Part One:
A Guide to Pressure Gauges, Depth Gauges, Timers and Compasses

It's A Control Thing

At Mavericks we firmly believe each diver should take responsibility for his or her own safety. This includes ensuring they are properly equipped. Owning your own set of instruments is essential to your independence and provides you with information vital to your personal safety. Depending upon another diver assumes you'll never become separated (something of an idealist view) and end up on your own with no hard information as to your location, depth, ascent rate or decompression obligations. Even extremely experienced divers get lost sometimes or find they've become distracted and have drifted deeper than they planned to or gone into decompression unexpectedly.

A commercial diver generally doesn't have a depth gauge, pressure gauge or compass. It isn't that they can't read them (although the physical conditions of the work site might very well mean they can't), it's just not relevant. Most professional divers aren't free swimming. They are dropped on the job site. They can be guided between work stations by a surface tender who communicates by intercom or by rope signals. Breathing gas is pumped from the surface, and ensuring there's enough for the diver is someone else's job. A special depth gauge, called a pneumofathometer, translates water pressure at the level of the diver to a readout on the surface. By depending on his surface team to control his position, depth, time and decompression stops, all the diver has to do underwater is his job.

Sport divers operate very differently to most commercial divers and perhaps have most in common with free swimming combat swimmers. Using self-contained breathing equipment and usually wanting to roam far and wide, we have to take care of ourselves. To do this we have to be informed as to how much breathing gas we have, where we are, how long we've been underwater or spent on the surface between dives, the time remaining before we go into stage decompression or how much decompression we have to do at what levels and how fast we are ascending. None of this can be left to guess. Instead we depend upon a range of specialist instruments for precise, up to the moment data. In this article we'll consider submersible pressure gauges, depth gauges, dive timers and watches and compasses. We'll offer some tips to keep you safe by pointing out flaws in equipment and techniques that can leave you open to incidents and the measures you can take to avoid, minimise or overcome them. And we'll touch on rigging---ways to stow your instruments so that they are easy to use, but unlikely to foul other equipment or damage coral reefs.

It's A Gas - keeping the breathing stuff flowing

The earliest sport divers controlled their gas supply by decanting, a system still in use by some military units. They carry two cylinders connected by an isolation manifold. One cylinder is breathed down at a time. When this becomes hard to breathe from as it nears empty, a valve is opened and air from the other cylinder flows into the first. Now both cylinders hold equal pressures and volumes of gas and half the gas has been used and half remains. The valve is closed again separating the cylinders. When the diver again finds it hard to breathe the process is repeated. With the air now down to one quarter, most divers would ascend. Obviously decanting does not work for single cylinder diving...

Soon sport divers moved on from decanting. Next they used a reserve or J-valve. Installed either in the tank valve or regulator first stage, reserve valves hold back a portion of air until the diver manually pulls a lever. The diver can breathe normally until the point of reserve gets near. Then a spring loaded valve begins to restrict the flow of air to him. Feeling this breathing restriction the diver opens the valve and the remaining air can be breathed normally. Reserve valves were once popular, but did have significant disadvantages. It's possible with some designs to trip them by accident or forget to set them. Then, when it gets hard to breathe it is because the diver's tank is empty! It is also not possible to accurately determine how much gas is in the tank at any given time during the dive. Because the amount of air held back assumes the diver will ascend directly to the surface, it isn't a practical gas management option for anything but the most limited stage decompression dives and completely impractical for exploring overhead environments. These dives where access to the surface isn't immediately available and exiting may require long or time consuming swims would probably require much more gas than the reserve mechanism would provide.

Today divers rely on pressure gauges to monitor and control their breathing gas. These let the diver see at any point in the dive how much gas remains.

The simplest pressure gauge attaches via a hose to your regulator. High pressure hoses can be ordered in different lengths and this can make it easier to create an instrument layout that works for you, rather than settling for an off the shelf compromise.

Mechanically they are fairly basic: gas from your tank enters a thin coiled tube, called a Bourdon tube. The tube is sealed at the other end. The high pressure gas causes the tube to flex and uncoil. As the pressure drops as the tank is emptied, the tube recoils. The uncoiling and coiling drives a needle around a scale from which you read off your pressure. This is shown as pressure, not volume, because the gauge does not know what size tank it's been fitted to.

