Mark_r & Michel,
Wand technique is something that needs to be demonstrated and explained at the same time but I will try to address the two specific questions raised.
Airflow is the most important factor in the removal of water from the carpet pile during cleaning and can also be used to supercharge the flushing action of the solution. The amount of energy in the jet-spray has an obvious effect on the physical disturbance of the soil and this energy can be enhanced by maintaining the flow of solution with the available airflow.
Having the wand at the correct angle so that air enters at high speed though the jet-side lip of the wand builds upon the momentum of the jetted solution to create a horizontal flow through the pile. This is the basic principal on which the CFR wand works.
This applies to AO and VO machines, but with a VO machine the gap needs to be finer in order to maintain depression in the system since it is the air speed that is critical.
The other question concerned long hose runs on a VO machine. Consider that the air-space within the tank and the hose can together form a vacuum reservoirs. When the wand tip is occluded the vacuum within this reservoirs increases to the maximum depression that can be provided by the vacuum unit.
As the occlusion is removed air rushes in by the only route possible - the wand mouth. This in-rush of air will decrease as the vacuum in the reservoir is relieved until it reaches a level which can be maintained by the vacuum units with partial occlusion of the wand mouth. We'll call this 'normal depression'.
We can use this by placing the wand mouth tight against the carpet and first allowing the internal depression to reach its maximum, then starting the cleaning stroke, controlling the airflow as above and ending the stroke at the point of normal depression. The wand is then moved foreword in the occluded position to allow the maximum development of depression in readiness for the next cleaning stroke.
This is rather like the storage of energy in an elastic band or coil spring - the total amount of energy output cannot exceed the energy input, what differs is the time-scale, so the energy output per millisecond can be several times greater.
The reason this second technique is less effective with an AO setup is that motor-for-motor it has much less vacuum potential, and is less necessary because the airflow will be more constant.
John.
PS. Michel, sorry the RH meter did not arrive (especially after such careful packing) I'll see it I can get hold of another. And, yes the device was similar the the one seen at Amtech, but about four times the size and a slightly different shape.