Finding a tiny hole in a tile pool?

[henningdalgaard]

Hello experts

I live in Denmark and took over a house with a pool 10 years ago. Unlike most pools here (Liner vinyl) it is a tile pool.
It is from the 70s but has proven solid up until two month ago when it started to loose water - about 1 inch per day.

I have closed all the valves for the pipes. The water level has sunk below the pool inlet valves. I also have a bottom drain but i am worried that the hole is in the concrete itself.

At first i figured the hole must be at the level where the water stopped but I understand now that the hole can be at pinhead size below that level because of the high pressure. To verify I filled the pool and the level dropped again but not to the exact same level as before. If it had stopped at the exact same level I would assume that the hole would be at that level but now I am afraid the case is that leaking stopped because the pressure no longer is high enough.

What do you guys think? Am I making the right conclusions here?

And what is the next step?

I have discussed the problem with a friend and we are planning to drain the pool completely and "jump in" to do a close up inspection of each square inch of the pool and knock on each tile to reveal loose tiles or concrete.

I don't suppose some miracle treatment or sealent exist that I can just apply to the hole surface or concrete joints to seal the hole?

What do I do if I can't find the hole?

Any ideas are very welcome...

Picture of pool:
https://ibb.co/iju1Aa

[Denniswiseman]

First of all I would seal of all pipework from the pool. You can use the rubber expanding plugs
If you are still losing water then it's in the shell
An Inch a day is quiet a lot and I doubt if a pinprick would lose that much water (800 litres)

[ChuckGeo]

Alternatively, you can fill the pool, then mark the water level and leave the pump off for 24 hours - measure the loss. Refill the pool to the starting mark and run the pump continously for 24 hours, then measure the loss. If the loss measurements on & off are the same suspect a non-plumbing leak such as a crack in the structure, at one of the spots where a pipe penetrates the wall, in the skimmer throat or skimmer or in the main drain sump. If the water loss is greateer with the pump on, suspect a return (pressure) line. If the water loiss is less with the pump on suspect a suction line (skimmer of main drain).
Non plumbing leaks are diagnosed with dye testing, pressure testing is used to confirm and locate pressure leaks.