LEARNINGCURVE
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any divers take for granted the
gas in their cylinders. We go to
the dive shop and ask for a fill,
pay up and go diving.
Occasionally, conversation may
turn to the cost of filling a cylinder, maybe
followed by a discussion on whether buying
a club compressor would be cheaper or
more convenient.
So, just what is involved in running and
maintaining a breathing-air compressor?
One way to find out is the Compressor
Operation Skill Development Course. Or if
you are an instructor, what better than to
teach this excellent SDC? It’s essential to
have experience using and managing
compressors; some background engineering
knowledge is helpful. Full engineer status
is not required. The focus is on teaching
an understanding of how compressors
operate, safely filling cylinders, and
managing compressors and the filtration
systems used – all aimed at producing air fit
for divers to breathe.
The practical aspects of compressor
operation and cylinder filling are, as you
would expect, a significant part of the
course. Servicing a compressor and its motor
is not part of the course and should be left
to those who are suitably trained. The
course, on the other hand, is open to all –
any students not holding Ocean Diver or an
equivalent qualification just need to know
about cylinder markings and safety
procedures before the course.
It’s a gas
Having spent many happy hours filling cylinders
on compressors, I have come to realise that in
general divers really don’t know too much about
the production and supply of compressed gases,
says National Instructor Marg Baldwin. As a
remedy, she recommends attending or
instructing on a Compressor Operation course
QLeft: Attaching the nitrox compressor whip to a cylinder RAbove: An overview of Southsea Sub Aqua Club’s compressor system
the whole filling process. It’s not necessary
for the students to remember the formula
for the gas law; it’s in the course materials,
but it is important that they know how
pressure, volume and temperature of a
gas are related. It’s always important to
bring classroom sessions alive so you could
use a bicycle pump to illustrate the
relationship between pressure and
temperature for example.
The law
The theory sessions may give students a
feeling of déjà vu to begin with, even for an
Ocean Diver. But there are frequent ‘light
bulb’ moments as the gas laws explain why
that annoying drop in pressure happens in a
diving cylinder that was warm immediately
after filling but has cooled.
The gas laws take on a whole new
meaning. After remembering Boyle’s law,
which describes how the pressure of a fixed
amount of gas changes when its volume is
changed, Charles’s law is used to explain
how compressing gases affects their
temperature. And Gay Lussac’s law,
sometimes called the pressure law, explains
why at constant volume, such as inside a
dive cylinder, if the temperature rises, the
pressure of gas increases; if the temperature
falls, the pressure drops.
The three gas laws mentioned above
interact to give the universal gas law,
understanding of which is fundamental to
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Stagebystage
RAbove: A 165-litre air bank
Compressor design comes next;
compressors used by divers are multi-stage
compressors with either three or four
stages. Each stage consists of similar
components, however as the size of each
stage reduces, in accordance with the
compression ratio, its mechanical strength
must increase to allow for the increase in
gas pressure.
Multi-stage compressors are more
efficient than single-stage compressors, as
the gas is cooled to dissipate the heat of
compression after each stage. As a course
instructor, you have an opportunity to take
the mystery away from adiabatic
compression, when all the heat stays within
the gas, and isothermal, when all the heat
Photographs of Southsea SAC’s compressor system by Martin Davies
of compression is dissipated so that the gas
stays at constant temperature.
After discussing compressor layouts, give
each student a copy of the visual aid
showing a compressor layout and, if
possible, go and look at the compressor to
be used in the practical session and find the
different parts.
Qualitycounts
Do you ever wonder what is in the filter
towers of a compressor, and what the
chemicals do? Find out as the air purity
standard BS EN12021, displayed beside
compressors, is explained. What do we
mean by clean air as defined by BS EN8478
and how is it achieved? Here is another
opportunity to remove mystery and
misunderstanding about the standard
of gases required for use in blends such
as nitrox.
Readithere
Before moving on to the practical aspects of
cylinder filling, there is an opportunity to
look in detail at cylinder markings and
labels. Exactly what do they all mean, and
what is their relevance not only to the
compressor operator but to the diver
g
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LEARNINGCURVE
Had your fill?
Although not essential, if students are
interested then you can work through
some practical examples of how
pressure changes in diving cylinders
from hot to cool, and cool to hot.
Remember to keep it simple and use the
Combined Gas Law in an appropriate
way for your students.
For those happy with maths and
equations it can be put as:
where
Vi = Initial Volume
Pi = Initial Pressure
Ti = Initial Temperature in Kelvin
Vf = Final Volume
Pf = Final Pressure
Tf = Final Temperature in Kelvin
Explaining that the Kelvin scale is a
measure of temperature where the
degree is the same as a degree
centigrade but the start of the scale is
offset by 273 degrees, so that 0°C is
273K
Always give a worked example.
