Identify the welding tank by the color of the gas tank. The most common colors in welding will be green for oxygen, maroon for argon, and red for acetylene and hydrogen. The Airgas website offers a more complete color chart. Do not rely on tank color alone, because many gas suppliers will repaint their gas cylinders a different color.
Read the label near the head of the cylinder. The label is usually a sticker with a bar code. The label is placed by the gas company after filling the gas cylinder. This label will tell you the date and month of the fill date, chemical symbol of the gas inside and the name of the gas. This is the most reliable and secure method of identifying the gas tank.
Read the manufacturing date of the tank. The date of manufacture is placed near the head of the cylinder and is written in month-year format, using one or two digits for month (M) and two digits for the year (YY). Look for a recertification stamp, usually marked with the month (M), location (L) and year (YY). If your welding tank is more than 10 years old and has does not have a recertification stamp, you must take it to a filling station to get it recertified. If it has a stamp but has not been recertified for five years, you must take it to get recertified.