How to switch from forward to back control

Switching from forward to back control

Sometimes applications will require control of the process ‘upstream’ of the Alicat instrument (a traditional back-pressure control situation, for example). For these cases, the valve needs to be in the downstream position.  Just make sure to let us know when you order, and we’ll take care of the rest.  If you need to change a forward pressure controller to a back pressure controller, this can be done in the field in two steps, one physical and one digital.

This guide will help you perform the necessary changes required to ensure your Alicat functions properly.  On devices with 7v firmware or earlier, this change will require serial communication with your device.

Step 1:  Switch Valve Position

Unplug your Alicat device and follow the valve switching video for your size valve, then proceed to step 2.

Tools Needed:

  • Your Alicat flow controller
  • Razor blade or utility knife
    T-8 Torx driver
  • 3/32 and 7/64 Hex drivers

Small Valves

Rolamite Valves


Step 2: Activate Inverse Control Mode

For forward pressure control, the valve will open more to increase pressure, and close more to decrease pressure.  When controlling back pressure with the valve on the downstream side, the valve will need to do the exact opposite, close more to increase pressure and open more to decrease pressure.  This functionality is called “inverse control mode” and can be enabled by adding 32768 to register 20 via the front panel or serial communication.


Step-by-Step Instructions

  1. Press the bottom right button for “MENU” selection.
  2. Press the top middle button for “ABOUT” selection.
  3. Press the top middle button for “DEVICE STATE” selection.
  4. Press the bottom middle button for “EDIT” selection.
  5. Press the top left button for “EDIT REGSTR” selection.
  6. Use the up, down, and select digit buttons to enter the register number 20.
  7. Press the lower right button for “SET” selection.
  8. Record the current value of the register for safe keeping.
  9. Add 32768 to this value to activate inverse control mode.
  10. Use the up, down, and select digit buttons to enter this new value in register 20.
  11. Press the bottom right button to “SET” this new value.
  12. Power cycle the device.

NOTE:  Registers can only be changed via the front panel on devices with 8v or newer firmware.  Older devices can only edit registers over serial communication.

  1. If you don’t already have a serial terminal application, download Alicat’s pre-configured application here: https://www.alicat.com/support/software-drivers/.
  2. Plug the device in and connect the communication cable to your PC. Open the serial terminal app.
  3. The terminal app will ask you for a COM port. You can find your COM port number in the computer’s device manager.
  4. Set the baud rate to match your Alicat (typically 19200) and hit enter. You should receive a message saying device was connected successfully. If you don’t, double check that the COM port number is correct.
  5. Query register 20 value by typing: AR20 and hit ENTER.
    1. Note: We are using a device with a unit ID of “A”. If it were “B”, the command would be BR20.
  6. Add 32768 to whatever value was given, and then write it by typing: AW20=[new value] and hit ENTER. (Example: My current value was 29719, so I would type: AW20=62487)
  7. Power cycle the device.

“Hello. My name is Michael Whiteside.  I’m an Applications Engineer here at Alicat Scientific.  In this video, I’m going to show you how to change your pressure controller to inverse control mode.  Now, let’s get started.”

As you can see with this pressure controller, the control valve is located on the upstream side.  This configuration provides downstream pressure control, as the pressure will be regulated to whatever side the sensor is exposed to (points to the downstream side of the controller).  We call this configuration a “forward pressure controller”.  A valve located on the downstream side, will result in upstream pressure control, as the sensor is now exposed upstream.  We call that configuration a “back pressure controller”.

In forward pressure control, the valve will open up to increase pressure, and close to decrease pressure.  It is the opposite for back pressure control.  With the valve on the downstream side, it will open up to decrease the upstream pressure and close to increase the pressure.  Due to this inverse functionality, the device must be told how to properly control the valve position (as in how far it is open) depending on the overall goal.

If you plan on reconfiguring your forward pressure controller to a back pressure controller, the first step is to switch the valve from upstream to downstream.  A tutorial on this valve switching process is located on our website (see above).  Once the valve is switched, the control algorithm must be changed to inverse control.

In order to change to inverse control, you will add 32768 to register 20.  In order to do this, we will hit MENU (bottom right button), ABOUT (top middle button), DEVICE STATE (top middle button), and we see a list of registers here.  We will hit EDIT (bottom middle button), EDIT REGSTR (top left button), and select register 20 (using the up, down, and select digit buttons).  We will hit SET (bottom right button), and we see register 20 currently has a value of 276.  Adding 32768 to this value results in 33044.  We will change this to 33044 (using the up, down, and select digit buttons), and we will hit SET (bottom right button).

That is how to properly change your pressure controller to inverse control mode.

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