Magnetic Stirrer. Straight, or One With Heating to a Specific Temperature Feature?

Mick you mentioned the capability of inducing a vortex in 2000 ml of fluid.

A vortex is the last thing you need when mixing developers.

That fascinating, spiral depression in the middle of the fluid is introducing oxygen into your liquid developer all the time.

Better to have the fluid moving around with the least depression in the surface. Not as nice to watch but, less chance of oxidising oxygen getting into your developer.

Mick Fagan said:

Something else I've learnt from this thread; I'll let you know how it goes.

Presumably the force required to properly stir various amounts of liquid is why various models have varying ability. I hadn't thought that it was the magnetic attraction losing attraction, if that is the correct understanding of what you are saying, being the cause.

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It should be fine. Magnetic stirrers work best for stirring liquids, and dissolving powders added gradually. I heat up solutions in a dedicated microwave oven, a hot plate would be nice to have.
My experience with heated magnetic stirrers is 30+ years old, as long as the drive magnet is sufficiently close, i.e. the hot plate elements are thin. Everything will be fine.
We used magnet stirrers for titrations either color or potentiometric worked great.

Well the magnetic stirrer arrived, was unpacked and taking my time I read the instruction manual, sparse as it was, it was an interesting read.

I purchased a stirrer with heating ability and a 2 litre liquid capacity. I was surprised when I read the literature, noted the actual model from the sticker on the side of the stirrer, and realised that the stirrer I had purchased according to the spreadsheet in the manual, has the ability to stir up to 5 litres. Happy with that.

Once I worked things out I quickly plugged it in, set the thermometer/temperature probe, then fired it up. Starting with one litre of water directly out of the tap at 20.5ºC I had the heating on and was stirring away and noted the speed with which the water was heating up; very fast.

Within a few minutes the temperature was climbing towards 30ºC, so I switched the heating element off and concentrated on slowly upping the speed in 100rpm increments until it was around 2,000 rpm. Speed works, heating works, temperature probe works. As for the accuracy of the temperature probe, not really a requirement, as long as it is somewhere where it's needed, it will suffice for my needs.

Everything worked out better than I expected and my largest beaker, 3 litres, sits comfortably on the platten. The flat area of the platten is approximately 160mm in diameter, the square platten is around 170mm square with a chamfered edge.

Downsides, well the temperature probe pole is not high enough for my 3 litre low beaker, so I'll look into making either a longer screw together addition, or find a 10mm stainless steel rod of 400mm length and fashion the base to sit into the holder. Other than that, it works perfectly so I quickly mixed up 500ml of D76; effortless.

I used a low 500ml beaker with around 400ml of water and set the unit to stir and heat to 45ºC. While the unit was stirring and heating I was weighing the chemistry, by the time I had weighed all the chemicals the water temperature was hovering around 49ºC, which would be the overshoot. Being a cheap unit it doesn't have the necessary equipment to reduce overshooting of the temperature to a small amount.

I then added the chemicals and noted the speed with which they went into solution, faaaaaaast. I then added water to 500ml then restarted the mixer for a few minutes before decanting into a jar with glass marbles to eliminate as much air as possible.

Great tool, glad I now have it.

The picture below shows the unit with a 3000ml low beaker on the platten, which I would suggest is probably the maximum beaker size to sit safely on this platten size. The beaker has a flat base of around 110mm diameter, for reference.

Mick Fagan said:

Being a cheap unit it doesn't have the necessary equipment to reduce overshooting of the temperature to a small amount.

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Managing overshoot in a device like this is a matter of PID control. It's a software thing, mostly, that doesn't require additional hardware apart from what's already there - i.e. a temperature sensor and a heating element. The PID parameters must be brought in line with the response time of the hardware, though. My guess is that this wasn't handled properly. Anyway, not something I'd worry much about as long as it doesn't run away.

Looks like a great little unit! Have fun with it

koraks said:

Managing overshoot in a device like this is a matter of PID control.

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And the tuning parameters (the numbers for the 'P', 'I' and 'D') are going to change with different water volumes.

My guess is the control is thermostatic - like a house thermostat. Putting the temperature sensor in the beaker is a great improvement over the old style (as in the one I have) with the temperature sensor in the platen.

One thing, the hot plate may have a simple bi metallic thermostat in the hot plate itself, the probe could be a thermometer

Right, a test for temperature and the speed with which it is gained, plus seeing how much overshoot occurs.

Reading the instruction manual and the little spreadsheet within the manual, this unit will control the temperature with the probe attached between 0ºC to 100ºC. With no probe attached, the temperature range possible is 0ºC to 300ºC.

It seems that I must have done something wrong when I mixed up the 500ml solution; possibly setting the temperature way higher unknowingly.

