Measuring Underground storage Tank Length

Look into ground radar surveying.

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I cannot claim to have tried this.... You get good "laser" tape measures for building work now at reasonable cost. If you could drop into tank on rod, 0.5 metre steps, rotate rod in steps and activate it somehow, it would build a picture. Could also try small camera. But if you want to KNOW, filling with water using dipstick is an option. Sorry for any offence if water is really costly to you.

You have received two great suggestions above but I have to ask, are there no drawings, specifications, or records available? Either through the facility that owns the tank or through local government?

Only other thing I can suggest is to pump out (or use) the remaining fuel and then purchase more fuel and request metered delivery. Depending on the size of the tank, metered delivery should get you within 2 - 3%.

Thanks guys for your suggestions ,I used the GPR (radar) but its not that accurate ,and tried to find the Tank chart by emptying the tank and put a metred fuel (however) with the aid of STICK meter ,its not also realiable 100% to be a correct volume deleivered inside that tank.

What Im currently thinking of , but it doenst have a precise description,is using the laser inside the tank but how ,that what I can see to be of invention to com.

Thanks to all of you...

If the tank is (or can be made) airtight up to a couple of bar, you could measure the volume by adding a known volume (mass) of compressed air (from a gas bottle), waiting until the temperature stabilizes, and measuring the pressure in the tank.