|pictured: Mares Mission 2D gauge consoles|
Depth and Time are vital for nitrogen and air management. A scuba diver needs to know how deep he has been and for how long in order to judge the necessity and length of decompression stops and to calculate residual nitrogen for repetitive dives. The time of a dive is easily tracked using a scuba diving watch and the depth is tracked using a depth gauge.
Scuba gauges are almost always sold as an integrated console. With a single console, the scuba diver has one piece of equipment, attached by a hose to the tank, which shows current depth and tank pressure. The console may be either encased in a rubber sheath called the "gauge boot" or embedded in a hard plastic shell. Sometimes the consoles are simply two dials - tank pressure and depth - embedded in a handheld device. Other consoles will integrate more than two dials: they might include a compass, timer, temperature gauge or other instrumentation. Consoles can often be disassembled and reconfigured so that the depth gauge can be replaced by a dive computer.
Even when nitrogen management is left to a dive computer air consumption is shown on a separate dial. If the diver is using a wrist-mounted computer the console may only have a tank pressure gauge.
In addition to the depth and time, the scuba diver needs to know how much air is in the scuba tank. At the beginning of the dive the diver starts between 2000 and 3000psi. The diver watches the air pressure gauge for several reasons:
If the valves are working properly and delivering proper air pressure the diver should be able to breathe from the regulator and the gauge will not move (except slowly, downward). If the pressure gauge dips with every breath then there is not enough pressure coming from the tank and there may be a problem with the scuba tank, valve, or hose.
Another reason to have a scuba tank pressure gauge: It is important to stop diving with air still remaining in your tank. A good recommendation is to get back on the boat with at least 100psi still in the tank. Not only does this make sense from a conservative safety point of view, but another important reason a diver should never decompress a scuba tank is that the air pressure prevents contaminants, water in particular, from getting into the tank. Water is not clean. When water gets into a scuba tank (if it is a steel tank) it causes rust, mildew, mould and bacteria to flourish inside. The scuba tank must then be sterilized, dried and refilled.
Scuba gauges are available in both metric and imperial units. If you learned to dive using imperial units (psi, cu.ft., depth in feet etc) then get yourself a gauge using those. Most American-made scuba gauges use imperial units and often those will be the ones in stock. However, if you learned to dive with metric units (meters, liters, kilopascals etc) you will be more comfortable with metric units on your scuba gauges. The unit of measurement has no effect on the quality of the device. For the manufacturer it's only a matter of producing the same device with different scale markings. Metric scuba gauges are most common in Europe. In Canada, even though everything else is done in metric, most scuba training and gear involves imperial units.
When shopping for scuba gauges for your scuba system, look for: