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

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Discussion Overview

The discussion centers around the potential effects of imperfections in a mercury barometer, particularly regarding the presence of air bubbles and the implications for accuracy in measurement. Participants explore whether adjusting the scale could compensate for these imperfections and discuss the temperature sensitivity of the device.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the presence of air bubbles in their mercury barometer and suggests that this may cause the reading to drop below 30 inches, questioning whether moving the scale down would be acceptable.
  • Another participant asserts that mercury barometers are inherently temperature sensitive, regardless of the presence of bubbles.
  • There is speculation that the expansion of mercury with temperature might be counteracted by the expansion of the imperfect vacuum, though this is met with skepticism.
  • Concerns are raised about the safety of handling mercury, with warnings about the dangers of mercury exposure and the environmental risks associated with improper containment.
  • One participant expresses a desire for numerical guidance on how to adjust the scale based on observed discrepancies, indicating a need for a better understanding of the error magnitude due to the imperfect vacuum.
  • A suggestion is made that if the barometer reads 29.5 inches instead of 30 inches, the scale should be adjusted accordingly, with a proposal for a mathematical approach to this adjustment.

Areas of Agreement / Disagreement

Participants express a mix of concerns regarding safety and technical adjustments, but there is no consensus on the best approach to compensate for the imperfections in the barometer or the implications of temperature sensitivity.

Contextual Notes

Participants mention various assumptions regarding the behavior of mercury and the effects of temperature, but these assumptions remain unresolved. The discussion also highlights the potential for differing interpretations of how to adjust the scale based on observed measurements.

ClamShell
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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?
 
Science news on Phys.org
Mercury barometers are temperature sensitive even in the absence of any bubbles.
 
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.
 
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
 
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
 
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.
 
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?
 

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