Why the pre-test is needed for some thicker zinc coated tanks...
I was having a hard time understanding why a Pre-test would help deliver a more accurate test. This page has a vid from Worthington that explains why the pre-test is needed for some thicker zinc coated tanks. Steel tanks expand and contract with pressure cycling as long as they are not taken beyond their elastic capabilities. A "surplus" thickness in the galvanized coating can prevent the steel from returning to the original state and this makes it look like a tank that is no longer elastic. They say the canadian DOT agrees with this assessment and that they actually changed the formal Canadian testing protocol because of it. The web page provides the changed rules for downloading. Take home is sInce thicker zinc has better corrosion protection the Canadian government changed their rules to prevent encouraging the use of thinner zinc. I am assuming the USA based rules do not prevent a pre-test but also do not require a pre-test. Maybe playing the vid for a hydro tester would make some headway. You can jump to 3:30 minutes to hear about the testing issue.
Makes sense that older tanks would be more likely to have a beefy '57 chevy like thick galvanized coating thicker zinc than a newer tank that saved some $$$ by using a thinner coating. I also expect older but well soaked tanks could have thinner zinc from the expected sacrificial corrosion. So, with no pre-test a well used vintage tank soaked in a lot of salt water might pass easier than a near mint vintage tank used only in fresh and maintaining its original thicker zinc. Hence, punishing the good guy. As I understand things the hydro does not push a tank beyond its elastic capabilities so you could hydro a steel tank hundreds of times and not change the tank in any way. This is why doing a pr-test is not cheating the test. Same concept as why it's safe for the wings of airliners to stretch up and down a million times.
I am hoping the engineers and metallurgists will step in and correct me if I have this wrong.
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