flasks and stir bar problems

[butisitart]

hi all, i've got a 3L erlenmeyer, and have problems with getting a stir rod to centre when spinning. can't see where anybody else seems to have a prob, but...
the flask (bought from a brewshop) is slightly concave (well, it seems flat across 90% of the base, then goes down to a slight gutter around the edge) at the base, so my 40mm rods slide into the 'gutter' at the edge of the base and just sit there and rattle.
so, are erlenmeyers supposed to have a flat base?? (in which case i've been sold a lolly)
or, is there some optimum speed where they should stay in the centre??
or is 40mm not a good fit with a 3L flask??
tried it in flat bottom jug and a glass beer mug. works fine in both but they are only 1L ea.
the concave guttering is the bit that worries me.

 

[MHB]

If its a home made stirrer, sounds like the magnets might both have the same pole up.
You need a North up and a South up opposite it, ideally about as far apart as the bar is long.
You can check easily enough, just put the bar on the magnets it should be pulling down on both ends, if not you might need to peel one magnet off and turn it over.
Pivoted (with ring) or egg shaped (olive) bars help to.

Like #2 and #3, much better than #5
Mark

 

[butisitart]

it's a brewshop bought stir plate model '78-1 magnetism heating mixer'.
looks like surplus from the russian military c1974.
works fine with 1L flat based jars, which are too small for what i want. i want to do 2L so i can drop 1L ea into 2 fermenters.
#3 above is the shape, and #2 and 7 both look good.
wondering if it's either the rod length or the very slight gutter that's the prob

 

[MHB]

If you use one end of the stir bar, you should be able to find the ends of the stirrer magnet, if they are too close together they could be the problem, 25mm is a pretty typical stir bar length for small heated units.
Mind you if the same bar is working in a 1L flask, it should work in the 3L flask to. If it isn't it might be the flask Erlenmeyer flasks usually are flat.
Mark

 

[butisitart]

thanks mark, i've got a 1L jug that i've just put on there. whirlpooling fine. tried the stir magnet thing before, but now it's in the drink. it gravitated straight to the centre of the plate and started spinning (when i switched the plate on), but couldn't sort out the ends of the magnet. try again later.
leaning to the flask as the problem here, and i would have assumed that all erlenmeyers are flat, partly for that reason. your bar skids off centre and never recuperates. maybe my brewshop (unknowingly, i would think), was selling imitations, although it heats and chills stove to cold water no problems.

 

[kadmium]

thanks mark, i've got a 1L jug that i've just put on there. whirlpooling fine. tried the stir magnet thing before, but now it's in the drink. it gravitated straight to the centre of the plate and started spinning (when i switched the plate on), but couldn't sort out the ends of the magnet. try again later.
leaning to the flask as the problem here, and i would have assumed that all erlenmeyers are flat, partly for that reason. your bar skids off centre and never recuperates. maybe my brewshop (unknowingly, i would think), was selling imitations, although it heats and chills stove to cold water no problems.

It's more to do with magnetism. If its a 40mm bar it will be too big for the stir plate. 1L will work because there is not as much resistance as 3L.

Also, the shape of the flask being very wide at the bottom increases resistance and decreases depth.

I would go a smaller bar, start on low speed and slowly work up.

Once the bar is thrown, it will get "pushed" to the side of the flask and held there by magnetism. If you slow the stirrer down the bar will start to wobble and then re-center.

I reckon bar is too big.

 

[Yes Dear]

I have had similar problems. I now start the bar at a slow speed and gradually increase the speed until a centre whirlpool. If the speed get to large the bar moves off centre and ends up in the gutter. I have made 4 starters i the last month and this process while fiddly has go them up and running.

