- Tracy Gunn
Equipment IDC Dive Theory
Updated: Jun 24
Need a little help with your IDC Equipment dive theory or just interested in knowing a little more about how it all works? Want to practice some more equipment exams? The following has been written to assist the divemaster or IDC candidate prepare for their final exams. Its also extremely helpful when working as a professional to understand the basics of the equipment you will be using.
TANKS
TANK MARKINGS
Self-Contained Underwater Breathing Apparatus
Tanks have various markings at the neck (they should never be stamped on the body). They may vary internationally but generally include
· Government Agency
DOT Department of Transport
CTC Canadian Transport Commission
· Alloy Designation
Steel 3AA
Aluminium 3AL
· Working Pressure
In Bar and/or PSI
A “+” after current hydrostatic test date means steel tank can be filled 10% beyond working pressure
· Hydrostatic Test Date
Indicates both the date and the testing facility. A “+” after the test date on steel tanks only, means tank can be filled to 10% beyond working pressure
· Stamped serial number and manufacturer designation
Tanks should always, at a minimum, have the following markings
Maximum working pressure
Maximum capacity
Hydrostatic/ pressure test date
These marking should always be checked before filling.
This is important even in different countries with different tank markings
* Before filling a tank, always - check the tank markings, specifically looking for a current hydrostatic/pressure test date and maximum capacity/pressure.
Tank markings vary from country to country – if they’re not familiar, don’t assume that they’re unimportant.
STEEL TANKS Vs ALUMINIUM TANKS
| STEEL 3AA | ALUMINIUM 3AL |
RESISTANCE TO CORROSION | Quicker | Slower Aluminium is less subject to structural weakening due to corrosion |
GALVANIC ACTION | Brass valves can react With Steel (rust) | Brass valves can react With aluminium (Al oxide) |
WEIGHT | Heavier | Lighter |
SIZE | Smaller (stronger) | Larger (less strong) |
BUOYANCY | Less buoyant (diver is not affected as tank pressure drops) | Can be more buoyant (more weight required as 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 |
Weight change in a steel cylinder from full to reserve pressure would be the same as in an aluminium cylinder – (air weighs the same, regardless of the cylinder). Steel tanks are less buoyant than aluminium tanks so you would be less affected by the change.
Tanks should always be left with some air in them. The pressure of the air will prevent moisture entering the tanks causing internal damage. In steel tanks moisture can quickly form Iron Oxide (rust) and in Aluminium tanks Aluminium Oxide, although aluminium tanks are less prone to rust and corrosion. So never completely empty a tank, always leave some pressure inside.
STEEL TANKS ARE STRONGER FOR THE SAME THICKNESS SO HAVE THINNER WALLS AND LARGE INTERNAL VOLUMES FOR A GIVEN SIZE.
= MORE AIR AT A GIVEN PRESSURE OR SAME AIR AT A LOWER PRESSURE
ALUMINIUM TANKS ARE WEAKER THAN STEEL AND REQUIRE A THICKER WALL AND LOWER INTERNAL VOLUME FOR A GIVEN EXTERNAL SIZE
=LARGER WORKING PRESSURE SO HOLD COMPARABLE OR SLIGHTLY MORE AIR THAN STEEL
VISUAL INSPECTION Vs HYDROSTATIC TESTING
VISUAL INSPECTION | HYDROSTATIC TESTING |
Once a year · Inspector checks internal and exterior of tank to check for damage or wear that might cause the cylinder to fail between hydrostatic tests · Not required by law in most countries but is an industry standard · Inside of tank is inspected for corrosion (rust OR aluminium oxide). Any corrosion is machine or chemically cleaned. If damage has occurred, tank may be destroyed · Check for buildup of contamination inside the tank · Removal of the valve to check for galvanic action between the threads of the neck and the tank. This also allows lubrication, reducing electrolysis. O-rings may be replaced as needed and valve examined · Thread of the neck is also examined for damage · If your tank has been empty for a while, feels heavy and you can hear something loose moving inside, but is within hydro test date, you need a visual inspection | Periodically as required by local law (generally every 5 years but not always)
Hydrostatic Test is done by · Immersing tank in water and measuring its volume · Fills tank with water and pressurizes it to more than its working pressure (7/5 times or 166%) and measures the tanks expansion · After releasing pressure, the tester measures its new unpressurized volume against its original volume · If it contracts to within acceptable limits, the tank passes, and tester stamps the tank with test date and tester identification symbol. OTHER CONDITIONS FOR A HYDROSTATIC TEST 1. Tumbling (or sandblasting) to remove corrosion 2.Damage due to impact 3.Exposure to heat more than 82ºC/ 180ºF Never repaint a tank using a heat painting process such as used on automobiles as heat can weaken the cylinder 4.Left unused for two years or more (Especially if with zero pressure) |
Visual Inspection - once a year
Hydrostatic testing - as required by local law
If you would like to test your knowledge on
Tanks
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VALVES
K valve and J valve are so named because it was their part number in a catalogue
K-valves are simply an on-off valve. (Most used today)
Will have either a yoke fitting or DIN fitting for the valve opening
Yoke most common for recreational diving
DIN (DeutcH Industry Norm) is where the regulator screws into the tank valve
DIN allow a better tank and regulator seal due to O-ring being trapped between two valves (the tank is “female” and the regulator is “male”)
Tank and valve are secured by threads, so the connection is much stronger than the yoke. This makes it more desirable for cave and wreck divers who may accidently strike the valve/ regulator on an overhead obstruction
Superior O-ring positioning and strength enables use of much higher air
J-valve is a valve with a reserve mechanism
Contains a spring-loaded mechanism that if activated (placing the lever in the “up” position), restricts the flow when the air pressure drops to between 20-40 bar/ 300-500psi
This alerts diver to low air and the pulls the lever into the “down” position, releasing restricted air flow.
