for a flowmeter, defined as; the number of pulses per unit volume. Often referred to as Meter Factor.
Kinematic Viscosity: The ratio of absolute viscosity to density, usually expressed as centistokes cSt.
Laminar Flow: Type of flow where the fluid moves in a parallel manner along the conduit and has Reynolds numbers below 2000.
Laminar Flowmeter: A type of flowmeter that generate laminar flow through its core, and so generates a linear output in terms of pressure drop across the meter.
Laser Doppler Anemometry: A high-performance measurement system that provides gas or fluid velocity at the laser intersection. Reference devices often used for the calibration of wind tunnels and anemometers.
Linearity: Is the deviation from a nominal calibration 'K' factor normally expressed as a percentage.
Limits of Permissible Error (of a measuring instrument): The extreme values of an error permitted by specifications, regulations, etc. for a given measuring instrument.
Load Cell: An instrument for the measurement of load or weight.
Master Meter: Typically a highly accurate flowmeter used as a reference device for calibrating other meters.
Measurement Uncertainty: The estimated amount by which the measured quantity may depart from the true value.
Measuring and Test Equipment: includes all of the measuring instruments, measurement standards, reference materials, and auxiliary apparatus that are necessary to perform a measurement. This term includes measuring equipment used in the course of testing and inspection, as well as that used in calibration.
Meter Factor: Defined as the number of pulses per unit volume. Also referred to as a 'K' factor.
Newton: A unit of force equal to the force that imparts an acceleration of 1 m/sec/sec to a mass of 1 kilogram.
Newtonian Fluid: A fluid whose absolute viscosity is the same for all values of shear stress.
Non-Newtonian Fluid: A fluid whose absolute viscosity varies with the rate of shear stress.
Nutating Disc Flowmeter: A type of positive displacement flowmeter that uses a disc with a slot in that "wobbles" rather than rotates. Often referred to as a Wobble Plate Meter.
Orifice Plate: A differential pressure type device consisting of a plate with a smaller hole in the center. Flow-through the hole causes a pressure drop. Orifice plates are often made to International standards and can be theoretically predicted for flow and pressure drop.
Peak Flowmeter: A meter used to measure the maximum flow rate for measuring the lung efficiency of asthmatics.
Pelton Wheel Flowmeter: A type of radial flow turbine meter.
Pick-up: A sensor used on a flowmeter to detect pulse measurements.
Pitot Tube Static: A differential pressure device for measuring point velocity.
Positive Displacement Flowmeter: A type of flowmeter that fills small volumes of space, i.e. in between gears as the fluid flows from the inlet to outlet, whilst turning the gears and generally providing a signal proportional to the flow rate.
Precision: The degree of exactness with which a measurement or quantity is stated.
Pressure: Force exerted per unit area.
Primary Standard: A unit directly defined and established against which all secondary standards are calibrated.
Probability: A measure of how likely it is that some event will occur.
Psychrometer: An instrument for measuring relative humidity.
Pyrometer: An instrument for measuring high temperatures.
Quality System: The organizational structure, responsibilities, procedures, processes and resources for implementing quality management.
Repeatability: The closeness of agreement among a number of consecutive measurements performed under the same operating conditions. Long-term and short-term repeatability are both important.
Resolution: It represents the smallest reading unit provided by an instrument. In other words, the digital value represented by one bit in a display in a digital measure.
Reynolds Number: A dimensionless number defining the ratio of dynamic and viscous forces of fluids in any condition.
Rotameter: A trade name associated with Variable Area Flowmeters that uses gravity to balance the force on a float in a tapered tube. The height of the float in the tube is indicative of the flow rate.
Sensitivity: Full-scale output divided by the rated capacity of a given transducer/load cell.
Short-Term Accuracy: The limit that errors will not exceed during a 24-hour period of continuous operation. Unless specified, no zeroing or adjustments of any kind are permitted. The transfer accuracy obtained with IET's transfer standards is a short term accuracy.
Slippage: A term referring to the passing of fluid through a flowmeter without registration. High slippage rates often occur on low viscosity fluids and can cause large errors
Stability or Long-Term Accuracy: The measurement that will predict the worst-case error for the period indicated, typically a year. To determine the worst-case error after one year, the initial accuracy is added to the one-year stability.
Standard Temperature & Pressure (STP): Standard conditions for experimental measurements, to allow comparisons to be made between different sets of data
Standard Uncertainty: Uncertainty of the result of a measurement expressed as a standard deviation.
Test Conditions: These comprise the assumptions and facts describing the environment, instrument, and sample to be measured. These will include temperature, relative humidity, power, frequency, etc
Thermistor: A semiconductor device made of materials whose resistance varies as a function of temperature.
Turbine Flowmeter: A type of flowmeter used in gas or fluid streams, where the movement of the fluid causes a turbine wheel to rotate. The rotation is sensed from a suitable pickup. These meters will be supplied with a calibration K Factor.
Turndown Ratio: Also referred to as rangeability. It is the ratio between the maximum and minimum flow rate of the meter, across its specified operating range. Turndown can often be increased at the expense of meter accuracy.
Turbulent Flow: A condition where the fluid is traveling in an apparently uniform stream but the individual particles within the fluid are traveling in a random manner in all directions.
Time: Measurement of duration.
Torque: Cause of rotary motion. It is equal to the applied force multiplied by the distance from the center of rotation.
Torr: 1/760 of an atmosphere.
Traceability: It is the path by which a measurement can be traced back to the source from which it is derived. Direct traceability implies that the laboratory has its primary masters calibrated directly by such an agency for reduced measurement uncertainty.
Transducer: An instrument that provides a voltage output.
Transfer: Standard used as in intermediary to compare standards.
Transfer Accuracy: A comparison of two nearly equal measurements over a limited time and temperature.
Transmitter: An instrument that provides a current output.
UKAS: United Kingdom Accreditation Service
Ultrasonic Flowmeter: A type of flowmeter that uses high-frequency sound waves to measure flow by either time of flight or Doppler methods.
Uncertainty of Measurement: It is a parameter associated with the result of a measurement that characterizes the dispersion of the values that could reasonably be attributed to the measurand.
Universal Viscosity Correction (UVC): A method of correcting a turbine meter performance when being used on oil at varying temperatures. The changing temperature and viscosity can be corrected for utilizing Strouhal-Roshko equations.
Vacuum: Any pressure below atmospheric.
Vane Anemometer: A type of wind speed measuring device, where a vane rotates in a housing, and the rotational speed is converted into an airspeed.
Variable Area Flowmeter: A type of flowmeter often referred to as a Rotameter (which is a trade name). The meter uses gravity to balance the force on a float in a tapered tube. The height of the float in the tube is indicative of the flow rate.
Velocity meters: A type of flowmeter that simply measures the velocity of the fluid usually at a single point, i.e. a pitot tube.
Viscometer: An instrument for measuring fluid viscosity.
Viscous Flowmeter: A type of flowmeter often referred to as a laminar flow meter, where the flow through the meter is under laminar conditions.
Viscosity: Resistance of a liquid to sheer forces.
Viscosity Correction (Air): Utilised when operating laminar flow meters away from standard calibration conditions.
Weight: The force of gravity acting on an object.
Wilson Flow Grid: A type of multiport differential measurement system for ducts and tunnels.
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