Understanding Intermediate Pressure

[LeadTurn_SD]

IP is the pressure required to close the 1st stage valve. It is working against the mainspring and ambient pressure to do that, so in that sense your statement is absolutely true.

But the pressure supplied to the first stage is tank pressure or, simply, supply pressure. I suspect you realize this. This pressure builds up in the IP chamber of the 1st stage, closing the valve and cutting off the supply. So the first stage is what's called a 'regulating' valve. Then when you take a breath, you lower the pressure in the IP chamber by drawing air from it, which opens the valve a bit, letting air in, until it builds back up to IP and then closes.

Nice explanation.

I think the concept of tank supply air closing the 1st stage valve is the thing that is not intuitive for a lot of people (or, it was not obvious to me until I started working on my own regs).

2nd stage valves work the way most of us suspect they do (or the way I supposed they would work). But the 1st stage was a little mysterious for me... and the design so elegant and simple that I was amazed when I finally bit the bullet and opened one up.

Beautiful in their simplicity.... maybe that is why I enjoy working on them.

Best wishes.

 

[axxel57]

What? MK11 and 17 are both balanced and have a steady IP.

Yes, okay, as you think.

For the rest, no worries,I will not start the discussion again http://www.scubaboard.com/forums/regulators/449842-aqualung-legend-lx-vs-scubapro-mk25-a700.html & http://www.scubaboard.com/forums/re...ung-titan-lx-supreme-scubapro-mk-17-c300.html

Promised.....

 

[eelnoraa]

I think the concept of tank supply air closing the 1st stage valve is the thing that is not intuitive for a lot of people (or, it was not obvious to me until I started working on my own regs).

I can give a very simple analogy. The toilet water tank we use everyday is on similar concept. When the tank level is low, there is no "floating" force pushing on the rod that close the valve, so water flow in. As water level increase, the rod with float at the end floats up slowly closing the valve. When the water is high enough, the valve will be fully close. No more water can come in. It is a very accurate way to regulate the water level in the tank. This system is what engineers call "negative feedback system".

 

[halocline]

What? MK11 and 17 are both balanced and have a steady IP.

They are a balanced design but do not have a uniform IP from full to empty tank. Try it, you'll be surprised as was I. There is a substantial 'upstream bias' in these regs, they are not fully balanced. When axxel first posted this, I checked the IP of my MK11 at various tank pressures, and sure enough, he was right.

 

[WhiteSands]

Reg Savvy has a few pages dedicated to definitions of various "pressure" related terms. How did you read that and come up with your definition??

From this:





---------- Post added June 5th, 2014 at 09:30 AM ----------

Ok now I think I understand better. Since the 1st stage blocks out the supply of gas from the cylinder once the valves are closed (IP), that is the pressure (IP) that is also delivered to the 2nd stage.

I think I have some questions regarding why IP increases as tank pressure drops for diaphragm regs. I have to go re-read and re-study the pictures to frame my questions properly.

 

[LeadTurn_SD]

I can give a very simple analogy. The toilet water tank we use everyday is on similar concept. When the tank level is low, there is no "floating" force pushing on the rod that close the valve, so water flow in. As water level increase, the rod with float at the end floats up slowly closing the valve. When the water is high enough, the valve will be fully close. No more water can come in. It is a very accurate way to regulate the water level in the tank. This system is what engineers call "negative feedback system".

That cannot be right!!!

You see, I really enjoy working on regulators, but hate working on toilets with a passion!

Therefor, your analogy must certainly be false ( ).

Besides, you did not specify whether the toilet tank was "balanced" or "unbalanced"!

Hmmmph. Amateurs.

Best wishes.

 

[axxel57]

They are a balanced design but do not have a uniform IP from full to empty tank. Try it, you'll be surprised as was I. There is a substantial 'upstream bias' in these regs, they are not fully balanced. When axxel first posted this, I checked the IP of my MK11 at various tank pressures, and sure enough, he was right.

Thanks, I will never be able to explain things as nice as you, at least not in English.:blinking:

 

[WhiteSands]

I don't understand why IP goes up when supply pressure goes down.





Can anyone explain the mechanics of why this is so? The supply pressure does not "resist" the closing valve in any way in a diaphragm, so why does the IP increase?

 

[halocline]

I don't understand why IP goes up when supply pressure goes down.

Can anyone explain the mechanics of why this is so? The supply pressure does not "resist" the closing valve in any way in a diaphragm, so why does the IP increase?

First, I really think that the Vance Harlow book has an explanation that is easier to understand than regulator savvy, even though the Wolfinger book has more detailed engineering information. But it's not easy to read.

Okay, so you need to remember that IP is the pressure required to close the 1st stage valve. The 1st stage valve is held open mostly by a big spring, this is true for both piston and diaphragm 1st stages. So, IP is more-or-less the amount of force needed to overcome the spring. But if there are other forces affecting the valve, they will influence the amount of force needed. In piston regs, the air pressure from the tank is working to push the valve open. (This is true for both balanced and unbalanced pistons, just in quite different amounts) So, that means when the tank is full, there's a higher amount of air pressure pushing the valve open, therefore more pressure is needed to close it. Thus the higher IP with a full tank with piston 1sts.

With diaphragms, it's exactly the opposite scenario, although it's a little trickier to visualize. The air pressure from the tank is pushing the valve closed. This means with a full tank there is more pressure from the tank trying to close the valve, already working against the big spring. This means the amount of air pressure in the IP chamber-also pushing against that spring- that is needed to close the valve is lower. As the tank empties, there's less air pressure pushing the valve closed, meaning you need higher IP to close the valve.

The balance chamber in diaphragm firsts provides an area of low pressure (IP) in the HP chamber that is theoretically equal to the area in which supply pressure (tank pressure) is pushing the valve closed. This negates the effects of tank pressure assisting with the valve closing, resulting in a more consistent IP throughout the supply range. Balanced piston first stages compensate for changes in supply pressure in a couple of different ways depending on the design. The simplest one, based on the MK5, just uses a design that minimizes any surface area present for the supply pressure air to push on the piston.

 

[redacted]

I don't understand why IP goes up when supply pressure goes down.





Can anyone explain the mechanics of why this is so? The supply pressure does not "resist" the closing valve in any way in a diaphragm, so why does the IP increase?

In the illustration you provided of the unbalanced diaphragm, there is a small hole in the yoke nut that the HP seat rides in. Think about what what pressures occur in that volume and what effect it has. Then look at what happens, pressure wise, in that same space with a balanced diaphragm design.