1- balanced, over-balanced or unparallel.... what does this mean?
A Balanced first stage just means the intermediate pressure is stable and does not vary as tank pressure falls. A balanced first stage will allow a non balanced second stage to be tuned for lower inhalation effort without having it freeflow at high tank pressure. The only unbalanced regs available today are flow by piston designs. Balanced piston regs and all diaphragm designs currently made are balanced.
A balanced second stage will do a better job of accomodating the IP change from an unbalanced first stage and provide a consistantly low inhalation effort throughout the dive. The irony here is that no one sells a balanced second stage with an unbalanced first stage.
Theoretically, balancing the design (first or second stage) allow the design to have a larger orifice and a higher flow rate. This is an issue with unbalanced flow by piston designs and they are limited to flow rates under 100 SCFM.
All currently produced second stages have a downstream bias to ensure they will act as an over pressure relief valve in the event of a high pressure seat leak. That means that none of them are perfectly balanced. But with a balanced first stage it is a moot issue as perfect balance is not required.
"Overbalanced" means absolutely nothing. The explanation of why it means nothing is long and complicated and I have covered it a few times before. If you do a search on overbalanced and cross reference with my posts it will show up.
Scubapro uses a piston stem that has an end that is slightly larger than the middle that passes through the high pressure o-ring. that slight difference allows the small (but still there) area of the kife edge that seals against the seat to be balanced so that there is zero IP change as tank pressure falls from 3500 - 500 psi. With a standard parrellel piston stem, there is a 4-6 psi change in IP the area of the knife edge is then not completely balanced.
2- what is the difference between piton and diaphragm?
A piston first stage used a piston that senses the drop in pressure as you inhale. It is "sucked" upward by the lower pressure acting on the piston head and the movement opens the valve and starts the air flow. Either the seat or the orifice is mounted on the end of the piston stem so it is in effect a one moving part system.
A diaphragm serves this same purpose. As you inhale the drop in pressure moves the diaphragm which is sucked inward where it then presses on a pad that presses on a pin that lifts the seat carrier off the orifice starting the airflow. There are more parts involved.
The other major difference is that the piston operates on the downstream side of the orifice while the diphragm operates on the upstream side of the orifice, which is not really significant as all diaphragm designs are now balanced.
3- adjustable vs. non-adjustable?
"Adjustable" usually refers to a knob that allows the inhalation effort to be increased or decreased to suit the divers position in the water or other factors such as current. Nearly all of them work by screwing in a knob that increases the spring pressure acting on the seat. It is a good idea to store them all the way out to minimize pressure and wear on the seat during storage. Another advantage is that as the seat wears and develops an excessive seating groove, a non adjustabel reg will start to freeflow slighty while the adjustable reg can have the knob screwed in a bit to stop the leak.
There are also flow vane adjustments. In a minimum or predive position, the flow vanve will develop positive pressure inside the case and stop a freeflow. In the maximum or dive positon, the flow vane will create a vaccuum in the case and cause the airflow to continue on its own. This makes the work of breathing lower for the reg.
4- 3/8" vs. 1/2" LP ports, does it differ what will be connected to the port or is it just the hose that will vary?
Half inch LP ports were all the rage 5-10 years ago and the idea was to increase flwo rates. They are a complete waste of time and money as the hose is not the limiting factor in the system. All they do is complicate your logistics, reduce your choice of hose options, and force you to use larger and stiffer hoses. Avoid them like the plauge.
5- I mostly dive warm waters, I wanna get my regulator setup which specs should I choose? I would also be diving single and double tanks ....
For warm water single tank diving I like a balanced piston reg like the Scubapro Mk 25. The layout of the design allows for streamlined hose routing if you use a long hose primary.
On doubles I prefer Scubapro Mk 17's as they allow the hoses t be routed straight down in a very clean configuration. It is also fully sealed making it very reliable in cold water and also easier to rinse after any salt water dive.
The Mk 25 also offers decent hose routing on doubles as well so if you want one reg to do double and singel duty - go with the Mk 25. But in the end it is much more practical to get three regs so that you can have dedicated single and doubles regs.
I am a big fan of the Scuabpro G250V second stage. It offers exceptional performance, is adjustable, has a metal air barrel (improves heat transfer in cold water and reduces dry mouth on any long dive) and is an updated version of a well proven high performance design dating back to the late 1980's.
