Dive Theory - Equipment
tanks / cylinders - tank markings
Govt. Agency Supervision / Approval DOT or DOT/CTC = Dept. of Transport or Canadian Transport Comm.
Metal Type
3AA = Steel / SP6498, E6498 or 3AL = Aluminium
Working Pressure
Maximum pressure lbs / inch2 (steel = 2250, al = 3000-4000)
Serial Number
12345 = Unique to tank
Manufacturer
PST = Manufacturer that made it
Hydrostatic Test Date / Hydrostatic Tester's Mark
6*96+ = Hydrostatic Test Date (Month, initials of tester, year)
+10% Overfill Allowed
+ = 10% overfill allowed on STEEL TANKS
These marking should always be checked before filling.
This is important even in different countries with different tank markings
tank metals
|
STEEL |
ALUMINIUM |
Resistance to corrosion |
Quicker (lasts longer) |
Slower |
Galvanic Action |
Brass values can react |
Brass values can react |
|
With steel (rust) |
With Al. (Al. Oxide) |
Weight |
Heavier |
Lighter |
Size |
Smaller (stronger) |
Larger (less strong) |
Buoyancy |
Less buoyant (diver is not affected as tank pressure drops) |
More buoyant (more weight required as tank pressure drops) |
Normal Working Pressure |
200 Bar (300 Bar tec. Diving) |
220 Bar |
Hardness |
Harder |
Softer |
Maintenance |
Harder to clean |
Easier to clean preferred by dive operators) |
Availability |
Less available |
More available |
Technical Diving |
More popular (especially DIN) |
Less popular |
Bottom |
Round |
Flat |
The Aluminium Tank with an International valve is the most common tank / valve combination in the world
visual inspection
A visual inspection test is required by international law to be conducted on all tanks (steel and aluminium) once (1) a year. A visual inspection is conducted by looking inside the tank for any corrosion (steel tanks = rust / aluminium tanks = aluminium oxide). If there is any corrosion the tanks will be machined and chemically cleaned. If the corrosion has done damage to the wall of the tank the tank will be destroyed. The treading at the neck of the tank (where the tank valve screws into) is also inspected for any damage.
hydrostatic inspection
Hydrostatic inspection = water pressure test
Laws differ from country to country
US / UK- required every five years Elsewhere according to local laws
A hydrostatic test is performed to ensure that the tank has no weak points in the body, and to make sure that the tank can withstand acceptable high-pressure air used for diving.
A hydrostatic inspection is conducted by placing the tank in a pressure container filled with water.
The tank is then filled with water, which will be used to pressurize the tank to approximately 7/5 of its working pressure.
Afterwards the tank is checked for any damage or disfigurement (shape change) that may have occurred during the test.
Certain circumstances can weaken tanks before a hydrostatic test is required.
Have tanks hydrostatically tested after exposure to any of the following:
Tumbling (or sandblasting) to remove corrosion.
Damage due to impact
Exposure to heat in excess of 82 degrees Celsius may affect the metals integrity. Never paint a cylinder using a heat painting process such as that used on automobiles.
If left unused for more than 2 years
tank valves
k valve
K-valves are our modern tank valves. The K-valve is a simple valve that will open or close the tank pressure coming out of the tank opening.
K-valves will either have a yoke fitting or a DIN fitting for the tank opening. This is for the different designs of first stages on scuba regulators. DIN screws into the tank valve.
J valve
J-valves (which are no longer used but are still found) are also called RESERVE VALVES. They were used before pressure gauges came into use. It's a spring-loaded valve that would close off a divers air supply once the pressure in the tank became low (between 40-60 Bar). The diver would then reach back and pull a pin, which would then re-open the valve. The diver would then know that he had a RESERVE of air and had to make his way back to the surface. When refilling the pin must be in the DOWN position.
DIN (Deutsche Industrie Norm) German
Has a better seal.
Is stronger.
