Can adjusting the scale compensate for imperfections in a mercury barometer?

[ClamShell]

I have noticed that when I fill the 36 inch glass tube of
my mercury barometer, that there are thin bubbles of
air stuck between the mercury and the inside wall of the
tube; can't seem to get rid of them. Maybe a vacuum
needs to be pulled before pouring in the mercury, but
that seems too extreme for my "frugal" means. I'm
thinking that the reason it steadily drops over some
days to less than 30 inches, when the value is being
reported as 30 inches, is because of these bubbles.
I'm thinking that the vacuum has been compromised.
So I wonder...is it OK to just move the inches scale to
the lower point? Does it become temperature sensitive?
Does the length of an inch on the scale need to change
too?

 

[dauto]

Mercury barometers are temperature sensitive even in the absence of any bubbles.

 

[ClamShell]

Maybe the expansion of the mercury with temperature
would even be counteracted by the expansion of the
imperfect vacuum. Maybe I've got just the right
imperfection to temperature stabilize the beast...
doubt it though.

 

[Chestermiller]

Are you sure you want to be messing around with mercury like this? Please tell me you're not doing it in your home. You have to be aware of how dangerous mercury can be for you and, if you lose containment, for the environment. Please be careful.

Chet

 

[AlephZero]

Most likely the tube and/or the mercury is dirty.

As Chestermiller said, messing about with mercury is dangerous if you don't know what you are doing, and a "badly designed" (by modern safety standards) mercury barometer is a permanent health hazard, continuously emitting mercury vapor.

You shoudn't be "pouring" mercury into the tube in any case. http://www.m-p.co.uk/muk/acrobat/0796000115.pdf

 

[ClamShell]

I was hoping that someone could answer my questions
above, so that I would not be tempted to re-do that
antique monstrosity. Thanks for the link Aleph, it was
especially nice to read that the author said "an effective
working reading" could be had without a 100% vacuum.
I have used these techniques on other project barometers
in the past and was never able to get the full height to
agree with the reported value. Never spilled any either;
you just need to go slow.

If I ever do re-do the beast, I'll seriously consider the
vacuum pump method. Have you a link on the vacuum
method?

Still would like to see some numbers though. Something
like "if it's 29.5 inches instead of 30 inches, then moving
the 30 inch mark to the 29.5 inch level will mean..."
That is to say if I could get a feeling for the magnitude of
the error due to imperfect vacuum, and correct it on the
scale, I would be less likely to consider messing with its
guts.

Thanks for the concern, I've always considered mercury
to be highly respected. I read that something like 17 tons
a year ends up in landfills in the USA.

 

[ClamShell]

Somebody out there MUST have a rigorous analysis mercury
barometers...

I'm thinking that there is a simple solution to the problem
of an imperfect vacuum in the top of the tube.

Namely that if 30 inches has been reduced to 29.5 inches,
that an inch on the readout scale should be reduced by the
factor 29.5/30.0. And if the mercury fell to 15 inches, then
an "inch" on the scale would be only a half inch.

Anybody got some better arithmetic?