Today I decided to get a feel for how the temperature rises and where it stops rising for a given temperature. I started with cold water straight from the tap, which was 23.6ºC. I set the temperature parameter on the unit to 50.00ºC, starting with one litre of water with the stirrer at 1500 rpm.

Within a few seconds the temperature started to rise, In the first picture on the left there are 8 horizontal bars, which I found out is the intensity of the heating element in action. Below the numeral 5, there is a three bar vertical wave, this I understand is lit when the unit is actual heating. The vertical two part bar to the left of the 1500 numerals, is a rotating bar (captured here in a vertical position) this tells me the motor is turning the magnetic stirrer.

The temperature after around 40 seconds has already started to rise, showing 23.7ºC






If you look at the speed that it took to start raising the temperature, then look at how long the intervals were from 45ºC on, it is obvious that it works quite well.

25ºC was attained at 2'.00"
30ºC was attained at 4'08"
35ºC was attained at 6'10"
40ºC was attained at 9'45"
45ºC was attained at 12'43"
50ºC was attained at 30'00"

I measured the temperature 5 minutes after it reached 50ºC with two independent units. My standard digital cheapy probe, 49.4ºC and my Aritherm very expensive and calibrated (some time ago) unit, 49.6ºC Over run didn't happen, as it essentially stayed at an indicated 50ºC.



The below picture at 45ºC shows the bars on the left are now down to two bars, the heating on indicator under the numeral 5 is not on. Stopwatch shows the elapsed time.





Final picture at 30'.00" showing 50ºC. At this stage the heating element status is down to one bar and there was virtually no change in the temperature readout in the ensuing five minutes.

I am very pleased with this unit.

That's pretty darn cool! I guess that's technology, amazing! Couldn't ask for more, especially at that price. Good deal.

BobUK said:

Mick you mentioned the capability of inducing a vortex in 2000 ml of fluid.

A vortex is the last thing you need when mixing developers.

That fascinating, spiral depression in the middle of the fluid is introducing oxygen into your liquid developer all the time.

Better to have the fluid moving around with the least depression in the surface. Not as nice to watch but, less chance of oxidising oxygen getting into your developer.

Click to expand...

I missed that, I'll slow it down for the next chemical mix.

Thanks.

I have a question for those with experience with magnetic stirrers.

I have now mixed up film developer five times and paper developer once, since purchasing. Plus, I've spent a bit of time fiddling (playing) with the unit to further understand it.

The stirrer bar is around 29mm long by 9mm in diameter, except for the central part where I presume there is the joining of the two pieces of plastic.

Now the glass beaker I have been using, is starting to get scratch marks in the centre where the stirrer bar rotates, this is something I hadn't thought about. I have seen some stirrer bars that appear to be flat without a central ridge, should I be looking at getting one like that?

I think I know the answer, but I thought I would throw it into the discussion of this thread for future readers of the thread.

The center ring is there to reduce bouncing with curved base containers, and also helps stop the 'flea' from catching on coarser materials. The flea coating is usually Teflon, but can be other plastics. That should not be hard enough to scratch glass - at least not labware or Pyrex. I have seen lab glassware scratched by breaking up sediments for geological testing, but not powdered photographic chemicals. If the grains you are trying to dissolve are harder than the glass, then it will scratch.

How much chemical do you add to the beaker at a time? Too much and it will settle at the bottom for a while and could lead to scratches until it dissolves. The other thing is that you do not want to over-spin the unit and get a vortex to the bottom. That will pull granules to the bottom first. It can also pull more air into the solution.

There are all sorts of stirrer bar designs. Some, like the triangular cross-section one, are intended for dense suspensions (eg muds). The cylindrical, or cylindrical with center rib are the general purpose ones. There are recommendations as to how long they should be in relation to the diameter of the beaker - around 1/3 to 1/2 - but the information varies.

Graham, interesting stuff.

When I mixed up the paper developer, 2.5 litres worth, I dumped around 190g of Sodium Carbonate and immediately realised I shouldn't have done that; it took a fair while before the solution cleared. Old mixing habits take a while to change.

It was also during this paper developer mixing I realised that maybe I need a longer stirring bar as I was using a quite wide beaker, you have confirmed my thoughts.

All of my beakers are flat bottomed and in my limited look on the offerings available, I see what I have with the central bump, ones without any bump, ones shaped like an American football.

And, yes, I have reduced the speed in all situations to virtually eliminate the creation of a vortex.

Thanks for your information, greatly appreciated.

If the vortex created by stir-bars really disturbed some people, for the air they introduce, those folks might buy some bread bags from Amazon or Walmart and pour the water and the chemistry in one, after fitting the bag into a Beseler print drum.

Securely tie the bag closed and closed again, put it on the motor base and set it to 'on'.

Come back in a few hours and check the bag, which will fit back inside the drum, if needed, to see if you have the results you're after.

If you want to keep the mixing solution warm or cool, aluminum foil can be taped to the outside or inside of the mixing tube.