This is a warning device and does not provide additional air volume
Tanks must be filled with the lever in the “down” position.
Advent of the submersible pressure gauge has made this valve uncommon
BURST DISC
Burst disc prevents an over pressurized or overheated tank from exploding
Required by law in many countries and installed in every valve
A thin copper disk that ruptures and allows air to vent when the pressure reaches 125-166% (generally above 140%) of the working pressure.
As the weaken over time, disks must be replaced regularly by qualified technician. Installing the wrong burst disk can result in tank rupturing before disk
If you would like to test your knowledge on
Valves
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REGULATORS
There are three types of Self-Contained Underwater Breathing Apparatus
OPEN CIRCUIT SCUBA
Scuba typically used by recreational divers. They are referred to as open circuit demand valve because they are activated by diver inhalation and exhaust is vented into the water. Thus, the circuit is open because none of the air is recycled
Much simpler in design, which makes it reliable and less costly. Closed and semi-closed are prone to malfunction
Much easier to learn to use.
It requires only a cylinder of air. Closed and semi-closed require chemicals and access to pure gases or enriched air
Much simpler to maintain and service
Ratio of gases in the breathing mix remain the same throughout the dive
SEMI CLOSED REBREATHERS
Drager Ray/ Dolphin
Diver inhales from a breathing bag that receives a steady flow of gas (usually enriched air). The diver exhales back into a 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.
CLOSED CIRCUIT REBREATHER
Buddy Inspiration
Diver inhales from a breathing bag and exhales back into a breathing bag. The gas has carbon dioxide chemically removed and electronic sensors control flow of oxygen and other gases as required, allowing the dive to carry much less gas. The circuit is closed because all gas is recycled, and none is released (except to vent upon ascent)
Modern closed circuit rebreathers constantly monitor oxygen levels in the breathing mixture and are able to adjust the oxygen concentration to a level that is optimum for the divers’ depth. The result is much shorter decompression times and much less risk of oxygen toxicity
Buoyancy is mainly controlled with the BCD using the LPI rather than the diver’s lungs as in the case of open circuits.
OPEN CIRCUIT REGULATORS
1st STAGE
Reduces high pressure from the tank (220bar/ 3000psi) to an intermediate pressure (around 10-13bar/ 140-190psi above ambient water pressure)
Will channel high pressure to the high-pressure hose (which connects to the pressure gauge so the diver can monitor air) and the intermediate pressure to the second stages and low-pressure inflator hose for the BCD
Diaphragm or piston valves are most likely found in the first stage of a scuba regulator
As the diver inhales, the air pressure drops in the first stage. This allows water pressure to flex a diaphragm or move a piston, opening a valve that releases air from the tank.
Air flows as long as the diver is inhaling, keeping the first stage reaching intermediate pressure.
When the diver stops inhaling, pressure rises in the first stage so that upon reaching intermediate pressure, the valve to the tank closes and air no longer flows
2ND STAGE
Reduces intermediate pressure to ambient pressure for breathing
As the diver inhales, water causes a diaphragm in the second stage to flex inward, depressing the downstream valve releasing air.
