My original controller was designed just for the RIMS system so when I went to an electric HLT I had to do a redesign. The new controller has circuitry for both the RIMS heater and the HLT heater and is mounted on drawer slides for easy access.
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The RIMS controller is basically unchanged from the original except for the use of a 10 amp solid state relay (SSR) in place of a 25 amp. The maximum current in the RIMS heater was seven amps so the 25 amp SSR I was using was an overkill.
The heart of the RIMS controller is a Watlow Series 93 auto tune controller using an Omega CF-000-J-2-60-1 type J thermocouple probe as the input sensor. The output power is controlled by a 10 amp ssr and switched through a surplus spst mercury displacement relay to insure absolute control in case of SSR failure and to permit running the PID without applying power to the heater. The heater relay is interlocked with the pump switch so heat cannot be applied unless the pump is running.
The HLT controller is identical except it uses a Watlow Series 965 controller in the on/off mode. A less complex controller could have been used but the 965 was available from a previous project. The control thermocouple is an Omega type J probe. The HLT element power is controlled with a 25 amp SSR and switched using another surplus mercury displacement relay in a dpst configuration. The heating element draws about 15 amps at 230 volts.
One float switch in the HLT is interlocked with the heater relay to shut off power when the water drops below a safe level and another shuts off the water inlet solenoid when the tank is full.