Only the bourdon tube itself is designed to hold high pressure air safely. The rest of the pressure gauge is only meant to keep water out under the low pressure experienced at normal dive depths. The deepest a diver has been, so far, is about 600 metres. At this depth the pressure is only 60 bar or so. Most recreational divers won't exceed about 5 or 6 bar in reality or 40 or 50 metres. Because high pressure gas might leak from the tube, perhaps due to corrosion weakening the tube after water entered the system from an improperly rinsed regulator or poorly maintained compressor, a blow out cap is fitted. This lets any gas escaping from the tube vent from the case safely. Without this feature gas might build up inside the case and blow the faceplate out causing injury.

Mechanical gauges require you to estimate how long your gas will last. Most divers become quite accurate at this as they gain experience. Usually they err on the side of caution and surface with a good safety margin. Formulas exist for predicting gas consumption and in the past some pressure gauges had calculators attached to help with gas management.

Electronic pressure gauges are sometimes built into dive computers. They may be connected to the regulator by a hose or electronically linked by a radio transmitter. Transmitters eliminate the hose reducing a possible point for entanglement. However underwater flash units and propulsion vehicles are documented to sometimes corrupt the signal. Electronic gauges normally calculate the time your air will last at your current depth and breathing rate by measuring the drop in tank pressure. They can then compare this with the time the computer thinks will be needed for the ascent, including any required decompression stops. All of this information is shown to the diver. If the diver runs the risk of running low or out of air before he can safely complete his ascent, the computer will warn him.

Gas management---by man or machine---is not an exact science. Swimming against a current to return to your entry point or getting chilled at the end of your dive are just two common factors you may encounter that increase respiration. A rarer example is having to share air with another diver. These situations cannot be foreseen by pressure gauges. The diver must plan ahead to allow a margin of safety to cover such events. For example selecting a higher capacity tank or turning the dive earlier. Gas reserves should not be set in stone. The 50 bar rule has been a surfacing standard for many years. It has not been changed to reflect that divers now use higher pressure tanks (typically, 50 bar used to represent a quarter of a tanks capacity - now it's likely to be between one fifth and one sixth). Nor does it draw a distinction between a deep dive and a shallow dive, or where the diver actually is when he hits 50 bar. If he's in shallow water right under the boat, some divers would argue he should stay in the shallows outgassing until the pressure has dropped to as little as 30 bar. Extending safety stops is thought to reduce the risk of getting a bend. The point is, divers need to think for themselves and not slavishly follow "rules". Running on Empty and Executive Action discuss coping with out of air situations.

Diving computers have largely superseded depth gauges. But if you don't own a computer or want a low cost back up to the ubiquitous electronic box, a depth gauge is a must have. Depth gauges let you see your current depth, usually record the maximum point of the dive, confirm you are ascending or descending, help you maintain the proper depth for stage decompression and safety stops and, when used with a timing device, accurately measure descent and ascent rates.

Most depth gauges are non electronic. The simplest is a capillary gauge. Little more than a scale and thin tube that is open at one end, capillary gauges work according to Boyle's Law. Water enters the tube under pressure compressing the air into an increasingly smaller portion of the tube. At 10 metres the tube is a 50/50 water/air mix. At 20 metres the air only occupies a third of the tube and the water fills the remaining two thirds. At 30 metres only 25% of the tube contains air---the rest is water and so on.

Capillary gauges are very accurate in shallow water. But as depth increases the increments on the scale are bunched ever closer together making them harder to read accurately. They are rarely used today, though they do have two things going for them over mechanical models. They automatically compensate for altitude diving and they are virtually unbreakable.

Bourdon tube depth gauges represent the lower cost model of mechanical depth gauge. Open bourdon tube gauges are constructed in a similar way to a cylinder pressure gauge. A tube is open at one end. In a depth gauge water enters here instead of gas. The pressure causes the tube to flex, driving a needle around a scale. Tank pressure gauges read off high pressure, while depth gauges usually read off no more than about 8 bar, translating to 70 metres.

The open bourdon tube can suffer from silting and corrosion affecting the mechanism. This will throw out the readings. To counter this some manufacturers use filters and non-stick surfaces to protect the tube.

Oil-filled depth gauges also use a bourdon tube. In this design the tube is sealed at both ends and enclosed in oil. Water pressure is transmitted through the oil, causing the tube to flex. In turn this counts off the diver's depth via a needle and depth scale. Oil-filled depth gauges avoid the problems open bourdon tube gauges are prone to.

