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peach
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I keep seeing small mountains of IKA plates turn up on eGay that look like someone has set about them with a hammer. Oddly, it always seems to be the
left hand dial that's snapped off.
Saying that, my plate now looks like Frankenstein's monster, but still functions fine (and I suspect those IKAs either work or can be coaxed into
doing so). In fact, my filthy plate makes less noise than it did when I got it (the magnet used to rub against the stand offs). I wish more lab gear
was quiet. Once I have the the recirculating cooler going and the hotplate, it's a constant buzzing for hours on end. The plate in particular
BuzzzzzzzZzes loudly when it's turned down low, with the PWM circuit chopping away.
The cooker hotplate idea is a good one. I used to use the kitchen cooker when I ran out of plate space.
This obviously isn't a guarantee against blowing yourself up, but computer fans are DC brushless motors. From the wiki on electric motors;
| Quote: | | Brushless motors have no chance of sparking, unlike brushed motors, making them better suited to environments with volatile chemicals and fuels.
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Hotplates tend to use ceramic / ferrite magnets, because the Curie temperature (the point at which they loose their magnetic appeal) is around 460C.
They're also not very strong.
You could use one of the high grades of neodymium magnets and simply place the fan further away from the heat source. If you're distilling solvents
and such, heat isn't going to be that much of a problem. My stir bar will sometimes jam because the motor can't turn against the friction, not because
the magnet can't hold the coupling force. And it's a good thing it does a lot of the time, because the alternative is that the flask starts trying to
spin.
Another option for a brushless design would be to directly couple to the magnet it's self and use that as your rotor. Pancake motors found in disc
drives and things like that do this. They have coils that produce the rotating field.
There are also 'mag drive' pumps, which are used a lot in the chemical industry, which use magnets in the rotor it's self and then produce the field
around the impeller. That way, the impeller can be made of something like PTFE and the pump head doesn't need dynamic shaft seals.
Even garden water features use mag drive impellers.
There are tons of guys using computer fans to stir things now, varying from just plugging them into an old computer power supply or a wall adapter to
building little LM317 circuits to control them. A lot of them are brewing alcohol. 
There's also a PDF floating around that someone on DIY Audio wrote about taking apart disc drive motors and using them to power turntables; as the
rotation has to be very smooth and at a constant rate to read the data correctly. Disc drive motors, as a result, have a lot of circuitry already
attached to control the coils. I think they're actually closed loop regulated (which means the circuit can tell how fast the element is actually
spinning, unlike most hotplate / stirrers). They have some crazy number of phases for the coils.
Cheapest PC fan on ePimp-wegotsyabankdetails.co.uk is £1.60, delivered. That's what IKA charge per sentence they answer a commoner's email with.
If you were running a small air powered motor, you might be able to drive it from a fridge compressor. Which is a lot quieter than a normal piston.
Again, the piston is powered by a brushless method. They'll easily produce 100psi and I've actually heard guys talking about hundreds of PSI from
them. But, admittedly, I've never really looked at the CFM for air motors. Other than when I looked at air tools for the workshop and discovered they
need an 8HP compressor (or something similar) to produce less power than a £5 budget line, no name, corded drill. That, combined with high pressure
spraying, wastes a monumental amount of energy industrially.
Here's the DIY Audio thread where they're rabbiting on about floppies
And this is an example of a fan stir plate from Mad Zymurgists (he has a photo guide on this link);
[Edited on 15-7-2010 by peach]
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zed
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Neodymium Magnets really can't take much heat. To my recollection they fail at 80 C or so. Well below the BP of water. Better to use massive
magnets inside your stir plate... Massive magnets that have a lot of heat resistance.
Samarium based magnets are pretty strong, but they are also pretty expensive. No can do.
Likewise, Dyna-mags and other sealed overhead magnetic-drive stirring units are prohibitively expensive.
Parr and their ilk, want thousands of dollars for a sealed magnetically coupled stirring unit.
The guts of these units, the neodymium magnets? Well, they're cheap as dirt.
Design an easy to build, cooled, sealed stirring unit, utilizing neodymium magnets.
It doesn't have to be incredibly durable. Just needs to be able to withstand high internal pressures, and corrosive chemicals. A unit that would run
a hundred hours or so before needing a rebuild, would be fine.
You will become a hero.
