EN 
NO 
The HPD is a level controller which works with sensors fitted to the Polaris heater. The sensor are Braude filaments which will not corrode in even the most corrosive of solutions.
The controller operated by passing a small ac current through the sensors which will detect the presence of a liquid. If there is no liquid present the controller will deault to the off position and the heater will be turned off thus ensuring safety of the heater and the tank.
HPD is best ordered at the time of ordering the heater ensuring the sensors are correctly positioned. It is possible to retrofit sensors where there is no other choice.