As long as the diver is inhaling, air will continue to flow
When the diver stops inhaling, diaphragm returns to relaxed position and valve closes
Exhaled air exits through one-way valves
In some second stage models, the diaphragm opens a small pilot valve, which creates a small pressure imbalance that opens the main valve
Advantage- less breathing effort
Disadvantage – more complex design, more difficult to service and adjust
If the regulator wet breathes it could be missing tabs or tear on the mouthpiece
Damage on a mouthpiece may be a potential stressor and can lead to more serious problems
PILOT VALVES
Are ONLY found in 2nd stages
Uses air pressure to both open and close valves
DOWNSTREAM Vs UPSTREAM VALVES
DOWNSTREAM VALVES | UPSTREAM VALVES |
Opens with the flow of air Air pressure acts to open valve Failure typically results in free flow Free flow is a fail-safe design | Opens against the flow of air Forced closed by air pressure Failure can shut off air |
ENVIRONMENTAL SEAL
Normal air flow causes regulator temperature to drop (expanding gases have a lower temperature. Look at what happens when you open a tank fully)
In extremely cold water, that temperature drop can cause the regulator 1st stage valves to freeze into an open, free flowing position.
To avoid this, some regulators have environmental sealing. This seals silicone grease or oil, which don`t freeze, around the first stage. This silicon or oil transmits the pressure from the water to the diaphragm or piston so the regulator can operate normally
BALANCED REGULATOR Vs UNBALANCED REGULATOR
BALANCED REGULATOR | UNBALANCED REGULATOR |
· Neither assists nor resists (DOES NOT AFFECT) opening of valves · Breathing unchanged with cylinder pressure · Is more capable of supplying air to accessories, such as low pressure inflators. · Is better able to supply the needs of two divers breathing simultaneously from the regulator. · Provides greater airflow and breathes easier at depth so depth is not a concern · Virtually all modern regulators are balanced | · Assists or resists (DOES AFFECT) opening of valves · Breathing changed with cylinder pressure (more difficult) · Breathing is more difficult at depth · No longer commonly found |
Alternate Air Sources
Generally the Alternate Air Source is brightly colored. Yellow is the most common and popular. It is widely accepted that the Alternate Air Source (AAS) should be attached to the diver somewhere within a "golden triangle" or "triangle of safety". This is an area formed between the chin and the two lower points of the rib cage.
This approach is favored by most divers because, being what most entry level divers are taught, in an emergency it is a familiar position located easily, built into the muscle memory of divers through repetition.
A panicked diver is focusing purely on locating a source of air. By having the octopus (AAS) identifieable by its color and placement, much confusion is removed from a panicked diver who is possible suffering tunnel vision.
Alternate Inflator Regulator
This combines a second stage with the BCD low pressure inflator
In an out-of-air emergency, you breathe from this yourself and pass your primary second stage to your buddy.
If you would like to test your knowledge on
Regulators
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GAUGES
CAPILLARY DEPTH GAUGE
- A simple piece of tubing sealed at one end and open at the other with depth increments indicated according to where the water column rests based on Boyles Law.
In accordance with Boyle’s Law, the internal air space will fill with water in a predictable manner — half the original volume at 10 meters, one third at 20m, one fourth at 30m, etc. By marking off where the water column will be at various depths, a crude but highly accurate gauge can be constructed with no moving parts.
Best for when diving at altitude.
They are inexpensive and reliable but hard to read accurately much deeper than 10mt/ 30ft. It will show a relationship of atmospheres rather than actual ambient pressure and is why diving at altitude requires special procedures and training.
A capillary gauge will give theoretical depths rather than actual depth.
If the atmospheric pressure is reduced (as in altitude), then the capillary tube will be filled based on the pressure. For example, if the atmospheric pressure is 0.8 ata (less than the 1.0ata at sea level) then doubling that pressure will decrease the volume by half.
2 x 0.8 = 1.6 ata.
This means the depth gauge will read 10 meters when the diver is actually at 8 meters. Capillary gauges read deeper than actual depth which is in contrast with other gauges which read shallower than actual depth when atmospheric pressure is reduced.
The indicated depth (ie 10 meters) is not the real depth but the equivalent sea level depth, which is the depth that divers would look at to find the no decompression limit (NDL) for an 8 meter dive at that specific atmospheric pressure. So capillary gauges are best used in conjunction with the use of dive tables.
Think ABC - Altitude, Boyle Capillary
At altitude will read deeper than actual depth (which makes it more conservative)
Capillary depth gauges have no moving parts and are generally very accurate in shallow water, making them excellent backup gauges
OPEN BOURBON DEPTH GAUGE
- Contain a curved/spiral (C) tube. Water enters the tube end and increasing pressure causes tube to straighten if the pressure inside the tube is greater than ambient pressure. The tube’s tip connects with rods and levers to an eccentric gear that, in turn, connects to a gauge needle. Thus, by increasing or decreasing the pressure inside the tube, the needle moves around the dial. This is how your mechanical, analog SPG works.
