How do you estimate the flow rate of one fluid into another like the Deep Horizon Oil leak?

How have experts estimated the amount of oil that was shooting out of that pipe in the Gulf? I bet there's some neat math or physics involved here, and some interesting assumptions considering how little concrete data are available.

One interesting fact told to me by my father, a chem engineer, is that if you have a high pressure gas leaking into a low pressure gas through a small hole, there is a upper limit to the rate of flow. That is, no matter how high the pressure gets on the high pressure side, the rate of flow does not surpass some finite limit. I suppose this relates to the speed of sound, but I don't know. I also don't know if there a corresponding phenomenon with liquids.

The following page should be of some help:

http://en.wikipedia.org/wiki/Diffusion