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Author: Subject: Melting point apparatus
veganalchemist
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[*] posted on 3-2-2013 at 08:33
Melting point apparatus


Hello all,

this is my first post but have been around for a while.

I want to build a melting point apparatus.
I plan to use a heating cartridge (about 200 W) but need to control the power input.
A Variac would but really need to control the heating rate.
Would a PID controller work?



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ScienceSquirrel
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[*] posted on 3-2-2013 at 15:35


You can do it with a big test tube filled with mineral oil.
To stir you want to use a coil of wire or if you want to get sophisticated a flea and a magnetic stirrer.
A thermometer of an appropriate range with a sealed capillary filled with the test substance is strapped to it and placed in the oil.
Heat very slowly with stirring using a small colourless gas flame.
If you want to be really traditional, puff at a long stemmed pipe and stroke your beard while doing the mp! :)
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mr.crow
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[*] posted on 3-2-2013 at 18:41


Well a PID controller would be great for setting a desired temperature. For a melting point you want a slow constant temperature increase, not a feedback loop.

To control the power to a heating element use a standard dimmer switch in an electrical box. This also works great for heating mantles.




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chemrox
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[*] posted on 4-2-2013 at 13:19


Squirrel describes an alternate to the Thiele tube. Thiele tubes can be had from ebay and most science supply stores. Their use is a time honored way to get melting and boiling points. The Thiele tube obviates the need for a stirring "flea" by providing constant flow of the hot oil due to convestion through the tube. However, to avoid the messiness of mineral oil and improve precision a melting point apparatus is preferred. However, you can't do boiling points with (most) melting point machines. Besides this is clearly a DIY adventure, like building a radio.



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veganalchemist
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[*] posted on 4-2-2013 at 13:39


I work at a local Uni and the undergrads use tubes filled with oil for melting points.
I want to build something a bit more sophisticated, where I can control the temp rate.
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watson.fawkes
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[*] posted on 12-2-2013 at 10:05


Quote: Originally posted by mr.crow  
Well a PID controller would be great for setting a desired temperature. For a melting point you want a slow constant temperature increase, not a feedback loop.
PID controllers can be used for a ramp, though the math is a bit more involved. The error term is calculated from the target temperature as a function of time, not simply as a constant. That said, it's desirable that the thermal mass of the sample holder be rather larger than the sample, to make it easier to get a consistent ramp, and also that thermal conductivity be high, so that temperature consistency is good.
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