Diaphragm depth gauges are usually the most accurate type of mechanical gauge. A diaphragm, open to the water on one side, moves vertically to operate a needle. These tend to be a little more expensive than other models.

Most modern depth gauges have a second, slave, pointer that is pushed around the scale by the main needle. This records the deepest point of your dive. It's a useful feature, as it is easy to drop a little without realising. Even a drop of a metre can throw out your dive schedule and suddenly make stops obligatory or much longer. A maximum depth indicator helps avoid confusion. It needs to be reset before your next plunge. Some mechanical gauges can be zeroed for diving at altitude---this type of diving is very specialist and requires precise modifications to most tables which assume you are diving at sea level or that you use special high altitude diving tables.

Depth gauge scales are a feature worth a little thought. One consideration is the actual depth range you really need. The less range you have on the gauge, the better spaced the increments can be. So a 45 metre scale may be easier for you to read than the same model gauge with an extended 90 metre scale. But also keep in mind you may want some extra range in case you end up deeper than you planned---it happens.

You might also like to have specific depths marked off boldly to make it easier to maintain your safety or decompression stop levels. So depending upon the tables that you prefer, you may want to think about choosing a gauge with clearly labelled stops at 3, 6, 9 and 12 metres for instance, rather than 5, 10 and 15 m.

The greatest accuracy can be gained by using electronic readouts. These are less ambiguous, presenting only your current depth as a main display, although your maximum depth may be shown unobtrusively elsewhere. Electronic depth gauges are usually combined with dive timers, explained next...

To dive safely you need to be able to accurately time your dive, safety stops, decompression stops and surface intervals. Your options for doing this include a computer (See Deep Thinking: choosing a computer), a dive timer or a watch.

Dive timers are usually water activated and automatically record your dive time and surface intervals. Often they also incorporate a depth gauge, presenting all essential timing and depth information in a single instrument. They normally have a maximum depth indicator, ascent rate warning (though this may differ from that used by the tables you've chosen) and log a number of dives.

Dive watches are sold in analogue and digital versions. Both are pretty much normal watches that are water and pressure proof with analogue models additionally fitted with a way of recording elapsed time. Usually this is a bezel, which measures one hour of dive time. The best bezels are marked in one minute increments allowing for precise timing of dives and stops. You must remember to set the bezel indicator against the minute hand of your watch before submerging. The minute hand is used to count off dive time against marks on the bezel ring. For safety bezels on dive watches only move in the counterclockwise direction. If knocked this means the timer indicates a longer dive time, rather than a shorter one. If the dive seems longer, worst case scenario is you'll end up coming up early or making longer decompression stops than are needed. If the bezel could turn the other way, inadvertently moving it would result in you appearing to have been down for less time than you really had. This could result in missed decompression stops or lead to making unsafe repetitive dives due to wrongly calculated surface intervals.

The crown, used to set the time and adjust the date on most analogue watches, screws down onto an O-ring. It's good practice to check it is properly seated before diving or the watch will leak. It's sensible to watch your second hand for a few seconds when checking your dive times---this will alert you if your watch stops!

Analogue watches may be automatics, driven by movement of your wrist, or battery powered. Automatics will need occasional servicing and battery models will need battery replacements from time to time. Even some very well known watch brands can be a problem to have serviced in the UK. The issue seems to centre on ensuring the watch has been properly waterproofed after being opened by the technician. This may mean your watch must be returned to the manufacturer overseas and this can create long delays and be expensive. It's worth checking on aftersales before buying.

Digital watches often include stopwatch functions that can be used to record dive time and surface intervals and a count down alarm that can be programmed to time safety stops. It can be easy to press or knock buttons by accident, which might cause your watch to stop recording vital information. To prevent this you may be able set your watch so you can only recall key information by selecting the relevant mode. This protects vital information from being lost, though it may mean it takes a few extra seconds to bring up the display you want.

Whichever type of watch you choose it needs to be easy to read, even in the dark. Confusing over-populated faces or functions that are tricky to set and recall can lead to mistakes, which could have serious consequences.

Some dive watches have built in depth gauges and dedicated dive timing functions that automatically kick in as you descend. Some dive computers can be used as normal wrist watches. Both normally have the benefits that you cannot forget to set them and important functions are locked in and cannot be cancelled or wrongly programmed accidentally.