[Edited on 15-7-2010 by zed]
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JohnWW
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If Nd magnets can only stand 80ºC or so, the temperatures inside many hard drives that use them may come perilously close to causing their failure;
unless they are of a different composition from those used in modern magnetic stirrers, the hard-drive ones being Nd2Fe14B according to another
thread, http://www.sciencemadness.org/talk/viewthread.php?tid=14145 , with some of the Nd replaced by Pr. They appear to owe their strong magnetic moment
to the two surplus 4s and one surplus 3d electrons from the Fe atoms (left with five unpaired 3d electrons) entering the vacant 4f orbitals in Nd
(which has three unpaired 4f electrons of its own, with four of the seven 4f orbitals vacant) (or Pr, with two unpaired 4f electrons of its own and
five vacant 4f orbitals), so as to maximize the number of unpaired electrons.
[Edited on 16-7-10 by JohnWW]
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zed
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Thermal Characteristics
Nd Material Type-------Maximum Operating Temp
N---------0.12----------------80ºC (176ºF)
NM-------0.12----------------100ºC (212ºF)
NH-------0.11----------------120ºC (248ºF)
NSH-----0.10----------------150ºC (302ºF)
NUH-----0.10----------------180ºC (356ºF)
NEH-----0.10----------------200ºC (392ºF)
NZ-------0.10----------------200ºC (392ºF)
The strongest magnetic materials are imbued with the least heat resistance. Too bad! The cheap, super-strong, Nd type magnets that are commonly
offered for sale are N-type.
Dyna-mag type units, must be equipt with a cooling water jacket, to keep their Nd magnets from frying.
Also of note, Nd magnets are very adversely effected by Hydrogen. Simple plating is not effective protection. Gotta be hermetically sealed, or
otherwise isolated from H2. They don't blow up or anything. They just moulder away.
[Edited on 15-7-2010 by zed]
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Contrabasso
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Heaters for a stirrer motor NEED to be torroidal so that the magnets can get close to the stirrer bar. with your disk shaped heater the rotating
magnets will be 20 - 30mm below the stirrer bar and the coupling will always be weak and delivered power may be too low to stir effectively.
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zed
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In stir plates, the strength of magnetic coupling is always a problem. Really vigorous stirring requires a very strong coupling. Even when using an
Nd "supermagnet" as a stir-bar, tweeking may be required. At a thousand RPM or so, the stir-bar tends to uncouple and break loose.
Solution? Closer proximity, bigger magnets, and maybe a bigger stir bar.
Torroidal heating sounds good. But, not too much heating.... Sigh! My Nd stir bar fails at high temperatures.
If very much heat, pressure, viscosity, or high RPM is involved.....Ya gotta look elsewhere. Overhead stirring, or sealed overhead stirring, in a
sealed reactor.
http://www.pressureproductsindustries.com/mixers/dynamag.htm...
http://www.parrinst.com/default.cfm?Page_ID=164
Crucible, I like the cement/nichrome hot plate quite a lot. It has a rustic, sort of pre-Columbian look. It might be difficult to make it perform as
a usable lab-type hot-plate, but it has real potential as kitchen "ART". It would make a damn fine coffee pot warmer, as is. Might be able to
sell millions of units, all over the world. Jump on it, before Martha Stewart steals your idea.
[Edited on 15-7-2010 by zed]
[Edited on 15-7-2010 by zed]
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Contrabasso
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At my old work lab, we used always to be short of stirrer magnets so we used to put a bit of steel wire in a glass tube, heat and crimp the ends. You
soon learned to bring a stirrer up to speed gently! or the bar flew off 
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peach
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Quote: Originally posted by zed  | In stir plates, the strength of magnetic coupling is always a problem. Really vigorous stirring requires a very strong coupling. Even when using an
Nd "supermagnet" as a stir-bar, tweeking may be required. At a thousand RPM or so, the stir-bar tends to uncouple and break loose.
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That's turbulence in the fluid. The same thing happens in fusion reactors. The goal is to keep the plasma flowing in a perfectly smooth circle, with
the ions spiralling around the field lines of the toroid. Real world turbulence ruins that and they fly off the field lines.
I also realized my point about stronger magnets further away is kind of dumb. The field strength drops by the ?cubic? law, so the magnets would need
to be ?four times? stronger per unit distance?
[Edited on 16-7-2010 by peach]
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zed
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The drop off in magnetic field strength/vs distance is shocking. I have a stainless steel vessel that has a 3/4 inch thick bottom. I would like to
use it with magnetic stirring. No way.
The stir bar, ends up being too far away from my stir-plate's drive magnets, for strong coupling.
No problem with thin-walled stainless steel vessels. The magnetic field ignores the stainless steel. So, magnetic stirring works fine.
It's the distance, not the material. Three quarters of an inch, might not seem like much, but in this case it's an insurmountable obstacle to
success.
