The Bubbling
Barometer
I like brewing beer. Anyone who has
watched home-brew beer fermenting will be familiar with the
'bloop-bloop-bloop' sound effects of carbon dioxide escaping from the
brew keg through a brewer's
airlock. One day whilst watching a
merry brew a'brewing, I pondered if one could make a highly sensitive
barometer by using a really large empty keg (say of a few hundred
litres capacity) and by allowing just
changes
of ambient
air pressure to cause air flow through an airlock. After
several months of 'thought experiment' on
how one might
construct such a barometer, fate stepped in one morning when I walked
into our laundry to find our 10-year-old 400-litre
mains-pressure electric hot water
cylinder leaking water all over the floor (400 litres ≈ 100
Gallons). I'm pleased to
report
that our highly effective local plumber had the worn cylinder
replaced within a couple of hours, leaving me with with a
large thermally insulated steel pressure cylinder, precisely what was required for my
much-pondered barometer project.
The first problem was to find the leak. After partially
repressurising the empty cylinder with water via a garden hose, I could
hear air
escaping through the pin-prick hole. By cutting away the
cylinder's outer
steel jacket with an angle grinder, I located the offending hole and
repaired it (using solder
and my large 250-Watt soldering iron). After repairing the
hole I patched the outer cladding with tape. Then after removing
the four
port holes on the cylinder, I left it in the sun for a week, to drain
and to dry completely on the inside.
As any student of chemistry knows, PV=nRT, which is a formal way of
stating
that for a closed system, pressure is proportional to absolute
temperature, and inversely proportional to volume. To have the
cylinder operating as a barometer, where gas volume is proportional to
pressure only, it is essential to have the system held at
constant temperature, say better than ±0.1°C per day.
But
where
does one find such extraordinary temperature stability?... the answer
lies right under one's feet, so the next task was to bury the cylinder
into
the ground,
deep!
A 400-litre electric water cylinder is a
cumbersome and heavy (≈110kg or 240lbs) piece of hardware, and moving
it around
and
burying it deep-down presented a few minor problems. Fortunately
it just squeezed into our Ford Laser station waggon. Even more
fortunately I already had a made-to-measure hole in which to bury
it. Our neighbour had kindly dug us a large hole for future use
as a
rubbish pit, but these plans now changed, and this large hole became
the final
resting place for our dearly departed hot water cylinder. After a
brief burial ceremony (pictured right) attended by baby Max and
grieving
relatives, and including a brief reading from the Good Book*, our
cylinder was finally laid to rest some 1.5m down (* The CRC Handbook of
Physics and Chemistry, 63rd Edition).
With all cylinder ports
securely capped, and an air-line attached, I
commenced a partial burial so that I could assess the behaviour of the
air flow in/out of the cylinder as atmospheric pressure changed.
As the the cylinder cooled and assumed the temperature of the
surrounding soil, air was predictably sucked into the cylinder via the
air hose, and the airlock bubbled (the airlock is actually filled
with olive oil, as I do not wish to humidify the air within the
cylinder). Upon reaching a moderately temperature stable state,
the airlock ceased bubbling and the liquid level came to be affected
by changes in ambient air pressure only. By keeping an eye on the
ambient air
pressure which is logged
hourly by a local Bureau of Met Automatic Weather Station, it
became clear
that there was a very close correlation between airlock fluid levels,
and the BOM readings.
The first test of the Bubbling
Barometer came a couple of days after I buried the cylinder, when
I
noticed on a satellite image (left) that a weather front appeared to be
moving
toward us. The corresponding Mean Sea Level (MSL) pressure chart
forcasted falling local air pressure
throughout the day until the
trough was encountered, and indeed between 9am and ≈3pm we observed a
steady fall in air pressure of around 6hPa (ie. hectoPascals, which are
equivalent to milliBars) Needless to say this
fall in pressure caused the barometer's airlock to start bubbling as
air was steadily sucked out of the cylinder. And it was no subtle
effect, with a bubble passing through the airlock every few
seconds. So if one did not have access to real-time satellite
images and MSL pressure charts, and had to rely on the barometer alone
to forecast approaching weather, the Bubbling Barometer
gave an excellent indication that a significant weather change was
approaching.
