Air-Integrated Dive Computer vs SPG: Is Hoseless AI Worth It?

What is the difference between air integration and an SPG?

The difference is where the pressure reading comes from. A submersible pressure gauge (SPG) sits on a high-pressure hose from the first stage and shows tank pressure mechanically, with no electronics. Hoseless air integration (AI) puts a transmitter on a high-pressure port of the first stage, and the transmitter sends the pressure to your dive computer.

Shearwater describes its system as one-way: the transmitter sends data and the computer does not answer, which is why two or more computers can be set to receive the same transmitter. Once the computer knows your pressure over time, it can work out a breathing rate and estimate how long your gas will last. That estimate is the main thing air integration adds. If you are still choosing your first computer, our dive computer buying checklist covers the other features.

How does a hoseless transmitter work?

It measures cylinder pressure at the first stage and broadcasts it every few seconds over a short-range link. Shearwater's Swift and Suunto's Tank POD use low-frequency radio (38 kHz and 123 kHz). Garmin's Descent T2 uses its SubWave sonar underwater, which gives it a longer stated range.

Placement matters because the range is short. Shearwater says to fit the transmitter on the same side of your body as the computer, and warns that holding the computer closer than 5 cm (2 in) can also drop the signal. It also suggests a first stage with at least two high-pressure ports, so a backup SPG can stay on the regulator.

Transmitters save battery by sleeping. Shearwater transmitters power off after 2 minutes without pressure, and the Swift keeps transmitting whenever the regulator holds more than 3.5 bar, so Shearwater recommends purging the regulator after the dive. Scubapro's Smart transmitter drops to a slow update rate when pressure has not changed for 40 seconds, and switches off at 14 bar or less.

What is gas time remaining, and how do makers calculate it?

Gas time remaining is how long you can stay at your current depth and still ascend directly to the surface with your reserve pressure left. The idea is the same across brands, but the name, ascent rate and reserve differ.

Shearwater publishes the most detail. The Peregrine TX averages your SAC over the last two minutes and ignores the first 30 seconds of each dive, when divers use extra gas to inflate a BCD or drysuit, so GTR shows "wait" for the first few minutes. It does not show GTR once decompression stops are required. Shearwater also notes that above about 207 bar (3,000 psi) its ideal gas assumption overestimates SAC, so GTR reads low early in a dive on a 300 bar cylinder and corrects as pressure falls.

Shearwater and Suunto both leave safety and decompression stops out of the figure, and Garmin states plainly that SAC and air time remaining are estimates that should not be your only source of information.

Worked example: gas time remaining at 18 m

The reserve you set changes the number more than anything else. Take a diver whose last dive went from 200 to 70 bar on a 12 L cylinder in 45 minutes at a 14 m average depth. Our SAC calculator gives 14.4 L/min, which is 1.2 bar per minute on that cylinder. Now put the diver at 18 m with 120 bar left, and follow Shearwater's published formula in simplified form:

• At 18 m the ambient pressure is 2.8 bar, so the diver uses 1.2 x 2.8 = 3.36 bar per minute.
• A direct ascent at 10 m/min takes 1.8 minutes at an average depth of 9 m and uses about 4 bar.
• With a 50 bar reserve, 120 - 50 - 4 = 66 bar is left to spend at depth, and 66 / 3.36 gives about 19.6 minutes of GTR.
• With Suunto's 35 bar end pressure, the same diver would see about 24 minutes.

Which reserve should you set?

Set it to your rock bottom pressure, because the computer's estimate covers only one calm diver going straight up. Our gas planner gives a rock bottom of 55 bar for the diver above: 18 m on a 12 L cylinder, with a stressed SAC of 28.8 L/min (twice the normal 14.4). That figure covers two divers sharing one cylinder through a minute of problem solving, the ascent and a 3-minute safety stop, which GTR does not model. With the reserve at 55 bar instead of 50, the diver in the example sees about 18 minutes of GTR instead of 19.6.

Recalculate when the dive changes. Deeper dives and smaller cylinders raise rock bottom, and a reserve left at the value from a shallow reef trip will overstate your time on a deeper wreck. Our rock bottom guide walks through the method.

What happens when the transmitter signal drops?

The computer warns you within a minute or two and then stops showing pressure. The timings differ by brand, as the table shows. Once the reading is gone, Scubapro's instruction is to use a backup instrument for pressure monitoring and make a safe ascent, and Garmin tells divers to end the dive and return safely to the surface when a tank pressure or battery warning appears.

Most dropouts have ordinary causes. Shearwater's troubleshooting list covers a dead transmitter battery, a closed valve (a transmitter only wakes above 3.5 bar), the computer too far away or too close, and radio interference from HID lights, scooters, suit heaters and photo flashes. Two transmitters with the same transmit interval can fall into step and lose data for 20 minutes or more, which is why the Swift actively shifts its timing when it detects other transmitters nearby.

Why do divers still carry an SPG with air integration?

Because the makers tell them to, and because some transmitter failures give no warning. Shearwater's Swift manual says a failing transmitter may report more or less pressure than the cylinder holds, or none at all, and that some failures produce no error message. Its advice is to carry a backup SPG and check regularly that the SPG and the computer agree. Suunto gives the same instruction for the Tank POD, and Garmin says the T2 should not be your only source of pressure information, listing a depth gauge, an SPG and a timer as backups.

The SPG also helps before the dive. Shearwater's pressure reading refreshes about every 5 seconds, so it tells divers to breathe from the regulator while watching pressure for 10-15 seconds before entering the water. A needle that drops while you breathe shows a closed valve straight away. An SPG keeps working if the computer's own battery runs flat, and a buddy or guide can read it in one glance.

How much more does air integration cost?

Several hundred dollars, and more if you keep an SPG as the makers advise. Keeping the same computer family makes the comparison fair. A Shearwater Peregrine ($580) plus an XS Scuba SPG with hose ($147) comes to $727. A Peregrine TX with a Swift transmitter costs $1,160 on Shearwater's US store, before you add the backup gauge.

Transmitters themselves list at $400 to about $500 at the makers' US stores. Prices change and differ by country. Budget for batteries as well. Shearwater rates the Swift's CR2 cell at 300 dive hours and Garmin rates the T2's CR123A at up to 100 hours.

Who gets the most out of air integration?

Divers who will use the data after the dive, and divers with more than one cylinder. The Peregrine TX logs pressure, GTR and SAC for each dive, pairs with up to 4 transmitters and handles sidemount by pooling two cylinders of the same size. Suunto's Ocean reads up to 5 Tank PODs. If you rent regulators on every trip, remember that the transmitter needs a free high-pressure port on each rental first stage, and a second one if you keep a backup SPG.