& instruments
Get to know your gauges and instruments. They’re what keep you informed and safe on your dive.

Scuba diving computers

Learn all about the gauges and instruments you will need to scuba dive


Today’s diving instruments are simple to read and use, and provide all the information the diver needs to safely plan and complete a recreational dive, and even to record the dive in their dive log. Divers need to keep track of some essentials while on a dive; depth, time, amount of air used, and direction. Both electronic (digital) and analog (mechanical) versions of most instruments can be found at your local professional dive retailer. Some electronic instruments combine all or most of the functions necessary into one or a few different instruments.

Today many divers use reliable diving computers to tell them how long and deep a dive can safely be. Dive computers are one of the first pieces of personal equipment a diver should purchase. Introduced in the late 1970’s, dive computers revolutionized diving because they made it easy for divers to keep track of how long and how deep they could safely dive, without having to make more complicated calculations using mechanical instruments combined with a “table” of safe depths and times. The old table method required considerable practice to use, and can still be used as a back-up if needed, but today’s reliable microprocessors help keep track of depth and time, and actually complete the safe depth and time calculations for you. By putting the depth and time data through a rather complex mathematical “algorithm,” a dive computer actually allows the diver to safely stay longer underwater than the older, harder-to-use table method.

While early dive computers were about the size of a small brick, and fairly heavy, they quickly followed the trend of most electronic devices and became smaller as the years passed. Today, it is not unusual to see dive computers about the size of a hockey puck. And many divers wear dive wrist-mounted dive computers so small that they can double as an everyday wristwatch.

Some newer computers even incorporate the submersible pressure gauge to that same compact unit. And some of these “air integrated computers” even use an electronic sender mounted on the regulator first stage to eliminate the hose between the first stage and the computer, further streamlining the diver in the water.

Types of Instruments

Dive watches are water-resistant timepieces with a means of measuring elapsed time underwater. For recreational open-water diving, a dive watch should be water-resistant to at least 140 feet of depth.

Depth gauges are analog or digital gauges that record current and (usually) maximum depth reached during a dive.

Submersible pressure gauges or SPGs display the amount of air remaining in a scuba cylinder. These can be mounted on hoses of varying lengths or – in the case of gauges used with back-up tanks – they can be mounted right on a regulator first stage.

Underwater compasses can be analog or electronic. For precision underwater navigation, many divers prefer a compass mounted on a combination slate and compass-board that is held in both hands while following a heading.

Depth/Time capsules show current and maximum depths reached during a dive, as well as elapsed time, and they often store such information for a number of dives. These instruments look similar to dive computers, but they do not calculate safety factors. Depth capsules are often used as backup timers, or as primary timers for divers using dive tables to calculate their own bottom times and surface intervals.

Dive Computers automatically track time and changes in depth and calculate safe time remaining in a dive, as well as planning information for surface intervals and allowable times and depths for subsequent dives. Some dive computers are air-integrated and include information on tank pressure, as well as estimated air-time remaining.

What Instruments Do You Need?

Divers should plan on having a reliable timing device such as a watch specifically designed for diving, along with a depth gauge, tank pressure gauge and compass. Diving computers combine the timer and depth gauge into one instrument that also warns the diver when and where to end the dive safely. Most manufacturers produce “consoles” that combine computer and compass into one into one, easy-to-use package.

Divers with additional training may also use additional instrumentation, but these basic instruments are all that are needed to get started.

How Instruments Work

The simplest form of timing device, the conventional dive watch, has a rotating bezel surrounding its face. At the start of the dive, the diver aligns the “0” on the bezel with the minute-hand on the watch, and at dive’s end, the minute hand will be pointing at the total dive-time, in minutes, on the bezel. As the bezel is designed to turn counter-clockwise only, it will add minutes rather than subtracting them, if it is bumped and turned during the dive. This makes such a watch a dependable and conservative timing device.

Digital dive watches generally have a push-button stopwatch function; pressing the button at the start of the dive begins the timer, and pressing again as one leaves the water stops it.

Depth/Time capsules are specialized digital devices that record both dive time and maximum depth achieved during a dive. These devices are water activated, begin recording data when you enter the water, and stop timing when you get out.

Analog pressure gauges use a mechanism driven by high-pressure air from the scuba cylinder to show air remaining in either pounds per square inch (PSI) or atmospheres (“bars”).

