Trying to understand Balanced Vs Unbalanced

[BeCool]

Hi everyone! This might be a bit technical question but if someone can explain it in the simplest way to help me understand that would be awesome. I'm going to use SP Mk2 as an example because it's simple. I understand that Mk2 is an unbalanced piston, but why and how an unbalanced piston will breath harder as the tank pressure gets lower? What is the physics/science behind this? And then 2nd question is what special design or features are used to make the 1st stage become balanced? Again, what is the science behind this? Thanks in advance first.


Edit: Pictures would be very helpful.

 

[DA Aquamaster]

All first stages are open when they are unpressurized. This is because it takes a certain amount of air pressure acting on the valve to close the valve.

In the Mk 2, the seat is on the end of the piston stem (think capital T with the seat on the bottom end of the T with the cross bar on the T being the piston head).

The tank pressure pushes on the bottom of the T and forces it off the orifice. Air goes through the orifice and enters the intermediate pressure section of the regulator. A small hole in the side of the T allows air up through the center of the T and out the top of the T. Air is then trapped in a compression chamber on top of the T and pushes it back down against both spring pressure and the air pressure from the tank.

This is where the "unbalanced" part comes in to play. When the tank is full there is simply more pressure pushing up on the seat, so it takes a bit more air pressure on top of the piston to push it down and close the valve. When the tank is close to empty, there is less tank pressure pushing on the seat and it takes less air pressure pushing on the top of the piston to close the valve.

What this means is that with a full tank, the intermediate pressure (IP) on a Mk 2 is about 145 psi and with a near empty tank it will drop t around 120-125 psi.

If you have an unbalanced second stage attached, it is essentially a rinse and repeat process as the higher intermediate pressure provides more assitance to open the valve in the second stage. So at lower tank pressures, you have less IP and less downstream force assiting in opeing the valve. That requires a slightly higher inhalation effort to open the valve. If you have a balanced second stage, there is not much difference in the downstream assitance and the inhalation effort is about the same.

In a balanced piston first stage, there is a sharp end on the end of the piston that acts as the orifice and it sits on the seat. The tank pressure comes from the sides of the long end of the T near the T and does not apply any upward force on the piston. The area of the piston stem/orifice on the seat is the same as the area of the piston stem where it passes through the first stage body, ans since the area is equal it cancels any differences in pressure.

In a second stage the same thing happens. The seat sits on an orifice just like in an unbalanced reg, except there is a small hole in the seat that lets air pass through to a sealed balance chamber on the other side of the valve. This puts equal air pressure on each side of the poppet assembly that holds the seat, so again the pressues are equal so changes in IP have no effect.

This link should clear it up for you:

http://www.scuba-info.com/scubaregulators.html

The first one is an unbalanced piston just like the Mk 2, the second one is a balanced piston like the Mk 25.

The third and fourth drawing are unbalanced diaphragm and balanced diaphragm (like the Mk 17) designs, but since it is really easy to balance the seat carrier on a diaphragm design, no one makes an unbalanced diaphragm reg.

 

[halocline]

You might have a look at Vance Harlow's book on regulator repair, or peter Wolfinger's book "Reg Savvy". Both of those have very clear diagrams that show the air paths and forces.

To present some more basic information on DA's excellent-as-usual explanation, the first stage in any scuba regulator lowers the tank pressure to "intermediate pressure" (IP) which it then sends to the 2nd stage. Overly simplified, the IP is somewhat inversely proportional to breathing resistance. (with unbalanced 2nd stages)

The ingenious way that 1st stages work is that IP is the amount of pressure it takes to close the valve, cutting off flow from the tank. This is how the 1st stage regulates (it is a regulator, after all) pressure. When you take a breath, you lower the pressure in the reg (IP) which opens it allowing more air from the tank until the pressure builds back up to IP until it shuts off. This cycle is completed with each breath.

With unbalanced regs, the IP is influenced by the amount of tank pressure exactly as DA explained. There are old diaphragm regs (the old double hosers) in which the IP goes up as the tank empties. These regs are easier to breathe with a near empty tank.

 

[BeCool]

Thanks for such great explanations. The part on balanced piston where the two surface areas become equal and cancelled each other was a bit hard to understand. I looked at the picture on the link and still couldn't figure out.

