stir plate questions.

I just got my stir plate done. I'm wondering how much of a vortex do I need in the starter. I don't have a flask yet so I have been testing it with a Qt mason jar. I will show a few pictures of the mason jar please tell me witch one you guys suggest.

Here is the first picture at 3 volts.

https://docs.google.com/open?id=0BybE14TdBD0JX2JMMzI1X0thZjg

here is the next picture at 4.5 volts

https://docs.google.com/open?id=0BybE14TdBD0JdlFYNkkzVExJOVU

This one at 6 volts
https://docs.google.com/open?id=0BybE14TdBD0JT3NyaHBfbktoVjQ

Or this one at a full 12 volts

https://docs.google.com/open?id=0BybE14TdBD0JTHFFOWF5QlBVblk

Sorry I couldn't get the pictures directly on the post.

Thanks,
Marcus

Just a small dimple on the surface of the liquid is all you need. That will provide adequate circulation to keep your yeast in suspension. So I guess I would go with number one, but to my knowledge it shouldn't matter how big the vortex is as long as the yeast stays in suspension.

The 6 volts is plenty, but why not wire in a potentiometer to adjust it as needed? Also, starters will act differently than pure water, so you will want some adjustment. I would actually disagree with Nuke that a small dimple is sufficient. A big benefit of using a stir plate is facilitate gas-oxygen exchange (getting CO2 out and oxygen in) as yeast need oxygen to multiply. So simple stirring to keep the yeast in suspension is not enough.

Thanks guys, I was going to wire in a potentiometer, I even wired one in yesterday but it did not seem to go low enough so I'm going to use the switch on my universal AC adapter. I will play with it once I get the proper equipment and get some real wort in there.

Marcus

A starter is a higher gravity liquid than straight water, so you will get less of a vortex with that than with straight water.

I have tinkered a lot with electronics so the idea of just using a carbon potentiometer to vary speed on a stir plate raises an eyebrow here, but more speed won't hurt.

I'm under the impression that a deeper vortex equates to greater gas exchange but after the first several hours on the plate i doubt there is much o2 in the neck of the flask, even with just foil over the flask.

TimpanogosSlim said:

I'm under the impression that a deeper vortex equates to greater gas exchange but after the first several hours on the plate i doubt there is much o2 in the neck of the flask, even with just foil over the flask.

Click to expand...

That's correct. I think of stirring as increasing the rate of gas exchange in two ways: 1) it increases the surface area for gas exchange, since a vortex will produce a concave surface with a greater surface area than a flat surface; and 2) it facilitates gas exchange by increasing the concentration gradient across the water-air interface. Since gas exchange occurs across a very thin layer at the water/air interface, the concentration of oxygen at that interface in a starter that isn't continuously mixed will ultimately depend on diffusion; the oxygen in the thin surface layer will become saturated faster than a mixed starter, and additional oxygen will only enter the liquid as the oxygen already in the layer diffuses out of the layer and into the rest of the starter. Mixing will homogenize the liquid, thereby decreasing the oxygen concentration in that thin layer, and increasing the concentration gradient of oxygen between the air and starter, thus facilitating diffusion of O2 in (and CO2 out). In reality, stirring really just minimizes the thickness of that layer, since there will always be a thin skin of starter liquid that is saturated with respect to oxygen. Hope this makes sense - I've had a few homebrew =)