The Walchem WBLWDB series controller is now obsolete and has been replaced by the WBL600 series controller. The new series has upgraded features; for reference, here is the description of the old series:
Boiler Controller: Single or Dual
Control: 2 powered relays or 6 powered relays
Output: 4-20mA
Single or Dual System Boiler Controller. With up to two conductivity electrodes, two analog outputs and six on/off relays.