A mass flow meter (MFM) measures flow. A mass flow controller (MFC) adds a control valve and regulation loop to maintain a requested setpoint. Choose the function first, then match the gas, flow range, pressures, wetted materials and interface.
Five inputs for selection
| Input | What to specify | Why it matters |
|---|---|---|
| Gas | Gas or mixture and its composition | Calibration and material compatibility depend on the gas |
| Flow | Minimum, normal and maximum values with reference conditions | Defines usable range and uncertainty at the actual operating point |
| Pressure | Inlet and outlet pressure and expected changes | The valve needs an appropriate pressure differential |
| Materials | Wetted metals, seals and cleanliness requirements | Determines compatibility with the process |
| Control | Analogue or digital interface, setpoint and readback requirements | Must match the equipment control system |
State the reference conditions
SCCM and SLM describe volume flow referred to stated standard conditions. They are not the actual litres per minute in a low-pressure chamber. Include the reference temperature and pressure when comparing instruments, quotations or process recipes. A gas correction must follow the manufacturer's calibration model; it is not a universal substitute for calibration on the intended gas.
Read the accuracy specification carefully
Accuracy expressed as a percentage of full scale behaves differently from a percentage of reading. For an illustrative 1000 mL/min full-scale instrument with a ±0.5% FS term, that term alone equals ±5 mL/min: 5% of a 100 mL/min reading, 1% at 500 mL/min, and 0.5% at full scale. This example explains the arithmetic and is not a specification for a particular product.
Also compare zero stability, repeatability, settling time, the permitted pressure range and the conditions under which these specifications apply. Select a range around the useful operating point rather than automatically choosing the largest available full scale.
Integration and commissioning
Confirm the flow direction, fittings, valve arrangement, power supply and signal levels. Leak-test the installed gas path, observe the specified warm-up time, and zero the instrument under genuine no-flow conditions. A zero command to a controller does not by itself prove that no gas is flowing. Gas, temperature and mounting orientation can affect zero and performance.
In vacuum service, chamber pressure also depends on pumping speed and conductance. A stable gas-flow setpoint does not independently guarantee a stable chamber pressure. Check the complete control arrangement and its response.
Prepare a model-specific specification
LeakLab-R, AMC2000, AS200 and AS200B configurations must be selected individually. Do not extend a gas list, pressure rating or accuracy statement from one model to an entire family. Record the gas, calibration conditions, range, inlet and outlet pressures, connections, interface, cable and required documents in the agreed specification.
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