Know your gear

Steel and aluminum tanks are both common. In the US, steel tanks have rounded bottoms whereas aluminum tanks are flat. 

Steel tanks are popular with cold water divers because they are usually more negatively buoyant than aluminum ones. Divers with a negatively buoyant steel tank don't have to wear as much lead. Aluminum tanks are common in warm water locales and as additional tanks for tec diving or as a backup gas supply. 

Check the shoulder markings (and bottom) for the capacity and service (fill) pressure - usually in PSI.  https://www.xsscuba.com/cylinder-markings 

Service pressures can be low pressure (2,400-2,640 PSI) "LP", standard (~3,000 PSI), or high pressure (3,300-3500) "HP".  3,000 PSI is most common for aluminum. Modern steel cylinders are often high pressure 3,442 PSI. 

Note the capacity, often marked on the shoulder. In the US it is customary to express capacity in cubic feet of gas. 80 cubic foot aluminum tanks are the most common. During your dives, log the capacity of your tank, your starting and ending pressure, and your average depth (logged by your dive computer). This will allow you to calculate your gas consumption rate. About 0.5 cubic feet (at the surface) is typical. Knowing your consumption rate, you can calculate if the tank you've chosen for your dive has enough gas to complete your dive as planned.

Verify the tank has been hydrostatically tested, "hydro'd",  within the last 5 years. Hydro dates are stamped into the metal on the shoulder of the tank. For example:  03/21 which indicates March, 2021. Month and date will be separated by a special mark of the tester. Find the most recent date stamp. Don't use tanks with expired hydros.

Verify the tank has been visually inspected or"VIP'd" within the last year. The VIP checks for cracks, corrosion, and a functioning valve seal. This is indicated by a sticker, usually punched on the month, year, and date of inspection. Do not use it if it is out of date.

Check for any obvious signs of damage (giant dents, big rusty scratches, mangled valve threads, bent valve etc.). Don't use if you suspect the tank or valve may no longer be structurally sound.

Check the tank markeings to verify the gas mix. Most tanks without special markings are for air. A green and yellow VIP sticker or tank bands in the US may mean the tank is used for an oxygen-enriched air, "nitrox" mixture. The visual inspection sticker may have this information. Do not dive a tank marked for nitrox unless you have been trained to do so, or it has been tested in your presence  with a calibrated oxygen analyzer and contains a normal oxygen percentage (~20.9%). NEVER dive tanks containing nitrox or pure oxygen unless you have received appropriate training.  This can be fatal. Taking them beyond their rated maximum operating depth (which may be as shallow as 20 feet for pure oxygen) can result in convulsions and death.

Smell your gas to check for contamination before every dive. Gas can become contaminated if the filling compressor is not functioning properly, if their are problems with the filters, or if the air intake is contaminated by boat or other vehicle exhaust. Turn the valve gradually and gently until it is barely open and smell the gas coming out. It should smell like nothing. Do not dive a tank with any moldy, smoky, or gasoline type odors.

Ask for a tank that is a good fit for your stature and strength. Here in the cold-water Pacific Northwest, steel HP 100 (high pressure 100-cubic foot) and HP 80 tanks are common. HP 80 tanks are much lighter (9 lbs of dry land savings when full) and are significantly shorter. They fit better on the torsos of shorter divers and are also easier to carry if the weight of your scuba gear is an issue. Most women, adolescents, and shorter men will be more comfortable in an HP 80 than an HP 100. If you are a tall, strong person who goes through air quickly, you might also ask for a larger tank (bigger than an HP 100).

When opening and closing a tank valve to go for a dive always either fully open it or fully close it and gentle does it. Partially opening or closing a tank can lead to confusion as to whether your air is on (and result in an inadequate supply of air at depth) and has contributed to several accidents and fatalities.

Your BC provides a means to adjust your displaced volume in the water so that you can achieve neutral bouyancy - neither sinking nor floating - at all times during your dive. Also, they should be inflated to float you on the surface before and after diving so your head is out of the water so you can breathe.

Check that your BC fits you. You should be able to adjust it to be snug, but not constricting. 

Locate all of the valves for deflating your BC and practice using them by feel / muscle memory when you dive and during your predive check. Know when to use which deflator. For example, since air rises in water, hip dump valves allow you to vent gas when you are horizontal or head-down in the water. 

Often, jacket-style BCs have 4 deflator mechanisms: the button at the very end of the inflator hose, pulling down/out on the inflator hose, pulling the string attached to the right shoulder dump valve, and pulling the string on the dump valve located near your hip/kidney/butt on the left or right side of the tank. 