Another question came to my mind is since the hp air is 3000psi coming from the tank, does it mean it's acting on the hp seat at 3000psi of pressure? that sounds like a lot! Similarly on the 2nd stage, the poppet spring has to resist 140psi of pressure from the hose?

 

[meesier42]

3000psi= 3000 lbs per sq inch. factor in that the hp seat is ~0.01sq inches and you have a whole 30 pounds of force. granted over simplified and not entirely accurate, but you get the point.

 

[DA Aquamaster]

Exactly - you have 3000 psi, but a small area on the seat. That pressure is however a limiting factor in the size of the orifice. If I double the seat size, I double the force acting on it, and I also double the change in force as the tank goes from full to empty. That means I also double the change in the IP as the tank empties - or it means I have to double the size of the piston head to maintain the same ratio of orifice to piston head area to limit IP change. It effectively limits the size of the orifice and indirectly the flow rate of the regulator.

---


In the first picture (unbalanced reg) the air presses on the seat and assists in opening the valve. In the second picture (balnced reg) the air enters the chamber from the side of the piston stem. The pressure there can push in all directions but since the size of the piston stem is the same at the seat and at the O-ring, the air pressure exerts no pressure on it.

 

[AquaNSun]

I'm not 100% sure but I'm gonna try here and please correct me if Im wrong.

For an unbalanced piston, the IP=closing force and is balanced by the spring force + force from HP air. So, if the spring has a closing force of 110psi and the Hp air is pushing on the seat at 30psi, the total closing force (IP) is 110 + 30 = 140Psi (at full tank, 3000psi). When the tank is down to 500psi, the Hp air is now pushing on the seat at say 10psi. The spring is still constant at 110psi, so the total closing force is now 110 + 10 = 120psi. As you can see the tank pressure is affecting the IP for an unbalanced piston in this example.

Now, for a balanced piston, the closing force is only balanced by the spring force because there is no HP air pushing on the seat. So, if the spring has a closing force of 140psi, that's all it needs to seal the seat be it at full tank or near empty tank. As a result the IP is constant for the whole range.

If what I think is correct above than a balanced piston spring is always stiffer than an unbalanced piston spring? Can someone confirmed that?

if the above unbalanced piston is used with an unbalanced second stage, at 120psi (IP) towards the end of the dive, you'll have to inhale a little harder to open the valve than you would if it were at 140psi because the extra 20psi would have help to push the seat open.

if the above unbalanced piston is used with a balanced second stage, the 120psi IP pushing on the seat is balanced by opposing force (120psi) in the balanced chamber so all you need is may be 2psi to open the valve and thus you don't need to inhale too hard. This also makes the balanced spring less stiff because most of the force is taken care by the balanced chamber.

With the above, I think it's still better to have a balanced 1st + balanced 2nd because a balanced 1st would give you more air flow volumn (higher IP) than an unbalanced 1st (if the difference is noticable at all).

 

[BeCool]

Cool! there's so much to learn

Some second stages also have the knobs to turn the resistance levels. Has this anything to do with balancing and is this just adjusting the air flow to breath better?

 

[elan]

Thanks DA Aquamaster for the information. Another question if in an DH Aquamaster regulator it becomes easier to breath when the tank depletes does it mean it's of an upstream design ?

 

[LeadTurn_SD]

Cool! there's so much to learn

Some second stages also have the knobs to turn the resistance levels. Has this anything to do with balancing and is this just adjusting the air flow to breath better?

Simple answer: The knob effects the loading of the spring in the 2nd stage (and thus seating force) that helps oppose the downstream flow of air (which is attempting to open the valve). The resistance knob therefore directly effects the "cracking pressure" (amount of inhalation effort to open the valve and start air flowing). Balancing allows a "lighter" spring to be used than in an unblanced 2nd.

I'm not sure if saying the resitance knob controls air flow is completely accurate, but I'd guess it would be possible to adjust the 2nd stage so that with knob cranked all the way down it would be very difficult to make the valve "crack" with a normal inhalation? But techs typically are careful to avoid this (they make sure that with the adjustment set to maximum resitance you can still breath from the reg).

Best wishes.

I'm sure someone will jump in with a better (more correct) answer.