How do RR measure grade?

[selector]

Too easy, Quentin....you trespass four more times over a 200 meter span and take an average of your calculations.

[Modelcar]

.....Yep, a solution to every problem....almost.  Would work if grade was intended to be consistent throughout that distance.  

[Anonymous]

I just really have to point out that the middle of the 100 foot tube DOES NOT have to be lower than the ends; it just has to have no air within the tube.

To justify or prove this, you can do so in the kitchen.  Set a pan of water on the counter,

Put one end of a plastic tube in the bottom the pan, the other end going up and over the pan edge and hanging down below the counter edge.

No water flows until you suck on the free end,getting the air out, and start the siphoning process.

Lift the free end of the tube up, leaving a loop at the bottom lower than the counter top.

The water will stop flowing if the free end is brought up to the height of the water surface in the pan, which interestingly enough, is the same height as the water surface. 

Now the tube in the pan has gone higher (over the top of the pan), lower than the bottom of the pan, and back up again; but the water level in the free end of the tube is at the same level as the water in the pan.

Basic Siphoning 101

Art