An SPG is a rather simple device that connects to the first stage regulator's high pressure port via a suitable high pressure hose.frogxman once bubbled...
Uncle Pug, can you explain how this device works?
The SPG allows one to instantly determine the pressure in the scuba tank just by looking at it!!!
I won't go into the details as to how the SPG actually measures and displays the tank pressure... just know that it does... unless of course the indicator needle should *stick*... but I will explain how to tell if that is the case.
How to use an SPG is the important thing here.
You should check your SPG reading at the beginning of a dive to see how much breathing gas you have. I will leave out a discussion of gas management practices here but we do have excellent threads on Scuba Board covering this topic.
Before beginning the dive it is a good idea to watch the SPG while taking a couple of breaths off of the regulator. This will confirm that you have actually turned and left your tank valve on. If you have turned your tank on and then back off... the needle on the SPG will fall to zero as you breath off of the regulator. This is why you should do this before the dive. BTW if you have turned the valve on and then almost off the SPG will drop as you breath and then rebound. Always open your valve all the way to dive the tank.
During the dive you should check your SPG every 5 minutes and consciously note your pressure. Do not stare incessantly at the SPG as this is counter productive. After several times doing this you should try to guess what your SPG will read before you look at it.
It won't be long and you will have developed the ability to *know* your tank pressure just by looking at your bottom timer! Of course you should still look at your SPG every 5 minutes just to make sure.
Thanks for asking this question Froggy... SPGs are a great adjunct to diving and have truly made the J valve with its reserve lever obsolete.
Oh, I almost forgot... about that sticking needle problem... it doesn't happen often... but if you have used your SPG as I recommend you will instantly know that it has a stuck needle when you look at your SPG and it reads significantly higher than you have guessed!! This is why incessant staring at the SPG is not good... it doesn't give you the same effect as, "Hey, I've been breathing off of this thing for 5 minutes and the needle hasn't moved!"