I've had some version of the following conversation rather frequently with colleagues in EPMA, so I'd like to visit it on this forum. Here goes:
Sometimes during the course of a flow counter's life, it will fail to function correctly, even though there is still P10 flowing and plenty of pressure in the cylinder. The reason that this happens, I am told, is that the gas mixture in the cylinder has settled out. That is, heavier argon has sunk to the bottom, lighter methane has floated to the top, and thus the cylinder must be remixed in order to work properly. So then, the conventional recommendation goes, one must disconnect the cylinder, lay it down on the ground, roll it around for a while, stand it back up, reconnect, and purge for a while. The counts should then recover.
Can someone here point me to any literature or data demonstrating that this settling actually happens and that roll-mixing actually works? I cannot find anything that convincingly supports this line of reasoning; what's more, it runs counter to everything I know about compressed gas cylinder use.
First and foremost is the safety issue. Every cylinder safety guide I've every read warns strongly against laying a cylinder down for any reason. Doing so greatly increases the risk of damaging the valve, potentially creating a ballistic hazard. Secondly, rolling cylinders on the ground can compromise the strength of the container, increasing the risk of rupture upon refill.
Even assuming we're all very careful and can perform this rolling operation safely, is there any reason to believe it would be effective? I don't believe that there is.
First, these are compressed gas cylinders, not condensed gas cylinders. Condensed mixed gases could indeed separate into separate fluid phases, provided the fluids were immiscible. If this happened, one would hear or feel them sloshing around inside the cylinder as it was being rolled and mixed. However, in a P10 cylinder, neither gas can condense. At full pressure of a new cylinder, the argon is supercritical (P=1800 psig > Pcrit = 705 psi while T > Tcrit = 151 K) and the methane is in the gas phase (P = 200 psig > Pcrit = 677 psi while T > Tcrit = 190 K). Thus, there can be no formation of phase boundaries.
One of the basics of gas physics is that gases expand to fill their containers, with the only density gradients corresponding to changes in gravitation from one end of the container to the other. Since the cylinders are under 2 meters tall in a field in which the center of mass is around 6,000,000 meters away, that gradient ought to be negligible.
Finally, thermodynamics generally doesn't favor spontaneous separation of any mixture, as that requires a reduction in system entropy.
So, barring a GC or MS trace counter to this, or if anyone knows a physical law that explains spontaneous gas separation (I sure couldn't find one, and if anyone can, I'm sure Praxair would be delighted to learn of it), I can't come up with any reason rolling a cylinder helps with anything. Yet I know people who swear by this dangerous practice of questionable value. After all, it is working for them. Why? I propose that when one disconnects a cylinder to roll it around, one inadvertently fixes the actual problem upon system reassembly -- poorly regulated gas flow.
In many circumstances, I have seen regulators attached to these systems that are poorly suited for the task. Our flow counters require just a slow gas flow (about a bubble per second), which is achievable at a pressure just above atmospheric. To do this most effectively, one requires a low-pressure (range from 0.1 - 2 psig) dual stage regulator. I haven't seen every system in operation so this is admittedly anecdotal, but the ones I have seen often have the regulator usually found in the general stockroom, which is a 125 psig single stage. The trouble with using this regulator is that it isn't built to control under 2 psig, and flow counters probably don't use 0.2 psig. So when these are placed in use, people open them just enough to leak, which isn't how they're designed and result in poor pressure and flow control, and perhaps even purity (this kind of regulator isn't sealed all that well). Worse, as the cylinder is depleted, single stage regulators need to be continually adjusted to maintain a pressure even within their operating range, let alone outside their range. So the longer they are allowed to operate in this unregulated condition, the worse things get. However, upon reassembly and purge, operators are usually very careful about resetting their unmaintainable conditions.
So naturally, everything works again after rolling the cylinder. I claim that, if the operator would do all of the steps they normally do when rolling the cylinder, *except for actually rolling the cylinder*, they would achieve the same result.
In conclusion, I recommend swapping regulators on P10 cylinders out for dual stage low pressure regulators. After doing this in my former lab, I never had another cylinder that "settled out," my cylinders lasted longer and could be used down to the last 100 psig, and my PHA control needed considerably less adjustment on my flow counters from session to session.
Again, I'd be glad to hear other opinions on this. I felt I should initiate this conversation and generate some thought, discussion, and new practices before someone gets hurt by doing something that is ultimately fruitless.
We depend on your feedback and ideas!