Controlling pressure while measuring mass flow (closed loop pressure)
For a step-by-step tutorial on changing the control loop variable, click here.
Closed loop pressure refers to using a feedback loop based on pressure instead of mass flow. A mass flow controller (MFC) will typically measure the mass flow and adjust the valve to increase or decrease the mass flow to the setpoint.
When controlling on pressure, the valve will instead adjust based on the pressure measurement. Imagine, for example, that you are controlling pressure downstream of the Alicat and want to reach 100 PSIA. If the pressure is at 80 PSIA, the MFC will open the valve further until the pressure reaches your setpoint. This is useful in applications where you want to simultaneously control pressure and measure flow rate. Some examples include leak testing, flow checking, and quality checking where you have to maintain a specific flow vs. pressure curve.
Applications engineer Alyssa Jenkins discusses the pressure control loop feature
Want more information? Alyssa discusses the pressure control loop feature in more detail in this interview.
Processes using MFCs to control pressure
Closed volume processes
Our bidirectional mass flow controller has three ports (inlet, process, outlet) and two valves (for incoming flow and exhaust). This device can be used to control a closed volume of pressure, with one valve adjusting to increase/decrease flow rate and the other exhausting to atmosphere or vacuum pump in case of pressure overshoot. A flow totalizer can be used to track total amount of fluid flowed while controlling pressure, so you can also measure how much of each gas enters the system.
Rotameters measure volumetric flow rate, and they are tuned at a very specific pressure to give the mass flow rate. While they may be marked in standard liters per minute (SLPM) or standard cubic centimeters per minute (SCCM), this mass flow reading is only valid at the pressure for which the rotameter was tuned. On the other hand, an Alicat has a pressure sensor that adjusts the reading when the pressure changes.
An Alicat mass flow device can be used to control the pressure downstream of the rotameter, while simultaneously measuring the flow directly on the mass flow meter and the rotameter. For this to work, the STP conditions and gas selection of the Alicat must be adjusted to match the rotameter. Then back pressure control point on the Alicat can then be set to equal the outlet pressure of the rotameter. This creates a condition in which the Alicat is controlling the rotameter’s pressure, standardizing the Alicat to the variable area flow meter. You can then introduce flow to calibrate the rotameter.