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JohnWW
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Although the attractive force due to electrostatic or magnetic monopoles, like gravity, falls off with the square of distance 1/r², with spherical
equipotential surfaces, with oppositely-charged mutually-attracting dipoles in close proximity (which is usually the case) the attractive force falls
off with the cube of distance, 1/r³, from a fair distance from the dipole, with equipotential surfaces planar cross-sections of which are "ovals of
Cassini", 4th-degree curves. This is dealt with in 200-level Applied Mathematics and Engineering Mathematics.
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Crucible
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| Quote: Originally posted by zed | In stir plates, the strength of magnetic coupling is always a problem. Really vigorous stirring requires a very strong coupling. Even when using an
Nd "supermagnet" as a stir-bar, tweeking may be required. At a thousand RPM or so, the stir-bar tends to uncouple and break loose.
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Hmmm, I'm not sure my PC case fan I'll be using as the stir motor gets up to 1000 RPM, I'm guessing it will top out at 350 RPM before I even mount
magnets on it (extra weight).
Based on the comments, I decided to do a quick test. I have 2 types of magnets to work with. Large heavy hard drive magnets and small light rare
earth magnets. The thickness of the hot plate is about 1.5" and I'm guessing I'll need strong magnetic coupling at a minimum of 2" to account for an
airgap and the thickness of the glassware.
When I position either type of magnet under a ferrous object (a screw in this case) I can get it to move (ie magnetic force > friction) from a
distance of no more than 1.5" which is too little. However, if I position the small magnet on top and the large one on bottom the effective range is
doubled and I get very strong coupling at 3" or more, which should be plenty. Assuming my stir bar is as strong of a magnet as my small rare earth,
which it probably won't be.
So this plan might actually work if I can find a suitably strong magnetic stir bar, or find a way to seal my little magnets in PTFE and make them into
a stir bar.
| Quote: Originally posted by zed |
Torroidal heating sounds good. But, not too much heating.... Sigh! My Nd stir bar fails at high temperatures.
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Yeah, I'd forgotten heat is the enemy of magnets. Thanks for the reminder. I think I will stick to trying the magnetic stirring for 60C or less.
| Quote: Originally posted by zed |
If very much heat, pressure, viscosity, or high RPM is involved.....Ya gotta look elsewhere. Overhead stirring, or sealed overhead stirring, in a
sealed reactor.
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If I can't get magnetic stirring to work or need high heat, I'll need to invest in a 2 or 3 neck RB and an overhead stirrer. Though I may experiment
with aluminum or mylar heat shielding around the magnets to see if that works.
| Quote: Originally posted by zed |
Crucible, I like the cement/nichrome hot plate quite a lot. It has a rustic, sort of pre-Columbian look. It might be difficult to make it perform as
a usable lab-type hot-plate, but it has real potential as kitchen "ART". It would make a damn fine coffee pot warmer, as is. Might be able to
sell millions of units, all over the world. Jump on it, before Martha Stewart steals your idea.
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LOL! Thanks zed. pre-Columbian huh? "Deep in the ruins of Maccu Pichu archaeologists have discovered an ancient heater which actually runs on
electricity, right next to some primitive glassware which surprisingly has standard 24/40 joints" Hahaha
I'll rush them into mass production just as soon as you tell me where to find people willing to spend $150-$250 on one. The cement alone cost me $50,
the wire and accessories another $35 and I don't even want to think about how much I paid for that solid state variac! But I still love her anyway,
and if I ever get the design perfected I may just sell them, to chem enthusiasts obviously. Not much potential in a $150 coffee heater!  And then I can start work on the perfect design for an enclosed overhead stirrer.
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densest
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The market for "concept" = insanely overpriced "decorative" = ugly house things is recovering. The too-rich are also too-dumb... The cement is
probably much cheaper in bulk or as a mix. I know that "furnace cement" can be expensive at a hardware store and much cheaper as "kiln cement" or
"castable refractory" in bulk. Solid state variac = dimmer 600W @ $15. Nichrome wire at 28ga (4.25 ohms/foot) is about $0.17/foot.
I've successfully fused magnets, etc., into PTFE tubing. The keys are temperature control and speed.
PTFE doesn't really melt well. It does become very soft and jellylike just below beginning to decompose. At that temperature it can
be molded and fused to itself. The window between too cold to fuse and decomposing is narrow.
Procedure: cut a length of PTFE tubing 3 cm longer than the object to be encapsulated. Get a pair of pliers with jaws wider than 2x the tubing
diameter ready where you can pick them up very quickly. Put the object in the tubing and center it. Using hot air (either a hot air gun or over -not-
in a flame or hot plate) gently warm one end over a 1-1.5 cm length. Turn the tube so it soaks in heat all around the diameter. It will lose opacity
to become translucent to almost transparent. At that time grab the pliers and compress the end of the hot area to seal it. Don't apply significant
force - the plastic is very soft. Leave a short length of hot plastic between the seal and the cold part to take up stress so that the result tapers
between the round area and the flattened end. Cool, then do the other end.
