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Discussion of diving methods and equipment available prior to the development of BCDs beyond the horse collar. This forum is dedicated to the pre-1970 diving.
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ScubaLawyer
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Mistral question

Sat Sep 05, 2020 11:59 am

Question for all you knowledgeable upstream single-stage Mistral aficionados.

At 2500 psi, at 2000 psi, at 1000 psi and at 750 psi my Mistral locks up tight (i.e. levers removed and nothing touching the pin). However, below 750 psi the HP seat leaks. Correct me if I'm wrong, but isn't the spring tension holding the seat in place constant regardless of tank pressure? It's a new spring BTW that came in a VDH kit. What am I missing (again showing my ignorance to basic regulator everything.  :) Thanks in advance.
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

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captain
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Re: Mistral question

Sun Sep 06, 2020 11:21 am

Ya got me stumped on that one.
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ScubaLawyer
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Re: Mistral question

Sun Sep 06, 2020 1:11 pm

Not sure how I fixed it but I resurfaced the HP seat (again) and now it locks up tight at all pressures. Go figure. :)
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

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Greg Barlow
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Re: Mistral question

Sun Sep 06, 2020 8:22 pm

Remember, the incoming gas pressure is pushing against the back of the seat. This force diminishes as the cylinder pressure decreases, while the spring tension remains the same. This is the basic reason why you will see a marked difference in the cracking effort with varying cylinder pressure. If the cracking effort is a measured 1.0” with 750psi, it will likely be around 2.25” with 2,500psi.

I can often get a couple of quarry dives on a steel 72. The first being on a downstream system like a Voit 50 Fathom; the second being on an upstream like a Voit Polaris 50. That way, I get the best performance from both systems.

Greg
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ScubaLawyer
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Re: Mistral question

Wed Sep 09, 2020 7:12 am

Greg Barlow wrote:
Sun Sep 06, 2020 8:22 pm
Remember, the incoming gas pressure is pushing against the back of the seat. This force diminishes as the cylinder pressure decreases, while the spring tension remains the same. This is the basic reason why you will see a marked difference in the cracking effort with varying cylinder pressure. If the cracking effort is a measured 1.0” with 750psi, it will likely be around 2.25” with 2,500psi.

I can often get a couple of quarry dives on a steel 72. The first being on a downstream system like a Voit 50 Fathom; the second being on an upstream like a Voit Polaris 50. That way, I get the best performance from both systems.

Greg
Thanks Greg,

I guess I knew that but it wasn't registering in my feeble dive-addled brain for some reason. I ended up taking needlenose pliers and rotating the HP seat on the volcano orifice a few turns while pushing in. That was all it needed to get more of a set and not leak at 300-500 psi tank pressure. Not sure I'd recommend the brute force method but it worked for me. :)

Mark
"The diver who collects specimens of underwater life has fun and becomes a keen underwater observer. .. seek slow-moving or attached organisms such as corals, starfish, or shelled creatures." (Golden Guide to Scuba Diving, 1968) :D

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Greg Barlow
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Re: Mistral question

Wed Sep 09, 2020 8:53 pm

Nothing like a little Yankee ingenuity! Another method that often works is to put it on a cylinder with around 2,500psi, and leave it pressurized for several hours. I’ve used that method when I’ve had a slight leak on the upstream valve systems.

Glad to hear it is working well.

Greg
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Chris
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Re: Mistral question

Sun Sep 27, 2020 2:55 am

ScubaLawyer wrote:
Wed Sep 09, 2020 7:12 am
Greg Barlow wrote:
Sun Sep 06, 2020 8:22 pm
Remember, the incoming gas pressure is pushing against the back of the seat. This force diminishes as the cylinder pressure decreases, while the spring tension remains the same. This is the basic reason why you will see a marked difference in the cracking effort with varying cylinder pressure. If the cracking effort is a measured 1.0” with 750psi, it will likely be around 2.25” with 2,500psi.

I can often get a couple of quarry dives on a steel 72. The first being on a downstream system like a Voit 50 Fathom; the second being on an upstream like a Voit Polaris 50. That way, I get the best performance from both systems.

Greg
Thanks Greg,

I guess I knew that but it wasn't registering in my feeble dive-addled brain for some reason. I ended up taking needlenose pliers and rotating the HP seat on the volcano orifice a few turns while pushing in. That was all it needed to get more of a set and not leak at 300-500 psi tank pressure. Not sure I'd recommend the brute force method but it worked for me. :)

Mark
At least you twisted it in the same spot the seat sits when its assembled. Would hate to have two grooves.

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captain
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Re: Mistral question

Sun Sep 27, 2020 11:51 am

It seat has to have enough clearance in the bore of the body to move up and down freely. The impression the volcano orifice makes in the seat will be to some small degree eccentric to the volcano orifice and the bore.
Any rotational movement of the seat will result is some misalignment between the seat impression and the volcano orifice and result in a leak. Sometime rotating the seat slightly stops the leak. Other times depending on how deep the impression is resurfacing or replacing the seat is needed.
Captain

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