Check that your inflator hose and valves are securely attached by twisting them clockwise to make sure they are snug. They often screw on to a fitting welded into the BC. They can come loose. If this happens during a dive, your BC may be unable to maintain sufficent air for your to establish neutral buoyancy - which can increase your risk of an uncontrollable descent, or make it hard to ascend to end your dive. 

Check that you can inflate your BC with both the power inflator (low pressure inflator hose) and the oral inflator.

Verify that your BC holds gas and does not leak when inflated. 

Check for wear and verify function of all buckles, straps, and releases.

Check the integrated weight system (if present) for your BC - verify that it securely holds weight, and make sure you know how to secure weight so it doesn't come loose during a dive and that you can easily ditch your weight if you have a problem at the surface. Many BCs have weight pockets that slide in and out. Verify the max capacity and do not exceed it.

Consider the lift capacity of your BC. The lift capacity measures how much negatively buoyant weight it can offset. You can look this up online or it may be printed on your BC. Many BCs designed for warm water diving have a fairly small lift capacity. Take care that when loaded with weight, and a full tank (which is usually negatively buoyant) your BC can still float. If not, get a BC with a higher lift capacity or move more weight to a different sytemt (weight belt or harness). If your BC cannot float when fully inflated it can contribute to problems getting in and out of boats, or in the event of an issue that requires you to remove your scuba kit at the surface.

Consider a backplate and wing style BC. Most divers learn in jacket-style BCs, but backplate and wing configurations (BP&W) are becoming more popular with recreational divers. This BC style is standard and required if you progress to tec diving - unless you choose to dive sidemount. Typically, a steel or aluminum backplate and nylon webbing harness form the structure of a BP&W and an air bladder called a "wing" with a standard inflator hose is screwed on to the back of the backplate. BP&W style BCs move some weight from your weight system onto your back (in the form of the metal plate) which puts it near your center of gravity and distributed nicely between shoulders and your lower back. This can make them more comfortable and stable - particularly in cold water where we carry a lot of weight. BP&Ws locate your air bladder above you when you are swimming - which makes it easy to stay in horizontal trim (but a bit harder to stay vertical at the surface). BP&W systems are also modular - so if a component fails, or you want to dive doubles instead of a single tank setup, you can switch out a few components and keep diving instead of having to buy a whole new BC. It is easier to carry a "pony bottle" as a redundant air source with a backplate and wing (clipped to the left chest and hip D-rings) than it is with many jacket-style BCs.

Service your BC inflator mechanism annually or as recommended by the manufacturer or if it begins to leak or stick. Often, replacing it with a new one annually can be easier and cheaper than servicing it. An inflator valve that sticks open can suddenly and rapidly fully inflate your BC leading to a dangerous uncontrolled, buoyant ascent. 

Consider a bright color especially if you dive in low-visibility water like we do here in the Pacific Northwest. 

The 1st stage regulator reduces tank "high pressure" to an intermediate "low" pressure of ~125-150 PSI. This is  delivered via "low-pressure" (usually labeled LP) ports in the regulator to the hoses for 2nd stages and BC/drysuit inflators. It also has tiny holes in the "high-pressure" (HP) ports that allow tank pressure to pass directly to your SPG or air-integrated (AI) transmitter so you can track remaining tank pressure.  

DIN/yoke - yoke adapter possible, reverse is not, DIN more secure

rated temperature / environmental seal , risk of cold water freezing and free flow

finger tight

check for o-ring, replace if leaking

filter clean?

serviced acccording to manufacturer, usually annually sometimes every 2 years or if it has a problem

negative pressure - breath in to check for leaks with dust cap in place

mouthpiece - worn, tears, cable missing, secure? replace with annual service - can come off and contribute to accidents

breathes easily? some are adjustable

no freeflow? some are adjustable

hose - no nicks, cuts, cracks, signs of aging, corrosion at crimp, leaks

replace hoses every 5 years to avoid failures which could lead to loss of breathing gas at depth

check underwater for leaks at beginning of every dive

serviced annually or as recommended by manufacturer (some are 2 years)

consider where  you will clip or stow your alternate 2nd stage or "octo"

consider a primary donate, long-hose configuration - gives about 7' of hose instead of 3' of hose for sharing air - can make sharing air more comfortable - giving both divers space. Panicked out of air divers are reported to often grab your primary regulator anyway. Keeps your alternate backup regulator where you can always easily retreive it, just below your chin on your necklace. This configuration is typically required for technical diving.

