I have a home-built controller for a HERMS brewing system (Heat Exchanging Recirculating Mash System). The purpose of this controller is to drive liquid (wort) from a large mashing vessel (a cooler box), through a heat-exchanging coil that is submerged in a reservoir of heated water, in order to maintain the temperature of the wort in the mash vessel. This temperature control is achieved through the use of a PID, which takes a temperature reading from the liquid wort as it flows out of the heat exchanging vessel, and drives a 10A kettle element (through an SSR) to maintain the desired temperature. This setup works extremely well, but there is a problem that sometimes occurs that could, potentially, be dangerous: the wort is pumped around the system by a 12V pump. If, for some reason, the recirculation stops (e.g. the pump gets blocked with debris/airlock, or simply breaks), then the measured temperature (just outside the HERMS vessel) does not rise with the temperature of the water within the vessel. This causes the PID to keep driving the 10A kettle element, up until the point that the water inside the vessel boils, and steam starts venting through some pressure release holes in the lid.
This exact situation has now occurred twice. The first time, I was lucky enough to spot it early and switch the system off. After this, I added a new PID to the system that measures temperature within the HERMS vessel. This PID (from Auber) has an alarm feature that can be triggered when a certain temperature is hit. The alarm feature triggers a small, internal, SSR (240v, 3A), which is connected to an alarm unit that flashes and sounds a loud siren. This works, and saved me on the second occasion.
What I want to do now is go further than simply sounding an alarm, and have the kettle element circuit shut off completely if this alarm condition is met (I can't always be within earshot to turn the system off). I am not well versed in electronics, although I do try my best to wire everything I have up as safely as possible. The only thing I can think of is a normally closed relay, that is driven by 240v to open it when the alarm condition is met. This would work, but would only open the circuit for as long as the alarm sounds. I also have some concerns that running this normally closed relay at 10a for, potentially, many hundreds of hours at a time (between alarm events) could see it fuse shut? Is this a reasonable concern? What I would really like is a way of switching a circuit off, in much the same way as a circuit breaker - i.e. the circuit switches off, a physcial switch is thrown and I have to manually intervene to enable it again. Ideally this would happen whilst the alarm siren continued to sound.
Can anyone recommend a solution please? As mentioned, the easiest option for me is to use the already existing 240v alarm signal, but if another way makes more sense I am willing to consider it. Bonus points if the solution can be DIN mounted, as that is the way this thing is currently built.
