Fermentation chamber fail safe

[Cthorbr]

I'm in the early stages of designing a temp controlled fermentation chamber. I'm about 75 percent sure I'm going the brewpi route. But regardless I know I will be using 2 10a SSR's for the heat and cool cycling.

One thing I have stumbled across a few times now is the possibility of an SSR failing closed and bad things following. Even if it is only a remote possibility of causing a house fire, I still want to try and protect against it.

This is where I was hoping the forum could help me out with some ideas. Has anyone built in a fail safe to there chamber? If so what did you do? I am playing with the idea of maybe putting time delay relays after each SSR. The idea being that if there was a longer than predicted heat or cool cycle it would trigger a normally closed contactor with a latching circuit that would kill power to both SSR's

I haven't really had any experience with time delay relays. Is this something that would even work? If so any suggestions for a type? Would I be able to get away with just one and some diodes to isolate each SSR system from each other?

Any and all suggestion, or pointing out of a flaw is more than welcome. Thanks everyone.

 

[SilverZero]

If you do the BrewPi route, I'd use one of the SainSmart relay modules (or similar). They're easy to set up and FuzzeWuzze has a great write-up on setting it all up. I did mine so it controls an outlet box and I just plug my chest freezer into it directly, unmodified. That's no worse than plugging it into the wall like normal. BrewPi also has refractory periods built in so it doesn't just fire straight to cooling or heating back and forth, there's a delay between cycles.

I don't see any need for a fail-safe system. IMHO it sounds like 50 times more effort or worry than necessary. I use a small chest freezer with a Lasko personal heater in it, and it's really only on for a few minutes at a time unless I'm crash-cooling or cold conditioning. Even then, it's a freezer. It's not going to run full-time unless there's a leak in the refrigerant system, and even then your only real danger is a bit of a bump in your electric bill.

 

[ehpower2]

I used a mcr on my unit tied to the outputs only. The key is you use a form of feed back so you know when each output is on for sure. For example on motor starters in the industrial world the motor starter has a set of aux contacts that i use that provides feed back to the controller. The MCR job which stands for master control relay is a special unit that will kill the power to all the outputs incase a fault develops. For example if the heatinng element is going and your ssr sticks on even though that ssr is stuck on the MCR will just cut power to everything so it will not matter.

 

[pocketmon]

Well, whatever can happen will happen.

Keeping the fridge, or the compressor, running won't burn anything(but freeze), so what you are worried should be the heating element. Fortunately, most heating devices have built-in protection mechanism which will be trigger when the temperature is too high. Choose your heating element wisely, and you will be fine.

I already change all my BrewPi to use ESP8266. The sketch that I am using can invoke a user specified URL in designated time period. I've also developed a simple server monitor script that can send mail when it is not updated for a long time.
That won't solve any problem but just give notice. When things go wrong, somebody has to handle it. At least, I can get a notice.

Below is my experience that you can skip, or learn a lesson from them.
My first brew of my first DIY BrewPi build failed at the fourth day when I was 300 mils from home, in Yosemite. The cause might be my bad hardware connection, but I don't know how to fix it but just add a watchdog timer to restart the system when it hangs.
My first brew of my second DIY BrewPi made of an Arduino Nano failed when I was in Vegas. I didn't notice that the Arduino Nano has a old version of boot loader which does not restart for watchdog timeout.
The Arduino BrewPi setups work pretty good with watchdog timer enabled, even with unstable temperature sensor connections. My temperature sensors seem to detach from time to time, but they survive through the failure.

 

[FuzzeWuzze]

As others said, just use the Sainsmart relays. They are rated for like 100,000 cycles. That is literally your fridge being turned on and off every 30 minutes for over 6 years for a relay PCB that costs like $5-6 for TWO relays. Honestly your fridge/freezer compressor or circuitry is more likely to go as fast or faster, as many i think still use mechanical relays you can hear click on when the compressor starts.

We arent switching high enough current to warrant a SSR, unless your using like an old 50's-70's chest freezer/fridge that may spike at a high current when its turned on.

Worst case the mechanical relay gets stuck on, and you freeze your beer. I suppose if it got stuck heating you could be in for problems, but thats why i use the Lasko MyHeat, because it has its own built in shutoff at like 110F or if it tips over, so i dont have to worry leaving the house for vacation,etc with beer in the chamber.

 

[ame]

I am using a 35W heater tape. If it gets stuck on it will raise my beer by about 1 degree C per hour. This is slow enough to detect and fix it, and not too much temperature change to really ruin the beer, even if I am out of the house all day.

I am using my Fuscus controller software (a clone of BrewPi). I could turn on the Pi watchdog timer, but I have found the Pi to be reliable over several months now.

 

[Cthorbr]

Thank you everyone for the replies. I'm not sure why I missed/didn't think of a heater with its own safety system. That essential relieves all my worries. A frozen batch would suck for sure. But only for the fact that it's frozen. Thanks again! I'm looking forward to starting this project.

 

[Thorrak]

Thank you everyone for the replies. I'm not sure why I missed/didn't think of a heater with its own safety system. That essential relieves all my worries. A frozen batch would suck for sure. But only for the fact that it's frozen. Thanks again! I'm looking forward to starting this project.

If you're using a kegerator or freezer, there is the possibility of using the built in thermostat as a backup as well. Just set it just above the freezing point, and the worst you'd have is cold crashed beer.

 

[day_trippr]

Yup, that will totally work for a fridge compartment thermostat.
But I wouldn't trust it for most standalone freezers as the thermostat scope is usually from well below 32°F to well below 0°F (hence, "deep freeze").

However, the typical bulb/capillary type mechanical thermostat on most freezers can be tweaked to shift the range upwards (indeed there are advocates of that in place of using external controllers) which could then provide a low end safety limit.

As for the whole fail-safe thing, actual need not withstanding, and price be damned, one could use a pair of single stage controllers (eg: MH1210, WH7016G, or even half each of a pair of stc-1k units) and daisy chain the switched AC through them.

Set the one on the Cool side in Heat mode with a Set Point somewhere above your warmest temperature, set the one on the Heat side in Cool mode with a set point comfortably below your coldest temperature, and co-locate their probes with the BrewPi probe. They'll keep a runaway from going critical...

Cheers! (where there's a will there's a way )