Mass Flow Terminology

We have put together a glossary to help guide you through the mass flow terminology we use today. This terminology helps expl
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​​​Generic Flow Terms:​​​​​​​​​

Mass Flow

​When measuring gases, mass flow is the amount of gas, measured in units of mass, flowing until of time. Examples of truss mass flow units are kg/hr and gm/sec. However, mass flow is often measured in standardized volumetric units.​
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Volumetric Flow​

When measuring gases, volumetric flow is the amount of gas, measured in units of volume per unit time. Examples include l/s and cfm. Mass flow can be determined from volumetric flow only if the density (ρ) is known. ​Qmass = Qvol* p.

Flow Rate

​Flow rate measurements are often described in terms of “standardized” volumetric flow units such as standard cubic centimeter per minute (sccm), standard cubic foot per minute (scfm), or standard liter per minute (slm). In a standardized flow unit, the number of molecules in a volume of gas, such as cc or cu. ft., is determined by defining the reference conditions (or “standard conditions” aka STP) and applying the Ideal Gas Law​.
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Thermal Mass Flow

In a thermal mass flow sensor, the amount of heat absorbed by the flowing gas is measured. This heat is a function of both the specific heat of the gas and the flow rate. A common sensor design includes a capillary tube; however, MEMS-style sensors uses flat heated surfaces have also been used.​

Standard Temperature and Pressure (STP) 
Also known as Reference Conditions.

In standardized volumetric flow units (sccm, slm, scfm,), the reference conditions, or “STP” temperature and pressure, define the amount of gas by determining the number of molecules using the Ideal Gas Law. In many cases, the selected reference conditions are 0°C & 760 Torr. But, other reference conditions are also used, such as 25°C & 760 Torr, or 70°F & 760 Torr. ​

Ideal Gas Law
Ideal Gas Law is often expressed using the equation PV=nRT, in this example, P is the pressure given in kPA, V is the volume expressed in L (liter), n is the number of moles, R is the ideal gas law constant 8.31 L*kPA/mol*K, and T is the temperature in Kelvin. In gas flow using standardized volumetric flow units (including sccm, scfh, slm), the ideal gas law determines the number of molecules in a particular unit.
​Mass Flow Meter

A mass flow meter is a precision instrument that measures the flow rate of a gas, often expressed using standardized volumetric flow units including sccm, scfh, or slm.​
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Mass Flow Controller
​A mass flow controller, or MFC, is a device that consists of a mass flow meter and a proportional control valve. The meter and control valve are components in a precision control loop.  In normal operation, the user will send a command, known as the flow setpoint, to the MFC and the device, in turn, will measure the flow rate and adjust the proportional control valve as required to reach and maintain the desired flow setpoint. ​
​Laminar Flow

​In laminar flow, the gas moves in a well-behaved fashion with more of the flow moving through the center of the pipe and the pressure drop is linearly proportional to the flow rate. Laminar flow is often critical for accurate flow measurement.​

​Turbulent Flow
​​In turbulent flow, gas behavior tends to be chaotic in nature and may lead to less accurate flow measurement.

​Purge is a process used to clear any gases or liquids from the flow instrument that may have condensed from previous use. This is typically done with argon or nitrogen due to their inert properties. Purging an instrument is good practice before new trials to clear out any contamination from previous results. In the case of a mass flow controller, a purge can be performed by fully opening the control valve.​

​Gas Blending

Gas blending is the process of mixing gases in a controlled fashion to create a blend with a known composition. For example, mass flow controllers attached to bottles of pure gas can be connected in parallel and joined at their outlets to create a blended gas. ​

Mass Flow Meter/Controller Components:

​Thermal Mass Flow Sensor

A thermal mass flow sensor detects changes in flow by measuring the rate of heat transfer carried by moving gas. There are several designs, including MEMS techniques. But generally mass flow meters and controllers employ a capillary tube design. There are different sensing techniques. In the ΔT approach, temperature changes upstream and downstream of a central heater are detected by using either thermocouples or resistive windings. There are also controlled temperature approaches in which sets of coils upstream and downstream coupled with ambient coils are utilized.
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​Thermal Mass Flow Transducer

A thermal mass flow transducer consists of a thermal mass flow sensor (see above) and supporting circuitry to power, filter, and amplify the flow sensor’s output.  ​