Can use higher air pressure
Preferred by Tech divers
Regulator has the o-ring as the tank does not have one
Can get adaptors to allow DIN regs on International Valves
burst disk
A BURST DISK is a thin copper disk that is located on the valve and will break if the tank pressure becomes too high (140% of working pressure). Disks are replaced every year, due to weakening of disk from filling / emptying. Instead of the tank exploding the air will vent (come out) through the burst disk. Newer disks, which vent air from both sides prevents the spinning of cylinder
regulators
We can describe the ‘most' common regulators in use as Open Circuit, Fail Safe and Balanced
open circuit scuba
Typically used by recreational divers. The diver inhales air from the cylinder via a demand valve regulator and exhales it into the water, thus the circuit is open because none of the air is recycled. Though open circuit doesn't recycle breathing gases, it is the main stay of recreational diving for several reasons.
It's a much simpler design, which makes it reliable and less costly (closed and semi-closed systems are more prone to malfunctions)
It requires only a cylinder of air (closed and semi-closed units require chemicals and access to pure gases or enriched air)
It is much easier to use
It is much simpler to maintain and service.
semi-closed circuit scuba
The diver inhales from a breathing bag that receives a steady flow of gas (usually enriched air). The diver exhales back into the breathing bag and the gas has carbon dioxide removed chemically – excess gas from the steady flow trickles out through a valve. The circuit is semi-open because part of the gas is recycled and part of it is released (except to vent expanding air on ascent).
closed circuit scuba
The diver exhales from a breathing bag and then exhales back into the breathing bag. The gas had carbon dioxide removed chemically and electronic sensors control flow of oxygen and other gases as required. The circuit is closed because all gas is recycled and none released (except to vent expanding air on ascent)
first stage
When high pressure (220 Bar) from a tank enters the first stage the first stage will do two things:
It will reduce the high pressure from the tank (220 Bar) to between 9-12 Bar. This new pressure is called INTERMEDIATE PRESSURE.
The first stage will channel, or guide, the tank pressure to the high-pressure hose (which goes to the pressure gauge, so that the diver can monitor air pressure). The intermediate pressure will go to the second stages and the low pressure inflator hose for the BCD
second stage
As the diver inhales, the diaphragm will move inward and press down onto a lever. This lever is connected to a valve. When the lever is pushed down by the diaphragm (when a diver inhales) the valve will move and allow intermediate pressure air coming from the first stage to enter the second stage where it will become AMBIENT / ABSOLUTE PRESSURE so that the diver may breath.
The pilot valve moves with the airflow
upstream valve
The pilot valve moves against the airflow
unbalanced
A regulator designed so that tank air pressure resists or assists (DOES AFFECT) the opening of valves in the first stage is called an unbalanced regulator:
Breathing will become more difficult as the tank pressure drops
Breathing is more difficult at greater depths
Unbalanced regulators are no longer commonly found
balanced
A regulator designed so that tank air pressure neither resists nor assists (DOES NOT AFFECT) the opening of valves is called a balanced regulator. The tank pressure does not affect the ease of breathing, even when two divers breathe from the same first stage, and inhale at the same time.
Depth is not a concern.
Virtually all modern regulators are balanced regulators
fail safe
Regulators are designed to free flow during a malfunction, which gives the regulator a fail-safe design. It will fail in a safe manner in that it continues to provide air. Obviously the tank will loose air faster, so the diver must ascend immediately.
environmental seal
In very cold water (such as cold water deep diving or ice diving) the temperature drop can cause water to freeze the regulator first stage valves into the open, free flowing position. To avoid free flow in extremely cold water some regulator first stages have environmental sealing. This seals silicone grease or oil, which doesn't freeze, around the first stage. The silicone or oil transmits the pressure from the water to the diaphragm or piston so the regulator operates normally
depth gauges
CAPILLARY GAUGE
Capillary depth gauges are a simple piece of clear tubing, sealed at one end and open at the other, based on Boyle's Law.
They are hard to read accurately at much deeper than 10m.
Accurate and mainly used when diving at altitude.
