All propane cylinders are not alike
CAUTION: USE PROPANE CYLINDERS AND TANKS IN PROPER POSITION
There are two basic types of portable propane cylinders – the vertical (upright) standard cylinder and the horizontal cylinder that comes with a special mounting bracket and which can be installed either vertically or horizontally. Both cylinders are manufactured in accordance with U.S. Department of Transportation (DOT) specifications.
When you purchase a new cylinder, be sure it fits the bracket you have. And if you use a cylinder exchange service, be sure the cylinder you receive is the same type as you turned in. Vertical cylinders are NOT designed for service while lying on their sides.
Propane appliances for cooking, heating, lighting, water heating and refrigeration are designed to operate on gas vapor only. Therefore cylinders and tanks designed for vapor service must be transported, installed and used in the proper position. Do not transport, install or use a vertical cylinder in a horizontal or upside down position. Proper care must be taken to position a horizontal container in the correct position for vapor withdrawal. Liquid propane could enter the system designed for vapor only, possibly creating a hazardous condition.
-copied from-
http://www.coxontool.com/index.php/Airstream/Propane