Mass flow techniques provide more RELIABLE, REPEATABLE AND ACCURATE gas flow rate measurements compared to volumetric methods such as rotometers and turbine meters. Mass flow measurements use the density of the gas to translate mass flow units (kg or lbs) to units of familiar volumetric terms(liters/min). However the measurements are still mass units and we call those units STANDARD liter/min (SLM) and they do not change with temperature and pressure changes. We do this by referencing just one, specific temperature and one specific pressure (STP). For a more detailed explanation, see Mass Flow vs. Volumetric Flow.

200 Series instruments have flow ranges from 0-10 sccm to 0-500 slm (Large flows to 15,000 slm with a Laminar flow element); use elastomeric seals, provide analog outputs, and are constructed of stainless steel. The controllers have normally closed valves. Corrosive gases, with the exception of the halogens, pressures up to 1000 psig (optional) are easily measured.