Q: a cylinder is filled to 200 bar at 15°C it
is then left in the sun where its
temperature rises to 35°C; what
happens to the pressure in the cylinder?
RAbove: Recording
compressor hours on a nitrox
compressor, full layout below
A:
It will rise to 214 bar
Or if such equations do not come
naturally try spelling it out the rule to do
the calculation like this, knowing that in
the case of filling a cylinder, the initial
volume is the same as the final volume,
so that can be taken out of both sides of
the equation, thus working out (final
temperature in °C plus 273) divided by
(initial temperature in °C plus 273)
multiplied by initial pressure in bar gives
you the final pressure in bar,
Always give a worked example.
Q: a cylinder is filled to 200 bar at 15°C it
is then left in the sun where its
temperature rises to 35°C; what
happens to the pressure in the cylinder?
A: using the rule, above
(35+273)/(15+273) x 200 = 214
It will rise to 214 bar
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g using the cylinder? The importance of
checking cylinders before filling is covered
in detail and should be given significant
emphasis. It’s best to demonstrate with the
real thing: steel, aluminium, different sizes,
different labels.
Really highlight the importance of
checking test dates and if a cylinder is ‘out
of test’ do not fill it – even if it is for a mate.
Also highlight the importance of checking if
a cylinder is oxygen clean. If it is then it
needs to be filled with ‘clean air’ to protect
its clean status.
Fill’em up
A major aspect of the course is the practical
side of using a compressor to fill cylinders;
including learning how to start, operate
and stop a compressor and how to use
a compressor or a gas storage bank
to fill cylinders.
A compressor operator can refuse to fill
any cylinder if it is corroded, rusty or they
are unsure in any way about its condition
even if it is ‘in test’. Cylinders that fail tests
have to be destroyed and often a test
station will have such cylinders cut in half g
Photographs of Southsea SAC’s compressor system by Martin Davies
LEARNINGCURVE
QLeft: Air system filtration
g – which make an excellent visual aid. Or
you can look at the British Diving Safety
Group web site
www.bdsg.org/safety.htm and download
the document ‘Advice for cylinder care
2011’. The very graphic photograph of a
pony cylinder that exploded during filling
will stay with your would-be compressor
operators for ever. This cylinder was in test
and had no outward sign of the corrosion
inside – emphasising the need for cylinder
checking procedures to be supported by
safe operating practices.
As instructors we know the value of
preparation, and hopefully practice, before
delivering any course – the compressor
operation SDC really needs thought and
rehearsal. Whether it’s a compressor system
you already know or one that you haven’t
used before, make time to go through the
compressor set up beforehand including any
supporting paper work such as the
procedures, risk management systems, and
PRight: Harmworthy air compressor
filling logs. Come the day of the course
there should be no nasty surprises such as
the sudden realisation that the authorised
operators list and filter management logs
discussed in the theory session are not in
place or are not being used, for example
if the change of a filter is long overdue.
To teach this, you really need to know
the compressor you are to use and any little
quirks or nuances it may have. Use it – fill
some cylinders and read its operating
manual – before the course. As you teach,
follow all the required procedures including
the detailed completion of compressor logs
and procedures do not be tempted to
take short cuts. Use personal protective
equipment such as ear defenders and
safety glasses.
The practical session can be lengthy
as each student needs the opportunity
to follow the required procedures and have
hands on cylinder filling experience. A
minimum of 45 minutes per student
VAbove: Nitrox
compressor whip
arresting system
Photographs of Southsea SAC’s compressor system by Martin Davies
is a guide. The ‘hands on’ sessions may
influence how you schedule the timing
of your course. Depending on compressor
location and numbers of students it may
be better to do the theory one day and
have a separate day dedicated to the
practical sessions.
It’s worth including some detailed advice
at this stage. Opening the cylinder valve
and giving it a little ‘blast clear’ before
connecting the filling whip, can clear
out any tiny particles of debris and water
droplets and prevent them from being
pushed into the cylinder during filling
and being a potential source of internal
corrosion. Valves should be opened gently,
then if anything is not connected properly,
or there is a slight leak, closing down again
is easy. Filling should be done slowly; keep
the heat to a minimum and the diver will
get a better fill.
At all times, we need to demonstrate
respect for dealing with high-pressure gases,
so be patient with students who are nervous
of it and be firm with those who appear
blasé about the risks.
What next? You can encourage students
to think about going a step further and
becoming a gas blender (Nitrox Gas
Blender/Mixed Gas Blender SDC) so that
they can fill nitrox and mixed gases.
My thanks to Coventry Sub Aqua Club
Branch 58 and Southsea SAC Branch 9.
Both clubs have run their own compressors
for a long time and have contributed to
the writing of this article, and to Frontier
Expeditions for the opportunity to teach
compressor operation in remote locations
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