So this is what I'm using.
Got it off fleabay for $78 shipped.
I'd thought about building a stirrer out of a computer fan but really wanted a heated one, and just didn't want to cough up $300 for one. When I say various versions of this one on amazon and eBay this made sense.
I liked the notion of it having a temp probe. There's a lot of hotplate stirrers out there, but you have to watch them with a thermometer, which precludes doing other stuff.
The manual for this was the absolute worst bit of auto translated from Chinese I've ever encountered. It took me hours of scouring the web to find a manual that was a little closer to comprehensible but it was pretty bad as well.
The problem I had out of the gate is it was horribly overshooting the set temp. It took a good bit of fiddling with the settings under the hood to figure out how to get it to not overshoot so much, and once I figured out what the various settings actually were I managed to get it to where it works pretty well on the temp side of things. I did have some challenges getting the temp probe into my boston rounds, as the bar they supply is a tad short of getting the probe to the absolute center of the plate.
It will retain your previous temp setpoint between uses, even when unplugged, which is nice.
It also has a timer function, so you can set it to hold temp for x minutes, and when the timer expires it will stop heating and drop to ambient. That's nifty.
As for the stirring bit, I've had no problems stirring right in the bottles. I'm using the two ounce VivaPlex Cobalt Bottles with the non child resistant eye droppers, and while they aren't completely flat, the stir bars work just fine in them.
I've gone to mixing with the caps on. After several batches mixed caps off I couldn't help but notice the flavors were all muted, and being as I don't have many flavors that are alcohol based, I'm guessing that they're probably all going up in thin air. Maybe I'm wrong and am all ears as to pro's of mixing caps off. I've read lots of breathing posts on this an other forums, but every time I've let em breath they've come out tasteless.
That brought another challenge in the temp probe has to be in in the fluid to tell the hot plate what to do, so I just got a 600 ml pyrex flat bottom beaker, put water in the that and put the temp probe in the beaker, then submerge the mix bottle with the stir rod in it in the water in the beaker. I put a couple hundred ml of water in the beaker and it's sufficient to transfer the head into the mix bottle without the bottle floating or moving around in the beaker, even with small 10 ml test batches.
I've got a second digital thermo that I use to calibrate the temp probe (it was off by several degrees from the factory, but there is a setting where you can adjust the offset from the real temp) and also to check the delta between the water bath temp and the mix fluid temp. I've found that the mix fluid temp will generally be about 4C less than the water bath temp.
My process now is to mix the PG/VG/Flavors by scale in the bottle, drop in the stir bar and cap it, put it in the bath and start the stirrer. Until it heats up a little you really can't crank up the speed, but once it gets heated up a bit I can crank it up. Once it reaches hold temp (I set the bath temp to 45C) you may have to drop the speed a bit as it will continue to speed up as it becomes less viscous, and if you don't keep an eye on it it can loose the magnetic connection to the stir bar. After I'd done several batches in it I've got a pretty good feel for what I can park it at and not have to worry about coming back later and finding it's decoupled from the motor.
When it's stirred, I pull the bottle out, run it under cold tap water to cool it down, put it back on the scale and add the nic, then put it back in the bath to stir in the nic for a couple minutes. My thoughts on that are since I've cooled it back down, in the few minutes it's' back in the bath it isn't going to heat up all that much, and I only run the bath at 45C anyway, and the max temp of the fluid in the bottle runs about 41C, so once the nic is added it might get to 32C for the few minutes it's on the stirrer.
Once that's done, I rinse the bottle in cold tap again to cool it back down, take off the mix cap, pull the stir bar, put the dropper in and put it in the cupboard.
A few questions for folks who've gone the stirrer route..
How fast do you run em? As fast as possible? Is the general idea to get as many air bubbles in the mix as you can?
How long should I be leaving it on? My first few mixes I ran it for half an hour. I've done a mess of single flavor test batches where I've only ran it for as long as it took to weigh up the next one.
Is there a general consensus as to how long to heat for and how long to stir for?
Any input would be welcomed.
And if anybody gets one of these and needs help figuring out how to get it to not overshoot temp wise and get the probe calibrated, I'd be happy to walk you through it. Took me hours to get it dialed in the first time because I had to figure out by inspection what each of the various settings actually did, as they aren't labeled and the manual is useless.