Inverse pressure control tutorial: Switching from forward to back pressure control

A forward pressure controller can be reconfigured into a back pressure controller by following two simple steps.

  1. First, you must swap the valve location (tutorials linked below).
  2. You can then activate inverse pressure control using either the front display or serial communications.

In this article, we first discuss the differences between a forward and back pressure controller and then provide instructions on making the switch.

Note: For devices with 7v firmware or earlier, this change will require serial communications.

Forward pressure control vs back pressure control

Forward pressure controller

  • Upstream valve
  • Downstream pressure control
  • Valve opens to increase pressure
  • Valve closes to decrease pressure

Back pressure controller

  • Downstream valve
  • Upstream pressure control
  • Valve opens to decrease pressure
  • Valve closes to increase pressure

Video tutorial

Video transcript

“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.

Step-by-step instructions (front panel)

  1. Switch valve position using the relevant valve swap tutorial for either rolamite valves or small valves.
  2. Press the bottom right button for “MENU” selection.
  3. Press the top middle button for “ABOUT” selection.
  4. Press the top middle button for “DEVICE STATE” selection.
  5. Press the bottom middle button for “EDIT” selection.
  6. Press the top left button for “EDIT REGSTR” selection.
  7. Use the up, down, and select digit buttons to enter the register number 20.
  8. Press the lower right button for “SET” selection.
  9. Record the current value of the register for safe keeping.
  10. Add 32768 to this value to activate inverse control mode.
  11. Use the up, down, and select digit buttons to enter this new value in register 20.
  12. Press the bottom right button to “SET” this new value.
  13. 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.

Step-by-step instructions (serial)

  1. Switch valve position using the relevant valve swap tutorial for either rolamite valves or small valves.
  2. If you don’t already have a serial terminal application, download Alicat’s pre-configured application here.
  3. Plug the device in and connect the communication cable to your PC. Open the serial terminal app.
  4. The terminal app will ask you for a COM port. You can find your COM port number in the computer’s device manager.
  5. 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.
  6. Query register 20 value by typing: AR20 and hit ENTER.
    • Note: We are using a device with a unit ID of “A”. If it were “B”, the command would be BR20.
  7. 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)
  8. Power cycle the device.

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