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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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Herman
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Re: USD Royal Mistral (again)

Mon Jan 25, 2016 9:39 pm

Ok guys, my mail man finally got around to delivering a set of RM seats today and I have spent some time looking at them...and frankly, if the balance chamber is the same as a RAM then no bleeping wonder it does not work. The hard seat stem that I have has a diameter of 0.104 inch (2.655mm) as opposed to 0.122 (3.107mm) for the standard USD HP balanced seat stem. The 006 oring (which is what is used in a USD balance chamber) has an ID of about 0.114 (2.90mm) or 0.010 (0.254 mm) LARGER than the RM hard seat stem....that is not going to work....at least for any amount of time. There will be some compression when it first pressurizes but with wear and the oring taking a set, it is not going to work long. Until a better match oring if found, the RM is not going to work short of making a new hard seat with a larger stem diameter....which creates balancing issues and a lot higher cost. IMO, a soft seat that is supplied with a replacement oring of the correct size might just work well...problem is finding that oring....
Herman

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Re: USD Royal Mistral (again)

Tue Jan 26, 2016 12:10 am

Maybe it would be easier to start with a -005 oring, and make a new balance chamber than to make a new hard seat. The 005 has about 0.101" ID... not exactly a tight fit on the stem, but making the balance chamber slightly small on the oring OD could make the oring fit snug on the stem.
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Ron
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Re: USD Royal Mistral (again)

Tue Jan 26, 2016 1:15 am

How many hard seats did you get? I'll check the stem of mine as well. Maybe only some were bad.


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Herman
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Re: USD Royal Mistral (again)

Tue Jan 26, 2016 7:41 am

I only have one but it looks in real good shape so it's not wear.
IMO, The easiest and best overall way to get RMs back working would be to make replacement hard seats in the same style as the standard USD/AL seat and use existing soft seats and balance chambers- new push pins may also have to be made. I know that is not keeping it as close to original as possible and is also fairly expensive but we also know that combination works. Plus soft parts will be easy to get for many years.
The best way to keep it as original as possible would be to make new soft seats and custom orings, this may be the least cost option depending on the cost of custom orings.
A third option and likely the most expensive would be to make both new hard and soft seats with modified stem and hole diameters. This one will likely produce a good product but it will cost a good bit to do and will require more engineering to get the balancing right.
The final issue is getting the parts made. I can't imagine there are enough RMs out there to make it worth doing cost wise. Unless you can sell thousands of them, the unit cost is going to be very high.
Herman

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luis
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Re: USD Royal Mistral (again)

Tue Jan 26, 2016 8:19 am

I don't believe there is a need for a custom O-ring. Last night when Herman and I were on the phone, we did find some Metric O-rings that would work (from McMaster Carr) but you had to buy a large sampler kit of O-rings. I am sure, with a little bit more search a different supplier would have a better selection.

Metric O-rings come in a large variety of thickness and ID combination.

The standard SAE O-ring is 0.07 inches thick. I have often used 1 mm or 1.5 mm thick for grooves that are too tight, and have used 2 mm or 2.5 mm thick O-rings, for O-ring grooves that were machined too loose. This is not exactly good technical practice, but when working with prototypes sometimes I have to push the limits to test something. With a big O-ring in a large groove, you have to be concerned with the potential for O-ring extrusion, but there are ways to account for it.
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Re: USD Royal Mistral (again)

Tue Jan 26, 2016 9:22 am

Custom O-rings can be made but its a very expensive process and you get no do overs....Balancing chambers believe it or not are much cheaper to make in small quantities than O-rings.......Ask me how I know :x

As Luis said, there are thousands of O-rings that are not standard size and would not be available from a distributor like McMaster Carr. Some of the O-rings I sourced for regulators had to come from a specialty manufacturer in Germany.
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USD Royal Mistral (again)

Wed Jan 27, 2016 10:21 pm

My poppet's shaft diameter is 2.60mm. Interestingly, with the -06 o-ring installed the rough inside diameter of this particular crown is :

Image

Image

It is a different part number than the RAM one. Is this the secret sauce?

My parts came (thanks VDH) and my RM works like a champ. I am going to dive it this weekend in the sound.

Image


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Ron
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USD Royal Mistral (again)

Thu Jan 28, 2016 12:38 am

The following are just my observations and experience, with a sample size of 1:

The magnahelic has the RM cracking at 1" of water on a full tank, and .8" of water on one with 300 psi. The highest pressure I could get by forcefully drawing on the RM on a full tank was 3", and I couldn't get it over 1.5" on a 300 psi tank.

