Posted By Jane Blunt Dear Taileuk You don't say what country you are in. The British Standard is BS EN 169, and there is a table in it giving the recommended shade numbers. Shade 4 is at the lower end and is suitable for acetylene flow rates less than 70 litres per hour while welding on steels. A higher shade number would be recommended for higher flow rates, and for cutting - and the shades recommended go from 4 to 7. The American shade selector is F2.2 from AWS. The shade numbers in this system range from 4 to 8 for gas welding. For arc welding recommended shades go from around 8 to 15, depending on process, metal and the current. The BS and F2.2 both have a selection chart. You can also find all this information in a book called 'Health and Safety in Welding and Allied Processes', published by Woodhead Publishing, along with loads of other information about welding. Regards Jane