Dive watches for recreational diving should be rated to at least 200 metres. This may seem excessive for sport divers. The reason is that the watch industry measures depth resistance under static pressure. This leads to some confusion, to say the least. Basically it means the watch is safe to 50 metres so long as it isn't moved. Since it will be on a divers arm, who, presumably will be moving about a bit, you need the 200 metre model... 50 metre watches are considered only to be safe for swimming.

It's not always necessary to find your way around underwater. You may be free to explore at will, then surface just about anywhere and a chase boat will pick you up. Other times navigation may be no more complicated than keeping the reef wall on your right. But often you'll want or need to have some sense of where you are underwater and the location of your area of interest, such as a wreck, or your exit point.

For diving in open water a diving compass is the usual tool for the job, sometimes backed up by underwater plotters and other specialist equipment. Overhead environments such as under-ice diving, caverns, caves, mines and wrecks require the laying of guidelines which will lead the diver back to their entry point and safety, even though compasses may still be carried as back ups or for map making.

Diving compasses fall into two main types. Needle compasses depend on a magnetised needle to point to magnetic north. Card types have a magnetised disc that spins to indicate magnetic north instead. Needle types are simpler and often lower profile than card models. Both styles of compass are usually filled with oil to resist pressure encountered while diving and to dampen down the mechanism, making them easier to read. A 360 degree scale is fitted for determining your bearing or heading and a direction of travel line is used as a reference to point the compass in the direction you want to go. Direction is calculated by knowing which way north is. You can then extrapolate any other heading or bearing from its relation to north. Your compass isn't broken if it always points to "N"!

Though needle compasses are low profile reducing snagging and drag, they need to be held so that you can look slightly over the top of them to see the display. They also tend to be unforgiving if you tilt them slightly which can cause the needle to stick. In turn, this can lead you off course. It's often advisable to pause occasionally and check the needle hasn't stuck when using a new compass until you learn its particular idiosyncrasies.

Card compasses are bulkier, but often include a side-reading feature. This is a small window set in the edge of the compass that lets you see your course without holding the compass below you. This means you can hold the compass at arms length, which may make it easier to read and will make you more streamlined. Although most card compasses will eventually lock up if tilted, they are more forgiving than needle models. Finning tends to roll a diver from side to side slightly and a card compass may tolerate this better.

Larger compasses tend to yield greater accuracy as the size of the scales makes it easy to check your progress. But if precise navigation isn't essential or you'd like a back up, miniature watchband compasses can be used. So called because they were sold to slip onto a wrist watch, these cost little and are fine for following a general heading.

A third type of compass reached the market a few years ago. These are sophisticated electronic models. In addition to providing simple course information like a mechanical compass, electronic compasses can make handling complicated routes much easier. A number of different headings can be pre-programmed into the computer and recalled underwater. Called waypoints, you can navigate from one waypoint to the next without relying on your memory or writing down a lot of headings. Time and depth information is also displayed on some models, which may be incorporated into dive computers.

Some divers like to mount their compass on a slate. Navigation information from the dive plan can be written down on this beforehand and easily followed underwater. This is an excellent option for dives that go beyond simple out and back runs and require several changes of direction. This could be to locate several specific areas of a large reef or find dispersed parts of a shipwreck. Swimboards are similar - these are used by combat swimmers and underwater orienteering competitors. Often a depth gauge and timing device are also fitted grouping essential information within easy view. Another useful navigation aid can be a plotter. It's used with a compass to plan your route. Even if your dive will be complex with many changes of direction a plotter will let you calculate the distance back to your entry point and provide you with the return heading. Because the return heading is calculated as a straight line that takes you back directly without retracing all the turns of your route out, it's fast and effective---so long as nothing blocks your route.

Sport divers don't always have to rely on their own navigation skills to find their way back to the boat or shore. Electronic beacons have long been used by commercial and military divers. Now available to recreational divers, two units are needed. A transmitter is placed near the point you want to relocate, often the anchor line, and a receiving unit is carried by the diver. It picks up the signal put out by the transmitter and the divers can follow the path back using the homing unit. They work well, provided a reef isn't in your way.

Part Two - Rigging:
getting your instruments under control

There's no set way to configure your instruments. It's up to you to experiment and find the layout that works best for you. During training many instructors like to equip their students identically. This makes a lot of sense while skills are being taught and mastered. However it isn't real world. Once you leave your course and begin diving with different equipment and gaining experience, you'll discover your own preferences. Your instructor, dive shop, other divers and this article can offer advice, but it's up to you to make the final choices. After all, you're the one diving with the kit.