Also known as the “bypass” or “bypass element,” the shunt facilitates measurement of large flows by allowing gas to bypass the flow sensor. The shunt size and geometry are selected such that the sensor is not over-ranged at the nominal full scale, but still utilizes the full range of sensitivity of the flow sensor.  The shunt is designed such that the ratio of the flow through the flow sensor and the shunt itself is a constant.  ​

​Laminar Flow Element

A laminar flow element, or LFE, is a special type of shunt in which the gas flow is conditioned such that flow stays in the “laminar flow” regime. In laminar flow, pressure drop as a function of flow rate is well-behaved and is therefore more consistent from run to run which allows the flow measurement to provide better accuracy. ​


The base of the flow instrument holds the sensor, valve, bypass shunt, and endcaps. The size of the base helps determine the maximum flow rate that can be handled by the instrument. ​


​Endcaps are the fittings that are on the ends of either side of the flow instrument and attach to the base. This is used to connect to tubing so that the gas may pass through to another part of the system.​


​In a mass flow controller, the valve regulates the flow rate through the instrument. Most mass flow controllers utilize a solenoid proportional valve design which includes the solenoid, an orifice, an elastomeric valve seat, and a valve spring. ​


​In a solenoid proportional valve, the orifice is a small round passageway with fixed diameter which, given the gas type and upstream and downstream pressures, determines how much flow can pass through the instrument when the valve is fully open. ​

​Valve Seat

​The elastomeric seal which, when pressed against the valve orifice, closes the valve and stops gas flow. When the valve seat is lifted off the orifice, gas is allowed to flow. Control of the valve seat over the orifice adjusts the gas flow rate. 

Valve Spring

The valve spring is tensioned such that the valve seat is sufficiently pressed against the orifice so that when no current is flowing through the solenoid, the flow controller’s valve is fully closed. This is also called the normally closed condition in that if power is removed from the flow controller, the valve is closed. Note that the valve spring also allows the valve to fully open and control at all setpoints between fully closed and fully open.


​Solenoid is a coil of wire which is used to generate a magnetic field for the control valve. The mass flow controller’s PID control loop regulates the amount of current to the solenoid and thus controls the distance between the valve seat and the orifice.​

​Two-stage Valve

​A two-stage valve in one in which a solenoid proportional valve known as the “pilot valve”, is used to control a larger plunger and valve seat. Two-stage valves can be used to handle very large flow rates. They can also be configured as normally closed or normally open.​

​Normally Closed
​A normally closed valve is configured such that when power is removed from the mass flow controller, the valve is closed, and flow cannot pass.
​​Normally Open​

A normally open valve is configured such that when power is removed from the mass flow controller, the valve is open, and gas will still move through the controller.

Descriptive Terms (Characteristics & Specifications):

​Full Scale
Also known as Range

Full scale is the nominal maximum flow rate that a particular thermal mass flow meter or mass flow controller will measure and/or control. In the example of an analog flow instrument with 0-5 VDC output, the range is the flow rate at which the instrument will give an output of 5 VDC.


​Overrange state occurs when the flow rate exceeds the range or full scale of the flow instrument. Many mass flow instruments can handle flow rates that are slightly overrange. Some models can even measure to 120% and beyond. However, the accuracy of the flow instrument in the overrange state will very likely deteriorate as the range of the model is exceeded.​

​Turndown Ratio

Turndown Ratio defines the usable range that a mass flow meter or controller can be used while maintaining its published accuracy:

Turndown Ratio =  Full Scale Flow / Minimum Flow

Most analog mass flow meters have an accuracy of ± 1% of Full Scale (FS) and have resolution better than 1%.  The usable range is from 1% to 100%.  They will have a turndown ratio of 100/1 or more commonly expressed as 100:1.  Digital flow meters will have an even greater turndown ratio due to their higher.
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​Accuracy as % of Full Scale
​Accuracy as % of full scale is the difference between the indicated flow rate of an instrument under test (IUT) when compared to the actual flow rate as measured by an accepted flow standard (Std) expressed as percent of the full scale:

% Full Scale = (IUT – Std) / Full Scale of IUT

For example, if a 100 SLM flow instrument is specified to be ±1% full scale, then the uncertainty is ±1 SLM across the entire range (0-100 SLM). 

