PhilipCarvel
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Can a smaller volume flask be used in a larger heating mantle?
I'm looking into different heat sources for my home labs, and for RBFs a heating mantle looks like a more efficient option than a hotplate and
water/mineral oil/sand bath.
But if I was to purchase a 1000 mL heating mantle, would I lose that efficiency with a smaller volume RBF, or does the concave shape allow 500mL or
250mL flasks enough surface area?
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Loptr
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I have seen larger heating mantles used with sand to accommodate smaller flasks. They also make smaller sleeves that can be used with mantles intended
for larger flasks.
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Praxichys
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I only use 1000ml stirring mantles. In my experience, they are fine for heating smaller flasks. I think the radiant heat makes up for most of the
losses caused by lack of contact. If you really need to get something hot, to the limits of the mantle, just put some foil over the top of the mantle
with the flask poking through it for an extra boost.
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Magpie
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I have only 100mL and 500mL mantles. I use them satisfactorily for 25, 50, 100, 250, and 500mL RBFs. When heating my 1000mL RBF I use a flattened
out 500mL mantle I bought off eBay that was misrepresented as a 1000mL mantle.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Boffis
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@ Philip Carvel; I routinely use 2L flasks in a 3L mantle without any problems and occasionally 250ml flasks in my 500ml mantle so yes. They are
perhaps slightly less efficient but the main problem I find, particularly with the 250ml flasks is that you seem to get a rim of dried solid above the
liquid on occasions because the heating takes place, in part, from above.
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Loptr
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I was under the impression heating larger mantles without providing someway for the heat to dissipate would cause problems, but what do I know, my
major was CS. 
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Chemosynthesis
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I've seen and done this before. It's not ideal, but if you are having contact issues, try using aluminum foil (works with a hot plate also). I have
seen, but do not recommend filling with sand. Have also seen glass wool insulation over the top portion of the flask.
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Dr.Bob
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You can usually mold/squish a mantle to fit one size larger or small flask pretty well, and then you don't need to do too much to make it work.
Plus, you must control the power to it, so that it heats rather slowly, I find about 35-45% power cycle (or ~30-50 volts with a variac) will keep them
from overheating, with the proper or mismatched size flask. Sand (or bbs, other conductive filler, etc) works, but once you have it in a mantle, it
is hard to remove it all. Then best thing I would suggest is to figure out what scale glassware you plan to use and try to buy that mantle first, if
you use 100 ml a lot, that will cover 50-100 well, maybe 150 ml. if you buy a 500 ml mantle, that will do 250-500 well, maybe 1000 if it is
flattened out (some come that way, it seems). Those two size cover most normal sized 24/40 flasks, as Magpie stated. If you are doing smaller
sizes in 14/20, you might need a 50 and a 100 ml or a 100 and 250 ml.
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BromicAcid
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I have used a larger mantle for a small flask. In those instances I've taken some glass blankets and put them around the flask to prevent losing all
of my heat. The only thing I caution you on is that if anything is spilled in the mantle, it has more access to oxygen than when a flask is sitting
flush against the surface. Inevitably a small amount of smoke or funk escapes during the heating due to something being in the mantle.
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subsecret
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Copper pellets (like the ones you might see at the sporting goods store) would work well as a conductive filler material.
Fear is what you get when caution wasn't enough.
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PhilipCarvel
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Thank you all, this has answered my question really well. I'm planning on purchasing a 1000mL heating mantle and working around volume issues, rather
than buying a hotplate and rigging various baths. Dr. Bob, you mentioned using a variac to control voltage with heating mantles. I was researching
different methods of voltage control and came across your discussion on Heating Mantle Controllers. I'm considering using smaerd's suggested design
from the pdf he posted, using a 600W dimmer switch. Would that be a viable alternative, or a dangerous shortcut?
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Dr.Bob
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A variac works fine, as does a fancy mantle controller, but a simple dimmer switch will also work fine. Just make sure that it can handle the
wattage of the mantle you are using, with a little extra to spare, but 600 w would likely work fine. You just need to control the power somehow,
either my limiting the voltage or the current, as long as you do it somehow, it will work fine. I have used a simple light dimmer in a metal box,
cut an extension cord in half and put half on each side, then you have a simple power controller. I like 10A ones, but you can get one at Home
Despot for about $10-20, I think. I still like variacs, and use them often, as they can be used for many other tasks as well, but they are large,
heavy and expensive.
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subsecret
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I use a dimmer. My mantle controller is in a double gang junction box, with a dimmer and receptacle. The dimmer is wired to one outlet of the
two-outlet receptacle, and the other is not connected through the dimmer. This lets me plug my heating mantle into one outlet, and my condenser pump
into the other. The box is fed with a cord and a plug.
Fear is what you get when caution wasn't enough.
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