How to Switch 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, you will need to physically switch the position of your proportional valve from the upstream side to the downstream side. When the valve is moved, you will also need to reverse the internal control logic to complete the control inversion process. Whereas previously the valve opened to ‘let in’ pressure, it will now close to ‘build up’ pressure. This guide will help you perform the necessary changes required to ensure your Alicat functions properly.

Switch from Forward to Back Control

1. Unplug your Alicat device and follow the valve switching video for your size valve, then proceed to the section specifically written for your software version.

Small Valves

Video: Changing valve position on any small valve Alicat controller

Rolamite Valves

Video: Rolamite Valve Swap

 

8V Firmware

Tools Needed:

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

2. After you have changed the valve position, it is time to adjust the valve’s control logic. This can be done by adjusting register 20 on the device. We will need to add +32768 to the current value. This new value will tell the device to control inversely.

a. In a default state, the valve will close to relieve pressure, and open to build pressure.
b. When we switch valve position, the logic will need to be reversed. The valve should close to build pressure, and open to relieve it.

3. We will be adjusting the register through the front panel.

a. Plug the device in and navigate to the register values by following these buttons: MENU→ABOUT→DEVICE STATE→EDIT
b. Set the register selection to 20 using the navigation keys.
c. From here, we will add 32768 to the current total. (Example: My current value is 29719, so my new value would be 62487.)
d. Ensure the new value is correct and press “SET”.
e. Power cycle the device.

4. Once the register has been changed, the device should function properly for backwards control.

 

Earlier Firmware

Tools Needed:

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

2. After you have changed the valve position, it is time to adjust the valve’s control logic. This can be done by adjusting register 20 on the device. We will need to add +32768 to the current value. This new value will tell the device to control inversely.

a. In a default state, the valve will close to relieve pressure, and open to build pressure.
b. When we switch valve position, the logic will need to be reversed. The valve should close to build pressure, and open to relieve it.

3. We will be using serial commands over RS232.

You can download the terminal app here:
https://www.alicat.com/support/software-drivers/.
Additionally, we will be using commands in our serial primer found here:
https://documents.alicat.com/Alicat-Serial-Primer.pdf

a. Plug the device in and connect the communication cable to your PC. Open the serial terminal app.
b. The terminal app will ask you for a COM port. You can find your COM port number in the computer’s device manager.
c. 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.
d. 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.
e. 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)
f. Power cycle the device.

4. Once the register has been changed, the device should function properly for backwards control.

*Questions? Send us an email at info@alicat.com or call us at 520-290-6060

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