I just built a stir plate and at full speed I only get a vortex half way down a 1500ml flask. I used a 5 volt power supply. Is that enough of a vortex or should I increase my power supply? I assume a vortex to the bottom of the flask would oxygenate more.
You do not need a ton of vortex for starters. Keep in mind that they still work of you just swirl them a few times a day, so any constant vortex is better that that.
That’s more than enough. Remember, the aeration you get from a stirplate is secondary. The primary reason to use one is to keep the yeast in constant contact with the starter wort.
For growing a starter you only need the speed high enough to get a dimple rather than a full whirlpool in the container. The biggest thing you are doing is keeping O2 introduced and degassing the CO2 that is produced. The more you keep O2 introduced the more yeast will reproduce vs full on fermentation.
And don’t use an airlock, you want some O2 to get in. I use a baggy over the top.