One could make a conical/hemispherical tool or a pair of half-cones or half-hemispheres on tweezers to make a rounded end which would probably be
stronger.
The procedure takes less than 5 minutes from start to finish.
[Edited on 17-7-2010 by densest]
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peach
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I don't wish to be a negative nancy, but have you tested those ideas everyday with numerous different chemistries? Like the seals on the ends of the
tubing via various heats and chemical agents? Does it stir properly? I can stain a genuine PTFE bar black without too much effort. And I've beaten the
shit out of my stir plate despite being careful with it. It looks like a piece of modern art.
Remember, crucible is talking about explosion proof gear. As I've said in another thread, I have enough problems of my own using genuine PTFE molded
bars or genuine plates; and I'm not dealing with explosion hazards. Trying to improvise explosion proof gear, doing acetone as your first distillation
and having trouble getting it to boil are concerning given the end goal. Someone doing that should easily be able to accomplish the former. That plate
doesn't seem to have been over 100C yet, the boiling point of the water that's inevitably trapped within the cement. Any potter or caster will tell
you, trapped moisture is an issue. When you have a litre or two of something highly flammable or explosive balanced on top of it, it's serious issue;
e.g. the cement burst, smashing the flask and exposing the contents to the red hot element.
If you've ever light a fire on concrete, you may know what I'm getting at. I've done this with concrete decades upon decades old and had it pop like
there were fireworks in it as the trapped moisture boiled.
I am genuinely proud of people DIY'ing gear themselves, it really is in the spirit of things. But I worry that crucible will assume it can perform in
the same environments all the certified gear can. There are so many variables to think of to jump from distilling acetone to explosions, "will that
seal break", "will my terminals get too hot", "will the cement burst", "will the variac spark", the list goes on for so long. To design that kind of
thing, you need to understand the fundamentals of what makes something explode. And knowing the difference between a flash point and autoignition
temperature is one of the first on that list.
Before I came to this forum, I spent about seven or eight years up each night reading high end electronics forums for six or nine hours a night. I
would still hesitate working with explosion hazards. You have to have everything nailed to the floor in terms of the variables.
Please take some photos of your PTFE encapsulated things. Not so's I can complain and pick holes. I've heat sealed things before myself and I'm
genuinely interested to see how it works with PTFE. Watch those HF fumes. 
I'd get rid of the feet on this design. If the cement ever fails, the feet will cause it to cave in on it's self, potentially breaking the flask. If
it were supported by a continuous surface, that wouldn't happen.
[Edited on 18-7-2010 by peach]
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densest
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@peach - you're asking about risks and risk evaluation. Edit: there's a "Bonding PTFE" thread on this site with good info.
I'll post pictures as soon as the *#@#$ box with the stir bars surfaces - no promises about when  because it's well & truly buried. The lab where I used it had to be shut down due to the landlord doing bizarre
things unrelated to the lab. We, the tenants, left very quickly when we heard about the problems, not wanting to be caught in legal crossfire.
Anyway - risks: I've done 3 stir bars the way I described. I examined all of them under magnification for cracks, wrinkles, and other evidence of
strains. The fused length was 1cm. I tested them by heating them in cold water to boiling while watching for bubbles, then cooling to room
temperature. If there were leaks, either air would bubble out or water would flow in. Neither happened. So, my judgment is that for temperatures
0C-100C, they're OK. One risk is acid reacting with metal inside. So in alkaline, anhydrous, neutral, or mildly acid environments, the risk is low.
Strongly oxidizing (especially chlorine) acidic environments are the biggest risk for metal reactions. Above about 100C, any water inside would
vaporize risking bursting.
PTFE is very inert. It is vulnerable to mechanical damage, very strong bases, and molten alkali metals. Dunno about molten Ca but I wouldn't risk it.
My homemade stir bars are weaker mechanically than commercially made ones, or so I must assume, so I won't use them if there's any risk of mechanical
damage through abrasion, impact, etc., or heating above about 95C, or in strong bases. I inspect them for damage after each use.
"Explosion proof" is about not providing activation energy for flammable mixtures and not propagating flames or sparks. It doesn't mean "invulnerable
to any damage" - mechanical & other chemical risks are different problems.
I think discussion about risks is very important - as long as risks are at least relatively quantified and carefully analyzed.
[Edited on 18-7-2010 by densest]
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Crucible
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@peach - I appreciate the concern, but that is the reason I posted this thread, to work out all the kinks and get input from others whose knowledge
exceeds my own. I think I have things pretty close to nailed down at this point. Unless there are some other variables I haven't considered. But
lets go down the list of things I've already taken into consideration.
