The Walchem WPH420-5NNU controller is now obsolete and the closest equivalent is the WPH600 controller. The new WPH600 series has upgraded features; for reference, here is the description of the old model:
pH/ORP controller, proportional control outputs, hardwired, cable glands, no 4-20 mA output, none, integrated datalogging and config file import/export via USB flash stick.