Because the tube is open, clogging can be a problem.
* At altitude will read shallower than actual depth (which could be unsafe)
OIL FILLED GAUGES
- Also use bourbon tube design but using a sealed tube in an oil-filled housing. Pressure transmitted through the oil causes the tube to coil straighten. This moves the depth gauge needle. It is not open to water and so is not prone to clogging
DIAPHRAGM GAUGES
- Function by connecting a flexible diaphragm to a series of levers and gears that move the display needle
DIGITAL GAUGES
- Are electronic gauges that read depth with a transducer, which varies the electricity it transmits depending on the pressure exerted on it. These provide a digital display. There offer the highest degree of accuracy, and are used in dive computers to determine depth.
SUBMERSIBLE PRESSURE GAUGES
Analog SPGs are based on the same principle as the Bourbon Tube Gauge
Electronic SPGs use a pressure transducer like those found in computers/ electronic depth gauges. These work by varying an electrical current depending on the pressure exerted on it
SPGs may be integrated with dive computers. Some designs have a transducer on the first stage that transmits to a wrist worn computer, eliminating the SPG hose
COMPASSES
North needle always points to magnetic north because the needle is magnetic, aligned by earths geomagnetism.
Divers read compass by direction directly with the needle, but electronic ones read heading digitally
Divers use liquid filled compasses so the gauge withstands pressure, and to dampen the needle movement for easier reading
DIVING WITH COMPUTERS
Each buddy has their own computer
Buddies follow most conservative dive depth/time
During a dive on which one buddy has a dive computer and the other is using tables, both divers should dive within the limits of the most conservative tool
All dive table guidelines and manufacturer recommendations apply
If a computer fails underwater ascend to 5 metres/15 feet and make a long safety stop, perhaps lasting as long as your air supply permits. This includes both buddies as a dive cannot continue sharing one divers computer.
Computer decompression mode – if you ascend before the required decompression time ends and the computer locks you out, stay out of the water for at least 24 hours, monitor yourself for DCS symptoms and only then re-enter the water
Dive computers with integrated SPGs or gas pressure features can track your depth, time, and gas consumption. An enriched air dive computer can also calculate your oxygen exposure
OPTIONS FOR CARRYING GAUGES
Gauges were first worn on wrist
Then went to console
Now returning to wrists
Wrist mount
Useful for compact instruments
Most accurate for compass
More streamlined, especially with overhead environments
May be prone to entanglement
Console
Combines several instruments into a package on the SPG or several instruments into one
Speeds up dive preparation
Keeps arms clear
Requires securing so it does not drag and damage itself or environment
Retractable mount
Clips to BCD with a spring wound retraction cord.
Popular for hoseless computers with divers who do not like wrist mount
Makes it convenient to hand hold compass for greatest accuracy
Diver should avoid diving to the no decompression limits because depth gauges, timing devices and dive computers may not be precise — even a slight variation can put a diver at risk if the limits are pushed.
Failing to monitor depth and timing device is the most significant equipment malfunction that can be directly linked to causing Decompression Sickness.
If you would like to test your knowledge on
Gauges
CLICK HERE
ENRICHED AIR CONSIDERATIONS
Because enriched air has more oxygen than air has oxygen, there is a greater potential for fire or explosion related to improperly cleaned equipment. Materials such as neoprene rubber, lubricants such as silicone grease, and contaminants including dirt particles, could pose a hazard with using enriched air.
Enriched air presents oxygen toxicity hazards not common to diving with air (21% O2) within recreational diving limits. Divers must know they are using enriched air and what blend they are using
INDUSTRY GUIDELINES
Most current regulators, alternate air sources, SPGs and BCDs can be used with EANx blends of up to 40% oxygen without modification. This guideline is in place due to the increasing risk of oxidative reactions with higher percentages
When using 40% oxygen or more, special cleaning or materials is recommended. (40% rule)
You need to oxygen clean equipment for enriched air use when:
Oxygen content will exceed 40%
The equipment manufacturer guidelines says so
Local regulation require it
FOLLOW MANUFACTURERS GUIDELINES WITH RESPECT TO USING EQUIPMENT WITH ENRICHED AIR
Any piece of equipment that will be exposed to more than 40% oxygen requires special cleaning, lubrication, and materials to meet oxygen service specifications. If such equipment is used with air from a standard source, it will need to be re-cleaned
ENRICHED AIR CYLINDERS REQUIRE SPECIAL MARKINGS
A 15cm band at the shoulder. The top and bottom of band should be a yellow 2.5cm band with the center 10cm green with the words “Enriched air” “Nitrox” or similar. Yellow cylinders need only green/label portion
Also requires visual inspection sticker stating that it has been specifically serviced for use with enriched air
A contents sticker or tag identifying the current blend, fill date, blends maximum depth and the analyser name/diver name
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. This is not appropriate with enriched air which practice calls for divers to use tanks they have personally analysed
THERMAL PROTECTION
THERMOCLINE is a steep temperature gradient in a body of water such as a lake, marked by a layer above and below which the water is at different temperatures.