Water in the cement - there was a good bit of moisture in the cement when it was first made. I always test my gear before using it "live". I spent
several hours calibrating the variac and testing the plate. Before I ever used it to heat anything, I heated it to 50C for 3 hours continuous, let it
cool, then heat it again to 90C for 3 hours continuous. There is a peculiar smell that moisture seeping out of cement carries, and I got an ounce or
two of water out of it before it "ran dry". Since then I used it to distill the acetone (also an explosion hazard) where it ran at 60+ C for greater
than 6 hours continuous. I think if there is any water left it would be very, very little. And since I won't actually need to push it to 100C to
boil flammable solvents, there is no reason to think that any residual water would cause a catastrophic blow out. Even if there was, it is pretty
unlikely it would break the flask, because I'm using a double boiler setup and all the glassware is supported on stands NOT by the hotplate. So if
the hot plate cracks, yes there will be exposed heating element but that's it. And as for structural integrity, I've done some informal stress tests
with the cement and it takes quite a bit of force to break it. The likelihood of it breaking under its own weight or the weight of the water pot is
quite slim IMO. The top is a full 1.5" thick and it took >10 lbs of force just to break a .5" piece.
As for the exposed heating element, I'm planning to dump some more cement on it just to cover it up. However, just on the chance that there might be
something left exposed or the cement may crack and expose some during operation, I'm also planning to use a fan pointed at the hot plate to blow any
fumes away. I'll have any potentially spark producing objects upwind and the great outdoors downwind. I can't conceive of any possible scenario in
which enough solvent and/or fumes could come into contact with anything capable of igniting them under these circumstances (remember I will have
turned off the breakers for the entire room also). And I don't think that because my equipment is DIY that it is unsuitable or inferior. If anything
I know what it is capable of and any potential hazards better than I would some second hand piece of manufactured labware which might have been
subjected to all sorts of dangerous conditions I'm not aware of, mechanical shocks that may have ruptured the internal seals, chemicals that may have
corroded the insulation on the wires.
But if you think there is any other thing I've overlooked, I'm all ears.
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peach
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@Denset
Sorry buddy, I was in a bad mood when I posted that and was complaining more than need be.
The reason I'm skeptical is that I know one of the major problems with PTFE is that it can't be thermoformed. Indicating that the bonds you've
produced are mechanical, as opposed to chemical.
I haven't read that thread, so I can't speak with much certainty, but I suspect industry would be using this method if it produced a true PTFE seal.
Instead, they've spent a huge amount of time and money, and induced strong risks, etching the surfaces and then gluing them together with less than
ideal seals.
If you want to do that kind of thing, look at PFA. That's basically Teflon, but thermoformable. I think FEP is as well. FEP is PTFE with a carbon
sticking out the branch, but it's fully fluorinated. PFA is the same but the carbon is linked to the chain with an oxygen. They're used as liners for
things like Tygon tubing when it needs very high chemical resistance. PTFE tubing is too rigid to use for similar applications a lot of the time and
can only be used as a pushed through liner membrane for tubing.
I'd use PTFE for corrosive gas handling, but it's fairly rigid stuff (so it'll be a pain to setup and try to topple the glassware) and it's opaque.
I don't think your stir bar needs a super strong magnet in it. Mine isn't all that strong and I can stall the motor in a commercial plate with thick
slurries. That's were overhead stirring comes in. And that's where you have to start messing around a lot more. It's easier and cheaper to use a
bigger flask and throw some more solvent in most of the time.
You are correct about risk concern where it's needed. If you see some of my other posts, you'll see I'm quite the opposite of a scaried cat and have
been suggesting the toxic label doesn't apply to things most other people consider toxic.
It's just, as I hint at, explosion proof gear usually means someone is doing something incredibly dangerous. Where a mistake will torch an entire
facility. Without be being clear what crucible is planning to do or what experience he has with it, it's hard to downgrade that on the basis of "I
know this guy, I know he knows and is asking to be sure, I know he can probably do it without asking". Also, I don't think crucible genuinely needs
explosion proof gear. Distilling acetone is something countless labs do with nothing close to explosion proof hotplates.
@Crucible
It's good to know you're using a bath, that'll solve the fire / explosion risk if the plate cracks.
But you are kind of not being very consistent, as you were requesting a thousand degrees on the plate and have now swapped to 100, an order of
magnitude less.
There are numerous solvents that are theoretically flammable, but next to impossible to actually ignite in the real world. Like acetone. It's very
easy to distill acetone with a normal plate and not have any problems.
If the solvent is building up in the atmosphere, you're wasting solvent, producing a health hazard and risking a fire or explosion. It means your
condenser isn't cool enough and / or doesn't have a long enough surface area.
