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Author: Subject: Temperature Difference between Hotplate>Heating Block> Flask
LuckyWinner
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[*] posted on 12-8-2020 at 00:24
Temperature Difference between Hotplate>Heating Block> Flask


there is a 40C temperature difference between my hotplate
and heating block.

could not measure heating block and flask temperature difference,
since my thermometer broke.

is a temperature differences of 40C between Hotplate>Heating Block (> Flask +xC) , common,
and does
-wind
-insulation (alu foil, ceramic wool)
play a major role in temperature difference.

I already broke 2 flasks in my heating block by removing the alu foil insulation too quickly
on a windy day...


what is the temperature difference between the heating block and a 5x insulated flask that is sitting inside it filled with 1/2 solution ?

should this be around 20C ?

for example, heating plate 200C = 160C heating block temperature = 140C flask liquid temperature? (when insulated in al foil 5x) ?
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draculic acid69
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[*] posted on 12-8-2020 at 06:50


Sounds like your heating block is a problem.just buy a heating mantle.or use an oil bath
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macckone
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[*] posted on 12-8-2020 at 08:35


The difference between a heating block and hotplate should not be that significant.
It sounds like your heating block is not well coupled to the hotplate.
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LuckyWinner
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[*] posted on 12-8-2020 at 11:21


Quote: Originally posted by macckone  
The difference between a heating block and hotplate should not be that significant.
It sounds like your heating block is not well coupled to the hotplate.


indeed there is a 1cm hollow place between the heating block and the flask
but that should not be dramatic?

broke my 3rd flask today... after heating to 260C.
strangely the flask was perfectly cut exactly 1cm above the heating block line...
like a laser cut it into 2 pieces...

I guess you cant even peak under the aluminium foil once you reach high temperatures...
otherwise your flask is going to break.
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macckone
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[*] posted on 12-8-2020 at 11:55


It actually sounds like you are heating too quickly.
Cheap glass doesn't like to be heated quickly.
With quartz and fused silica you can get away with a lot of sins, not so much with cheap borosilicate.
There really should not be that much space between the block and the flask.
It sounds like it is primarily heating around the rim of the block and that is causing a lot of stresses.
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LuckyWinner
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[*] posted on 12-8-2020 at 12:20


Quote: Originally posted by macckone  
It actually sounds like you are heating too quickly.
Cheap glass doesn't like to be heated quickly.
With quartz and fused silica you can get away with a lot of sins, not so much with cheap borosilicate.
There really should not be that much space between the block and the flask.
It sounds like it is primarily heating around the rim of the block and that is causing a lot of stresses.


all my glass is borosilicate from duran, branded. I would not trust chinese glass for any hotplate activity.

now that all my RBF are broken , is it safe to heat a DURAN erlenmeyer flask
directy on a ceramic hotplate with aluminium foil to 170C, slowly?

over 30 min heating to 170C is slow enough for a 250ml DURAN erlenmeyer flask?
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macckone
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[*] posted on 12-8-2020 at 21:00


I think you will have more success with your erlenmeyer directly on your hotplate.
I would say the heating block is giving you more problems than it is worth.
Maximum heating rate of borosilicate glass is 10C/min but 5C/min is safer.
Thermal shock resistance is supposedly 160C but 100C seems more reasonable.

The standard test is to put it in boiling water and let it heat up then shove it in an ice bath.
This is done on a certain number out of a production lot.
Those that are tested are not sold, they are recycled.
Custom and cheap glass isn't tested like that.

One suggestion is to by a roll of fiberglass pipe wrap instead of foil.
It works a lot better than foil but is irritating to the skin.

I would slow down with the heating and use an oil or sand bath instead of that heating block.

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[*] posted on 12-8-2020 at 22:57


Heating block sounds clean and convenient but aluminum has so high thermal conductivity compared to water, oil or powder baths it is like hitting the glass with hammer instead of wiping it with feather.

Btw, where the heating rate is based?
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macckone
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[*] posted on 13-8-2020 at 06:53


That is from a lab glass website.
I would have to do a web search to find it.
Those are ideal heating rates.
Borosilicate should withstand sudden temperature changes up to 100C provided it hasn't been stressed.
Ie. just because you can doesn't mean you should.

And yes a heating block has to have good transfer in an even manner.
There is a reason professional labs still use water, oil and sand baths even when fitted heating mantles are available.
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[*] posted on 13-8-2020 at 08:10


I have used heating baths for almost a year now and will not heat by any other method, excluding very high temp reactions.
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S.C. Wack
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[*] posted on 13-8-2020 at 14:37


Putting sand in the block, and not heating to 260C at all, much less cooling with a blast of air, may reduce the number of these threads.



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[*] posted on 14-8-2020 at 04:04


Quote: Originally posted by S.C. Wack  
Putting sand in the block, and not heating to 260C at all, much less cooling with a blast of air, may reduce the number of these threads.


Using an appropriate sized block might help aswell.
As others mentioned, the block should make perfect contact with the heater and the flask.

I have a 250ml one for my older heating plate and would get other sizes if they werent so damned expensive.




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