Magnetic Stir Bar

Bruce Osgood · Oct 27, 2010

I bought a magnetic stirrer through EBay and it came with a 1 inch stir bar. Trying it out in a 4 quart glass Pyrex container it has fits and starts and rolls off to the edge of the pot and just lays there.

Trying it in an 8 Oz. plastic measuring cup it spins like a cyclone.

The question is: Is a one inch stir bar too small or a 4 quart container too large? The diameter of the plate is 5-1/2 by 5-1/2 inches.

Thank you,

keithwms · Oct 27, 2010

The 4 quart container has a thicker base? The guiding field is typically only strong enough for 1/8 to 1/4 inch or so.

Photo Engineer · Oct 27, 2010

I have no problem with 1 Liter (~1 quart) glass containers, but cannot get much over 300 - 700 rpm depending on the solution (density and viscosity). If the bottom of the container is arched upwards as some are, the stirrer cannot maintain its center and it will move to the side. This in addition to what Keith says above.

PE

keithwms · Oct 27, 2010

Yep, the shape of the bottom plays a big role.

Bruce Osgood · Oct 27, 2010

The 4 qt does have a thicker base. It is 1/2 of a double boiler and the bottom is slightly convexed.

I just tried 1 Ltr of water in a glass coffee pot and the problem seems to lay with the container, not the stir bar. In the coffee pot it works as I think it should. It spins on center at level 1 and develops the vortex about level 5-6 and builds to level 8 and the next level is Max.

Thanks

Photo Engineer · Oct 27, 2010

Bruce;

Stir bars are sold in a variety of lengths to suit different containers. You may want to use a 3" bar for the larger containers. They are not all that expensive. Wards Scientific sells a variety of these handy little objects.

PE

glbeas · Oct 27, 2010

When they get spastic like that you just need to slow the speed till the bar can keep up with the magnetic field.

Dan Henderson · Oct 28, 2010

Photo Engineer said:
Bruce;

Stir bars are sold in a variety of lengths to suit different containers. You may want to use a 3" bar for the larger containers. They are not all that expensive. Wards Scientific sells a variety of these handy little objects.

PE

My problem is that some of the chemical settles around the bottom edge of the graduate and I have to stir it manually to get it into the vortex. I emailed the manufacturer of the stirrer asking if a longer flea would solve the problem. They replied that I should only use the size (1") that came with the stirrer. I inferred they meant that a longer flea would overtax the stirrer, but that does not make sense to me. The magnetic field is going to go around regardless of whether nothing, the 1" flea, or a longer one is being affected by it, right? And there is no reason to think that a longer one would damage the stirrer?

eclarke · Oct 28, 2010

I use a 4 liter beaker with my stirrer but it has a perfectly flat bottom, if there's any dome, none of the stir bars will stay in the middle. If there's an American Science and Surplus store near you, they sometimes have 4 liter beakers for a good price. I bought mine from the local lab supply but it was $75.00 but also one of my favorite darkroom possessions...EC

Photo Engineer · Oct 28, 2010

There are limits to the size of the bar that should be used. Only the Mfgr. can say for sure and that would be in the instrucions. Generally though, a 1" bar is much too short for that size top.

PE

zesbaugh · Oct 28, 2010

Dan Henderson said:
My problem is that some of the chemical settles around the bottom edge of the graduate and I have to stir it manually to get it into the vortex.

I avoid this issue by sliding the container around on the plate, the stir bar will remain at center of plate no matter where you move the container. Move the vortex to the chemicals instead of the chemicals to the vortex.

Speaking of vortex, should these be avoided? With a deep vortex you expose more surface area to oxygen, but is it enough to significantly impact shelf life of chems?

Photo Engineer · Oct 28, 2010

A vortex is mixing oxygen into the solution and can lead to oxidation of sensitive chemicals. A vortex should be avoided.

PE

Dan Henderson · Oct 28, 2010

I will never cease to be amazed at the information that can be learned at APUG, by asking questions and sometimes by just being quiet and reading. Thank you both.

Mike Wilde · Oct 29, 2010

Take the hood off, and take a look, The size of the magnet assembly under the hood will tell you how far apart the drive magnets are. The stir bar in mine is 3" and the under the hood part is about 2.5" long, so a little longer seems to be ok.

Neil Poulsen · Oct 30, 2010

You can get bigger. I have about a 2" for my stirrer that works well. I also have a 4" rod that's about a half inch in diameter. That's a little big.

You can get these at a scientific supply outlet. I use American Scientific in Portland, and they do a great job. Their prices are reasonable.

Bruce Osgood · Oct 30, 2010

This has been a very productive thread for me and I would like to thank all who replied.

mts · Oct 31, 2010

When the chemical components accumulate near the bottom edge of the beaker I use a glass stirring rod to help them along. With a little assistance everything goes into solution.

Dan Henderson · Oct 31, 2010

Bruce Osgood said:
This has been a very productive thread for me and I would like to thank all who replied.

Me too! I am one of those "too much is never enough" kind of guys, so I always cranked my little magnetic stirrer to 100%. So I was especially interested to learn that even though it looks cool, the vortex is to be avoided.

glbeas · Oct 31, 2010

You can also get a cross shaped stir bar which seems to be able to move more liquid while stirring, working a bit like a turbine blade.

craigclu · Oct 31, 2010

If you need to buy stir bars, you may want to consider getting a variety pack. I recall some years ago that a very generous pack of many sizes was not much different than 2 or 3 singles. I believe I used VWR at the time but I think they've gotten harder to deal with as an individual.

Mike Wilde · Nov 1, 2010

Then there is a DIY approach for any sized bit of ferrous material.

There is a product available at electrical trade wholesalers for re-insulating tool handles whose rubber/plastic has become damamged.

From time to time I have seen it, but am not sure of its trade name.

You clean item x, dip it, and then wait for the carrier to evaporate away, I believe, leaving a tough rubber coating behind.

Photo Engineer · Nov 1, 2010

We used to seal an iron bar in glass tubing and we could make any length stir bar we wished. Yes, they were fragile, and yes the seal had to be good. With practice with a torch, you can do it.

PE

Dan Henderson · Nov 1, 2010

Mike Wilde said:
Then there is a DIY approach for any sized bit of ferrous material.

There is a product available at electrical trade wholesalers for re-insulating tool handles whose rubber/plastic has become damamged.

From time to time I have seen it, but am not sure of its trade name.

You clean item x, dip it, and then wait for the carrier to evaporate away, I believe, leaving a tough rubber coating behind.

I think I might have a container of that stuff in the basement. I was also wondering why heat-shrink plastic tubing would not work well. Not as elegant as PE's solution, but less fragile. Which, according to my girlfriend, is a good thing where I am concerned.

Photo Engineer · Nov 1, 2010

Heat shrink tubing can leak at the ends unless sealed with epoxy.

Plastic tubing can work too if heat sealed or sealed with epoxy.

Some epoxies can react with some photographic solutions.

PE

Wade D · Nov 2, 2010

Mike Wilde said:
Then there is a DIY approach for any sized bit of ferrous material.

There is a product available at electrical trade wholesalers for re-insulating tool handles whose rubber/plastic has become damamged.

From time to time I have seen it, but am not sure of its trade name.

You clean item x, dip it, and then wait for the carrier to evaporate away, I believe, leaving a tough rubber coating behind.

That would be the best method. Getting an even coating is the most important thing and a 2x coating will insure no thin spots. I don't know the trade name either but it can be found at Home Depot and other hardware stores for very little expense.

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