When conducting deeper dives, especially those deeper than 18meters and especially in freshwater lakes where thermoclines are more common, divers must consider thermal protection adequate for the temperature at depth.
DRY SUITS
If a dry suit seal that has recently been replaced, or on a new suit, feels too tight, you can trim it with scissors in small amounts until you get the correct fit. Latex seals are tapered and have a series of trim lines (small, raised lines on the outside of the seal) that start at the seal opening and progressively get bigger. When trimming, use these to keep the lines even and use a pair of good scissors. Trim in small amounts until you get the correct fit.
WET SUITS
Wetsuits are made of closed-cell foam neoprene, a synthetic rubber that contains small bubbles of nitrogen gas, when made for use as wetsuit material. As you dive deep, the gas bubbles in the neoprene compress and the diver will have less insulation
WEIGHT SYSTEMS
A diver needs to wear sufficient weight to that they are neutrally buoyant at the surface
All weight systems should be quick release, regardless of type
Remember, you should do a buoyancy check
When we change our dive equipment
When we change our dive environment
When I haven’t been diving in a while.
(As you may recall, to do a buoyancy check, you adjust your weight, so you float at eye level with a deflated BCD and holding a normal breath. Then add two kg/ five lbs if you do this with a full scuba cylinder to account for the weight of the air you use during the dive.)
LIFT BAGS
Use a lift bag for objects heavier than 4kg/10 pounds but not more than 45kg / 100 pounds
Choose a lift bag with the lifting capacity as close to the object’s negative buoyancy as possible .
If bag capacity is close to objects negative weight, expanding air will bubble out the bottom, making runaway ascent unlikely. If using a larger capacity, then if a run-away ascent occurs it will accelerate as expanding air increases buoyancy and when the bag pops the surface, it will spill and sink
Put a puff of air in, just enough to make it stand up and pull tightly on the rigging
Using your Alternate, slowly inflate in short bursts. Do not use primary because it is unnecessary task loading. Hold inflator so you can pull free easily and so it cannot tangle in rigging.
After each burst, pull up to see if you can lift the object. Continue until object rises gently when you pull up and hovers off the bottom. You are trying to make NEUTRALLY buoyant, not positively buoyant
Position yourself beside and level with the lift bag
Ascend at a maximum of 18mt / 60ft per minute
You need to vent air from both your BCD AND the lift bag
REELS, SMBs and DSMBs
Dive reels
- can be used for multiple purposes.
Search Patterns
Towing a dive flag or float
Measuring things
In Tec Diving they’re standard equipment in open water to
Use with a lift bag,
Critical in cavern, cave, and wreck penetration diving to provide a continuous guideline back to the entrance and surface
SMBs and DSMBs do exactly the same thing and are intended for the same purpose, to let people at the surface know where divers are located.
SMB | DSMB |
Surface Marker Buoy | Delayed Surface Marker Buoy |
· A long bright collared tube deployed at the surface after a dive · Can be attached to a line and towed during the entire dive | · DSMB is only deployed at the end of the dive, on the ascent or during the safety stop. · The DSMB is deployed to alert the surface that divers have started to ascend. · Generally deployed at the safety stop so have lines that measure 5mt and up. · Always tied to a line · DSMB has an over inflation valve and no spill design |
Would you like to test your knowledge on
Enriched Air, Reels, SMBs and DSMBs, Lift Bags, Weights and
Exposure Suits
Here are all the equipment exams
If you would like to test your knowledge on
Tanks
CLICK HERE
If you would like to test your knowledge on
Valves
CLICK HERE
If you would like to test your knowledge on
Regulators
CLICK HERE
If you would like to test your knowledge on
Gauges
CLICK HERE
Would y ou like to test your knowledge on
Enriched Air, Reels, SMBs and DSMBs, Lift Bags, Weights and
Exposure Suits