Just sent you a PM...
---------- Post added March 13th, 2014 at 11:52 AM ----------
Hmmmm, for some reason my PM didn't get sent, so I'll post your answer here.
There clear faceplate on the front should unscrew, and there should be an o-ring in there. You will remove the gauge face through the front.
To change the battery:
1. Remove the hose.
2. Unscrew the metal hose connector from the gauge body and take out the spool. At this point you should see the gague face inside become loose.
3. Remove the clear faceplate on the front by unscrewing it.
4. Pop the gauge face out throught the front, the battery is on the back of the gauge face.
I forget what type of battery it is. Mine had an old one still inside, so I was able to find a replacement. If there isn't one inside yours, let me know and I will get the number from mine.
FYI - The sensor on the back is a switch that turn the digital display on when you submerge. So, the display will not come on when you replace the battery until you press the button underneath the sensor.
Good Luck!
Bill