The Walchem WDT410-5N6U controller is now obsolete and the closest equivalent is the WCT600HSSNN-AN controller. The new WCT600 series has upgraded features; for reference, here is the description of the old model:
Cooling tower controller -bleed, feed, 2 biocides, ph/ORP, & alarm, hardwired, cable glands, no 4-20 mA output, graphite electrode, ph electrode & tees (in-line or submersion), 20 ft cables, integrated datalogging and config file import/export via usb flash stick.