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February 23rd 2014

22 Feb 2014 ="post-tag" > Written by  ="post-tag" >

Oxygen tanks?
scuba-cylindersQuite often when teaching an open water course, students refer to the diving cylinder as the oxygen tank. Not really that surprising as we need oxygen to survive, but walking around in our day to day lives we don't breathe pure oxygen. We breathe air, which is rounded up to be 79% nitrogen and 21% oxygen. The gas you breathe when you go diving is the same air as you are breathing now (unless you are somehow reading this on mars), it's just compressed into a cylinder that you breathe from during your dive. So here's a few facts about the thing that enables you to be underwater in the first place.
You may have dived all over the world, and used cylinders of varying sizes and shapes. But could you tell the difference between them? There are three types of cylinder; Steel, aluminium, and carbon fibre. You'll never ever see a carbon fibre cylinder as they are very expensive and pretty new. Dive resorts could never afford them and they'd have no need to use them anyway. Steel and aluminium are the types you'll always encounter. Steel is heavier than aluminium, but it's also stronger so the walls of the tank are slightly thinner. Because steel is stronger, air can be compressed to a higher pressure. So you can have a smaller cylinder, such as the so-called stubby. Steel is more liable to corrosion than aluminium, so steel cylinders have to be painted, and re-painted due to their exposure to saltwater. In tropical areas such as Thailand, you'll only find aluminium being used; they are less affected by saltwater and cheaper than steel.
The standard cylinder as used by recreational divers is the AL80. The AL stands for aluminium and the 80 means that it can hold 80 cubic feet of air. Things can get a little confusing as cylinder sizes can be described by internal volume of gas or water capacity (how much water it can hold), or by the nominal volume of gas stored. Being all fancy we use metric everywhere except the US, so it's common to describe an AL80 cylinder as an 11 litre (10.94l to be exact), that is rated to be filled to 210 bar of pressure. Tanks need to be periodically inspected to ensure that they are fit for purpose. A visual inspection involves looking inside the cylinder for corrosion, metal fatigue, and especially corrosion around the neck of the cylinder where the threads that house the valve live. A hydrostatic test is a pressure test, undertaken to determine that the metal is not fatigued (metallurgists call it creep or cold flow- I used to be one- a metallurgist, not a creep).. Cylinders are stamped with the latest hydro test date, and when they need to be performed depends on where you are in the world. In Thailand it's every five years.
In recreational diving, as a fun diver using a dive resort's cylinder you don't really care too much what type of tank you have, as long as it will give you a decent dive time. But with technical diving we need to know exactly what we're dealing with, as we plan our dives using software and have to calculate the gases that we will need. Anyone who has done a course with Big Blue tech will be able to tell you that 11 litres at 200 bar equals 2,200 litres of air. As tech divers have two tanks on their back that's 4,400 litres (not as is commonly thought, two tanks equalling 400 bar!). By calculating your Surface Air Consumption (SAC) rate, you can determine exactly how much gas you will need on a given dive. This means two things, tech divers clearly have OCD, and you can tell a tech diver out on a recreational dive as they'll be sat on their own in a corner of the boat, rocking back and forth with a calculator in hand, trying to look busy to hide their lonely tears.
As a customer of a dive resort you don't have to worry too much about diving cylinders. It's probably a good idea to check that they are within their hydrostatic test date, the valve is in good condition and the o-ring that will create the seal with your regulator is good to go, and generally that the clyinder and valve don't look they were recently purchased from an antiques shop. Apart from that, don't pick them up by the handle.. yes that means you divemasters, instructors and DMTs, don't bash them about, and never let them get completely empty (water can then get in and corrode them). Just connect them to the breathy thing and your air-filled diving cardigan, and away you go.
If your interested in learning more about diving cylinders, regulators, BCs and other diving equipment, Big Blue Tech runs regular equipment service technician courses. Have a look on the Big Blue Tech website and get in touch to find out more.

Lowest of the low
Continuing the theme of creepy crawlies, here's some facts about every tourist and restaurant owner's nemesis, the cockroach:

- Cockroaches could survive a nuclear war. For humans, a dose of 800 or more rems would be lethal. The lethal dose for the American cockroach is 67,500 rems and for the German cockroach it is between 90,000 and 105,000 rems
- A cockroach could live a long time, perhaps a month, without its head.
- Number of legs on a cockroach: 6
- Number of knees on most cockroaches: 18 at least
- Number of minutes cockroaches can hold their breath: 40
- Time that cockroaches spend just resting: 75%
- Cockroaches can run up to three miles in an hour.
- Male cockroaches transfer sperm to females in a “gift-wrapped” package called a spermatophore. Some males cover the package in a protein-rich wrapping that the female can eat to obtain nutrients to raise her young.
- The New Zealand Y2K Readiness Commission gave out a recipe for cockroaches in case the world ended on New Year’s Eve, 1999. “Simmer cockroaches in vinegar. Then boil with butter, farina flour, pepper and salt to make a paste. Spread on buttered bread.”
- Cockroaches can make up to 25 body turns in a second – the highest known rate in the animal kingdom.
- Cockroaches can respond remarkably quickly – after around 29 milliseconds – to the sensory cues that their antennae deliver.
- Blinded and deafened cockroaches were able to navigate completely normally, even if their average speeds were lower than their sighted and air-current-sensitive counterparts.
- Female cockroaches prefer males at the bottom of the social pecking order, and dominant males try and stop them from having their way. But when females do get the low-ranking man of their dreams, they produce fewer sons, apparently in an effort to avoid passing on his wimpishness.
- Scientists claim some female cockroaches prefer weaker partners because they like gentle sex. A University of Manchester team has concluded stronger male cockroaches are too aggressive and often injure their partners.
- Most species give birth to live young — highly unusual for insects — but a sure way to prevent other critters from feeding on their eggs.
- If food is scarce, adolescent cockroaches can live on a very reliable resource — their parents’ feces.

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