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This was originally posted on the Lego page, and closed because it's not actually about Lego itself. I thought I'd give them a chance first, in case someone had done this already and had some application-specific knowledge. But it looks like not.

Anyway, I have a Lego Pneumatic Engine (LPE) set up as a "steam" locomotive, and I want to power it with a HPA, paintball, auxiliary SCUBA, or whatever-someone-decides-to-call-it tank:

enter image description here

I think a 6.1cm diameter, 0.22L or 0.35L tank will fit nicely in place of the 0.5L plastic water bottle. I can redesign things and move some structure around if I have to, but the overall size is pretty well fixed.

The air inlet is a sports inflator needle pushed into the black rubber hose at the front, with a bicycle pump currently attached to it. The maximum pressure there is about 35psi (~2.3bar).
I can use a longer hose there if needed too, but that's about as short as it can be. The straight section with a slight kink is the inflator needle.

So, is there a compact way to regulate the HPA tank from 3000psi (200bar) down to 30psi (2bar) with a schrader-valve type connection at the end?

enter image description here

I see that a fair number of HPA tanks (but not all) come with a factory-attached pre-regulator, that can apparently be adjusted by taking it apart and changing the spring. But it seems like a stretch to me, to take something that is designed for 3000psi in / 800psi out, and get it all the way down to 30psi out instead. Maybe it's possible, and if so, that's great! But it seems like a bit much to me.

It seems more likely to me, to just barely get into the range that a shop air regulator can handle, and use that for the final delivery as well as a complete shutoff. But there are still adapters to be found, and perhaps some creative folding to make it all fit in the available space.

I also found this, which says it takes 4000psi in and is fully adjustable from 4000psi out to 0psi out. It looks a bit cumbersome at first glance, compared to the space that it would go in, but no real size reference except for the gauges. Also, with such a wide range of adjustment, it might be difficult to have it "barely on" and stable at my required 30psi. Not to mention a rube-goldberg chain of adapters, probably on both the inlet and the outlet sides of it.

Is there an elegant way to get the required 100x drop in pressure, with decent regulation (there's a throttle valve after the final regulator), in the space shown, and with the right connector at the end? (imitation schrader valve)

AaronD
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  • Do a search for pressure regulators and consider intermediate storage. – Solar Mike Jun 13 '20 at 07:16
  • @SolarMike I've been away for a while and just now came back. Anyway, I did that search, and showed some of the results in the question. I was just wondering if anyone else had some additional knowledge that is harder to search for. – AaronD Jun 18 '20 at 23:10
  • @SolarMike As for intermediate storage, I don't think I have the space, except for what volume there is in the connections between regulators. I'm already looking at the smallest refillable HPA tanks that I can find, and that pretty well takes all of the space by itself. I suppose I could add a tender behind the locomotive, but I'd rather not if I don't have to. I *am* thinking of putting a Lego air tank (3cm dia x 6cm long, max 35psi) *after the final regulator*. Would that satisfy what you're thinking? – AaronD Jun 18 '20 at 23:15
  • How about a couple regulators in series? One of those 0-4000 psi gages attached to say a 0-50 regulator. Dial your first regulator down until the needle starts to move on the 0-50, then use your second, much finer gage to dial it in closer to 30 and give you some better resolution by your working pressure. – Corey Jun 19 '20 at 19:13

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