Chemgineer
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Magnettic stirring is surprising
My heating mantle has a magnetic stirrer which I always thought was a little weak and when I started it I would just turn it on slowly and the stirbar
would often decouple and cascade around the flask.
Well I've just realised that magnetic coupling increases with velocity/acceleration so if I just whack it on faster it actually couples more strongly!
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jackchem2001
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Interesting. In my experience using round bottom flasks higher speeds may decouple overtime, especially if solids are present. I generally turn up
stiring to a moderate speed (maybe this is the effect you describe?), then turn down to find the minimum that consistently stirs and go slightly above
that.
I am not sure how much mixing benefit there is to faster stiring speeds at lab scales anyway. I would speculate that (especially for viscous liquids)
a bigger stir bar would do you better than faster speeds. Or for jointed glassware, an empty thermometer well could be used as a baffle.
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MrDoctor
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is it possible that instead of a high temp resistant magnet that instead it has been constructed with an electromagnet that is in series or parallel
with the motor, receiving the same (albeit rectified) regulated power?
i have never observed what you describe, any kind of change in the coupling strength with speed. At most there is, where a particularly thick/heavy
solution once spinning produces far less resistance compared to when stationary, but im sure you dont mean that
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Chemgineer
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Quote: Originally posted by MrDoctor  | is it possible that instead of a high temp resistant magnet that instead it has been constructed with an electromagnet that is in series or parallel
with the motor, receiving the same (albeit rectified) regulated power?
i have never observed what you describe, any kind of change in the coupling strength with speed. At most there is, where a particularly thick/heavy
solution once spinning produces far less resistance compared to when stationary, but im sure you dont mean that |
I suspect you could be right! Just tested on 500ml nearly full flask of ethanol and I can get it stirring right up to top speed but then when I slow
it down at some point it decouples!
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Twospoons
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You are probably seeing two effects:
1] at faster speeds there will be a thicker film of liquid between the stirbar and the flask, reducing friction between the stirbar and flask
2] at faster speeds the viscous drag on the stirbar will result in an increased angular lag behind driving magnet in the hotplate, which in turn
shifts the magnetic force vector on the stirbar to be more sideways and less down, so once again less friction against the flask.
Helicopter: "helico" -> spiral, "pter" -> with wings
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