Watertightness

Compressed air or nitrogen will leak more readily than water and does not leave puddles. If your tank won't hold 5 PSIG better start over because you can get that much pressure difference driving from San Fransisco to Vail, Colorado

Plug open ports with Fastest plugs. Pressurize to 5 PSIG (Precision Regulator +/- .1 and test gauge, analog, digital or transducer) Block flow and start timer. No pressure decay would be perfect. You didn't give the size or volume of your tank but it is easy to calculate leak rate by pressure decay.

This process can be automated with PLC and pressure switch (Go/No Go) if manual testing is too labor intensive. Several tanks may be tested at the same time to make the test period more efficient.

Beware. 5psi overpressure is enough to knock down a brick wall. Is 5psi dangerous? Yes - if it is propelling a manhole cover towards you.

Use an incompressible fluid instead.

Use water. add a dye to show up any leak.

the test carried out after filling water to brim is best way to check but do not do this test immediately after completion of tank for it may mis lead you there is every possibility of entrapped water comming out for a while generally it takes about 3 weeks to come out so if you want to fill water immediately then do not come to any conclusion for first three weeks and decide if it leaks the vulnarable area can be pipe out let or joining part or if there is any foriegn material like wood nail or wire or cloth leads are available if they are available then remove it so that leakage problem can be solved in simple way

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