Monday, June 15, 2009

Life under pressure, part II

It turns out that Blogger cuts your message after the picture, so
here's the main bit of writing from yesterday:


First, let's set the scene. Picture yourself standing just inside the
doorway of your regular, 20 foot refrigerated shipping container.
Throw in a bolted-together green Unistrut steel internal frame (you
can see a horizontal member on the top of the back wall) with
varnished plywood bolted up along the sides. Now throw in some heavy
shelves, a few benches, and a big stainless steel tool chest. Set the
noise from the reefer unit to 'periodically earsplitting.'

The main event here, now that we've got the lab itself built, is the
high-pressure aquarium. These are basically scuba tanks with a
removable lid. There are three such aquaria in this picture, wrapped
in insulation in the left foreground. These particular vessels are
turned from solid titanium and have a pressure rating of around
4500psi. They have an internal volume of about 3L, and thanks to the
Ti, weigh in at a relatively svelte 60 pounds or so.

Next, we need to get water into the aquaria. These are flow-through,
meaning they have a constant supply of clean water coming in to them.
Our water is piped in through a hose from the ship, filtered, and
collected in a reservoir at the back of the van, just off the left
frame of the picture. The seawater is then pumped to the clear plastic
equilibration column in the top center of the photograph. Here, a
mixture of gasses is bubbled through the water to adjust the chemistry
to desired levels -- oxygen, hydrogen sulfide, and so forth. Finally,
the equilibrated water is pumped through the big blue high pressure
pumps into the vessels. A backpressure valve on the outflow keeps
things at a cozy 3500psi.

On the other side of things, we need to measure what's coming out of
the vessels -- things like oxygen consumption, carbon dioxide and
hydrogen sulfide uptake, pH, and so forth. We do this in two ways. The
first is an analytical stream, including pH and oxygen sensors, a
cyclic voltammetry electrode, and a mass spectrometer, to which a
stream selector valve switches each vessel for about 30 minutes at a
time. The analytical stream gives us real-time data on what's going on
with vessel chemistry.

In addition, we take and preserve water samples for later analysis.
That's what the fancy-looking bank of valves above the pressure pumps
is for, allowing us to easily divert water from the waste stream of
each vessel into a test tube or sample vial.

At the end of the day, presuming check valves don't fail, backpressure
valves don't fail, pumps don't fail, motors don't fail, instruments
don't fail, and operators don't fail, all this adds up to some
interesting and unique data about amazing critters from the bottom of
the sea. Pretty neat!

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