Hi Guys, just a few updates...
Firstly I'd like to thank everyone again for all their interest and willingness to help out testing the stirplates. We've had a few teething problems with the fan mounting and I'll be very interested to hear if anyone else has the same troubles.
A couple of stirplates haven't shipped yet as I'm still waiting on their payments. I'll send another PM to these guys this afternoon.
Recently I mentioned I would discuss the topic of throwing the stirbar. This occurrs at high speeds when the stirbar succumbs to the forces of friction and chaos and the magnetic pull from the magnets in the stirplate isn't strong enough to hold it in place. At high speeds the stirbar can come "unstuck" at which point it will stop stirring and just bounce around in your wort (this is where stirbars get their nickname - "fleas"). There is no automatic recovery from this scenario. Once it's bouncing around it won't start stirring again until you manually reset it. To do this you have to stop the stirplate, reposition the stirbar so it's held back in place by the magnets, then gradually ramp up the speed again. This can take a bit of a knack if you've already pitched your yeast and can't see the stirbar inside your flask.
As you can imagine, if you're asleep or away from your stirplate when the stirbar is thrown, you might come back hours later to find that it hasn't been stirring the whole time. Not. happy. jan.
Anyhow, the trick to reliable stirring is to choose a safe, moderate speed.
If you stir too slow, you risk the motor staling if for some reason it encounters more friction.
If you stir too fast, you risk throwing the stirbar and it has also been suggested that this may stress the yeast. There is a very wide envelope of speeds that are acceptable and reliable and I usually just aim for a medium speed where there is about an inch of vortex in the surface of the wort.
After choosing a speed, it pays to make sure you're safe by moving your flask around and even lifting it a few milimeters from the plate while it's running. If it is still stirring after this test it's very unlikely that it will stop working while it's unattended.
So why build stirplates that can spin so fast?
The speed of the stirplate is relative to both the characteristics of the fan motor and the voltage it is run at. By changing one of the resistors in the regulator circuit it would be possible to limit the maximum voltage supplied to the fan, restricting its speed to make it harder to throw the stirbar. However there may be some cases where a bit of extra power is required, especially if larger stirbars are used. Knowing the information above, I'm confident that any user can select an appropriate speed for highly reliable stirring.
There is also another reason why the stirplate speed is left unrestricted and I'm open to speculation about this one. I found that when using a smaller flask, it is possible to stir it so fast that the vortex in the surface of the wort can reach all the way down to the stirbar at the bottom of the flask, churning air into the wort. I found this useful as a sanitary way of aerating the wort before pitching yeast.
So it goes like this:
1. Add water and malt extract to your flask.
2. Bring it to a boil in the microwave to sanitise.
3. Cool it to desired temperature in an ice bath.
4. Drop in sanitised stirbar and aerate with stirplate at high speed.
5. Slow down the stirplate to a safe speed then pitch yeast.
I've used this method a few times for reactivating yeast cultures, however it may be overkill.
Keep the feedback coming guys,
Cheers,
Mick.
