SPG — Submersible Pressure Gauge

An SPG, or submersible pressure gauge, is the tool that tells you how much air remains in your cylinder.

How it works in practice

The device attaches to your primary regulator console via a low-pressure hose. Most modern gauges show readings in bar or PSI so that you can track consumption as you descend. I remember diving at Anilao in 2018 where the visibility was only 5 m. Because the visibility was poor, I found myself checking my SPG every 3 minutes to ensure my air supply stayed predictable.

You must understand your specific unit. Some analog gauges use a needle while digital models show exact numbers on an LCD screen. Digital gauges are helpful because they often include depth and temperature readings in one small housing.

Always check your hoses for cracks before every trip to places like Koh Tao. A tiny leak can drop your pressure by 10 bar in just 15 minutes.

Why it matters for safety

A reliable SPG prevents unexpected out-of-air emergencies. You should begin your ascent when you hit your pre-planned reserve limit, which is often 50 bar in many recreational settings. If you dive to 30 m, your air consumption increases significantly compared to a shallow reef dive.

Planning is essential. I once saw a student panic because they misread a 50 bar reading for 150 bar. This mistake happened because the gauge was tilted at a strange angle during a heavy surge.

Track your rate. If you use 30 bar in 10 minutes, you know your remaining time.

Common mistakes and errors

Divers often forget to check their pressure during high-excitement moments. In Komodo, seeing a reef shark can distract you from your air supply for several minutes. While the shark is interesting, your pressure might drop from 140 bar to 80 bar without you noticing.

Misreading the scale is another frequent issue. Some older gauges use different increments that confuse newer divers who prefer metric units. You must confirm if your gauge uses bar or PSI before you jump into the water.

Check your equipment. A faulty SPG is a silent danger.

Connection to air consumption

The SPG works closely with the concept of Surface Air Consumption (SAC) rates. Your SAC rate tells you how much air you use at the surface per minute. When you descend to 20 m, you breathe air at 3 ata, which means you use your tank twice as fast.

This relationship dictates your dive profile. If your SPG shows a rapid drop, you must ascend immediately to stay within safe limits. I always carry a backup octopus or a redundant pressure gauge when diving deep sites in Tubbataha.

Keep your eyes on the numbers. Check your SPG every 5 minutes.