Balanced Second Stage Adjustment (G250)

[Aloha Joe]

As I started typing this the importance of reading one of the recommended books became quite obvious.

The short question: assuming some ideal bench setup (max flow, tank pressure, first stage performance, etc) what outside conditions affect G250 flow performance? Assuming I set the adjustment on the verge of free flow at max open, is there some condition where it could end up flowing freely (environment, when wet, dirty, worn, etc.)?

Stop reading here if you like to keep it simple
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For me thinking this through and anyone else who likes this stuff as much as I do... I'm not asking or expecting people to answer every question. Feel free to comment on what you want, or just read and think, or be thankful you don't end up in so many rabbit holes

The VDH video mentions 2 theories for adjusting the G250: 1) set to not flow freely when screw is fully open to prevent accidental freeflow, and 2) set to just flow freely when screw is fully open to allow on-site adjustment for maximum breathing performance. I liked the conservative idea - it seemed simple at first, and probably 'good enough' for most, but it raised the question: what external conditions affect this adjustment?

As I started typing I came up with more questions than answers, and now I see why one might prefer leaving some adjustment rather than the conservative 'no free flow' setting. (probably the short answer)

1) Tank and rig performance: there must be some ideal or max flow tank/rig condition. Again it seems simple: set G250 with IP at max working pressure with my rig. But thinking deeper, it seems like tank volume and pressure could have some affect, and how does that affect the first stage too? I imagine an 80cf tank at 3000 psi would have some pressure drop off versus a 117cf at 3400psi - maybe that doesn't affect the cracking pressure of the secondary, but it could affect the pressure (IP) at which the G250 closes, or exaggerate any flow that leaks past (sensitivity. So the topic becomes 'balanced secondary performance as a function of tank volume/pressure and first stage performance' - which still is several topics on their own (tank flow as a function of volume/pressure, first stage performance as a function of tank condition, and... how does the balanced secondary affect IP at varying tank/rig performance.

2) Secondary flow performance: how does it change over time? Worn seals - do they promote more flow (leakage), is the G250 more likely to free flow as it gets worn in? Or does corrosion typically dominate, causing flow performance to degrade over time?

3) What's happens in the water?? Obviously you don't want to free flow when you surface so this probably doesn't have much to do with the 'knob' setting, and has more to do with the vane position (I assume the reg is set with vane at 'min' regardless of knob adjustment, so there's some adjustment at depth...)

 

[tbone1004]

1.
It's really pressure more than volume, but it depends on how well the first stage is balanced and what that means for your second stage.
I dive Poseidon Jetstreams which are what I like to call mostly balanced. At 3600psi the IP is usually around 123, but at 300psi, it climbs to about 145. I have to be really careful to make sure that I adjust the IP when the tanks are full because of that swing. The Jetstream is a partial upstream valve, so if I tune it to 123 at lower than full tank pressures, when it gets on a full tank, it won't lock up. On a downstream second stage like the G250 with that first stage, you would have to make sure to tune it at the low pressure side because you'd have a slight freeflow at the bottom.
Diaphgrams in general have a more stable balance than pistons, but you do need to make sure that when you set the IP and tune the second stage, that you are doing it at the "worst case" IP on the regulator. I believe most pistons have their IP directly proportional to tank pressure, so for those you need to make sure again that you are tuning the second stage at both extremes.
This impact is mitigated by the balance mechanism of the second stage which makes it less sensitive to IP changes coming from the first, but it is still something to think about.
This can also be made worse by sealing diaphragms in first stages that boost the IP relative to depth which is why you really don't want to tune a sealed diaphragm to slight freeflow in my opinion. Freeflow will get progressively larger as you go down and while you can turn it back as you go deeper, it's annoying to some people.

I personally tune all G250 and similar regulators a smidgen before they freeflow. I will listen with the adjusting tool for the freeflow then back it off just until it stops.

Short answer to your tank size is that you can't think of the hoses like a fire hose. You have to think of them more like a lung which @victorzamora and I were talking about the other day. When you breathe, you are removing gas out of all of the LP hoses and the IP chamber which is the pressure drop, then when you stop breathing, the first stage replenishes the volume. Depending on the size of that chamber and the length of the inhale, the pressure drop will change and in that case it is up to the first stage flow characteristics. Pretty much all can flow more than the valves though so tank size is pretty irrelevant. It's all about supply pressure and how that impacts IP.

2.
Depends on the specific design. The seat in the G250 will get worn in and that will make it harder to breathe. Sometimes they can feather when that happens, and contamination can come in which is when the small freeflows start to happen. I have not seen any real corrosion inside of regulators that still performed properly and there shouldn't be any water inside the actual barrel itself.... If there is, it gets blown out so it shouldn't sit.

3.
The vane position is the venturi assist and it lowers the effect of the venturi. This is what causes the regulator to keep freeflowing once it starts. When you are using the regulator, the vane should be in +, and when you aren't breathing on it, it should be in -. +=dive, -=predive. Pretty simple.
Water flow can act to assist the breathing so if you are facing into a strong current, kicking fast, on a scooter, etc. it can induce a freeflow. In those situations you turn the vane to -, and usually turn the knob in to prevent that freeflow.
If you also have a G250 as a secondary, then that should stay with the vane in -, and the adjustment knob all the way in. When at the surface, you can leave the vane in whatever position you want, but should open the adjustment knobs all the way out to release pressure on the seat