You don't actually measure the volume of water required to increase the pressure. If the system were perfectly hydraulic that volume would be 0. You would simply be increasing the pressure placed upon the uncompressible liquid. All systems will have some compressable air space trapped in them. And so, require a small volume of water be added by the pump. The closer to hydraulic the system, the quicker the pump would increase the pressure at any given drive pressure and flow rate.
It's not so much the initial expansion that is of concern, but the permanent expansion. Some but not all tanks will have an acceptable maximum expansion. What is of concern is how much of that expansion remains after pressure has been relieved(permanent expansion). 10% for most al and 3aa steel, 5% for most fiber wrapped tanks. Our data base has the individual parameters for probably over a hundred different tank ratings and dimensions.
In the case of most dive tanks: Ideally, the tank returns to it's original volume with no permanent expansion. Permanent expansion upto but not including %10 is still a pass. However, a hydro tech who is looking at a tank with 9% permanent and any other telltale signs that it is not fit for service may exercise the, "or any reason to believe the tank is not fit for service" clause.
Imagine the old rubber band in your desk drawer. We're concerned with finding the ones that no longer return to their original width and thickness after having been stretched.
On average an Al 80 will displace around 70-75 cc of water at 5000psi.
Does this answer your question?
For the purpose of hydrostatic testing the water is assumed to be incompressible (and all the air should be evacuated). The volume you are pushing into the tank should be the exact amount (or very close) to the volume the displacement measurement from the outside jacket. You are indirectly measuring tank wall deflection with the volume change.
As a mater of fact, you actually can take direct measurement of the internal fluid volume change as pressure is increased. It is allowed per CFR 49, but it is acknowledge that the test is not as precise (and therefore, not acceptable for + stamping on the appropriate tanks).
About the 10% residual permanent expansion:
Per CFR49 section 180.205:
(i) Cylinder condemnation. (1) A cylinder
must be condemned when
(iv) For a DOT specification cylinder,
other than a DOT 4E aluminum cylinder
or a special permit cylinder, permanent
expansion exceeds 10 percent of
total expansion.
"Exceeds" means that up to and including 10% is passing and of course %9 and even %9.9 are passing with a margin.
I had three tanks erroneously condemned (at the same time). IMHO the operator made several mistakes, but the only thing in writing was the 9.2 % and 9.6% permanent expansion in his records. His letter of explanation said that he condemned the tanks due to the permanent expansion. By law he had to replace my tanks, stamping XXX under these conditions was a violation of the codes. Hydro test is not a judgment call; a VIP is to some extent.
There is some good information on the internet (when I get home I will try to add a really good link I found once), but I agree with Scared Silly: visit your local hydro facility and tell them you want to learn about the process some more.
