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Author: Subject: Heating mantle temp question
captainC11
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[*] posted on 22-4-2019 at 02:14
Heating mantle temp question


Hi guys

If one placed a 1L or 500ml rbf in a three litre heating mantle would the heating mantle digital temperature be same as internal temp or only be few degrees off ? I notice a lot of the heat comes from the sides of the mantle . If so would insulating wool or metal bb bullets be good to fill sides to ?

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happyfooddance
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[*] posted on 22-4-2019 at 14:04


It is definitely a good idea to fill empty space with metal shot or even thick Al foil balled up, but you DON'T want to use insulating wool: that would have the opposite effect from transferring heat, and would quickly burn out your mantle.
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captainC11
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[*] posted on 23-4-2019 at 19:57


Quote: Originally posted by happyfooddance  
It is definitely a good idea to fill empty space with metal shot or even thick Al foil balled up, but you DON'T want to use insulating wool: that would have the opposite effect from transferring heat, and would quickly burn out your mantle.


Thanks but the real question I wanted to know is the digital temp on mantle would be close to internal temp regardless of rbf size if one put 1L or 500ml in 3L mantle ?

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happyfooddance
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[*] posted on 23-4-2019 at 20:15


I would use a thermometer or thermocouple and find out yourself. I use an infrared thermometer almost religiously in the lab and in the kitchen.

What kind of mantle is it? The heating algorithm from one board to another varies significantly.

Most chemists that do a lot of fine control heating will use a variac (best) or PID (ok) over a digital setting for most applications.
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BromicAcid
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[*] posted on 23-4-2019 at 20:36


It depends, if you are just heating a flask of liquid to some point below the boiling point and you hold it there, eventually you may come to some equilibrium. However, in most cases this will not be the outcome. If you are distilling something your contents will hold at the boiling point but the mantle will heat hotter and hotter. Think of a pan of water on an electric stove. That electric element is glowing red hot but your water is holding steady at about 100°C. If you're distilling under vacuum or anything like that the temperature differential will be even more extreme.

As others have said, don't insulate around the flask. I have wrecked mantles by heating them too hot with a flask that does not fit them correctly. In one of those cases I was stripping off water, water takes a lot of energy to distill, the mantle was glowing red hot, the whole thing 'glassed' and became useless afterwards. I have never tried metal beads but I can see how it could work.




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captainC11
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[*] posted on 24-4-2019 at 18:09


Quote: Originally posted by BromicAcid  
It depends, if you are just heating a flask of liquid to some point below the boiling point and you hold it there, eventually you may come to some equilibrium. However, in most cases this will not be the outcome. If you are distilling something your contents will hold at the boiling point but the mantle will heat hotter and hotter. Think of a pan of water on an electric stove. That electric element is glowing red hot but your water is holding steady at about 100°C. If you're distilling under vacuum or anything like that the temperature differential will be even more extreme.

As others have said, don't insulate around the flask. I have wrecked mantles by heating them too hot with a flask that does not fit them correctly. In one of those cases I was stripping off water, water takes a lot of energy to distill, the mantle was glowing red hot, the whole thing 'glassed' and became useless afterwards. I have never tried metal beads but I can see how it could work.

Lets say it’s for refluxing purpose would pointing an infrared thermometer at the flask while heating give accurate internal temp ? Or would throwing a thermometer inside flask be better or no different ?
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MrHomeScientist
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[*] posted on 25-4-2019 at 07:44


Pointing an infared thermometer at the flask would give you the temperature of the external surface of the glass. If you're ramping the heat up slowly and/or have let the flask sit in the heat for a while, you might come to equillibrium and you might read close to the internal temperature.
A thermometer in the flask in direct contact with the liquid would, of course, be the most accurate method.
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RedDwarf
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[*] posted on 26-4-2019 at 13:01


I don't think you have a hope in hell of the mantle display showing anywhere near the internal flask temperature. Even if you use ball bearings within the mantle the point of contact between individual bearings for direct conduction is small so a lot of the heat transfer will still be radiative/convective or conductive through the air pockets and the external flask temperature will vary significantly from the mantle temperature. Using an infrared thermometer runs the risks of inaccuracies due to the size and "placement" of the measuring area - basically if you are too far away or point it in slightly the wrong direction you'll be measuring mantle or environment or something other than the flask wall.
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captainC11
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[*] posted on 26-4-2019 at 18:41


Quote: Originally posted by RedDwarf  
I don't think you have a hope in hell of the mantle display showing anywhere near the internal flask temperature. Even if you use ball bearings within the mantle the point of contact between individual bearings for direct conduction is small so a lot of the heat transfer will still be radiative/convective or conductive through the air pockets and the external flask temperature will vary significantly from the mantle temperature. Using an infrared thermometer runs the risks of inaccuracies due to the size and "placement" of the measuring area - basically if you are too far away or point it in slightly the wrong direction you'll be measuring mantle or environment or something other than the flask wall.


What’s the best way to measure internal temp a thermometer in the flask ? I guess would be the best
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[*] posted on 27-4-2019 at 05:36


Yes thermometer or thermocouple inside the flask
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