ACCURACY: The combined error of nonlinearity, repeatability, and hysteresis expressed as a percent of full-scale output.

AMBIENT PRESSURE: In mass flow controllers, the absolute pressure of the medium surrounding the device.

BYPASS: A mass flow controller component that maintains a constant ratio of gas flow through the device”s sensor and main flow path, dividing the gas stream precisely over the entire calibrated flow range. Enables the total flow to be determined by measuring just the portion of gas that passes through the sensor. Also known as the flow splitter.

CALIBRATION: The comparison of transducer voltage outputs against the outputs of a reference standard.

DRIFT: A change of a reading or a set point value over long periods due to several factors, including change in ambient temperature, time and line voltage.

FLOWMETER: A device used for measuring the flow or quantity of a moving fluid.

FLOW RANGE: Actual speed or velocity of fluid movement.

FULL SCALE: The maximum measurand that a transducer is designed to measure within its specification. Full scale is also known as Rated Capacity.

GAS FLOW: Gas flow, as we use it, can be expressed in: Volume (f.i. l/min); Standardized or Normalized flow (f.i nlpm or slpm); or Real Mass flow (f.i. Gr/min or Kg/hr).

HEAT TRANSFER: The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are: Conduction – the two bodies contact; Convection – a form of conduction where the two bodies in contact are of different phases, i.e. solid and gas; and Radiation – all bodies emit infrared radiation.

MASS FLOW: The mass that passes through a cross-sectional area (flux) per unit time.

NORMALIZED: Referred to predefined conditions. In case of gas it means predefined Pressure and Temperature, because those are variables that relate Mass to Volume. Normalized conditions are typically used in the metric system and are defined as 0 C and 1 atm.

RANGEABILITY: The ratio of the maximum flowrate to the minimum flowrate of a meter.

RESPONSE TIME: The time required by a sensor to reach 63.2% of a step change in temperature under a specified set of conditions. Five time constants are required for the sensor to stabilize at 63.2% of the step change value.

REYNOLDS NUMBER: In fluid mechanics, the Reynolds number is the ratio of inertial forces (vsρ) to viscous forces (μ/L), and consequently it quantifies the relative importance of these two types of forces for given flow conditions. Thus, it is used to identify different flow regimes, such as laminar or turbulent flow.

STANDARDIZED: Referred to predefined conditions. In case of gas it means predefined Pressure and Temperature, because those are variables that relate Mass to Volume.

Standardized conditions are typically used in the English system and are defined as 70 F and 1 atm.

THERMAL MASS: Thermal mass flowmeters generally use combinations of heated elements and temperature sensors to measure the mass flow of a fluid. The fluid temperature is also measured and compensated for. They provide a direct mass flow readout, and do not need any additional pressure temperature compensation over their specified range.

VISCOSITY: The inherent resistance of a substance to flow.

The above glossary of terms and definitions was contributed by Sierra Instruments Inc. (, a manufacturer of mass flowmeters and controllers for gas, liquid, or steam.