Control loop variable tutorial: Changing between mass flow, volumetric flow, or pressure control

An Alicat mass flow controller is usually set up to control mass flow rate, but it can instead be set to control pressure or volumetric flow rate.

See below for instructions on how to change the control variable.

Video tutorial

Video transcript

“Hello, my name is Michael Whiteside. I am an applications engineer here at Alicat Scientific. In this video, I’m going to be showing you how to change the closed loop variable on your Alicat device. Now let’s get started. 

In order to change the closed loop variable on your Alicat device, we will hit menu (bottom right button), control (top left button), advanced control (bottom middle button), loop setup (top right button), and loop variable at the top left here.  

As you can see, there are three variables in this list: mass flow, volumetric flow, and absolute pressure. Whatever variable is selected here, the device will actively control to. Say we would like to set this to absolute pressure (top left button for ‘up’, top middle button for ‘down’), hit set (bottom right button), and now this mass flow controller is actively controlling to pressure while measuring the mass flow. This specific mode is called closed loop pressure, or CLP.  

During CLP, we can establish a CLP max flow rate. This can be done by clicking menu (bottom right button), control (top left button), advanced control (bottom middle button), control options (bottom middle button), and CLP max flow (top right button).  

We click this top left button and we see the current CLP max flow is 2000 SCCM. We can really change this to whatever we would like. Say we change this to 1500 (top three buttons for ‘up’, ‘down’, and ‘select digit’), hit set (bottom right button).  

Now what this means is as the device is controlling to pressure, it will make sure to never exceed that 1500 SCCM or 1.5 SLPM. This limit gain here in the top right is a limiter P gain for the CLP maximum flow rate. What this does is it helps guide the valve behavior as the actual flow rate approaches the CLP max flow rate limit.  

That is how you change the closed loop variable on your Alicat device.”  

Step-by-step instructions (front panel)

  1. Press the bottom right button for “MENU” selection.
  2. Press the top left button for “CONTROL” selection.
  3. Press the bottom middle button for “ADV CONTROL” selection.
  4. Press the top right button for “LOOP SETUP” selection.
  5. Press the top left button for “LOOP VAR” selection.
  6. Use the up and down buttons to select your desired variable.

When switching from flow to pressure, it may be necessary to adjust the valve tuning parameters.

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