To connect the quick disconnect (QD) fitting - push the hose onto the male valve fitting while pulling the female flange backwards toward the 1st stage end of the hose. The same process will allow you to disconnect - push the hose into the fitting while pulling the flange away from the fitting, then pull the hose off the fitting once you feel it disconnect. Connecting and disconnecting is  easier when there is no pressure in the line. If the line is pressurized, pushing the inflator button while disconnecting may  help you to disconnect. 

metal valve connection easy, or difficult and worn?

practice disconnecting quickly by feel, while swimming down and venting from your hip dump in event of a stuck inflator which could lead to uncontrolled buoyant ascent

hose - nicks, cracks, signs of wear

replace hose every 5 years

verify reads 0 when not pressurized, if not do not dive it

point plastic face away from others when pressurizing in case of crack/failure

verify tank pressure is full and matches ~ service pressure before diving

as part of pre-dive safety check test breathe while watching pressure on gauge or air-integrated computer to verify air is flowing without restriction and tank valve is fully on

consider where you will clip or stow SPG so it does not dangle

replace hose every 5 years

Verify it turns on and battery level is sufficient to complete the dive

Verify it is set to the correct gas. If you are diving air it should indicate "air" or "21%" FO2 (fraction oxygen) depending on the manufacturer. 

Verify time and date are correct. This lets you track the time of day underwater (e.g. "return to the boat by 11 am") and log your dives accurately. 

Read the manual before diving it. In particular, know how to read depth, dive time, no decompression time remaining (often "no deco" or "NDL" - no decompression limit), emergency decompression stop instructions, and safety stop instructions.

When purchasing, consider the type of diving you intend to do - do you need a multigas computer? Almost all computers will calculate nitrox and air dives on a single tank. If you have trouble with close vision, consider screen size and font size. Will  the computer you dive be the same as your buddies - if not, be sure to familiarize yourself with each other's computers. Rechargeable or easy to change battery?

Air integration (AI) is a convenient and increasingly common feature that allows you to track your tank pressure on your computer. AI computers can usually estimate gas time remaining based on your consumption rate, remaining pressure, and depth. Diving with an AI computer may allow you to dive without an analog SPG and hose - increasing your streamlining. Not all computers support air integration. Air integration always requires a transmitter that is screwed into one of your 1st stage high pressure (HP) ports.

Alerts / vibration / beeps / compass

Visibility can be limited underwater, so a compass is standard dive equipment.

Verify that you have one and know how to use it before each dive. 

Many computers and even some underwater cameras (e.g. the Olympus TG-5, TG-6) have built-in electronic compasses. You may choose to use these instead of an analog version. 

weight integrated BC

trim weights (BC and/or cyclinder band)

weight belt

weight harness - keeps from falling off hips

cold water considerations - learn more

hard, soft

threaded, lead exposure

backplate, v-weight

drysuit best here to stay warm enough

semidry wetsuit is next warmest

regular wetuits are usually uncoformtably cold in our local waters

for additional warmth, use a drysuit with dry gloves, a thick (10-11 mm) hood, and additional undergarment layers

heated vests also an option 

Consider a bright color especially if you dive in low-visibility water like we do here in the Pacific Northwest. 

consider stainless steel spring straps - durable, no adjustment to make when you take them on and off, rarely fail

fin style - will they work well for all types of finning including frog kick, helicopter turns, and back finning/back kicking?

locally we use open heel fins that can fit over thick wetsuit or drysuit boots

get scuba fins, not snorkel fins

try your fins on with the boots you intend to buy for your wetsuit or drysuit

Consider a bright color especially if you dive in low-visibility water like we do here in the Pacific Northwest. 

appropriate fit most important, should not leak if you inhale, try with regulator in mouth or snorkel

shave moustache or stubble to reduce leakage under nose

can get prescription lenses, or reader inserts

SMB/DSMB or other visual

whistle or other auditory

TIP: buy and take with you, small, travel easily

lights - 2+ for night diving, standard here even in daytime

cutting tools - belt cutter, EMT shears, dive knife - for fishing line or similar entanglements or to cut diver out of gear for rescue

best stowed in a pocket or clipped onto your BCD

O2 kit - standard 1st aid

way to reach emergency services - cell phone to call 911, marine radio or satellite phone if out of cell range

Emergency Action Plan - have a plan for what to do in an event of an emergency, know the phone numbers for DAN (Divers Alert Network) and local emergency services (e.g. 911). This can be as simple as "we will call 911" if diving from an urban area. Remote areas may require more planning/equipment.

AED - heart attacks are a leading cause of diver injury and death, particularly among older divers. Access to an AED may save someone's life. Know where the closest one is. Many local dive sites have AEDs nearby.