The key principles to be guided by are that access to your instruments should be easy and quick and that they must be easily readable. They should not be allowed to create a snagging hazard that could cause them to become caught on projections on a reef or wreck and become damaged (nor should they be able to damage the environment by accidental contact). Thought needs to be given to ensure the gauge won't interfere with other equipment---you need to guard against an instrument snagging a line when you send up a delayed surface marker buoy or catching on a guide line for instance. A bulky computer worn on your inside forearm might have the potential to interfere with getting a proper rescue grip while administering resuscitation.

The usual places to carry your instruments are on your wrists, in a console or attached to your harness. Many divers use a combination.

Wrist instruments are usually provided with depth compensating straps. If you are using a foam neoprene exposure suit, it will compress with depth. As it thins out it will cause the instrument to "hang" unless the strap has a way of automatically taking up the slack. Simply retightening a strap will work until you ascend. Then it may cut off your circulation unless you loosen it again. There are several ways to make a strap that automatically adjusts for changes in your suit. Some buckles are spring loaded. Other straps may have corrugated flex sections. Simple rubber straps, which have some give in them, work well. Expanding metal bracelets are found on some watches. Whichever type is chosen, the important point is that it can contract and expand steplessly. Not all straps do---many diving watches have metal straps which only have a fold out section in the clasp. This lengthens the strap to fit over a dive suit sleeve, but it doesn't satisfy the requirement to take up slack during descent.

To guard against loss of an important instrument, such as a watch, some divers use straps that pass through both retaining pins. With the strap secured through both pins, a pin failure isn't going to cause loss of the instrument---and the vital information recorded and displayed by it. Dive computers may be provided with a wrist lanyard for additional security. Another option is a neoprene band that fits over the instrument and has a cut out to let you monitor your display.

If your compass is on your wrist you may want to outstretch the other arm and then grasp it with your other hand near or on the elbow. This brings your compass in line with your face and puts it around 15 to 20 centimetres away. This may be to close to focus on, depending on your eyesight and may mean you have to consider some type of correction (see panel later on eyesight correction). It can also be a difficult position to maintain, especially when wearing a bulky exposure suit. Bulky suits bend your arm off your centreline over time and tend to tire you. For accurate navigation runs, any tendency for the compass to get out of alignment with the centre line of your body is likely to introduce errors and may mean you miss your target. Some divers prefer to hold their compass in both hands at arms length. This can be more comfortable and it can also be easier to keep the compass properly centred. You can do this by slipping the compass off your wrist, although, of course, you might drop it. An alternative is to secure the compass using a retractor. This can be connected to your harness and will keep your compass against your body and largely out of harm's way until you need it.

Grouping instruments together has long been popular. Divers often baulk at festooning their wrists with timer, compass and depth gauge. Not only does it take time to put them all on and take them all off, you may have to split instruments across both arms meaning not all of your information can be read at once.

Consoles solve this problem for many divers. Usually a console combines a pressure gauge with a depth gauge or computer. A triple console normally adds a compass. In the past consoles might also carry dive timers, knives and thermometers, but these are uncommon today.

When selecting a console you'll face several choices of layout. Double consoles generally place the pressure gauge and depth gauge or computer on the same side. Triples may also place the compass on the same side as well. This means all of your key information can be read simultaneously. During navigation runs confirming your depth can be very important, enabling you to check you haven't missed your target, for instance. But this style of triple console can be long and bulky and some divers prefer a triple console with two instruments on one side and the third mounted on the reverse. This results in a smaller unit but means you'll need to flip the console to have access to all of its displays. Double consoles will often accept a compass module later as an add on.

Some divers like to cross their consoles, and sometimes safe seconds, across their waist. Usually they are then slotted into a hose clip. A downside to this method is that it adds more items of equipment that must be released in an emergency to remove a casualty's set.

Wrist instruments need little consideration. They are inherently well protected themselves and unlikely to cause problems. Pressure gauges and consoles are our main concern. For most divers the BCD becomes part of the solution to uncontrolled instruments. Modern BCDs usually have instrument mounting points designed into them. These include hose runways as well as D-rings to which clips, retractors and bungee cord can be attached.

