Bezaleel
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Correct position of thermometer in Claisen adapter
The supplier I bought from, indicated that this (see photo) should be the correct position of the thermometer bulb, but to me it feels as if the
region in the green circle would be a more correct position. (On the simple adapter, I always put the bulb before the middle of the outlet to the
condenser, but with the ground joints I cannot adjust its position.)
Any thoughts?
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Fyndium
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It's a matter of degree or less, but the bottom should be at takeoff level.
I torched one thermowell shorter myself to match the depth.
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Bezaleel
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Thanks, Fyndium. Just to make sure I understand you correctly, you say that the bottom should be where I drew the green line, right? So it should
ideally be a cm higher than where it is now?
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RustyShackleford
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I usually place it so its just below the line like it is in the image you posted. I dont think it matters if its below the line (by like 1-2cm, but
above can be inaccurate with low flow rate.
[Edited on 25-3-2021 by RustyShackleford]
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Sulaiman
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I agree with your supplier,
and you should 'calibrate' your thermometer in the position that you will use it,
distill water and, if practical, a pure sample of your target liquid.
'Ideally' the thermometer should be in contact with the vapour over its specified immersion depth,
and the adapter should be thermally insulated to reduce any temperature gradients.
This is rarely practical so in-situ calibration and consistent location is recommended.
CAUTION : Hobby Chemist, not Professional or even Amateur
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Fyndium
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What is said above is what I agree with.
The very specific position is not super-accurate, unless you are measuring tenths or hundreds of a degree differences, as long as the head is well
immersed in the vapors and is at least somewhat in line with the takeoff, it will give correct readings.
For more accurate measurements, like said, you will have to calibrate the thermometer and calculate the barometric pressure, your elevation, and such
things. When I distilled ethanol, I got consistent high readouts and did everything else, after I calculated the barometric pressure difference and it
was absolutely just on point at azeotrope. 1010bar barometric pressure can cause one % difference, which makes 78.2C show up as 78.9C.
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Fery
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I did some tests here:
https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
I found, that when the flask was almost empty, the vapor was further heated above boiling point very likely by hot flask walls.
e.g. I recorded 106 C for boiling water
so not only the thermometer bulb position is important but also the fulness / emptiness of the flask, whether the pot with hot oil is covered by Al
foil, how much hotter bath is used, whether the level of the liquid in distilling flask is above the level of oil bath or already below and how deep
below (so bigger glass area is heated above boiling point) etc
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Bezaleel
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Thank you all for your responses! This gives me a very clear idea on what to take into consideration when reading off my thermometer and setting up a
proper distillation. I already recognise a lot of the things you have pointed out - thanks for your explanations.
@Fyndium @Sulaiman, so far my applications do not require very high accuracy, but I wanted to make sure I do not introduce any unnecessary error by
wrong positioning.
@Fery, thanks for the link. A very nice read about your pineapple ester also.
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