I was curious to know just how
temperature-stable the buried cylinder really was, located at its mean
soil depth of 1-metre. Before burial of the cylinder I placed a
50mm dia PVC pipe (see right) down into the
cylinder pit so that later I could lower down a thermometer to check
the soil temperature. Recently I purchased a small
temperature logger from Jaycar.
This device, looking like and overgrown lipstick container, is perfect
for the task, and the first results are shown on the plot to the left
(click on plot for full-sized version). These data, showing
around four days of 1-hourly temperature measurements, indicate a very
slow increase in soil temperature at 1-m depth (≈1/6 °C per
day). The data logger has a measurement resolution of
±0.5°C, and within this accuracy there is not the slightest
indication of a diurnal (i.e. daily) variation. Which is what we
want to see, and got me thinking further about our local soil
temperature profile.
Having established the that barometer worked as
intended, the next task was to bury an air-line up to our house so that
the airlock display could be mounted somewhere convenient inside the
house. The
distance between the buried cylinder and the house is about
100m (≈330'), and for this link I used 4mm* polythene tubing, of the
kind commonly used for garden watering systems (*ID=4mm,
OD=6.2mm). To protect this
thin tube
from damage I placed it inside 13mm polythene pipe.
These polythene
pipes, and their various joiners and fittings, are surprisingly cheap
(eg. 100m of 13mm tube= AUS$27). The only big issue with laying
the air-line was the required 100m of trenching. We are located
in Scribbly
Gum (Eucalyptus Rossii) open woodland within a bushfire
prone region of NSW contiguous with the vast Pilliga Forest,
so burial of the air-line to a depth of 100mm is advisable.
The volume of air contained within the 100m of air-line was a concern,
given that it is buried near the surface where it is not in a
temperature stable environment. It's a concern until one
considers that the total volume of this thin air-line is 1.25 litres,
which is negligible compared with the buried cylinder's 400-litre
volume. On the subject of volume, if the ambient air pressure
changes by
1-hPa, what would be the volume of air that would flow in/out of a
400-litre cylinder? The elevation of our house is 541m (1774'),
where the mean air pressure is about 950hPa (compared with standard
sea-level pressure of 1013.25hPa). Since volume is proportional
to pressure at constant temperature, then a change of 1-hPa in
sea-level pressure would displace around
(400/1013)×(950/1013)≈0.37 litres of air in/out of our 400-litre
cylinder. So a modest air pressure change of say 10hPa would
displace
around four litres (≈1 Gallon), which
explains why the airlock display generally starts bubbling rapidly when
a significant change in the weather is occurring.
The buried cylinder is located in a very temperature stable
environment, but if for example the temperature did change by 1°C,
what
volume of air that would move in/out of the
cylinder? The absolute temperature of the cylinder is currently
around 290°Kelvin, so a 1°C change of temperature would
displace around 400×(1/290)≈1.4 litres of air. This is
equivalent to the
amount of air that would be displaced by a 3.7hPa ambient air pressure
change, which reconfirms the need to hold the cylinder temperature very
constant.
Eventually I made a nice job of mounting
the brewer's airlock on an attractive
wooden base made of Tasmanian
Oak , and this simple instrument now takes pride of place on my
wall
of meteorological instuments. Compared with all other pressure
measuring instruments I own, it provides the quickest visual indicator
of what the air pressure is doing; rising or falling, and the rate.
The liquid I chose for the airlock is
glycerine, and a 1hPa change in air pressure causes the fluid level
[between the two bulbs] to change by 8.1mm. It takes a pressure
change of
around 5hPa to get things bubbling. To smooth out
the rate of bubbling, so that there are many small bubbles rather than
a periodic monstrous "glurp", the rate of airflow through the
instrument is controlled by a hyperdermic needle placed through the
rubber bung at the top of the airlock.
'H' and 'L' labels indicate a Higher or Lower pressure tendency.
One curious feature I have noticed during times when the barometer is
bubbling, is that the bubbling may periodically stop for a few minutes,
and then resume. What I think is causing this effect is the
passage of microbaric pressure waves, which present a tiny pressure
moduation on top of the general barometric trend. Which has now
got me thinking about the measurement of microbaric variations, and
their causes... and which may well spawn a new instrument when I can
think of a good design.
