True for ASME(motorhome tanks for over 30 years) and DOT Portable cylinders. BUT, on DOT portables it is built into the Pigtails connected to the tank valve in conjunction with the tank valve. On older DOT portables where you have to use a wrench (7/8) to install the POL end and tighten the pigtail, with NO large quick connect threads---NO. Doug
Maybe not on older tanks. But yes on newer tanks. Often times if your tank is shut of and you turn it on too fast that safety feature will block the valve. That happened to me just a couple days ago. I couldn't get it to re open by turning the tank back off. So with the tank off I thumped the bottom of the tank with a 2x4. That worked.