All depth gauges sense the pressure of the surrounding water and translate that to depth in feet or meters. Analog (mechanical) depth gauges use a device that expands and contracts under varying pressure, and pushes a needle through a spring mechanism; this needle points to current depth, much like the hand of a clock. Digital depth gauges use an electronic pressure sensor to drive a digital display that reads depth out in numerals (meters or feet).

Digital pressure gauges use a sensor, quite similar to that used in a depth gauge, to drive a numerical display. Digital gauges can usually be set up to read in the diver’s choice of PSI or bars.

All dive computers contain three elements: a timer (usually water-activated), a depth sensor, and a microcomputer that calculates the absorption and release of breathed gasses through the use of a mathematical model called an algorithm. Dive computers have become very sophisticated and some algorithms take into account predicted changes in the diver’s metabolism due to cold or hard work underwater. These instruments can also be adjusted by the diver to allow more conservative (more safety factor built in) or more aggressive (less safety factor built in) dive profiles, and can even be adjusted for use with breathing gasses other than air. These multi-gas computers are used by technical divers, who often use special gas mixes at depth.

Which Kind is Best?

A top-quality dive watch is an instrument that can serve you well throughout your diving career. Even if you are using a dive computer it is an always-available backup timing device. When you’re out of the water, it’s a wristwatch – and one of the ways that divers recognize one another on dry land!

Similarly, quality analog SPGs and depth gauges can serve you well for decades. When you buy a dive computer, these items become backup gear or (if you take up technical diving) gear used with the multiple tanks involved in that type of diving.

Electronic depth capsules and dive computers represent the cutting edge of dive technology, and help you wring every available moment out of your allowable time underwater. That said, dive computers get better and more versatile with each passing year, so when you buy a dive computer, you should get it for the type of diving that you are doing right now. If you are planning to take up technical diving, in say, five years time, don’t buy your multi-gas computer until just before you begin that training – the “tek” computer available half a decade from now will almost certainly be more fully featured (and probably more compact) than the one you can buy today. That’s one of the exciting things about the way that dive equipment is evolving!

When to Buy

Most dive professionals will advise you to buy your first set of instruments at the same time you buy your first regulator – this way you will have a complete and usable regulator set. In fact regulators are often sold in “packages” that include a first stage, a second stage, an octopus, and an instrument package.

Since a dive watch can also be used as both a fashion accessory and a timepiece, a dive watch can be purchased at any time, and many divers own more than one.

Because each model of dive computer operates a little differently, it’s best to get training in the use of your first computer. This doesn’t mean that you cannot buy a computer as part of the instrumentation for the gear you’ll use in your open-water training; the computer can also serve as a timing device and a depth gauge, and so will serve you well in that regard. But you should try to get further training before trusting your dive planning to a computer – either that or buy a computer just before you take your computer diver specialty class.

If you wind up with two dive computers – say because you get a watch-size one as a gift (or a gift to yourself!) – consider taking both computers along when you go on an extended dive trip. This way you have a backup computer to use in the event that your primary computer is lost or damaged. When diving with two computers, simply use the more conservative of the two for ending and planning your dives.

If you aren’t going to go into technical diving for a few years yet, it’s probably best to get a computer that works well for your current diving. When you are ready to move up to technical diving, the dive computers available then will almost certainly offer features and advantages not available on current models.

How to Buy

When buying instruments, look for name-brand gear that can be serviced just about anywhere in the world. This can be a life-saver if one of your instruments sustains minor damage while you are away on a dive trip.

As with most dive gear, your best source is your local professional dive retail store. The people there not only stock what you need; they know how to use it and they are your best source of advice when it comes to picking out exactly the right instruments.

About the only instrument that you should ever consider purchasing outside a dive store is a jeweler’s-quality dive watch. In many localities, such high-end watches can only be purchased at a store specializing in such timepieces. Even so, check before you buy – you might discover that your local dive center carries even these high-end watches.

Instrument Maintenance

Instruments should be rinsed with fresh water after each dive with the same care that you would use to rinse a high-quality underwater camera. When drying, be sure not to leave them in the sun – on digital instruments in particular, this can darken the display and render them unusable for a short time.

Consider using a gauge protector to keep the faces of your instruments from getting scratched up during or after your dive. Always store instruments in padded cases specifically designed to protect them from damage. When storing instruments with user-replaceable batteries, consult your owner’s manual to see if the batteries should be left in place or removed during off-season storage. If batteries are removed, attach a tag saying so, so you aren’t unpleasantly surprised during your first dive of the following year!