​Accuracy as % of Reading

​Also called Accuracy as % of Point. It is the difference between the indicated flow rate of an instrument under test (IUT) when compared to the actual flow rate as measured by an accepted flow standard (Std) expressed as percent of the actual flow:​

% Reading = (IUT - Std) / Std


​Uncertainty in measurement is a value that describes the spread in measurement values that is related to the measurement technique itself. Standard deviation is often used to characterize uncertainty.  ​


Hysteresis is the difference in the accuracy of a measurement when approached from above the setpoint and the accuracy of a measurement when approached from below the setpoint.​

​NIST Traceable Calibration

​NIST traceable calibration is using metrology instrumentation that has a calibration trail leading back to standards residing at NIST (National Institute of Standards & Technology). Note that a NIST traceable calibration does not necessarily mean that the calibration has a low-level of uncertainty. An optional NIST documentation package includes all documents (i.e. the paper trail) linking the calibration back to NIST standards. ​
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Linearity is the difference the ideal straight line from zero flow to full scale is compared to the actual flow indication of the instrument under test.  Linearity is often expressed as a percentage of full scale.

​Repeatability is an instrument’s ability to produce the same outcome given the same operating conditions using the same instrument. It is often expressed as a % of full scale.  ​


​Reproducibility is the closeness of agreement between the measurements of a value among different instruments, usually over longer time periods with typical variation in environmental conditions. A measurement is made with multiple instruments at a setpoint. The standard deviation of the values obtained is the reproducibility. It is usually specified by the worst case setpoint as a 3σ value​.


​In a flow instrument, a totalizer sums the amount of gas that has passed through the instrument over a period of time. It may be a separate device or an internal register within the flow instrument. This is convenient for applications that require the recording of total gas use, not only the flow rate. ​

​Gas Correction Factor (GCF)

​The GCF is a multiplier that allows the user to convert the calibration of a thermal mass flow instrument from one gas to another. The GCF will vary by gas due to the differences in density and specific heat.

​Nitrogen Equivalent Flow Rate

Nitrogen equivalent flow rate is a number used to describe the nominal flow rate that a user would expect if the flow instrument was used in nitrogen gas. Nitrogen equivalent flow, when expressed in sccm or slm, will typically have reference conditions of 0°C & 760 Torr. The number is very useful for determining the correct size of the instrument’s body, fittings, and other components.

​Calibration Gas

​Calibration gas is the actual gas used during the calibration of a mass flow meter or mass flow controller. However, instruments are often calibrated for use in gases other than the calibration gas by the use of calculations and conversion of the transfer flow standard. 
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​Warm Up Time

​Warm up time is the time required for the flow instrument to reach thermal equilibrium such that it performs within its published specifications, including accuracy and response time. 

​Maximum Allowable Working Pressure

​The maximum pressure that a device can be exposed to without causing permanent damage to the operation of the unit. 

​Working or Operating Pressure

The pressure range over which the device is designed to operate. For pressure devices, this would be the measurement range. For flow devices, this is the line pressure that the device is intended to operate.

​Burst Pressure

​This is the pressure at which the device will rupture due to the application of internal pressure.

​Settling Time

​Also known as response time. In a mass flow instrument, it is the amount of time elapsed from the moment the flow command is changed until the flow instrument reading stabilizes to within an acceptable error band of the desired flow.  

​Response Time

​Response Time can be defined in several ways. The first way is by the time constant of an exponentially rising output after a step change in input. One time constant (τ) is the time for the output to reach (1-1/e) or approximately 63.2% of its final value. A second way is the Rise Time, or the time it takes the output to go from one level to another. Frequently, this is 10% to 90% or 0% to 100%. The first two methods can be misleading if a system response is not exponential or exhibits overshoot. A third way is the Settling Time, which is the time required for the output to reach and remain within a given error band around its final value following a step change in setpoint. We typically use a ± 2% error band and a step change from 10% to 100%. Using 10% removes some variability due to valve preload and pneumatic forces. Other step changes can be used as well, such as 0% – 100% or 10% to 90%.

​Wetted Materials

​Wetted materials are the materials in the instrument that are exposed (i.e. direct contact) to the gas medium during the operation.

