15 Essential Diving Terms and What They Mean

15 Essential Diving Terms and What They Mean

SCUBA diving is an activity that requires technical knowledge of the underwater breathing apparatus and the ocean environment. If you’ve taken an open water diving course then these terms should be familiar to you. If you’re interested in the basics of diving or just need a refresher, the acronyms and terms below are essential for every diver to understand.

1.) SCUBA - Self-contained Underwater Breathing Apparatus

The acronym SCUBA, for Self-contained underwater breathing apparatus, was coined by Christian Lambertson in 1952. Although term is relatively self-descriptive, it simply refers to the equipment used to breathe underwater being either open or closed circuit rebreathes.

2.) ABT - Actual Bottom Time

Actual bottom time is calculated as the time spanning from the descent at the surface to the first safety stop upon ascent from the bottom. Bottom time (BT) is important to the no de-compression limit and effect measurements of how long a diver can stay at the bottom without making periodic decompression stops.

3.) BCD - Buoyancy Compensator Device

The buoyancy compensator device (or commonly called BC) is an essential component in the underwater breathing apparatus. The BC is an inflatable jacket worn by divers to regulate buoyancy. A BC jacket is inflated using gasses from the dive tank and is used to attach the regulator and tank to a diver’s body as well as maintain neutral buoyancy at a desired depth.

4.) AP - Absolute Pressure

Absolute pressure is a measure of the total pressure exerted on a diver by both the atmosphere and the water column above the diver. Total pressure varies between salt and fresh water as salt water weight more than fresh and exerts progressively higher pressure on a diver the deeper they descend.

5.) ATA - Atmospheres Absolute

There tends to be some confusion around the difference between Atmospheres absolute ATA and Atmospheres ATM. Atmospheres absolute is the measurement of atmospheric pressure at various depths underwater in addition to the atmospheric pressure at the surface. For example if a diver is at 66ft below the surface the ATA would be 3 atmospheres - 1 ATM at the surface plus 2 ATM (33ft = 1 atmosphere).

6.) DCS - Decompression Sickness

Breathing compressed gasses, both oxygen and a mixed gas such as nitrox, increases nitrogen in the body. Nitrogen is absorbed by the body tissue faster the deeper those gasses are inhaled. Ascending to fast makes the increased nitrogen in the body expand rapidly. Decompression sickness, commonly known as “the bends” usually manifest itself within minutes after ascending to rapidly however in some cases can occur prior to surfacing or up to 12 hours after completing a dive. For more information see this in depth article on Divers Alert Network.

7.) DV - Demand Valve

The demand valve is the second stage of the regulator that allows divers to breathe compressed gas at ambient levels. The demand valve is coupled with a mouth piece and connected to the first stage of your regulator via tubing that transports air from the dive tank to your mouth.

8.) NDL - No Decompression Limit

Decompression limits are important to calculating dive times, surface intervals and allowable bottom times for multiple tank dives. The No decompression limit is the amount of time a diver can stay at a given depth. Nitrogen level incurred from breathing compressed gasses build incrementally y with depth and amount a time a given dive. No decompression limits are based on the specifics of a dive (depth and time) to render the amount of residual nitrogen in a diver’s body.

9.) PSI - Pounds Per Square Inch

PSI is a measurement of force per square inch relative to a vaccum or confined space rather than atmospheric pressure. PSI is not specific to SCUBA and is a label of measurement you’ll see on many inflatables such as basketballs to car tires. PSI is particularly important to diver’s tanks for either air or nitrox. If a diver’s tank is an 80 cubic foot cylinder that means that the tank holds 80 cubic feet of air and the PSI of a filled tank would measure the pounds exerted per square inch inside the tank based on the quantity of air left.

10.) CCR Closed Circuit Re-breather

There are two basic systems that can be used when breathing underwater – Open circuit and closed circuit re-breathers. Closed circuit rebreathers look similar to a traditional recreational SCUBA set up but differ in that they have the ability to extract carbon dioxide from exhaled air allowing the diver to re-use this air. The design of many CCR components differ from OCR components however the basic components such as a upper torso harness to house gas tanks and gas regulation breathing tubes are the same.  

11.) DCI Decompression Illness

Decompression illness encompasses both DCS decompression sickness and AGE arterial gas embolisms. The cause of decompression sickness is a reduction in ambient pressure (a diver surfacing to fast) whereas an arterial gas embolism occurs when gas bubbles are created in the circulatory system often as a result of lung overexpansion. Both DCS and AGE are fundamentally caused by decompression and categorized under the broader definition of DCI.

12.) PADI Professional Association of Diving Instructors

PADI is the largest SCUBA certification agency in the world. PADI is based in the United States and operates throughout the globe having certified just over half of all SCUBA divers. While there is not one central governing body that regulates SCUBA certification, PADI is the gold standard. PADI offers a variety of dive certifications and their list of certification courses can be found on their course catalog here.

13.) TDT Total Dive Time

Total dive time is calculated as the time spanning from descent at the surface until you breach the surface at the end of your dive. Total dive time is different from bottom time as bottom time is the metric used for no decompression limit calculation however Total Dive Time may be important to a boat captain that is expecting you to surface within an expected time period for pickup.

14.) Surface Interval

A surface interval in the amount of time between two dives. The surface interval varies depending on the depth and length of a dive and is important as it allows built up nitrogen in a diver’s body to diminish. Surface intervals are charted when planning a dive by using dive tables or a dive computer.

15.) FSW – Feet of Seawater

Feet of seawater is a measurement of pressure that one foot of sea water exerts on a diver. FSW is also a component of ambient pressure defined as ATM atmospheres. 1 atmosphere is equal to 33 feet of seawater or 14.7 PSI.