I promised some pictures of my single-handed water level, here they are:
The water level has a small end and a big end, here is the small end on its staff, aka 'wooden batten', (supported for the photo by my glamorous but shy assistant!) and a close-up:
The tube is 5 mm bore PVC, as used for motor car windscreen washers. The liquid is ordinary tap water with a pinch of fluoroscein to enhance visibility and a drop or two of washing-up liquid to ensure the fluid wets the PVC tube.
Here is the big end:
The bottle is approx 160 mm diameter and is about 3/4 filled with the tinted water. There are two tubes fitted to the screw-on lid. One is just an air vent, its tube points downwards to exclude rainwater - the other tube leads to the little end. Because the only opening into the bottle is at the top, the liquid siphons in and out of the bottle as the little end is raised or lowered. At the start of each working session, the tube is 'primed' by carefully sucking at the open little end. If you 'park' the staff during a working session, you need to be careful that it doesn't fall or get knocked over - all the working liquid could escape! At the end of a working session, all the liquid is returned to the bottle by raising the staff and connecting tube progressively hand-over-hand.
Years ago I originally tried a water level that had two identical ends. The problem with that is that movement of one end affects the level in the other end so if you're working single-handed, you have to keep going back and forth to check that the reference end is properly at its level mark.
Enlisting the aid of the glamorous but shy assistant to hold the reference end and call out the level information has been known to cause domestic discord!
I arrived at this system independently, I'm not consciously aware of ever seeing anything like it elsewhere. Still, it's such a simple scheme, I wouldn't be surprised if I'm not the first.
With this system, you set the big end at such a height that with the foot of the staff on the working surface the liquid level at the small end is at a comfortable height up the staff to be viewed without stooping or other contortions!
As the foot of the staff is moved between lower and higher points, so more or less of the PVC tube is filled with liquid. This liquid can only come from or go to the big end bottle. However, the surface area of the fluid in the bottle is much greater than the cross-sectional area of the PVC tube (in my current set-up, (160/5)^2 or 1024 times). So in my set-up, a movement of 1" liquid level at the small end results in less than a thou change in the reference level in the big end bottle. Simple but effective!
In the second photo, you can just see a pencil line on the staff a few inches above the liquid level. That corresponds to the liquid level when the foot of the staff was on the bottom of my excavation, before the scalpings were added. The level operation involves a reversal - as the staff is lifted, the liquid in the tube falls. There is also a second line on the staff, more or less at the liquid level - that corresponds to my guess where the surface of the scalpings is going to be once they are raked-out flat and level. A draw-back of this water level implementation is that it would be cumbersome to use up a ladder! However, I've found it to be a great help when working at ground level.