Controlling pressure with an Alicat laminar DP mass flow controller
Closed loop pressure refers to using a feedback loop based on pressure instead of mass flow. Mass flow controllers (MFC) typically measure the mass flow and adjust the valve to increase or decrease the mass flow to the setpoint.
However, Alicat’s line of laminar DP mass flow controllers use pressure to calculate mass flow. This allows the user the option to control the fluid based on a pressure setpoint, instead of a mass flow setpoint.
Controlling on pressure is useful in applications where you want to simultaneously control pressure and measure flow rate. Some examples include leak testing and flow/quality checking, where you have to maintain a specific flow vs. pressure curve.
Imagine, for example, that you are controlling pressure downstream of the mass flow controller 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.
Watch the video below to hear VP of Applications Engineering, Alyssa Jenkins, go into more detail on the pressure control loop feature.
Application: Calibrating a rotameter with an Alicat MFC
Rotameters measure volumetric flow rate, usually given in units of standard liters per minute (SLPM) or standard cubic centimeters per minute (SCCM). While these units indicate the output is a mass flow rate, this is really only true at the pressure for which the rotameter was tuned.
In contrast, an Alicat MFC has a pressure sensor that adjusts the reading when the pressure changes and gives an accurate mass flow rate at any temperature or pressure. This in combination with the pressure control loop allows an Alicat MFC to be used for rotameter calibration.
The basic steps are as follows:
- Connect the Alicat device downstream of the rotameter.
- Set the pressure setpoint to match the pressure at which the rotameter was tuned.
- Compare the flow reading on the Alicat device to the reading on the rotameter.
- Adjust the rotameter to match the mass flow reading on the Alicat.