 

[Coalminer]

I use a 30mm x 7mm cylinder shape stir bar on a KLand stir plate in all sizes of flasks from 250ml to 5 litre flasks with no problem.
The only time there is a problem is if you let the vortex reach the bottom of the flask, this will throw the bar off balance.
No need for a vigorous stir. All flasks are Schott Duran brand apart from the 5 litre which is a KLand one.
None of the Schott Duran ones are perfectly flat but are almost flat. The KLand one is flat in the centre only.
Yes definitely start slow and build up speed slowly

 

[butisitart]

thanks so much guys/ladies if any,
it's counter-intuitive but i think the 40mm is the problem, or certainly a major part if not entirely (spin speed may be another factor).
i've got a 25mm that i threw into the erlenmeyer with 1L and 3L water to see. stuck to the middle on highest spin with a huge vortex and didn't look like moving. (you can see things in water). the 40mm is at the bottom of a starter at the moment, so i can't run comparatives yet.
counter intuitive cos i would have thought that a longer rod would have been more effective at churning through that much starter.
only variable now is the difference on the 25mm in starter rather than water, but it looks the goods.
owe you all one of my finest when next you're in brissie - i was about to go on the hunt for a flat bottomed erlenmeyer, which is starting to sound like they might not exist anyway.
thanks for pointing me in the right direction (magnetic pun - bad dad joke). might drag it out on my daughters for fathers day

 

[kadmium]

Easy way to get your 40mm magnet out of the starter is to use your other stir bar to grab it. You can then slide it up the side of the flask and grab it from the neck.

Then sanitise your 25mm and drop it in. Start the stirplate slow and it will center itself.

Will be a 4minute job and you will have a better stir. As said before, no need for a super vortex. Good movement is all that's really needed!

The reason is because the 40mm bar will be longer than the magnetic stirplate. So it can't grab it by the "ends" of the bar. So its holding on loosely as opposed to gripping the 25mm bar.

Once it becomes unbalanced, the North and South of the stirplate magnets then fight the stir bar and flick it off, like when you try to hold two magnets with the same poles together.

Once it's off, the magnetic stirplate is spinning so fast the little stirbar can't grab on cause its going "North South North South North South" too fast. Once you slow the stirrer, the magnet starts to wobble before aligning and sticking itself in.

Thats my understanding, im sure if I'm wrong someone will correct me. Cause, magnets. How do they work?

 

[butisitart]

thanks kadmium,
i wasn't on here, so didn't see your post. (keep it for reference though)
what i did do was another 500ml LDME into the erlenmeyer, and since that was sterile, tipped the 1L jug in (which had 500ml starter), with the 25mm rod.
now it's working a treat, whirlpooling nicely. i'm growing a 4 year old refrigerated unused 25% of a wyeast kolsch pack, so incrementing up, and now 1L. it's experimental, but smells fine, looks normal - will see if it can deal with a fermenter on monday. the only sad thinig about yeast is that we have no way of measuring its efficacy.
but yes, everything you're saying makes sense. appreciate
thanks to all for really good info
i have the science acumen of a lobotomised frog

 

[Andy Graham]

hi all, i've got a 3L erlenmeyer, and have problems with getting a stir rod to centre when spinning. can't see where anybody else seems to have a prob, but...
the flask (bought from a brewshop) is slightly concave (well, it seems flat across 90% of the base, then goes down to a slight gutter around the edge) at the base, so my 40mm rods slide into the 'gutter' at the edge of the base and just sit there and rattle.
so, are erlenmeyers supposed to have a flat base?? (in which case i've been sold a lolly)
or, is there some optimum speed where they should stay in the centre??
or is 40mm not a good fit with a 3L flask??
tried it in flat bottom jug and a glass beer mug. works fine in both but they are only 1L ea.
the concave guttering is the bit that worries me.

I purchased one of those flasks a couple of years ago. The stir bar worked well initially. After sterilising wort starter on gas burner a few times in the flask, the base became concave. This resulted in magnetic bar refused to centralise at any speed , as the bar stayed in the trough around the edge. Broke it yesterday, so ordered a new one. I won't boil wort in flask again, that way the base will stay flat ( flattish anyway )Hope this helps.