Metal Sealed

​Metal sealed is a mass flow instrument (meter or controller) which contains no elastomeric seals with the exception of the valve seat. Metal sealed versions offer higher purity than elastomeric instruments and fewer wetted material issues when using reactive gases. ​

​Elastomeric Sealed

Elastomeric sealed refers to the use of o-rings in a mass flow instrument to ensure that the unit does not leak. Seals are typically made using Viton®,  but other elastomeric material such as Buna-N, Neoprene, and Kalrez® may be available. 


Kalrez® is a Perfluoro elastomer (FFKM/FFPM) manufactured by DuPont™. This elastomer is known for its resistance to even the most corrosive gases, such as Hydrofluoric acid (HF).


​Viton® is a fluoropolymer elastomer, trademarked by DuPont™, and designed to be highly versatile against various environments and commonly used in o-rings. It is resistant to most gases and this makes it highly favored in chemical, petroleum, and aerospace industries.


​Buna-N a nitrile rubber that is commonly used today. It is often used in flow controllers that handle carbon dioxide. It is less expensive than other elastomers such as Kalrez® and it does not allow the CO2 to absorb into the o-ring which may cause explosive decompression.


Neoprene is a synthetic rubber that is known for its chemical stability and flexibility over a wide temperature range.

Oxygen Cleaning

​Oxygen cleaning is a process in which the flow instrument is made safe for use in oxygen by ensuring that all hydrocarbon and other combustible materials are removed. Typically, the instrument is thoroughly dissembled into its individual components and then each is cleaned. During reassembly in a clean area, special lubricants may be required. Upon completion of cleaning, the flow instrument is bagged to ensure that it does not become contaminated. 

​Monel is nickel alloy that has excellent resistance to corrosive gases particularly halogens.
​Viton® is a registered trademark of the Chemours Company

Kalrez® is a registered trademark of the E.I. du Pont de Nemours and Co

Mass Flow Controller:


Upstream Pressure

Upstream pressure is the inlet pressure of the flow instrument. For flow controllers, an understanding of the range of upstream and downstream pressure conditions allows the instrument to be configured and built for optimal response and stability.​
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​Downstream Pressure

Downstream pressure is the outlet pressure of the flow instrument. For flow controllers, an understanding of the range of upstream and downstream pressure conditions allows the instrument to be configured and built for optimal response and stability. 
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​Also known as the command. The setpoint is the desired flow rate. The flow controller’s setpoint can be provided by either analog signal (e.g. 0-5 VDC, 4-20 mA…) or by digital signal, depending on the capabilities of the mass flow controller. In a mass flow controller, the actual flow is measured and then the valve is adjusted such that resulting flow matches the user’s desired setpoint. In the case of analog control using 0-5 VDC, a setpoint of 5 VDC commands the mass flow controller to control at 100% of the stated full-scale flow of the instrument.  

​Digital Control

​In digital control mode, the mass flow controller receives its setpoint (or command) via some form of digital communication such as RS232 or RS485.  

​Analog Control

In analog control mode, the mass flow controller receives its setpoint (or command) via some form of analog signal such as 0 5 VDC, or 4 20 mA. In the case of analog control using 0 5 VDC, a setpoint of 5 VDC commands the mass flow controller to control at 100% of the stated full-scale flow of the instrument.

​PID Loop (Proportional/Integral/Derivative)

PID Loop refers to the closed loop feedback system that is used to determine the overall response of a flow controller. In a PID control loop, the amount and rate of change of error between the actual flow and the user’s setpoint are inputs to the control loop. Based upon these inputs, the control loop generates a valve signal to continuously adjust the valve to maintain the flow rate at the desired setpoint.

​Soft Start

The soft start feature in certain mass flow controllers allows the user to increase the flow rate at a certain percentage of full scale per second to reach the desired setpoint. In most cases, this ensures that the flow controller does not overshoot the desired flow setpoint. 

​Auto Zero

​The auto zero feature, when enabled in certain digital mass flow controllers, allows the instrument to automatically rezero itself after a period of time in which the flow controller has a closed valve.

​1% Shutdown

​​This 1% shutdown feature, when enabled in certain digital mass flow controllers, allows the instrument to interpret any analog setpoint signal that is less than 1% of full scale as a zero flow command which in turn causes the flow controller to ensure that its control valve is fully closed.  ​