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Post by duckbill on Jun 27, 2020 13:56:49 GMT -8
In the past I have considered the orifice in a banjo fitting to be "restricted" in the sense of being sufficient for safety in the event of a catastrophic hose failure. Now I'm seeing that the pin hole in a standard restricted HP hose fitting is even smaller than that in a banjo fitting, so I'm wondering what your opinion is on this. I don't have pin gauges to accurately measure the orifice in my banjo fitting, but I can say it is quite a bit less than .066" in diameter, the smallest drill bit I have on hand. What do you think? Would you consider this orifice sufficiently small to affect an "acceptable" margin of safety in the event of a hose failure?  |
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Post by SeaRat on Jun 27, 2020 17:17:13 GMT -8
In the past I have considered the orifice in a banjo fitting to be "restricted" in the sense of being sufficient for safety in the event of a catastrophic hose failure. Now I'm seeing that the pin hole in a standard restricted HP hose fitting is even smaller than that in a banjo fitting, so I'm wondering what your opinion is on this. I don't have pin gauges to accurately measure the orifice in my banjo fitting, but I can say it is quite a bit less than .066" in diameter, the smallest drill bit I have on hand. What do you think? Would you consider this orifice sufficiently small to affect an "acceptable" margin of safety in the event of a hose failure? I don't know! This seems a logical place for a test. Maybe I can sacrifice a full tank of air to see if, from my banjo, the flow is sufficiently restricted to reach the surface from a reasonable depth. John |
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Post by vance on Jun 28, 2020 9:40:48 GMT -8
I wonder if the new reproductions have the same size orifice as the originals?
EDIT: I just looked at one I got from Seahunt Jerry (not sure if it is an original or an early reproduction), and a later reproduction I got from VDH. The orifice in Jerry's is much smaller than that of the repro's. Besides the orifice, the two banjos are clearly of different manufacture.
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Post by vance on Jun 28, 2020 9:52:53 GMT -8
Unless it's an o-ring failure at the banjo end of the hose, wouldn't the hose fitting restrict the air volume regardless of the banjos orifice?
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Post by james1979 on Jun 28, 2020 10:08:21 GMT -8
I would think so... but I have happened across some old 3/8 HP hoses that had a much larger orifice than modern ones.
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Post by duckbill on Jun 28, 2020 12:04:35 GMT -8
I would think so... but I have happened across some old 3/8 HP hoses that had a much larger orifice than modern ones. Which is precisely why I'm asking. |
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Post by duckbill on Jun 28, 2020 12:07:11 GMT -8
I wonder if the new reproductions have the same size orifice as the originals? Good question. I have an original, but it's in storage right now. Either way, the question still stands. |
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Post by snark3 on Jun 28, 2020 14:09:01 GMT -8
I saw either here or VDH somebody figured out how long it would take to drain a tank through the restricted HP hose. It took quite a while for the tank to empty.
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Post by luis on Jun 29, 2020 2:55:18 GMT -8
From what I have read, if you can drill it, it is way too big to be a significant HP air flow restrictor. I haven’t actually done the calculation and now you got me curious. The orifice flow calculations are relatively easy for a specific pressure drop (delta P).
My understanding is that they don’t make a drill bit small enough to even come close to be a significant flow restrictor for high pressure air.
I remember reading somewhere and maybe even seen a You Tube video on how they make the very small HP scuba air restrictor orifices. To make a HP air restrictor orifice they actually drill down until there is a relatively thin wall obstruction and then the final orifice is made by a tightly focused laser cutter.
I am going to have to search to see if I can find that YouTube video.
If you look at restrictor orifice at the end of a modern HP hose, you can actually see a tiny little orifice with your bare eyes, but I am not even sure that is the actual smallest restriction point in the orifice.
In the case of the bleed air for the dry chamber in a Sherwood regulator they don’t even use an orifice. They use a porous sintered metal stone type of restrictor and they are only restricting LP air. They also only need a very small leak to fill the dry environmental chamber. You always see champagne bubbles leaking out of those first stages. They are intentional and it works well.
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Post by luis on Jun 29, 2020 3:05:21 GMT -8
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Post by luis on Jun 30, 2020 8:47:49 GMT -8
Well I did some very simplistic calculations. A small drill bit (number 80 = 0.0135” diam, or 1/64” = 0.0156”), can slow down the flow to roughly about 7.6 cfm or 10.2 cfm respectively.
Therefore it could take about 10 minutes or less to empty a steel 72.
Personally, I think that is “significant” so it seems theoretically possible to drill a restrictive orifice. This is probably not as good as what modern regulator or HP hoses offer for restriction, but it is not insignificant.
But, if you are able to make produce an orifice that is, let’s say about 0.005” in diameter, then the flow rate drops to about 1 cfm. That is a huge flow reduction.
The opposite is also true. A slightly large orifice will quickly become useless as a restriction. It is an exponential function.
I have not found where I saw how they make the miniature HP restrictive orifice, but I think it was part of a You Tube video on how Scuba regulators are made.
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Post by nikeajax on Jun 30, 2020 9:49:25 GMT -8
... I have not found where I saw how they make the miniature HP restrictive orifice, but I think it was part of a You Tube video on how Scuba regulators are made.  The tech used to do that has been around for over 100-years: I'm pretty sure it's the same to create the jets for lanterns. It's my understanding that the orifice is too small to drill, so I've been wondering just how the heck do they make those holes  JB |
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Post by luis on Jun 30, 2020 11:08:07 GMT -8
Hi JB,
I am sure there is more than one way to accomplish a tiny orifice. But the video that I remember seen was using a laser technology which is relatively new.
When I lived on my sailboat, I did most of the maintenance on my diesel engine. The injector orifice on those is so small that any moisture (water droplets) in the diesel fuel would blow the orifice tips out. The injector pressure is very high and somehow water droplets do not form small enough to go through the orifice.
Ok, I never question the minimum size of water versus diesel… something else I am going to have to look into later.
In any case, I don’t know how they made those orifices, but diesel engine have been around long before laser machining was available.
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Post by nikeajax on Jun 30, 2020 13:40:20 GMT -8
Hi JB, I am sure there is more than one way to accomplish a tiny orifice. But the video that I remember seen was using a laser technology which is relatively new.
 Sorry, but ya gotta know that I'd go there  |
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Post by duckbill on Jun 30, 2020 20:05:57 GMT -8
Thank you. I thought there must have been a way to make small orifices before laser cutters were used. Are those calculations linear, or do they incorporate a curve to reflect reduced flow as pressure is drained?
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