Acetone is miscible in water. So, venting your receiver bend through a wash composed of water would trap the vapours before they got airborne.
Explosion proof gear is usually used in industry were an accidental escape would result in the entire warehouse instantly going up. The goal is to
stop it getting airborne in the first place. It's the old prevention over cure theory.
Don't rely on things to prevent problems once they arise (like fume hoods and explosion proof gear). Get rid of the problem before it becomes one.
A good way to stay safe is to look at the concentrations of the chemical that can be readily smelt, then at the concentration needed to create a fire
or explosion. That way you'll know "I can smell it, so it can ignite" or that it's getting close to a problem.
Flammable chemicals have a range of dilutions they'll ignite in, and others that won't. You can often need quite a lot of the fuel present in the
atmosphere for it to start burning, far more than is needed to smell it. And, once it gets concentrated, it often won't burn anymore. You can fill a
room with highly flammable chemicals and, provided they've displaced a good percentage of the oxygen, you could throw a light blowtorch in there and
nothing would happen.
The fire triangle is a good thing to remember. But add to that, the fuel, oxidizer and ignition source have specific parameters set on them as well;
e.g. the difference between flash points and autoignition. It's not a static thing. With chemicals and odd environments like chemists experience, you
can't simply tick them off. You have to think about each and how it varies, because sometimes they're ticked on or off when they wouldn't be in the
normal world. It's quite dynamic.
[Edited on 21-7-2010 by peach]
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densest
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@peach - Thanks, yes, there are bad days and bad weeks... sometimes bad years.
Agreed, yes, the bonds are mechanical. I'm only asking that the joint be water/solvent tight for very low pressures without other mechanical
stresses. Making my own NaNapthalide is a project for some other day. I wouldn't make a beaker the way I suggested, no, it would be a disaster!
PFA is nice... I use PFA fittings to attach my PTFE tubing to various pieces of equipment. They're only mildly exorbitant from McMaster-Carr. Yes, I
use the PTFE tubing for corrosive gases like Cl2, HCl or HNO3/NO2 & friends. I got some thin-wall tubing so it's not as stiff or opaque as it
might be. It's often necessary to use a couple of extra feet of tubing and loose coils of it to get around the stiffness. And lots of extra horizontal
& vertical rods & clamps to make sure I don't get another bath in aqua regia. Two were enough.
I've found that the motors used in stirrers/stirring hot plates are very badly matched in torque and speed to the requirements of the job. The motors
are used at speeds well below their optimum. One wants 60-600 RPM or so, and the motors are 1800/3600 RPM AC motors slowed down by SCR controllers. To
fix this would require either a multipole motor (expensive), gearing (expensive), or a DC motor and control (also expensive).
One can get surplus DC gearmotors for $5 - $25 with speeds in the appropriate range(s) very easily. For a DIY overhead stirrer they work very well.
Getting a brushless one is a little harder if one needs explosion-proof... and if I needed explosion proof equipment, I should very carefully
reexamine what I was doing!
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peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
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I found your perfect chrimbo present Densest....
>>>>>>>>>>>>>Christmas for Densest<<<<<<<<<<<<
I spent a few hours hunting for some FEP or FPA tubing to fulfil my corrosive gas needs and found some really cheap (less than $700 a roll), but I
don't think it's a wise choice for glassware. I contacted one seller who confirmed this for me (that it'd kink all the time and not bend very easily).
I also did some significant editing of the wiki's on PTFE, FEP, PFA and ETFE, as they didn't really explain which was better for what.
You're right about stirrer motors as well. By the time I have mine spinning at full speed, the bar will frequently detach it's self due to turbulence.
A big stir bar rocketing off it's axis inside a flask full of something corrosive, nasty, capable of staining the ass out of something or pretty much
anything other than water is not a fun thing. Not counting the cost of replacing any glass it breaks.
My no name plate was about £150 - £175 I think. When it arrived, the magnet was skuffing against the baffles it went through, making a 'chif, chif,
chif' noise. That went away.
But now something seems to be going wrong with the stir circuit. When I set it to full, I can hear something that sounds a lot like arcing.
I'm a little concerned about it actually. I'm not an electronics god, but I know that when I turn something off, the neon lights connected to that
switch shouldn't keep glowing (like they're leaking to earth) and the PWM circuit was making a questionable amount of noise before the arcing sound
appeared.
Saying that, it's worked fine up until now and still does, for now. It will go hotter than an IKA (which is handy for drying salts out) and cost a
whole lot less.
[Edited on 21-7-2010 by peach]
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densest
Hazard to Others
Posts: 359
Registered: 1-10-2005
Location: in the lehr
Member Is Offline
Mood: slowly warming to strain point
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Oooooh! Must have Thanks!
