TD:
You asked:
How empty is it when it is drained?
I will answer this by saying depends on the instructions, but here goes:
1. If a normal defuel is carried out there is fuel that remains in the tank and there will be vapours.
2. During normal usage when the tank contents have been used, like your car, there will still be fuel that remains in the tank and there will be vapours.
3. When a tank is drained through the normal drain points, there will be a considerably smaller amount of fuel left in the tank and vapours.
4. If the tank is drained, depuddled and vented, there will be no fuel or vapours.
If the instructions where to carry out 1, 2 or 3 then in degrees there will be fuel and vapours, if the intruction was to do 4 then there will not. To quantify this you have to consider how this information might be relevant, i.e. having fuel and vapours in any fuel tank is not a problem, it only becomes a problem if there is a source of ignition within the tank as it is effectively isolated from the system by all the valves, or there is a big hole in the tank/supply pipe to the tank (allthough if the tank shut off valves are shut then again the tank would normally be deemed safe) this also has to have a nearby source of ignition. As aircraft fuel tanks on a lot of aircraft are in the vicinity of an engine then it is normally assumed that heat alone on the outside of a fuel tank wall would not cause an explosion regardless of the amount of fuel/vapours within that tank.
I hope this is ok, but as with all these things I will no doubt be corrected on any technical errors/assumptions in the above from those who are current on type or more knowledgable on fuel systems than my memory is.