Gas phase during water sampling

I have a question concerning simultaneous sampling of water and gas phase.


The situation is a pipe (diameter somewhat around 15 cm) where a mixture of water and gas is flowing through (two-phase flow). Assuming that the flow is separated and the gas phase is moving on top, what will happen if I open a sampling tap at the bottom of the pipe? The flow is induced by a pump and the pressure in the system is above ambient (20 bar). 


My assumption is, that I only sample the water and the gas phase on top will not leave the pipe unless I allow the gas phase to "connect" with the tap by extracting a big amount of fluid.


The problem is maybe comparable to a bottle of sparkling water, where I drill a small hole at the bottom. Will I only sample the fluid phase or will I sample a mixture of both?


I hope somebody has an answer. Thanks in advance!




That's a tricky question.


Given your setup, it follows that you will only be sampling water (*not* simply "fluid", mind you; fluid is a common term for both gas and liquid). But gases might be more or less soluble in liquids. Chances are that your water is saturated with some gas (say, $\ce{CO2}$) and pressurized with the same gas in the pure form. Then, as soon as you take your sample and put it in a sealed (or worse yet, *unsealed*) container, it will release some gas, and so the composition of the sample will become **different** from that of the liquid in the pipe.


Whether or not this issue is relevant to your situation, is up to you to assess.


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