That Nuvair drier is one nice unit, I like it.
The only down side to it is it does not have PMV (pressure maintaining valve). The compressed air needs to be held back to around 1700 psi before exiting the drier because the drier requires a dwell time to dry the air. But I know a lot of people don’t do it and are happy with the air quality delivered.
Here are a few thoughts on the need (or not) for a PMV with these filters that might help . . .
First, congrats on correctly pointing out that the key factor in the function of these desiccant driers is all about dwell time, which boils down to a rate of molecules flowing through the media. That flow rate is usually expressed in standard CFM (although you often don't see the word "standard" used - just CFM). Many people falsely think that these do not work at lower pressures, which is just
not true (although there is an important point about this that I will include in a moment). If they are used within their design parameters - which for this unit is a flow rate of about 6 CFM - then there is no issue with it doing its job as intended. 6 CFM is a LOT more air flow than most of us use - it takes a good sized dive compressor to beat that. A 6 CFM compressor would fill one of the "huge 100 cubic foot" tanks from zero to 4500 psi in under 20 minutes . . . .
Now, there is an advantage that results from having these only work at higher pressures, such as what happens with a PMV - highly compressed air simply can't hold as much water vapor as "uncompressed" air, and a desiccant filter is actively adsorbing the water vapor up t oits limits. So if we use a PMV before one of these filters, we get to "squeeze" more water vapor out of the air by forcing it to condense out into liquid water. The advantage is that we end up "asking" the desiccant to adsorb less water vapor, so it will last longer this way. It is simply
not true that these desiccant filters "do not work" until the pressure rises to a high enough level - saying so is simply misinformation, or possibly marketing via scare tactics if the truth really is known by whoever is saying such things. But it
is true that they can work more
efficiently once the pressure rises . . .
So a PMV can help us extend the life of the desiccant - but what happens if we don't use one? Well, it turns out we airgunners get
most of the advantage of using a "PMV" without even using one! How is that? It works out because as airgunners, we rarely fill from empty, which is what that Nuvair filter is designed for anyways (divers use almost all their air before refilling, while we only use roughly the top third of a fill). So if no PMV is used when this filter is connected between a compressor and a gun, only the first little bit of the fill is done "without a PMV" as the line and filter pressure rises up to open the gun valve. From that point on, the filter is pressurized to the same level and the gun and gets the benefit of behaving exactly the same as having a PMV for the rest of the fill (since it is above the PMV limit anyways). So probably over 90% of the air pumped basically "sees" a PMV in the system.
If filling a tank, one can put a simple check valve (that can be made for about $20 from Amazon parts) in front of the drier, and the filter can be "backfilled" with pressure from the tank, thus the check valve won't open until that pressure is reached - and that is probably higher than most PMVs anyways. So the only time that the filter would work without a "PMV-like" pressure setting is when we fill an empty tank (and it is not the whole fill that lacks the benefit, just the portion up to typical PMV pressure levels). Just be sure that if you do it this way, that you have a way to vent either side of the check valve!
Bottom line, I would not be opposed to having PMV on my filter, but I would not consider one to be a great benefit - and I certainly would have no issue with one that did not have a PMV (but since I fill tanks, I would probably do the check valve trick with it if it did not have a PMV). In practical terms, for most shooters a PMV will only improve the efficiency of vapor removal from less than 2-10% of the total volume of air pumped; and at the extreme end of the scale (like for divers) it helps on about 30-60% of the air pumped (depending on fill levels and the tanks being filled) - it simply can't impact all the air pumped, as once the pressure in the tank/reservoir rises to the PMV's set point, it ceases to actually do anything further . . .
