Mass flow controllers (MFCs) have gone through an evolution since they were introduced in the 1970s to replace simple ball and tube rotameters, evolving from the first analog 5-second thermal sensor-based (TB) devices, to digital 1-second TB devices adding pressure sensors to dampen the flow spikes from supply pressure sensitivity. The evolution continued in early 2000, with the introduction of more primary flow measurement sensing using pressure. These pressure-based (PB) MFCs using sonic orifices or compressed laminar flow elements (CLFEs) and millisecond response pressure transducers eliminated the historic issues of 1- to 3-second first time constants, supply pressure sensitivity and attitude sensitivity of the TB MFCs. The PB MFCs introduced new issues of slow bleed down at shutoff and did not solve the low-flow requirements or the fast response needs of less than 100ms response.
Reno’s FlowNodeTM technology literally evolved outside the box of the traditional MFC. The first step was the choice of the CLFEs. The CLFE has two main benefits over the thermal sensor and sonic orifice in that it is primarily viscosity-based. Viscosities in gases vary significantly less gas to gas when compared with the specific heat and thermal conductivity of the TB and the densities of gases in the sonic orifices. This smaller variation in gas-to-gas viscosity in the CLFE simplifies multigas calculations. The CLFE has another attribute in that it has a beneficial nonlinearity at low flows, creating larger signal-to-noise ratios and allowing accurate control to wider ranges.
Reno’s FlowNodeTM technology uses the downstream pneumatic shutoff valve location, putting the CLFE and temperature sensing into the valve. The near-zero internal volume position at the diaphragm of the valve opens up capabilities such as accurate, repeatable and pressure-insensitive flows of <0.01 sccm while being able to turn on and off in sub-100 milliseconds. Multiple FlowNodeTM elements can be put next to each other, enabling the widest range achievable today (<8,000 to 1 @ 1% reading accuracy with three elements and 100,000 to 1 control range). The FastGasTM commands allow flow to turn on and off as fast as the valve can actuate (50ms).