Gauges and dive computers
0 of 4 hands-on
01 Introduction
Underwater, you cannot rely on feeling or guessing. You need to know how deep you are, how much gas is left and how long you have been down. This lesson looks at the instruments that tell you, and how to treat them as aids to good judgement, not substitutes for it.
Objectives
- Describe what a pressure gauge, depth gauge, compass and dive computer show
- Explain what a dive computer measures and what it calculates
- Describe the limits of a computer and how to use one responsibly
02 Watch
Reading the dive
4 min · Video in production
- 1
A diver's wrist computer showing depth, time and the no-decompression time remaining.
- 2
A cylinder pressure gauge needle falling slowly through the dive, with the reserve zone marked.
- 3
A compass held level in front of a diver and a line being followed across a reef.
- 4
A diver glancing at the computer and gauge together before signalling to the buddy.
03 Learn
The classic instruments
| Instrument | What it tells you | Notes |
|---|---|---|
| Submersible pressure gauge (SPG) | How much gas is left in the cylinder, in bar | Attached to the regulator by a hose. Check it often |
| Depth gauge | Your depth, and often your maximum depth | Divers check it to stay within the plan |
| Timer or watch | Elapsed dive time | A basic backup for a computer |
| Compass | Direction of travel | Used for navigation, and held level and away from metal |
The dive computer
A dive computer measures pressure, from which it works out depth, and it measures time. Using a mathematical model of how nitrogen enters and leaves the body, it calculates how long you can stay at your depth without needing decompression stops. It can also show your ascent rate, safety stop timing, water temperature and a record of the dive.
- No-decompression time (NDL): how long you can stay at the current depth before a decompression stop is needed.
- Ascent rate: how fast you are rising, with a warning if it is too fast.
- Safety stop: a timer for the stop that many divers make at around 5 metres at the end of a dive.
- Alarms: warnings for depth, time, ascent speed and, on some models, gas.
- Air integration: some computers receive pressure from a transmitter on the first stage and show gas remaining.
- Dive log: a record of each dive, which can often be downloaded.
What a computer cannot do
A decompression model is a mathematical estimate based on averages. It cannot know your age, fitness, hydration, fatigue, how cold you are or how hard you worked, all of which affect how your body takes up and releases nitrogen. Computers use cautious assumptions, but they do not guarantee that no problem will occur.
Using one responsibly
- 1.Learn how your computer works, and read the manual. Practise reading it before you need to.
- 2.Check the battery and settings before each trip, including any gas mix, altitude and conservatism settings.
- 3.Use your own computer for the whole dive and the whole trip. Do not share one between two divers.
- 4.If you dive with a group, follow the more conservative reading.
- 5.Keep a backup, such as a second computer or a depth gauge and a watch, for longer or more remote trips.
- 6.Observe the ascent rate and safety stop, even if the computer does not insist.
Predict first
Two buddies have different computers, and one shows 6 minutes left to the no-decompression limit and the other shows 9. What should they do?
Reading gas
A pressure gauge is as important as a computer. Check it regularly, and plan to end the dive with a reserve. A common approach is to turn the dive when about a third to half of the gas has been used, depending on the dive, and to start the ascent while there is still a comfortable reserve. Gas planning is covered in more detail in other courses.
Take a guess first
A diver starts a dive with 200 bar. Their plan is to start the ascent at 70 bar to keep a reserve. How many bar are they planning to use?
04 Explore
Try it yourself
Match each instrument with what it shows.
Match each instrument with what it shows.
- Pressure gauge
- Depth gauge
- Compass
- Dive computer
- Air integration
05 Apply
Scenario
On a dive, your computer shows plenty of no-decompression time, but your pressure gauge shows 60 bar, which is lower than your planned turn pressure. You are at 18 metres, and your buddy points at an interesting wall further on.
What should you do?
06 Check
07 Summary
Key points
- A pressure gauge, depth gauge, timer and compass are the basic instruments.
- A computer measures pressure and time, and models nitrogen to give no-decompression time, ascent rate and safety stop information.
- Computers use averages and cannot know personal factors. Stay well within limits.
- Each diver has their own computer, follows the most conservative reading and checks gas regularly.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.