The same person is selling 1500W stovetop heating elements for $15(?) or so. Dunno how thick they are or whether the top plate is magnetic... still,
it's about the right size & power.
I have another (non-stirring) hot plate which is infrared/radiant heat. Not suitable for metallic or reflective containers. Works very well with
glass. It's a non-contact approach.
I'll keep my eyes out for FEP or PFA. As I said, the PTFE tubing (0.031" wall) is quite usable if you're willing to observe its minimum bend radius.
It was very inexpensive.
I saw huge quantities of fluorocarbon pipe go for a song about 5 years ago I
had no place to put it.
[Edited on 21-7-2010 by densest]
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quicksilver
International Hazard
Posts: 1820
Registered: 7-9-2005
Location: Inches from the keyboard....
Member Is Offline
Mood: ~-=SWINGS=-~
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A close friend had access to a damn nice lab and had some very expensive, quality equipment. One thing I noticed was that really quality hot-plates
were not integrated with stirring mechanisms: they were stand-alone items. Another issue was that the heating surface was QUITE THICK (& there was
a was a label upon the hot-plate that directed the user to wait before using so as to find stability at proper temp). The electronics of the
hot-plate were also not cheap & very high wattage. Checking Mouser [Electronics] for the cost of the rheostat found it to be enormously expensive
for that single item. Other parts were uniquely high-end. No cost cutting venues were in place what so ever; it was impressive. Which leads me to
believe that one can have standard quality & decent utility or one can have first rate quality but need to pay the price.
I've own several hot-plate-stirring devices and I really don't think they ever were so well made that I wasn't forced to check (& re-check) the
temp constantly as changes of as much as 10 degrees were not uncommon. I've owned YellowLine & Corning plates but the better, high-end Barnstead
plates can be found to also have schematics available.
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Crucible
Harmless
Posts: 22
Registered: 14-6-2009
Member Is Offline
Mood: No Mood
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@peach I don't know that I'll ever get my plate up to 1000C, but I designed it so that I would have the option. I'm a big fan of having few pieces of
equipment that have a wide range of capabilities. Sorry if that confused you.
You might be right that I don't need a super strong magnet, but I'll need a regular stir bar to test the magnetic coupling and have the option to seal
some strong magnets into PTFE if I need to. I'm all for using the option that works with least cost/effort.
My main concern with explosions was indeed volatile solvents (ether, petrol distillates, etc). And my concern with that is joints leaking. Having
some fumes leak, ignite, and flash back shattering all my precious glassware, spread flaming solvent everywhere, and shower me with broken glass would
really ruin my day. I always try to prepare for the worst case scenario, so that's why I went for explosion proof. Isn't that what it's for?
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peach
Bon Vivant
Posts: 1428
Registered: 14-11-2008
Member Is Offline
Mood: No Mood
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The following will read as nitpicking and bitching. It is not intended as such, I'm curious about the details.
| Quote: | | One thing I noticed was that really quality hot-plates were not integrated with stirring mechanisms: they were stand-alone items.
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I've seen this as well. I think it's that they expect a lot of their buyers will be making use of overhead stirrers. It could also be laziness. I've
cracked open a plate that cost $1k a piece. It wasn't anything that exciting inside.
| Quote: | | Another issue was that the heating surface was QUITE THICK (& there was a was a label upon the hot-plate that directed the user to wait before
using so as to find stability at proper temp). |
That again sounds like they don't have faith in their own product. Do you mean wait for it to heat up and then put the glass on it? That's asking for
it to pop. The thickness of the plate would only add stability to the temperature if they haven't added it electronically.
| Quote: | | The electronics of the hot-plate were also not cheap & very high wattage. |
I'm not trying to be rude, but how long ago did you observe the insides of these? I can now buy a transistor that'll dump the entire load of my house
through it's self for not very much money at all. My own cheap plate is a higher wattage than the $1k one I bought.
We just got a fancy induction cooker top that's got a fan cooled PWM supply in it that'll regulate something like 6kW (ten times more than a normal
lab plate) and it's full of alarms for when the surface is hot, timers and locks to stop kids burning themselves. That was about £200 I think. The
miracle of solid-state.
| Quote: | | Checking Mouser [Electronics] for the cost of the rheostat found it to be enormously expensive for that single item |
This is where I worry and what made me question the age of the plates, as a rheostat is (to my knowledge) and massively outdated method of controlling
a load like that. There are linear regulators and PWM now. I can put together something that'll control similar loads with a handful of components
that cost a few dollars. A rheostat also wastes a ton of energy when small powers are needed at the load, and inherently wastes power whatever load
it's set at as it dumps some of the current through it's self.
