Sulaiman
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Mercury Barometer construction advice please
To make a mercury barometer,
I already have 980g of ‘double distilled’ mercury and a 1m stainless steel ruler,
I have two 1m long, 8mm i.d., 2.5mm wall thickness borosilicate glass tubes on order.
I have read a few articles on barometer construction, but have some points not clear yet,
So I am asking for advice;
1) What would be a good way to clean/condition the inside of the borosilicate glass tubes so that they are super clean and do not contaminate the
mercury?
2) Does anyone who has made a mercury barometer have any practical advice ?
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j_sum1
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My opinion.
Buy a barometer. Keep the mercury.
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Sulaiman
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I want to have an 'accurate' barometer for atmospheric pressure (b.p. etc.)
using the typical mercury in a tube sealed at one end design.
(with a correction vs. temperature lookup)
I also want to use it as an absolute vacuum gauge
by attaching the vacuum to the un-sealed tube (other half of U-tube)
allowing pressure measurement of Atmospheric to zero.
I could use an aneroid barometer and calibrate/set it vs. known atmospheric pressure,
but I like the idea of a guaranteed accurate (primary reference ?) instrument.
[Edited on 3-8-2015 by Sulaiman]
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annaandherdad
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I would imagine that if the glass were reasonably clean, then any small contamination left would not affect the accuracy of the barometer appreciably,
since it would have to be enough to change the density of the mercury by about 0.1% or more, and that sounds unlikely to me.
Or put it this way. Suppose there is a layer of "crud" on the inside of the glass of unknown, small, possibly very small, thickness. Now the crud
may or may not dissolve in the mercury. If it does not, and if it remains stuck to the side of the glass, then it would not affect the height of the
column. If it does not dissolve in the mercury, and if it comes loose, then it will float to the surface, where it will exert some force downward on
the mercury column because of its weight. Now suppose that the density of the crud is something like 1/4 that of the mercury (maybe 3 gr/cm^3).
Then in order to suppress the top of the mercury column by 1mm, the layer or crud floating on top would be 4mm in thickness, and easily visible. In
fact, even a fraction of 1mm of crud floating on the surface would be visible.
Supposing we had 4mm of crud floating on the surface, thereby depressing the mercury by 1mm, then the volume of crud is pi x (4mm)^2 x 4mm, which if
spread evenly over the walls with an area of 2 x pi x 4mm x 760mm would mean that the layer of crud on the walls was .04mm thick. Do you think you
have that much crud?
If the crud does dissolve in the mercury, then of course it would be invisible, and it would cause the level of mercury to rise (because its density
with dissolved crud would be lower). But the amount of crud needed to raise the level by 1mm would be comparable to the amount calculated above, so
the question remains roughly the same, do you think the crud on the inside of your tubing is as thick as .04mm?
You could always treat the inside of the tube with piranha solution.
Any other SF Bay chemists?
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annaandherdad
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Actually maybe outgassing from the layer of crud on the walls above the mercury would be a bigger effect.
Any other SF Bay chemists?
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Sulaiman
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my assumption is that if the mercury stays shiny and the meniscus shape remains constant,
then I will mainly be concerned with loss of vacuum and thermal expansion effects.
I will undoubtedly empty and refill the barometer too many times,
so the fewer starting problems the better.
as I skimmed through various barometer related web pages I saw mention of cleaning and conditioning the inner glass surface,
also the use of a thin layer of liquid above the mercury to .........
I must learn to keep pointers to references.
I guess now is a good time to start.
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annaandherdad
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I'd be curious to know more details about this, how the conditioning is done, and what the liquid is.
Any other SF Bay chemists?
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j_sum1
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I wasn't meaning to be condescending or dismissive here. It is just not a project that I would take on. A kg of mercury has so many interesting uses
without locking it away in a glass tube. And, as far as I know there are good devices that are able to measure atmospheric pressure to within 0.1kPa.
I seriously doubt that any device I constructed could attain that accuracy.
One thing to consider. What is the diameter and thickness of your glass tube? Both of these factors will affect the ease with which you can read the
level of the meniscus against a scale.
As for impurities, as mentioned there are two things to consider. One -- any impurities that significantly alter the density of your mercury. i
think you will be ok here. The second -- anything that contributes to the vapour pressure in the tube. I guess you want your glass clean and dry but
you don't want any residue of any solvents clinging to the glass. I think I would oven dry the glass. By way of comparison, you want it as free of
volatiles as a GLC tube. I think that would be easy to attain.
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Sulaiman
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The glass tubes are 8 mm internal diameter, 2.5 mm wall thickness,
Design = bid on eBay mercury listings for months, when finally won some, just under 1 kg, order glass tubes to suit.
I would have liked more mercury / wider tubes to reduce the meniscus depression effect
but this will be OK as I am using a U tube so errors will subtract out.
Mercury + Glass = £35 + £16 = £51 + misc extras.
I do not think that I can buy an accurate aneroid barometer for the same cost
and this one will also allow measurement of vacuum.
I like the idea of oven drying the glass, not sure if our oven will fit 1m glass tubes, t.b.d.
So how do you clean tubes for GLC ?
[Edited on 4-8-2015 by Sulaiman]
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j_sum1
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Well, GLC runs hot and the vapours pass through eventually as a dry inert gas goes through it. And that is when it is packed with a medium that the
gas molecules adsorb onto. So, I would guess that after thorough mechanical cleaning, passing hot dry N2 through for several minutes would probably
eliminate traces of any solvent that remained.
Let me stress that I am no expert here and am running on pure instinct. If someone has a better idea or more qualified opinion, you better listen to
them.
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Sulaiman
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mmmm ... hot dry N2 ... I'll pop down the local shop and see what they've got 
The glass tubes arrived today ... smashed!
contacted seller as not only smashed but wrong size 
must be bad glass karma,
I recently broke a B10 25 ml quickfit pear shaped flask
and a B10 air condenser that I was using as a fractionating column.
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Sulaiman
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update
Two (1=spare/OOPS) new Simax borosilicate glass tubes arrived safely,
each 1.5m long x 16mm o.d. x 3.5mm wall thickness.
presently filled with 0.1 M HNO3 as (I have read) new glass has an alkaline surface.
(since it took almost exactly 100ml each to fill the tubes the i.d. is c9.2 mm ... within specifications)
next = thorough rinsing with distilled water then a rinse with acetone and hot air drying.
(now that I have 1.5m tubes they will not fit into our domestic oven)
Based on my tests with the broken borosilicate tubes, I need to get a bigger hotter flame so I will have to get a gas torch ...
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