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The comparison between fuel gages and scrubber life seems to come up
regularly however a more appropriate comparison is with the fuel filter.
Like the scrubber it removes some component in the medium that passes
through it. In both cases the components removed, CO2 and crud, are present
in varying concentrations and for the most part unknown concentrations. The
manufacturer recommends replacing the filter at some interval which is
based on the normal level of contamination in typical fuel samples.
Changing the filter as per this recommendation will normally prevent the
car from stalling due to a clogged filter. But it won't prevent it from
clogging earlier if you have the bad luck to have your tank filled with
sludge at some third world gas station. Similarly a couple dives spent
swimming against a current will give you early scrubber breakthrough.
Lets consider some other variables:
Your new fuel filter was defective due to manufacturing errors, shipping
damage, improper installation. (I've actually seen mechanics remove an old
filter, wipe it off and reinstall it facing the opposite direction.)
Your CO2 absorbent faces all of the same risks.
One of the greatest risks in scrubber breakthrough is also similar to the
fuel filter. That fuel filter or scrubber that has been in use for longer
than it's recommended duration may seem to be working just fine until you
have to swim back to the ascent line against a current or accelerate to
pass that slow moving truck. This is when you will find that the scrubber
that was working so well a few minutes ago won't cope with your higher
exertion rate. Just like your partially clogged filter won't provide enough
fuel to get past the truck.
The best CO2 sensor in the world will not be able to warn you against this
kind of problem. There are simply too many stacked variables involved.
In the case of the fuel filter you can replace it according to the
manufacturers recommendation and you probably won't ever have a problem
with it, until you are unlucky enough to get that tank full of sludge. Or
you can ignore it until it plugs up and get towed to the garage. (Unless
you had a head on collision while attempting to pass the truck.) In either
of these situations there are at least two subsequent courses of action.
You can change the filter, pay the bills, and drive away. Or you (or your
estate's attorney) can write irate letters to the car manufacturer and oil
company complaining that the fuel was dirty, the filter was defective, the
recommended maintenance schedule was inadequate, the filter should have
been bigger, the car should be able to run on sludge, etc.
The options in the case of the scrubber are similar. You can change the
scrubber at the recommended interval and you probably won't have a C02 hit.
Provided you use good absorbent, pack it properly, pre-breathe (warm up)
the scrubber, and don't spent the entire dive swimming into a 2 knot
current. If you do have a CO2 hit it shouldn't be anything more than a
minor inconvenience, provided you are carrying adequate bailout. If you do
decide to push the limits of your scrubber you need to be aware of the risk
you are assuming. This means a constant awareness of your exertion rate,
the ability to recognize hypercapnia symptoms, and above all adequate
bailout. Without adequate bailout you may just have the aquatic equivalent
of a head on collision with a truck. Again there are at least two courses
of action. You bailout, surface, dump the used up scrubber, refill it, and
keep on diving. Or you (or again you estate's attorney) can start writing
irate letters to the rebreather manufacturer and the absorbent manufacturer
complaining that the sorb was defective, the scrubber was too small, the
recommended duration was too long, there should have been a CO2 sensor
installed, etc.
Gordon Smith
http://www.jetsam.ca
Kiss Rebreather
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