Bronkhorst EL-FlowVary-P control valves were already introduced by Bronkhorst in 1985 for controlling high-pressure gas flow with high pressure differences of up to 400 bar (later on extended to 700 bar!). The control valves—usually integrated in a mass flow controller (F-230 series)—are a more compact and much cheaper alternative to flow meters, which are combined with valves that are controlled pneumatically or using a servomotor. For controlling process pressure, Vary-P valves are often also applied in conjunction with Bronkhorst pressure meters. The Vary-P valve comprises a flow control section, along with a mechanism that keeps the differential pressure over this valve at a constant approx. 6 bar. As a result, the pressure at the inlet and outlet of the mass flow or pressure controller may vary without affecting the control performance.

For gas flow control up to 200 bar, Bronkhorst has recently developed a new, directly driven control valve; i.e. without a Vary-P mechanism. This development makes mass flow controllers fitted with this valve (F 220 series) even more compact, offering even faster control. Industrial versions (IP65 rating) of all mass flow controllers can also be supplied.

High-pressure mass flow controllers are often used to control gas flow in chemical processes, both in research and in production environments.