CH4 or H2 diffusion through schedule 40 PVC pipe  

Posted by Charles Barker on May 14, 2004 at 18:40:59:

In studies of methane content in coalbeds, a fresh gas bearing coal sample is put in a water-filled pressure tight container of made of 5 mm thick schedule 40 PVC pipe. Another method is to put the same type of coal sample in a water-filled container made of 5 mm wall thickness aluminum pipe. the desorbed CH4 or H2 accumulates in a small headspace volume above the water fill. volume measuremenets are at first made every few minutes, but after gas content and pressure declines, the measurements are made daily. Hence gas pressure inside the sample container is not allowed to rise much. Typically the initial pressure inside the cylinder
<5psi (35KPa) above ambient atmospheric pressure. The desorbed gas volume measurements can go on for days to months depending on the physical diffusion/desorption character of the coal sample. Typically the pressure of the desorbed gas constantly declines and the experiment ceases when the internal pressure equals atmospheric. >

Those workers that use PVC have been attacked because of conjectured diffusion of methane and hydrogen through the schedule 40 pipe versus the supposed rates of diffusion through aluminum pipe. Does anyone have an grasp on how diffusion of CH4 or H2 in PVC pipe would effect these measurements?

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