Retractors are available in different lengths. To support the weights of heavier equipment, different degrees of strength are also offered. For instruments a lightweight retractor will usually be fine. Stronger models are preferred for large torches. Some retractors have a detachable clip. These can be an advantage with pressure gauges and consoles. With this convenience feature you can leave your retractor attached to your BCD while disconnecting your console or SPG easily for storage with your regulator after your dive.

Part Three:
Safety Considerations For Instrument Diving

There are a few tips of the trade that can help you use your instruments safely and prevent you getting into trouble. Some are discussed below.

Passive Partners: why your best friends won't tell you what you need to know

Almost all diving instruments are fundamentally flawed. The flaw is not in their design, manufacturing or quality control. The flaw is that to be of any use at all the diver has to be bothered to look at them. Most experienced divers, including me, have overlooked their instruments at some point. Dropping beyond planned depths, overstaying no-decompression limits, crashing stops and running out of gas are the wages of this particular sin of omission. Developing the habit of regularly checking your instruments is crucial to avoiding dangerous mistakes. It also helps confirm they are working.

Strictly Need To Know: sharing information

If you are buddy diving responsibly, you really do need to share information with your partner. Firstly, it's unlikely either of you will run low or out of gas or into a decompression situation unexpectedly. While divers should look out for themselves, the truth is most experienced divers have gotten lazy or distracted at some point and gotten into some degree of trouble from not properly monitoring their gauges. Cross checking helps avoid this. Secondly, it lets you compare your depth and timing instruments. This should alert you to any inaccuracies.

Diving with a good margin of safety means working around the needs of the diver with the lowest amount of gas or with the shortest no-decompression time remaining. You might need to head for home earlier if one of the team is a heavier breather than you are or is using a smaller tank to ensure you reach shore with a reasonable reserve of gas. Someone who has dived deeper than you during your dive, has dived more often or made more aggressive dives previously or is using different table or another brand of computer may be much closer to going into decompression than you are or need to make much longer stops. Comparing gauges every few minutes is a good idea to keep you up to date about your buddy's dive status.

It's vital you can read and understand your buddy's instruments. Computer displays especially can be hard to interpret if you aren't familiar with them.

Some divers have developed unofficial signals to indicate, for example, reaching half their tank pressure. It's important to make sure everyone in the party knows the protocols and signals being used if they are new to the group.

They might seem to be your loyal friends on any dive, keeping you out of trouble so long as you consult them regularly, but sometimes instruments lie to you. Basing your dive and your safety on misinformation clearly isn't good. Gauges can go out of calibration, meaning that your depth, direction and gas levels may be very different to what you think they are. Discovering problems will almost certainly mean you are underwater at the time and the realisation is likely to be unexpected, swift and disconcerting.

It's good practice to check your instruments' accuracy regularly. Depth gauges should read zero at sea level and pressure gauges empty until the tank valve is cracked open. Depth gauges should be compared with your buddy's during your dive. Comparing pressure gauges on the same tank to test accuracy before a dive is a good idea.

Gauges are rarely 100% accurate. Depth gauges especially may be out by several percent, with the error becoming most marked at greater depths. It's simply too expensive to make them perfect. The pragmatic approach, and while less than scientific, it seems to work, is to follow the most conservative gauges. Errors should be reported to your dive shop who can arrange to have gauges recalibrated.

Compasses can also fail, despite their simplicity. More likely an error will be the result of other equipment such as knives or torches affecting the magnet. A little thought in how you position influencing equipment will avoid this. Leaving a fluid filled gauge, like a compass or depth gauge, in the sun can cause the liquid to overexpand and damage the gauge. Some models have an expansion diaphragm to allow for this.

To counter the problems of misleading spin from gauges, some divers, especially those who don't like to have to rely on someone else (bearing in mind that if you don't have a regular buddy you trust to hand, you may be saddled with an unknown, under-trained and under-equipped liability by way of a "partner") or dive solo carry a duplicate set of instruments. This also helps overcome some of the issues below.

Over time, sport divers have adopted safer diving practices. Many, such as using BCDS, carrying alternate air sources and surface signalling devices are taken for granted today and rarely questioned by newer divers. Carrying back up instruments has yet to become universal. Even though they are indispensable!

It's worth thinking about how an error during your dive could become compounded and how you might plan ahead to minimise the problems that could occur. Planning for a gas failure is discussed in detail in Running on Empty and Executive Action. Preparing for overstaying your dive times or overshooting your depth is discussed here.

