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  1. #1
    Timothy's Avatar
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    Problems with Hot Plate Stirrer

    For mixing my PC-TEA, I recently splurged on a Corning Hot Plate Stirring machine. It heats up real fast and seems to work real slick but every time I turn on the stir function with the magnet in the beaker, it starts going around and just when it builds up speed, the magnet bounces or somehow becomes un-synchronized with the motor and just jams up against the side of the beaker vibrating. I have tried moving the beaker to the most central position but after dozens of readjustments it still does the same thing. Is there some magic to this ? I am getting so frustrated !
    Tim N. Roscoe

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  2. #2
    Ken Nadvornick's Avatar
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    Sometimes the center of the stirring bar does not coincide with the rotation point of the magnet inside. Then the bar tries to spin off-center, which only works at slower speeds before speeding up makes it go berserk. Inexpensive bars seem to have this problem more frequently. And it seems more of a problem with larger bars where more off-center mass is involved.

    I have a small bag of stirring bars, but only a couple are perfectly balanced.

    Another problem is that the bars can become decoupled from the rotating magnetic field if the solution is so viscous that it retards the spinning of the bar.

    Ken
    Last edited by Ken Nadvornick; 05-07-2014 at 07:29 PM. Click to view previous post history.
    "Hate is an adolescent term used to stop discussion with people you disagree with. You can do better than that."
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  3. #3
    Bruce Osgood's Avatar
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    Another source of the problem may be the bottom of your beaker. If it is rounded up, where the center is higher/thicker than the sides, the magnet will spin off from the center.

  4. #4
    Photo Engineer's Avatar
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    The viscosity of some TEA solutions is too high for rapid stirring. OTOH, magnetic stirrers are not designed for high speed stirring anyhow so you are probably going too fast for the conditions.

    PE

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    Quote Originally Posted by Photo Engineer View Post
    The viscosity of some TEA solutions is too high for rapid stirring. OTOH, magnetic stirrers are not designed for high speed stirring anyhow so you are probably going too fast for the conditions.

    PE
    You'd be surprised. Some of today's stir plates can reach amazing speeds. I use a stir plate all day long when formulating at the bench. IKA makes some stir plates that can mix very high viscosity oil products at tremendous speeds.

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    Hello,
    in our lab we have many of the blue stirrers from IKA. They reach up to 1200 rpm. For greater volumes and viscous liquids use longer and thicker magnets. There are many special forms on the market with oval or cross forms or in the form of dumb-bells. High viscous media are always a problem. Start at low rpm and accelerate slowly until the magnet begins to jump and then reduce the speed until it turns normally. Very comfortable if you can connect a contact-thermomter to keep the temperature constant. For boiling 5 liters of water the heating output of 650 W is a little weak.

  7. #7

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    jochen's tip is right on the money. I have a Corning, but his advice for starting slow works for most. I start my Corning out so it just barely turns and then increase the speed in small steps until I'm where I want to be. Go to fast and the magnetic rod will dance all over the place. JW

  8. #8
    Photo Engineer's Avatar
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    I am aware of all of these high speed hot plates, but knowing the Corning stirrer inside and out (I have 2 and use 1 at GEH), I know that in this price range, the units are far less "robust" than the high end units. I have a 3" stir bar that does a reasonable job at up to about 500 RPM or higher. The 1" and 2" have problems with 10% gelatin at much over 300 RPM.

    Now, we are talking the Corning unit which is the subject of the OP. Throwing in those high end industrial lab units is a curve ball to the OP.

    PE

  9. #9
    Timothy's Avatar
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    It must be the viscosity for sure. If I try starting it at all with the heat off, it will not even start. Only when the TEA gets warmed up and thinned out, will it start to spin. This unit starts spinning at 60 rpm and that is all I have ever set it to. As soon as the spinning has made a vortex in the liquid, the magnet starts bouncing. I am using a 2 1/2" octagonal bar, but I just ordered a star shaped one. Sure hope that works better.
    Thanks for the suggestions.
    Tim N. Roscoe

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  10. #10

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    Hello,
    I'm working in the lab of a lubricant manufacturer. For producing lab samples of higher viscous oils we never take a magnetic stirrer but an electrical stirrer with a little propeller and a normal electrical heating plate. This is similar to a hand drilling machine with a drill chuck. The speed is adjustable mechanically or electronically depending on the model. A DIY hand drilling machine with controllable speed will work but is very loud and the life time of this motor and gearbox is only about 50 to 100 hrs.

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