Thank you everyone for the suggestions. The method by Raja Swaidan worked great for my system. The pressure change method was very accurate. I just used a 100ml stainless steel with a know pressure (80-40psi). The change in pressure on connecting to the unknown volume gas line gave consistent values. Very easy and quite reliable for me. I cross checked it by connecting to another stainless steel vessel of known volume instead of my gas line. It gave the exact volume of the second stainless steel vessel. The volume measured by this method was higher than the water method by about 15ml. But I didn't use water at higher pressure or flow. Also as I mentioned earlier there are numerous up and down bends in my line with rapid changes in tube diameter as well. I guest the bends and the changes in diameter are the reason for the inaccuracy in the water method.

You could fill it with a gas A (flush it long enough so the concentration inside the line is 100% A) and then purge it with a gas B. If you integrate the outlet signal of gas A (measured by e.g. IR or MS) and you know the flow rate of gas B, you have the volume.

We did this once with MS detector and compared with your method using liquid water, and got the same results +/- 0.2 ml. So I would not worry too much about voids.

If I were to do it, I will do it with a simple way: close one end of the line and fill water with pressure from the other end. the volume of water will be the volume of the line.

if you apply enough pressure I do not think there would be any voids and this is a reasonable method.

Assume that gas line is a cylinder, measure the large and then the intern diameter (regularly is 1/8 or 1/4") and calculate the volume.

If you want to know what is the exact void space of the line the way suggested in Qaiser Khan will solve your problem.

But if you wanna to know the volume of the gas coming out of the line this way doesn't solve the problem in this kind of problem there is a way to solve in a book named Gas conditioning and processing by Cambell I have forgotten the details so you can find the solution there

Raja Swaiden,

you have indicated a valid problem in the water method. The water needs to be completely drained or flushed out and inside walls should be dry.

The other method then will be to evacuate all the air inside with an inert gas like helium or nitrogen and then fill the tube with that gas. by reading simply the pressure, you can calculate the volume but again in this method the pressure gauge should be of very high accuracy otherwise you will get considerable measurement error.

I agree with Raja.

Have a container of N2, with low pressure at room temperature (assume ideal gas), with no leaks. measure pressure (P1) and Volume (V1) for container only.

If the tube is evacuated, and closed from other end, then N2 is allowed to flow into tube from the container, then the pressure will come to a new equilibrium (P2). Then use simple ideal gas laws. P1XV1(container) = P2XV2(Container + tube) assuming constant temp, for ideal gas,

After measurement, all parameters are known except tube volume.

Continuation to my comment above,

Low pressure doe not mean very low. Perhaps 0.5 Atm will be good.

Assuming the tube internal surface is soundly clean, the technique gives soundly good. However, if surface has bad texture of thick deposits, you need to clean it properly.

If soundly clean, you should not worry about loss of some adsorbed N2 molecules (which may liquify at surface by adsorption)

Volumetric flow rate of a gas line can be measured by soup-bubble column. Even very smal volumetric flow rates.

If you just vant to measure the volume of a line: vacuum the line and than fill it with water, measure the water volume using a burette.

Thank you everyone for the suggestions. The method by Raja Swaidan worked great for my system. The pressure change method was very accurate. I just used a 100ml stainless steel with a know pressure (80-40psi). The change in pressure on connecting to the unknown volume gas line gave consistent values. Very easy and quite reliable for me. I cross checked it by connecting to another stainless steel vessel of known volume instead of my gas line. It gave the exact volume of the second stainless steel vessel. The volume measured by this method was higher than the water method by about 15ml. But I didn't use water at higher pressure or flow. Also as I mentioned earlier there are numerous up and down bends in my line with rapid changes in tube diameter as well. I guest the bends and the changes in diameter are the reason for the inaccuracy in the water method.