OPEN BOURDON TUBE GAUGE
Open bourdon tube gauges contain a spiral shaped tube. Water enters the tube and increasing pressure causes the tube to straighten. The straightening of the tube moves the depth gauge needle.
Because the tube is open, clogging can be a problem with these devices.
OIL-FILLED GAUGE
Oil-filled gauges also use bourdon tube design, but using a sealed tube in an oil filled gauge housing. Pressure transmitted through the oil causes the tube to coil more tightly. This moves the depth gauge needle.
The depth gauge is not open to the water and therefore not prone to clogging.
DIAPHRAGM GAUGE
Diaphragm gauges function by connecting a flexible diaphragm to a series of levers and gears that mover the display needle.
DIGITAL GAUGE
Digital gauges are electronic gauges that read depth with a transducer, which varies the electricity it transmits depending on the pressure exerted on it. They provide a digital display.
These offer the highest degree of accuracy and are used in dive computers to determine depth.
SUBMERSIBLE PRESSURE GAUGE (SPG)
The SPG works on the same principle as the bourdon tube gauge. Electronic SPGs use a pressure transducer similar to those in dive computers / electronic depth gauges. SPGs may be integrated with dive computers. The most recent design is a transducer on the regulator first stage that transmits the air pressure to a wrist-worn computer, eliminating the SPG hose.
COMPASSES
Needle is magnetic therefore always points north.
Liquid filled to damped movement and to help withstand pressure.
OPTIONS FOR CARRYING GAUGES
Wrist Mount (oldest and most efficient
Console
Retractable mount
Pressure
Atmospheric Pressure |
The pressure exerted by the weight of the atmosphere at sea level. It is 14.7 psi or 1 tm or 1 Bar. |
Gauge Pressure |
Is the pressure designation that uses atmospheric pressure as its zero point. Expressed as ‘g' i.e. psig. |
Absolute Pressure |
Is a pressure that uses a vacuum as its zero point, or its equal to Atmospheric Pressure + Gauge Pressure. Expressed as ‘a' i.e. psia or ata |
Ambient Pressure |
Same as Absolute Pressure |
Hydrostatic Pressure |
Water pressure |
What is the absolute pressure (ata) at 18m in salt water ?
18m / 10m = 1.8 atm; 1.8 atm + 1 atm = 2.8 ata
ENRICHED AIR DIVING CONSIDERATIONS
equipment considerations
Because enriched air has more oxygen than air has oxygen, there is a greater potential for fire or explosion related to equipment that has not been properly cleaned.
Industry guidelines involving equipment used with enriched air:
Most manufacturers require their equipment to be cleaned to oxygen service specifications if tit will be exposed to more the 23 percent oxygen. When using 40 percent oxygen, or more, special cleaning materials are recommended. This is called the '40 percent rule'. Any piece of equipment that will be exposed to more than 40 percent oxygen requires special cleaning, lubrication and materials to meet oxygen service specifications. If such equipment is used with air from a standard source, it may need to be re-cleaned. Follow the manufacturers guidelines with respect to using equipment with enriched air.
special markings
A 15cm band at the tank shoulder. The top and bottom of the band should be a yellow 2.5cm band. The centre 10cm band should be green and contain the words ‘Enriched Air', ‘Enriched Air Nitrox', ‘Nitrox', or similar. Yellow cylinders need only the green portion.
A visual inspection sticker stating the cylinder has been cleaned to oxygen service specifications, or not if enriched air will not be blended in the cylinder partial pressure blending in the cylinder requires putting pure oxygen in the cylinder, even if the final blend will have less than 40 percent oxygen).
A contents sticker or tag identifying the current blend, the fill date, the blends maximum depth and the analyzer / divers name.
analazing cylinders
Enriched air divers must personally analyze the contents of their cylinders before using them. On some dive boats the normal practice is to grab any full cylinder available for the next dive – this isn't appropriate with enriched air, which practice calls for divers to use the tanks that they have personally analyzed.