 

[butisitart]

I purchased one of those flasks a couple of years ago. The stir bar worked well initially. After sterilising wort starter on gas burner a few times in the flask, the base became concave. This resulted in magnetic bar refused to centralise at any speed , as the bar stayed in the trough around the edge. Broke it yesterday, so ordered a new one. I won't boil wort in flask again, that way the base will stay flat ( flattish anyway )Hope this helps.

as above, i have the science acumen of a lobotomised frog, BUT
are you sure the flask became concave from repeated heating? i wouldn't have thought glass could do that.
i thought mine was flat until i looked at it carefully about 2 weeks ago, but having said that, the 25mm bar that i threw in last night went 15 hours high speed vortex, didn't budge off centre. i suppose the next question there is, if your bar refused to centralise, is it up around the 40mm length?? cos that seems to have been my problem.

 

[kadmium]

Considering it's boroscilicate and the melting point of glass is 600c plus, not saying it's impossible but I would doubt very much that you heated it enough to alter it's shape. I agree with butitsart, that it was probably always concave. Kind of like the mandela effect

 

[Stuart99]

Not sure about borosilicate but ordinary glass behaves like a fluid. If you set a sheet of plate glass on edge and leave it there, and come back and measure the thickness the bottom will be thicker than the top. Flip it over and the glass will slowly flow back to the bottom.

 

[kadmium]

Not sure about borosilicate but ordinary glass behaves like a fluid. If you set a sheet of plate glass on edge and leave it there, and come back and measure the thickness the bottom will be thicker than the top. Flip it over and the glass will slowly flow back to the bottom.

That's not true. It's an old wives tale that gets perpetually passed on because no one bothers to check it. Extract from an article debunking the myth as nonsense

"In fact, ancient Egyptian vessels have none of this sagging, says Robert Brill, an antique glass researcher at the Corning Museum of Glass in Corning, N.Y. Furthermore, cathedral glass should not flow because it is hundreds of degrees below its glass-transition temperature, Ediger adds. A mathematical model shows it would take longer than the universe has existed for room temperature cathedral glass to rearrange itself to appear melted. "

and also

"The team’s calculations show that the medieval glass maximally flows just ~1 nm over the course of one billion years. That’s just 0.000000001 nm per year—which, although is theoretically measurable, would be practically impossible to achieve."


found here:
https://www.scientificamerican.com/article/fact-fiction-glass-liquid/
and here:
https://www.cmog.org/article/does-glass-flow
and here:
https://ceramics.org/ceramic-tech-t... students, I,actually flowing, ever so slowly.

 

[butisitart]

so i won't be peeking over the top of my neighbour's sagging opaque glass bathroom window any time soon

 

[kadmium]

so i won't be peeking over the top of my neighbour's sagging opaque glass bathroom window any time soon

By the time you're ready, the sun will have burned out, the planet frozen over and sucked into a black hole. By then, the glass won't be the only thing sagging...

 

[Andy Graham]

as above, i have the science acumen of a lobotomised frog, BUT
are you sure the flask became concave from repeated heating? i wouldn't have thought glass could do that.
i thought mine was flat until i looked at it carefully about 2 weeks ago, but having said that, the 25mm bar that i threw in last night went 15 hours high speed vortex, didn't budge off centre. i suppose the next question there is, if your bar refused to centralise, is it up around the 40mm length?? cos that seems to have been my problem.

No, 25mm bar. When I first used the flask I checked base with straight edge and was quite flat, not perfect but worked well. Last week I checked again because stirrer would not centralise and the concave was 2 -3 mm. Problem solved when it fell on brewery floor, no longer concave base. I improved the stir plate spinning magnet as well
. The electro magnet was roughly 8 mm from underside of stir plate. Seemed too far to me to work well. Removed base of stir plate box and gently bent motor drive bracket until spinning magnet is around 3 - 4 mm from underside of stir plate. Spins stir bar spins easily, even on low setting.