Scientific project: Rotary mortor help!

This is a really nice forum!

Ok. I am a Ph.D. researcher and I do not know much about electronics.

I need to mix water in a sealed incubation chamber (sort of canister). I have a magnetic stirrer, that is placed underneath the incubation chamber and which rotates a magnetic stirrer bar inside the chamber.

The problems I have are:
- the speed is not constant (the stirrer would speed up at time, or even stop for a couple seconds)
- the speed, even at the lowest setting is too fast.

So, I would like to make my own magnetic stirrer. (A magnetic stirrer is just a rotary motor, with a controller that rotates a magnet, the stir bar is also a magnet and is placed inside a container. Once the rotary motor spins, it spins the stir bar as well).

I have found sources to buy magnetic bars.

I would like to buy a 12 V DC rotary motor (I believe a gear head one is probably the cheapest, cheaper than a stepped motor) coupled with a controller (I believe a potentiometer would work) so that I can set the rpm between 1-60 rpm.

I do not know what I need as a potentiometer? What resistance should it have?

Any links welcome. I generally buy from jameco, but other links are welcome.

Thank you immensely.

Serge

BTW. there is that motor (probably high rpm), but the controller could be taken and assembled to the low rpm motor.. very cheap.
Heavy Duty 12VDC Motor with Regulator-The Electronic Goldmine

Here are some potentiometers:
Standard Potentiometers-The Electronic Goldmine

Here are a couple of suggestions:
1) All of the magnetic stirrers I have disassembled used a shaded pole AC motor. Shaded pole motors, while low torque, are easily controlled for speed.
2) I would advise against anything with brushes. Although you might not be working in an explosive atmosphere, and most mag stirrers are not explosion proof, the brushes do add a bit of risk. Second, in an enclosed incubator, they will produce ozone from the arching as well as brush dust.
3) DC brushless are controllable. AC three-phase are also relatively easily controllable using single-phase converters. And of course, you have shaded pole. Some other single-phase, brushless (i.e., not universal AC/DC) motors are also controllable, but I have always had problems with them.
4) If you are in the northern hemisphere, consider making the magnet rotate CCW looking down from the top.

John

Hello John.
Thank you for your answer.

I am working in a non explosive atmosphere.
The motor will not be enclosed, but outside the incubation chamber.

Here is a little bit more about my setup:

I have 6 chambers.
A chamber could be compared to a canister.
The top of the canister has a Dissolved oxygen probe (low flow probe, polarographic).
The bottom of the canister has a stand on top of it = inside (made out of of a galvanized chicken wire). The stand sits above the bottom of the chamber.
On the bottom of the chamber, there is a magnetic stirrer.
The stand receives a sediment core (from the bottom of a lake).
The canister is filled with filtered lake water. No bubbles are enclosed.

All the chambers fit inside an acrylic container (30 gallons aquarium).
The aquarium sits on its stand.
In the table top of the stand, there are holes where currently the magnetic stirrers are mounted.
Each magnetic stirrer sits just under each incubation chamber. so that the stir bar spins.

The aquarium is filled with water. The water temperature is maintained steady since it is circulating, via a peristaltic pump pumping water in and out of a thermostated bath (I can either chill or warm up the water, depending on the lake water bottom temperature).


My only problem with that setup is the speed of the magnetic stirrer being not constant and too fast even at low setting.

This is why I need a good, but still cheap alternative (money is running thin now), so that I can stir at low rpm with no change of the speed.

I need to be able to adjust the rpm (so buying a fixed rpm motor, e.g. 30 rpm ) is out of question.

I hope this clarifies my needs.

Serge

I've got one with a motor in it, too. Seems like the wrong way to do it - why not just put 3 coils under it? No moving parts ecept the stirrer itself, fewer things to go wrong. Use Hall sensors the same way a brushless DC motor does, and that would also give you the ability to have a nice little readout for viscosity as well as speed.

Press a button, and it releases the stirrer.

Intentionally lower the efficiency of the coil drive algorithm, and the coils could do double-duty as heaters to heat the solution.

Sounds good. After 40+ years in the lab, I never saw one like that.

As for cheap and easy, I have seen variable speed drills and record players cannibalized for stirrers.

John

You completely have lost me here. This is very likely because we do not use the same jargon. Talk to me as if you were talking to a dummy.

In simple terms, what would I need?

When you say coils are you referring to the coils of a rheostat?

It looks like you both advise me against a brush motor, and more into a brushless motor. Are you thinking about a stepped motor?

Would a 60 rpm stepped motor coupled to a rheostat work? One rheostat would control the DC feeding all my 6 stepped motors?

Would I be able to control the rpm from 1 to 60 rpm with such a design?

I am unsure why I got it wrong?

What about viscosity now? For now, I just need to have enough mixing in my chamber so that there is no Dissolved oxygen gradient building up: the sediment core indeed consuming oxygen (SOD= sediment oxygen demand).
I need to set the stirring so that enough mixing is provided, but not too much to create resuspension of my core sitting on the stand. I do nor want to also have too much mixing to boost up artificially my SOD.
We measure water velocity (doppler current meter) in the lake, but need to set a standard in terms of velocity in my chamber so that I can compare the rates over time.
In other words: over time, different sediment cores, different temperature sin the chambers, but same velocity each time.
I measure SOD every 4 weeks per grant agreement with the funding agency.

Serge

PS. I think I know enough about viscosity (since I teach it, Reynolds number, stokes law etc.), but for my experiment, I do not need this for now.
I am aquatic ecologist working on bottom microalgae (periphyton).
I have no knowledge of electronics, this is why I am picking your brain.

Since the motor doesn't start under load I think you can use a series voltage dropping resistor.
If you run the motor off of 120vac, use an incand. lamp to get the approximate speed you want, then measure V across and I through the lamp to size your resistor.

I was suggesting a completely different way of doing it, get rid of the motor entirely and just use coils to directly drive the magnetic stirrer.

For what you're doing, you really need a PWM drive (Pulse Width Modulated) with a low frequency.

A 12V cordless electric drill has a really nice one in it. Try one at the hardware store, and look at what a low speed you can acheive. See if this might me suitable.

A brushless dc motor from an RC car with a speed controller driven by a servo tester instead of a reciever might be another option. One of the advantages to brushless is they have long life, because there's no brushes to wear out.

Thank you for your help

Guys, thank you for your help. I built my incubation chambers 4 months ago and they do wonder. This forum is fantastic. So fantastic that I am asking again for help.
https://www.electro-tech-online.com/threads/help-a-ph-d-scientist-infrared-sensor.94771/

Now, I guess I need to attend to an electronic bootcamp.



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See video of the setup here. Works great.
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Thanks to you all.
Great board and good folks here.
Hydro.

Hey thanks for posting the results of your project. Many people do not come back or share the victory/defeat/satisfaction and we are left hanging.

I'm curious, and i've read your posts... what does this do exactly?

Well,
it measures how much oxygen is respired by the sediment (the mud on the bottom of a lake). It is important to know as if the SOD (sediment oxygen demand) is too high, this could potentially has adverse effects on the water column if the sediment is being resuspended subsequent to wind events.

Hydro