Make sure you understand that your blower and the wastetank on your machine are designed with certain limits. The lift limit on a blower is such that it will not overheat and distort the structure, leading to failure. As well, your waste recovery tank has a structural limit that when lift is maintained under this limitation, collapse or structural damage will not occur.
For most equipment on the market, the limitation is around 14"-15", and is maintained through the proper setting of your vac relief valve. Your's is set at 14" and may be appropriate for your blower and wastetank.
That is not to say that the entire setup is optimal. First of all, typical spring loaded vac relief valves leak quite a bit of CFM at partial lift settings. During cleaning, it is at these partial lift settings that air flow should be conserved through the wand, and not through the vac relief valve, if possible. On most valves, you cant do much other than keep the valve poppet lubricated, so the action will be smooth. Some valves utilize a quality construction, with a bushed poppet that reduces friction, allowing the spring to maintain a precise, reliable setting. However, partial opening at partial lift settings is an inevitability. That is unless you use a vac relief valve, specifically designed to retard or eliminate creep leaking. An example of this type of valve is the Bayco, or Kunkle valve used by some independent cleaners, and a couple of manufacturers (MasterBleand, Powerclean).
The second figure you tested, was the "open flow" vac reading. In actuality, the desired reading is as low as possible. This is because this figure generally represents the amount of restriction in the vacuum side of the blower, between the blower and the exit orifice of the wastetank. 5" is not a 'bad" reading, but it could be better. make sure your vacuum filter is not impeded by debris, which would raise the amount of restriction, and thus the reading.
You will notice, that as you add vacuum hose, and then a wand, your open flow reading will rise as you add more and more restriction. If you have a Kunkle or Bayco type of relief valve , the amount of CFM loss is negligible, but you still want to reduce the restrictions due to hose or wand as much as possible to permit a greater disparity once the final orifice size is introduced. The final orifice size is the amount of restriction that then exists when the wand contacts the carpet.
This wand-carpet interface is one of the most important aspects to maximizing extraction capability. Ideally, you want to have the air moving across the fibers and nearest the wand opening to have the greatest velocity with what the blower is able to generate. velocity of the air gives it the motive force to move or conduct water and soil off the fiber, and into the wand plenum(head). If you have done all you can to reduce restriction from the blower to the wand plenum, and you you have also reduced or eliminated the inefficiencies of a stock relief valve that leaks at partial lift, then all that is left is to maximize the air velocity at the wand-carpet interface with a good wand lip design.
Wand lip design is woefully simple in a traditional wand. The plenum is narrowed to a simple slit to raise the velocity as it enters the narrowed opening. However, there has been relatively recent advances in tuning the openings of wands to create an even better wand-carpet interface. This is accomplished with an attached wand orifice device (a
glide) that does two things. First, it optimizes the profile of the wand contact point(s), such that the wand oping is buried deeper into the pile, with the same or less exertion of the operator. this done by virtue of the rounded tip profile of the glide, and a low-friction material (teflon). The glide also further narrows the slit orifice either into short variated slots, or round holes. The latter having a greater influence on actual velocity-motive force.
