I had a water pressure gauge that wasn't working. I decided to take it apart and was a little surprised at what I found. Attached is a picture of a Teleflex Amega series water pressure gauge, 0 to 30 PSI
It appears that pressure enters the tube and acts like a little pendulum. Attached to that thicker tube through a few hinges is a serrated Hammer type mechanism. As the pressure increases in the tube it lifts moving the hammer which is attached to a cog which moves your gauge needle. Pretty interesting