Solenoid?

spacemanspiff1974

I know that I have a long way to go in designing a rebreather. Still educating myself, and yes I will take a class and yes I will make sure everything is checked out beforehand. I just want to understand the entire process.

Which leads to my next question. If I am purchasing a solenoid, what brand/model number do people here suggest?

Not sure if I want an ECCR yet, I may opt for kiss, but want to know my equipment options regarding this.

AD_ward9

A normally closed dual coil solenoid from Lee:

Run it at 10bar (it is rated to 3000psi). They are designed for high reliability applications, unlike most machine automation pneumatic solenoids.

Check solenoids very carefully to find the min pressure they will work at and the max pressure: just drive it with an oscillator and check that the mean rate it fills a balloon can be sustained for long periods. It is vital you measure this so you do not get any surprises.

In dive rebreathers, there is usually a bleed route over the solenoid, so the pressure of the gas helps it to shut. This bleed route is not present on the Lee solenoids, so they do not tend to stick on like Inspo solenoids do, and the high pressure rating allows them to work over a much wider range than most dive solenoids. I

If you look at the datasheet you may be tempted not to use a first stage valve and put in a flow regulator orifice made of Iconel right up on the cylinder valve. However, that is risky. The Lee valve materials are not O2 compatible. If you replace them with a bronze, that will work though.

Saying all that, we prefer variable orifice valves - they are fail safe. At $450 a shot they are not cheap: the cost is because they contain 3 sapphires, option of micro DC motor or stepper motor, a 64:1 gearbox, Hall sensor to detect position, and have an off point when overdriven). If you need some, email me. Given big volumes the price could come down a lot.

Whatever valve you use, it is a good idea to have a 15 micron filter in front of it. These are sintered bronze filters, but I am sure you know that anyway.

Cheers

Alex

Genesis

I'm going to take a completely different tack on this from AD_Ward.

IMHO, you need to consider a few things here FIRST:

What is the consequence of a stuck open (or failed closed) valve? This comes down to how much time you have from the event until something really bad (e.g. too high PO2) occurs. It also comes down to what sort of redundancy (is it manual, electrical, etc) do you have in place for such failures. Basically, is a failure a nuisance or a life-threatening event? This is determined by a number of things, such as the IP you're running at, the flow rate of an open valve, etc. Much of this is interdependant with other things (a valve that flows little must be "on" for more time overall, thus draws more power and has more impact on battery life, etc.) Price is also a part of this, because at a low price a diver keeping a spare valve around is not a big deal (if it can be swapped easily); at a high price or with limited availability that's unreasonable, thus, you need much higher reliability.

Once you figure THAT out, you can then determine if you can tolerate failures or not, and how often you can tolerate them.

BTW be aware that there's more than one risk with solenoids - if they or any part of their wiring are inside the loop you have to pay very close attention to electrical issues because at depths under 70' you're likely to be running a FO2 in the loop that will support greatly accelerated combustion. Thus, avoiding potential ignition risks becomes a critical matter.

This is very similar to most of the other design decisions, really. Mechanical items break - this is a reality. The questions come down to how often do they break, how much do they cost to fix, what risks come when they break, etc.

I recommend looking at the entire system and doing this analysis piece-by-piece..... with the solenoid as part of it......

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AD_ward9

That was a good post, with a very important punchline. Search for HAZOP to see how it should be done.

Following the same line, EN14143 has some other useful safety tips on injectors. O2 injectors must be able to supply 6litres/min of O2, otherwise they may not be able to inject enough gas if you ascend fast.

On the oversupply risk, if you are using a compensated reg, it is easy to put in a restrictor so they cannot go above 6litres/min. If you go without a first stage, an orifice acts as a sonic mass flow limiter.

Do you know what the error will be, from your injector controller? The Rebreather model posted on the Open Revolution area lets you work it out. Usually a few %, but in some designs is a lot more, due to the lag and response between the injectors and the sensors. Designs that inject O2 onto the sensor faces "to keep them dry" can suffer large errors in this department, enough to increase the risk of a bend.

Cheers,
Alex