This is kind of an apples and oranges kind of question. It would make just as much sense to ask "What's the difference between the pressure in a scuba tank and absolute pressure?"
Ambient pressure is the pressure of the environment around you. Similarly, my ambient temperature right now is the temperature of the room that I'm sitting in. There are different ways to express that pressure.
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Absolute and gauge are two different ways of expressing pressure measurements (whether the pressure you are talking about is the ambient pressure, the pressure within a scuba tank, or any other pressure).
Absolute pressure is the total pressure as compared to a vacuum.
Gauge pressure is another way of expressing pressure. Gauge pressure is most commonly defined as the pressure compared to 1 atmosphere absolute. To put it another way, gauge pressure is the difference betwwn the meausured pressure and 1 atmosphere absolute. So gauge pressure will be 1 atmosphere less than the absolute pressure. The reason this is called "gauge pressure" is that many pressure gauges show the pressure DIFFERENCE between two ports on the gauge. One port is connected to the thing to be measured; the other port is left exposed (via a tiny hole) to local atmospheric pressure, which is more or less 1 atmosphere-absolute.
In a few cases, gauge pressure is pressure (such as that of a tank) as compared to the ambient pressure. When the pressure being used as the reference differs significantly from standard air pressure, this is more often called "differential pressure".
Charlie Allen
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