If you are using tables with a depth gauge and timing device, it's a good idea to plan your maximum depth and time and write them on a slate. Because going over your planned limits can easily lead to confusion as you try to recalculate your dive profile, possibly while narked and in low light, making it hard to read the tables, pre-planning overshoot profiles is a useful precaution. Typically this might mean writing down the shortened no-decompression limits or depths and times of stage decompression stops needed if you end up on the table's next two time and depth schedules. Having this information immediately to hand also saves you time---if you have to work out your new decompression situation after crashing your original dive plan, the time taken can increase your penalties severely. Slates can be kept in pockets on BCDs or suits or worn on your wrist.

Computer diving makes it very hard to calculate bail out plans for overshoots. Provided your computer is working it will automatically calculate and display the information you need to surface safely if you violate your dive plan. But if your computer crashes, you might be in trouble. This is because the traditional approach to dealing with this would be to decompress for the deepest depth and maximum time reached. However multi-level diving could mean that you might have spent over an hour underwater without entering decompression because you worked your way from deep water into shallow. Taking deepest depth and maximum time would quite possibly take you off the scale of some sport diving tables. In any event, the decompression penalty, even if you had the gas to carry it out, would be wildly over the top. If you have been monitoring your computer closely and were within your no decompression limits before your computer crashed, you are probably still within them or have only limited decompression to do. Using your back up timer and depth gauge, or your buddy's instruments if you have stayed together, ascend to six metres and try and stay at that depth for as long as your gas and other factors will safely allow.

If you are making a computer dive involving stops and it crashes, you can get into real difficulties. Any planned decompression dive using computers should involve a written down decompression schedule as a back up or a second computer should be carried. It's hard to see how else you can carry out the decompression stops in safety following a computer failure.

If you have monitored your dive properly, including any violations, using dive tables you will be able to work out when it will be safe for you to return to the water. Some tables will penalize you for entering into decompression and keep you on the surface until your residual nitrogen levels have fallen enough to match the tables design parameters. If your computer crashed, the manual will advise you on how long you need to wait before you can dive again. You can then transfer to using your back up gauges and tables to continue diving.

Most divers will, at the very least, dive at night. Others will dive in low visibility, in reduced daylight or will enter overhead environments and leave all sunlight behind. In all of these situations you will still need to monitor your information displays. Electronic instruments often have a backlight to make the screen easy to see. Analogue gauges usually rely on luminous batons. It's useful to shine a light on these periodically to re-energize them. In any situation in which you need a light in order to see, you should be carrying a minimum of one back up light. See Beaming Down for more advice on selecting lighting equipment. Bright lights can actually make it hard to read instruments as they can cause glare or make LCD panels "disappear". A small torch may be more useful. Some divers mount these on their mask strap to make reading instruments easy while leaving their hands free to operate dump valves or hang onto a shot line while monitoring safety or deco stops.

It's very easy to become distracted endlessly reading your gauges. Particularly during ascents when you are often multi-tasking controlling venting from your BCD or dry suit and perhaps deploying a delayed surface marker buoy, it's all too easy to take your eyes off your buddy. Separations are a real hazard. Try facing your partner during ascents and keeping your gauges positioned so you can read them while still monitoring your partner. Nearing the surface both divers can scan the area behind and above each other for hazards, such as boats. Each diver turning 360 degrees means there will be times you lose sight of each other---it's best to avoid this.

Mixing tables can lead to problems. Author Steve Warren relates an example:

Figures That Confuse

In the UK metric units are the standard measurement of tank pressure and pressure gauges are normally marked in bar. Metres are used to measure depth. At USA dominated destinations imperial units might be more common. Pressure is read in pounds per square inch (psi) and depth in feet. This can lead to confusion if you are using a rental gauge as the unfamiliar readout may mislead you. If your buddies or dive supervisors aren't aware of what measurements you are working with other problems can arise. A real example is an instructor who issued a student with a borrowed electronic pressure gauge. Both instructor and student misread 200 psi for 200 bar. It was the difference between a near full tank and a near empty one.

It's important that you, your buddy and others in your dive team are fully aware if you are mixing and matching metric and imperial instruments.

Choosing diving instruments can take a little time and effort, but it's time and effort well spent. Learning how to use them effectively can take practice, especially when experimenting with rigging.

Good diving...