Mixing Unbalanced 1st Stage W/ Balanced 2nd

[rab1412000]

Diaphragm first stages, regardless of brand are "upstream" designs as the seat and seat carrier are located upstream of the orifice and the tank pressure acts to hold the seat on the orifice. Piston first stages are "downstream" designs as the seat is located downstream of the orifice and tank pressure acts to push the seat off the orifice.

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does this mean that in all diaphragm 1st stages, in case of valve failure, air will stop?

 

[herman]

Depends on the failure mode....what failed/broke/how and when in the breathing cycle but that is also true of a piston reg as well. The current USD/AL design (the one in your Legend) has been in almost every reg USD/AL (and a host of others) since 1965 (and an unbalanced but very similar design since the mid 50s), the possibility of no gas due to mechanical failure is pretty slight considering the number of them that are/have been in use over the last 50+ years... No gas due to mechanical failure of a properly serviced reg is not something I would worry too much about .....you are more likely to get eaten by a shark.

 

[rab1412000]

thanks that downstream no air comment kinda had me worried.

 

[psiborg1812]

I'd be more worried about the unchecked LP hose bursting or the air that you got from the gas powered compressor than a mechanical failure of a diaphragm reg.

 

[rab1412000]

yeah diaphragm regs have less moving parts and are environmentally sealed by design making them sturdier than pistons afaik.

 

[herman]

yeah diaphragm regs have less moving parts and are environmentally sealed by design making them sturdier than pistons afaik.

They actually have a good many more moving parts than a piston reg. Your Legend has 3 springs as opposed to a piston reg which has 1. The diaphram reg has 4-5 moving parts if you include the diaphram as opposed to 1 (the piston) in a piston reg.

 

[rab1412000]

i see, thanks for the correction.
i always thought a diaphragm reg was a simpler design.

 

[Gilldiver]

They actually have a good many more moving parts than a piston reg. Your Legend has 3 springs as opposed to a piston reg which has 1. The diaphram reg has 4-5 moving parts if you include the diaphram as opposed to 1 (the piston) in a piston reg.

While springs breaking are not unknown, I have USD double hose regs from the early 50's that have the same springs as the day they were assembled and have never replaced a spring due to breaking. The 1st stage design on the original Aqualung from 1948 was only slightly modified (the only change was the orifice diameter) and used on all USD/Aqualung two stage regs until 1968 when the balanced 1st stage came out with the Royal Aqua Master double hose reg. As shown above, this 1st stage design is essentially the same as you are getting on a new Aqua Lung reg bought in 2010, for a 50+ year basic design run.

In the last 30+ years of my diving I have never seen any 1st stage fail in a closed mode, but have heard of it once. I have seen and had 1st stages fail in an open mode due to crud getting between the high pressure seat and orifice. Somehow crud from a dirty tank got past the 1st stage filter holding open the seat.

What I do see a lot of on piston regs, and almost all regs, are O-rings going causing a free flow. All in all, the less O-rings the better.