I am planning on using the manometer to measure how out of level a foundation is - basically as you have explained it Bruce.
I fully understand that a single measurement is actually not very meaningful and can be misleading as far as finding out if a foundation is
moving. For me and I think “home inspectors” in general, it is more of a matter of finding out how level the foundation is and how large the variance is between two points.
For example, if there is a “large” difference of 4 inches between two points 40ft apart, that may not be to a buyers liking and is an indicator of possible movement (along with other signs of course) or the foundation may not have moved at all and it could just be horrible workmanship on the part of the contractor who laid the foundation (pretty unlikely). Either way a 4 inch variance over 40ft is not up to generally accepted standards, not too many people want to pay full price for that.
There are a few inspectors out here who use this device as "home inspector" and not as "engineer". Maybe I am wrong but for the example above I don’t think it is crossing the line from “inspector” to “engineer”.
Here is a first choice for myself
WatrLevel - Home Page
And another - quite pricy for basically the same thing
Pro-Level Manometer Overview - Function, Uses, Advantages, Accuracy