I posted this elsewhere, but it's relevant here:
"Lift" is the mirror image of "pressure". That being said, it's the "ability to do work", regarding vacuum. When you put your hand (or some other body part) across the opening of a vacuum port (blower port, hose end, etc.), it's not "called lift", but "resistance" to airflow.
"Lift" is a way to measure vacuum, like, "temperature" is to heat or cold. "Available lift" is a better way to think of it. Because lift is limited. If you start out with 15 inches of "Mercury lift", that's how much force is available to pull airflow with. If you have a live vacuum reel, or even, vacuum hose curled up on the ground as you're backing out of a job, the airflow has to constantly change direction as it makes each turn. As this happens, "lift" is used, resulting with less available lift to pull with at the end of the hose. So, of you have 15" and use 8" to overcome the resistance along the way, you actually only have 7" remaining to pull with.
"Mercury lift" vs. "water lift" is two ways of saying the same thing... the amount of available force to pull the airflow with. It's like Farenheit and Centigrade are two ways of saying the same thing. Mercury weighs a lot more than water. So, the force to pull a 1" diameter column of Mercury is a lot more than that required for a 1" diameter column of water. 15" of Mercury ("Hg.") = 210" of H2O.
"Lift" is more of a contributing factor for dryer carpet.
Once again, not necessarily correct.
Lift without airflow is useless. Try cleaning carpet with 20" Hg. @ 1 CFM and you'll see. They're both important.
Airflow without lift is useless. Try the same thing with 2" Hg. and 500 CFM. Unless you're cleaning at the truck, you won't have enough lift to get through to the end of the hose, much less, move air and solution with it.
There is an important consideration here. Look at your vacuum gauge often. With nothing connected to the system, and the system running at normal working speed, the gauge, ideally, should read "0", or as close to zero as possible. The more it reads with nothing connected, the more lift is consumed before you even connect hoses to the system.
Same is true for a filter. If you connect it to your system, and notice an increase in Hg. indicated on your vacuum gauge, then that filter is interfering with airflow and consuming available lift. If your vacuum gauge increases its indicated level by 2", you lost 2" of available lift when you added it.
So, the vacuum gauge is a great diagnostic tool.
It also tells you if your recovery tank filter is getting clogged. Check it daily.
Does the above make sense...?