A recent industry memo told of an accident involving an M25x2 threaded valve installed in a G3/4 threaded cylinder. And it recommended taking G3/4 threaded cylinders out of service. I was confused by the G3/4 terminology because most of our cylinders are 3/4" threads... More accurately they are 3/4" NPS threads... Which are not the same as G3/4.
This needs further clarification - I did some reading at Dive Gear Express and BOTH G3/4 and M25x2 are metric threads of Europe and not common in the USA.
G3/4 - An old metric BSPP 3/4" thread used mainly in Europe
- It' not common over there anymore.
- It's not common at all in the USA
3/4 NPS - This is the common USA scuba thread for old and new scuba tanks.
- It's NOT the same as G3/4 (above)
- It's NOT the thread that this memo recommends taking out of service.
M25x2 - This is a metric/European thread
- It's stamped on the cylinder neck
High Pressure Cylinders 3442 and 3500 psi use different valve threading but you can't mistakenly thread one into the other.
- 3442 psi uses USA 3/4 NPS thread.
- 3500 psi uses USA 7/8 UNF thread.