Is this regulator not fully balanced? Is this a side effect of there being no IP? I submerged the mechanism on a full tank and a near empty one, so I am fairly confidant that there is no leaking.

What do you gentlemen think? The engineering part of this does escape me somewhat.

Also, I think the soft seat is the crux of the regulator's issues. Tom had one that was flaking and would not seal. Mine is minty, despite the regulator being put up wet with salt water. My regulator was perfect inside the body and crown area. I wonder if the seat just held up poorly. The hard cone part of the seat must sit inside the orifice of the seat, because I looked at it with a jeweler's loupe after running the regulator for a while and there is no impression of the poppet on the face of the soft seat. I guess it seals with a wedge fit?


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Herman
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Re: USD Royal Mistral (again)

Thu Jan 28, 2016 7:31 am

Measuring small inside diameters is difficult with calipers, esp of a soft parts like an oring. Even if the crown block is compressing the oring enough to get a seal early on, it will take a set and eventually leak without enough margin to account for that. Based on both our measurements and the specs of a 006 oring, it might seal for a while but not for long or reliably. To achieve long term, dependable operation IMO the oring has to be changed.

As for balancing, it is a sure thing that it is not perfectly balanced, the simple measurements tell us this. To be balanced, the stem and orifice hole have to be the same diameter, these are not, 0.104 vs 0.118. I have not did the math but that much is significant.

IMO, the reg has issues with both the soft seat and the balance chamber. The soft seat may be improved with a different material and the balance chamber can be improved with a different oring.
Herman

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Ron
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Re: USD Royal Mistral (again)

Thu Jan 28, 2016 1:13 pm

Yeah, the whole balancing thing sort of escapes me on a single stage. With a 2 stage I mostly get it, as you want the forces on both ends of the HP seat to be the same. That's when exposed to an intermediate pressure though, so I'm not so sure how the single stage version works.


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Herman
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Re: USD Royal Mistral (again)

Thu Jan 28, 2016 2:16 pm

Same thing, your IP is 14.7psi....at the surface anyway. :)
Herman

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luis
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Re: USD Royal Mistral (again)

Thu Jan 28, 2016 8:29 pm

Herman wrote:Same thing, your IP is 14.7psi....at the surface anyway. :)
That is very true when the valve is closed, like when you are exhaling.

But during inhalation the gas dynamics get a bit more complicated. Even do these regulators have a single mechanical stage, they actually have two very discrete pressure drop points where pressure discontinuities form (actually pressure drop planes). These sudden pressure drops actually form supersonic shock waves in the regulator at the pressure drop planes.

In these single stage regulators, the first pressure drop occurs (as expected) at the valve, in the orifice. The second actually occurs at the venturi nozzle (or the exit from the pressure body into the can/ horn).

There is a small dynamic pressure chamber surrounding the push pin. The boundaries are from by the valve (supplying the gas pressure), the O-ring around the pin (sealing), the venturi nozzle and the other bleed ports (bleeding the pressure).

I call it a dynamic pressure chamber, because the pressure only builds up during dynamic flow.

I have measured the pressure inside the chamber and when taking a breath the pressure fluctuates depending on the amount of inhalation. The more inhalation, the higher the pressure. With a very deep breath I have seen it reach 180 psi, but on average it normally only goes up to somewhere between 70 and 120 psi.
Luis

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Ron
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Re: USD Royal Mistral (again)

Thu Jan 28, 2016 11:53 pm

That's really cool Luis. Maybe someday when you aren't super busy you could do another one of those cool drawings where you color code the airflow again. You did that with the RAM, and that's how I understood its function.
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Bryan
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Re: USD Royal Mistral (again)

Fri Jan 29, 2016 9:19 am

This is a good idea and it would also be nice to revise the Royal Mistral drawing that is not quite correct. Ed in the Philippines has done some really nice animations for us in the past. Maybe we can task him to help as well?

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Ron
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USD Royal Mistral (again)

Sat Feb 06, 2016 7:41 pm

I took my RM diving today. I had it down to 51 feet in the 50 degree water of the sound. It reminds me of a DW Mistral, but with far less of a crappy WOB at the beginning. It breathes relatively easy the entire dive from 2475 to 300 PSI, but it does breathe noticeably better as the tank empties. I gather this is a function of not being fully balanced. I would have taken the regulator deeper, but I don't like to solo dive deeper than I can do an emergency swimming ascent, and I've never done one from deeper than 60 feet in practice. It was a great, 30 minute long dive. I love this regulator.

Image

Image
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