Tommyjoking,
The aluminium tape has been used in New Zealand for years by quality companies who specialise in welding stainless steels for the dairy industry. We had to wear disposable overshoes and change our overalls when entering the workshops so if there was even the slightest chance of contamination it would not have been used.
As I said it is more expensive than masking tape but as I have mentioned in previous postings regarding welder quals / WPSs etc if all the companies in an industry use it, all the companies factor it into there tender price then it is not an issue.
Regards,
Shane
I typically allow the purge gas to flow until I've displaced the volume of the system six to seven times. That is, if the volume to be purged is one cubic foot, than the purge has to be sufficient to allow five cubic feet of gas to flow through the system. At 15 cfh, it would take about 30 minutes. That sounds like a long time, but it is necessary to assure the oxygen level is reduced to a sufficiently low percentage. Look at it this way. The system starts with 100% air (20% O2 and 80% Nitrogen and assorted other gases), displace the volume once, the percentage of air is reduced to 50%, displace the volume again, and the air is down to 25%, one more time and the air is at 13%, once again, 7%, again and the air is at 3.5%, again and the air is at 1.25% (oxygen is about 0.3%) and one last time the air is down to .6% of which O2 is 20% or about 0.12% by volume. The math may not be to the first of second decimal point, but so what, you get the idea.
The oxygen level influences the quality of the weld in the system. Too much oxygen and you'll have discoloration. You want a maximum of 0.5% oxygen in your system when you start to weld. As mentioned by some of the other knowledgeable folks, tape the open joints to stem the flow of shielding through the open roots. It is for that reason I prefer to use U-grooves with a 0 root opening and a 1/64 to /32 inch root face. It reduces the chance of aspirating oxygen into the system and it reduces the flow needed to maintain proper purge.
I encounter many jobs where insufficient purge time is allowed to properly displace the oxygen in the system before welding and then the contractor fails to properly maintain the purge while welding by taping the open root joints.
As for maintaining the proper pressure, a magnehelix can be used to monitor the internal gas pressure to prevent blowing the root pass out of the joint. You want any bleed holes to be located at the highest points in your system because the argon is heavier than the air. The water soluble dams and removable bladders do a good job of reducing the volume to be purged and reduce the amount of gas required and the time it takes to obtain a proper purge.
Good luck - Al