Tare Weight(TW)
The tare weight of a container is what the container weighs when it is empty. This weight includes the weight of the valves, and is important to know when filling containers by weight.

The water capacity of a container is the weight of water needed to completely fill the container. DO NOT CONFUSE WATER CAPACITY OF A CONTAINER WITH ITS PROPANE CAPACITY. To determine the propane capacity of a container, simply multiply its water capacity by 42% (.42).

DOT containers are required to be certified for safe continued use at specific intervals.
The first test is required 12 years after the manufacture date. After the 12-year test, the container must be tested every 5, 7 or 12 years, depending on the type of examination. There are three methods of retesting and requalification of DOT cylinders:

1. Visual Requalification
The recorded visual requalification is documented on a DOT form. If suitable for continued use, the month and year of the visual requalification is then stamped on the container near the original date of manufacture or the previous requalification date, followed by the letter "E" indicating visual (external) retest. This type of requalification is valid for a period of five (5) years.

2. Simple Hydrostatic Test
The simple hydrostatic is performed by pressurizing the ICC/DOT cylinder with water with at least two times the design working pressure of the cylinder. This test is also documented on a DOT form. If suitable for continued use, the month and year of the test is stamped on the container near the original date of manufacture or the previous requalification date (with the testing company's federal DOT identification number between the month and year), followed by the letter "S" indicating simple hydrostatic test. This type test is generally valid for a 7-year period.

3. Water Jacket Hydrostatic Test
As with the simple hydrostatic test, the ICC/DOT cylinder is pressurized with water (with at least two times the design working pressure). During this test, however, a metal jacket filled with water surrounds the cylinder. This water jacket is used to measure the amount of expansion of the cylinder body when under pressure.

Once this test is completed, an extensive inspection of the cylinder is conducted. After this method of requalification and if the container is suitable for continued use, the date only is stamped on the cylinder (no letters) with the testing company's federal DOT identification number between the month and year. The hydrostatic test is valid for a 12-year period.

These periodic testing procedures are designed to detect leaks or damage to cylinders, which could pose a safety hazard if the cylinder is allowed to remain in service.