Welding shielding/purge gas shall be ultra high purity (argon plus hydrogen content of at least 99.9997%). Argon and mixtures of argon with up to 5 volume % hydrogen may be used. Mixtures shall be pre-mixed and certified for purity and concentration by a dedicated gas supply company. Particle filtration efficiency shall be no less than 99.99999% for 0.01 m particles. The filters shall be replaced regularly and a maintenance record shall be documented.