A resistive divider was literally the first circuit we were introduced to in secondary school, when I was 14-16.
A rheostat also means no active, closed loop feedback and therefore, no temperature regulation. Which goes back to the point about plate thickness and
waiting for it to warm up.
| Quote: | | No cost cutting venues were in place what so ever; it was impressive. Which leads me to believe that one can have standard quality & decent
utility or one can have first rate quality but need to pay the price. |
I'm saying this entirely willing for you to tell me it's totally wrong, but that sounds a lot like a very lazy method of designing things. And then
passing the price for the high cost components onto the customers. A lot of companies still do this now. There's certainly a sweet spot between the
high end prices and the China town price, as there is with everything. The former means you're probably being exploited, the latter means someone else
is being exploited and you're getting a piece of shit as a present for your low price.
| Quote: Originally posted by Crucible | | @peach I don't know that I'll ever get my plate up to 1000C, but I designed it so that I would have the option. I'm a big fan of having few pieces of
equipment that have a wide range of capabilities. Sorry if that confused you.
|
I did do a fair amount of complaining. But, now I know a bit more about what you're trying to do, I'd say it's a fair attempt. Don't think I'm getting
all nice on ya, that's never going to reach 1000C.
Not because you've built it poorly, but creating a large surface area exposed to the atmosphere and electrically heated to 1kC isn't so easy.
At first, I thought you were up to something pyrotechnical or highly explosive with it.
| Quote: Originally posted by Crucible |
My main concern with explosions was indeed volatile solvents (ether, petrol distillates, etc). And my concern with that is joints leaking. Having
some fumes leak, ignite, and flash back shattering all my precious glassware, spread flaming solvent everywhere, and shower me with broken glass would
really ruin my day. I always try to prepare for the worst case scenario, so that's why I went for explosion proof. Isn't that what it's for?
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If yar be usin' jointed glassware, grease it and add keck clips. I know you're trying to cover all the possibilities with the plate, but it might be
causing you to overlook the more simple.
I've distilled lots of flammable solvents and things like nitromethane, and I smoke; sometimes stood next to it. It'll take maybe two or three minutes
to grease and clip the joints and there's no way enough vapour will escape from those to ignite. I could hold a lighter against the joints and not see
anything.
If you're worried about flashpoints and autoignition, reduced pressure distillation will lower the BP, so it'll be away from those temperatures.
Flooding it with N2 first will mean there's essentially no way the fuel can ignite, because the amount of oxygen present will be so far below the
ignition requirements and the volume will be saturated with fuel (which also won't burn in high mixes). If you reduce the pressure enough, you won't
even need to synthetically heat the flask, it'll boil as it absorbs heat from the air around it.
It's a good idea to be safe, but those joints and exhausts are by far and way the first port of call.
Saying all this BUY A BIG FIRE EXTINGUISHER AND HAVE A HOSE HANDY
I've had to extinguish someone who set themselves and a plastic tank alight with petrol. The screaming people make is instantly recognisable from
hollywood or anything else, it is animalistic and communicates pure fear. And every fraction can make the damage exponentially, irreversibly worse.
He was using the petrol to light a BBQ, and all I had to hand was a small kitchen powder extinguisher. It ran out in seconds, and I had to fill
buckets of water to put the tank and garden out. When people are on fire, the most disheartening thing you'll feel is the extinguisher running out.
The tanks are designed not to flashback, it's not like in the films when they explode.
And, luckily, he'd only set his feet on fire. He had some blisters and the marks stayed for a few months, but they got better.
I laugh remembering him running around with his feet on fire, but I still graphically remember the noise, the fear and adrenaline. It would be
absolutely horrible to have that happen and for it to be worse, with no extinguisher.
He set himself on fire after managing to ignite the fuel tank's open lid. Thinking it'd explode, he'd kicked it to try and put it out. Where upon the
petrol sloshed out all over the floor and his feet.
A prime example of panic making things worse.
I would also draw a conclusion from this that you are most likely to set yourself a light when you're handling the flammable solvents at the start or
emptying the still, rather than a freak accident during distillation.
Let it cool right back to room temperature. Before opening the joints.
If the glass ignites, the flame will burn outside the glass where the oxygen is, not inside. The biggest risk is that the glass shatters and empties
the fuel onto the floor in a gigantic puddle. To this end, put the glass in a big aluminium pan that'll catch it if it tries to escape. This will be
very easy to control, low risk and extinguish by comparison. You can by them cheaply from commercial food preparation warehouses, they sell them for
making big trays of dinner for people at school and in work.
A big tank of liquid CO2 costs next to nothing. They are wired to plastic hoses that burst when a fire breaks out in commercial, high risk areas.
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