I have built an adapter myself based on the instructions floating around. I use it all the time and I pay 80 cents to refill my tank to 2100 PSI. I control the pressure of the refill, being aware, a hot fill will result in lower pressure when the tank cools, so always fill a cold tank as slow as practical (by throttling a needle valve). My modification consists of the addition of a needle valve to control the flow rate to the tank and thereby the ultimate fill pressure. I fill at local vehicle fueling stations but you must be VERY careful as they are all 3000/3600 PSI - way over the limit for the steel tanks. You must start with the needle valve cracked open very slightly. You do NOT want full fill rate going into your tank. If you start with the fill valve closed it wont work, it expects some amount of flow. Once filling at the VERY SLOW rate you can then open the needle valve to fill until you reach the 2250 pressure. Vehicle fueling goes FAST - you could blow up the tank if you are not controlling the fill pressure. That is why you should have a needle valve, pressure gauge (any good local hydraulic supply company) and seal the threads with the high pressure sealant - again, hydraulic supply company should give you a few drops no charge make sure you tell them you are working with CNG.

I dont know anything about using aluminum tanks (typically 80 CF and good to 3000 PSI), but due to moisture/corrosion potential I would probably stay away from them myself. When its time for a new tank, I will by a NEW steel tank or a proper CNG tank. The thing about tanks made for CNG is that CNG has a potential to have moisture content and you have no way of knowing it, and thereby can corrode the the tank - no problem as long as you never let it go to 0 PSI, and always inspect it. The new CNG tanks are epoxy coated or fiberglass coated to eliminate that issue but they have limited lifetimes and dont allow hydro after 10 years..... However, you will still have the 2250 PSI issue unless you can confirm your regulator on your boat will handle 3000 PSI.

The fact is the steel SCUBA tanks (72CF) work fine (up to 2250 PSI you can get higher, but look for the rating. 2250 is orig equip.) as long as they are hydrostatically tested and properly maintained. When they do get corrosion, they roll it with steel pebbles and clean out the corrosion, then Hydro it again every 5 years. Outside of that you are playing roulette if you dont.

I refill my CNG tank at the local CNG Vehicle refill station However, I must caution you that you should have some understanding about gas laws, pressure and mechanics. If not, get it from the suppliers, or just buy the refilled cylinders. While this is safe if you follow safety rules (as long as you fill only to the rating of your pressure vessel and the adapter parts) it can also be lethal if not assembled properly and used properly.

I give no warranty to the safety or acceptability of the parts listed below.

Parts list for CNG Adapter
1) brass nut (LH thread) CGA 350 NPT
2) Brass nipple CGA 350
3) hydraulic Tee high pressure** 1/4 inch (5000 PSI)
4) gage 0-4000 psi (Note: must be not oil type. Oil filled gauges may explode)
4.5) Needle valve (rated to 5000 PSI) to control fill and reduce pressure fill in tank
5) NGV1 Sherex/OPW LE-36 (3600 psi) receptacle
6) Adapter joins Sherex /OPW to Tee. The thread on the Sherex end must be 9/16 straight thread.
Note: all fittings must be rated at 5000 (preferably 10,000) psi working pressure or higher. More is better.

Sources:
High Pressure: Thread Adapter and needle valve, hydraulic Tee high pressure
Hydraulics and Pneumatics, Inc. (505) 884-1110
3207 Claremont Avenue Northeast
Albuquerque, NM 87107

Any welding supply store. I used:
Argyle welding: gauge, brass nut CGA 350, Brass nipple CGA 350
1575 Candelaria Blvd, NE Albuquerque, NM 87197
Phone: (505) 345-8101

Best place to get this item:
Sherex/OPW LE-36 (3600 psi) Manufacture specifications:
http://www.opwglobal.com/Product.aspx?pid=145
Actual Source/distributor:
http://westechequipment.com/product/...185_2588_2592 and

CGA 350 denotes CNG fittings. Oxygen and other gases have a different CGA number. The fittings for different gases use different threads on their connections. Purchase these fittings and the gauge from a welding supply shop. Total cost = $20.00. Most shops dont stock CGA 350 fittings and will have to order them for you.

** High pressure Tee can be purchased at a hydraulic supply or repair shop. Cost = $8.00.



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