Problems with Hot Plate Stirrer

Timothy · May 7, 2014

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 !

Ken Nadvornick · May 7, 2014

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

Bruce Osgood · May 7, 2014

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.

Photo Engineer · May 7, 2014

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

RattyMouse · May 8, 2014

Photo Engineer said:
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.

jochen · May 8, 2014

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.

JW PHOTO · May 8, 2014

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

Photo Engineer · May 8, 2014

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

Timothy · May 8, 2014

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.

jochen · May 9, 2014

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.

edcculus · May 9, 2014

I've only ever used a stir plate to make yeast starters for brewing, but everyone touched on the big points. Make sure you have an actual flat bottomed beaker or flask designed to be used in this situation. In my experience, you have to start out slow, and can't just crank up the speed once it starts going or else it will throw the bar. Get it going at a slow speed, but give it 30 or more seconds to get all of the liquid swirling at the same speed as well. bump up the speed again and give it a little more time.

Also, depending on what youre mixing, do you really need an extremely fast speed? sometimes just a good ripple at the top is enough for good mixing.

JW PHOTO · May 9, 2014

I keep my speed fairly low when mixing developers for one simple reason "Oxygen Induction". If I kick the speed up and get that giant, whirling vortex tornado I'm pretty sure I'm introducing more oxygen into the mix than I really want, which is none. I keep it just fast enough to mix the ingredients and just long enough to get that job done. JW

cmacd123 · May 10, 2014

JW PHOTO said:
I keep my speed fairly low when mixing developers for one simple reason "Oxygen Induction". If I kick the speed up and get that giant, whirling vortex tornado I'm pretty sure I'm introducing more oxygen into the mix than I really want,

Indeed, I have a fairly cheep hobby mixer and if the Vortex gets going you can HEAR the bubbling sound when the tip of the vortex reaches the bottom of the container (this with plain developers like D-76)

The Mix bars that come with mine have an small o-ring in the middle, that may help with uneven containers, I normally use a rounded bottom Paterson Graduate, or a big Automotive type graduate.

Gerald C Koch · Dec 29, 2016

Photo Engineer said:
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

+1

Exactly my experience. Bring up the speed slowly. If it's too fast you will get a hint before the magnet starts to dance, then slow down a bit.

GRHazelton · Dec 30, 2016

I have a KitchenAid immersion "blender" which has a whisk attachment, in addition to the little shielded chopper thing. I'll have to try these in plain water to see if they can run slow enough to do the job without introducing air. Just gotta wash thoroughly after using....

Gerald C Koch · Dec 31, 2016

GRHazelton said:
I have a KitchenAid immersion "blender" which has a whisk attachment, in addition to the little shielded chopper thing. I'll have to try these in plain water to see if they can run slow enough to do the job without introducing air. Just gotta wash thoroughly after using....

Any device that may be used for food should not be used for stirring chemicals. Also avoid any mixer that introduces air into the solution.

GRHazelton · Dec 31, 2016

Gerald C Koch said:
Any device that may be used for food should not be used for stirring chemicals. Also avoid any mixer that introduces air into the solution.

Certainly one should avoid mixing methods which introduce air. But, wouldn't stainless steel implements, spoons and the like, or glass vessels, thoroughly rinsed and run through a dishwasher be safe for food use after mixing chemicals? Wood or plastic might well be a different matter.

When my Father (PhD in ChemE from Michigan) and I mixed Beutler's formula and other "soups" we pressed into service big glass measuring cups and stainless steel spoons, etc. Everything was washed carefully in hot water and detergent before returned to the kitchen.

I recall when I took Qualitative and Quantitative Analysis years ago (back in the days of chain balances!) many of us cooled down with a 500 ml beaker filled with ice and water....Mr Bell, the grad student supervising the lab, had no problem with the practice as long as we avoided the ethyl alcohol....Just kidding.

Photo Engineer · Dec 31, 2016

Washing carefully often does not remove organic chemicals or metallic chemicals that are harmful.

PE

Problems with Hot Plate Stirrer

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