Controlling pressure with an Alicat mass flow controller

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 and flow/quality checking, where you have to maintain a specific flow vs. pressure curve.

Watch the video below to hear applications engineer, 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:

1. Connect the Alicat device downstream of the rotameter.
2. Set the pressure setpoint to match the pressure at which the rotameter was tuned.
3. Compare the flow reading on the Alicat device to the reading on the rotameter.
4. Adjust the rotameter to match the mass flow reading on the Alicat.