If you should have inclination (and space!) to build an instrument
similar to this, please feel welcome to contact
me with any
queries.
A 400-litre electric water cylinder is a
cumbersome and heavy (≈110kg or 240lbs) piece of hardware, and moving
it around
and
burying it deep-down presented a few minor problems. Fortunately
it just squeezed into our Ford Laser station waggon. Even more
fortunately I already had a made-to-measure hole in which to bury
it. Our neighbour had kindly dug us a large hole for future use
as a
rubbish pit, but these plans now changed, and this large hole became
the final
resting place for our dearly departed hot water cylinder. After a
brief burial ceremony (pictured right) attended by baby Max and
grieving
relatives, and including a brief reading from the Good Book*, our
cylinder was finally laid to rest some 1.5m down (* The CRC Handbook of
Physics and Chemistry, 63rd Edition).
With all cylinder ports
securely capped, and an air-line attached, I
commenced a partial burial so that I could assess the behaviour of the
air flow in/out of the cylinder as atmospheric pressure changed.
As the the cylinder cooled and assumed the temperature of the
surrounding soil, air was predictably sucked into the cylinder via the
air hose, and the airlock bubbled (the airlock is actually filled
with olive oil, as I do not wish to humidify the air within the
cylinder). Upon reaching a moderately temperature stable state,
the airlock ceased bubbling and the liquid level came to be affected
by changes in ambient air pressure only. By keeping an eye on the
ambient air
pressure which is logged
hourly by a local Bureau of Met Automatic Weather Station, it
became clear
that there was a very close correlation between airlock fluid levels,
and the BOM readings.
The first test of the Bubbling
Barometer came a couple of days after I buried the cylinder, when
I
noticed on a satellite image (left) that a weather front appeared to be
moving
toward us. The corresponding Mean Sea Level (MSL) pressure chart
forcasted falling local air pressure
throughout the day until the
trough was encountered, and indeed between 9am and ≈3pm we observed a
steady fall in air pressure of around 6hPa (ie. hectoPascals, which are
equivalent to milliBars) Needless to say this
fall in pressure caused the barometer's airlock to start bubbling as
air was steadily sucked out of the cylinder. And it was no subtle
effect, with a bubble passing through the airlock every few
seconds. So if one did not have access to real-time satellite
images and MSL pressure charts, and had to rely on the barometer alone
to forecast approaching weather, the Bubbling Barometer
gave an excellent indication that a significant weather change was
approaching.
I was curious to know just how
temperature-stable the buried cylinder really was, located at its mean
soil depth of 1-metre. Before burial of the cylinder I placed a
50mm dia PVC pipe (see right) down into the
cylinder pit so that later I could lower down a thermometer to check
the soil temperature. Recently I purchased a small
temperature logger from Jaycar.
This device, looking like and overgrown lipstick container, is perfect
for the task, and the first results are shown on the plot to the left
(click on plot for full-sized version). These data, showing
around four days of 1-hourly temperature measurements, indicate a very
slow increase in soil temperature at 1-m depth (≈1/6 °C per
day). The data logger has a measurement resolution of
±0.5°C, and within this accuracy there is not the slightest
indication of a diurnal (i.e. daily) variation. Which is what we
want to see, and got me thinking further about our local soil
temperature profile.
Having established the that barometer worked as
intended, the next task was to bury an air-line up to our house so that
the airlock display could be mounted somewhere convenient inside the
house. The
distance between the buried cylinder and the house is about
100m (≈330'), and for this link I used 4mm* polythene tubing, of the
kind commonly used for garden watering systems (*ID=4mm,
OD=6.2mm). To protect this
thin tube
from damage I placed it inside 13mm polythene pipe.
These polythene
pipes, and their various joiners and fittings, are surprisingly cheap
(eg. 100m of 13mm tube= AUS$27). The only big issue with laying
the air-line was the required 100m of trenching. We are located
in Scribbly
Gum (Eucalyptus Rossii) open woodland within a bushfire
prone region of NSW contiguous with the vast Pilliga Forest,
so burial of the air-line to a depth of 100mm is advisable.