Joe Morgan: Thanks for clearing up that dips in tubing to a sight glass should not affect the level readings. On the port tank there is a horizontal run fwd of about 5 feet, then a splice to clear vinyl for another 2 ft, then vertical to a valve on a Y. This is perhaps only a couple of inches above the top of the tanks. From the stbd tank, a horizontal run fwd of 5 feet then crossing under the engine about 3 feet then 2 ft to clear vinyl then vertical to the Y. The output line from the Y then goes aft horizontal about 2 ft to a strainer and then to the Sureflow diaphram pump. To observe the water level I open up the strainer and the valves to one or both tanks. A couple of times I have tried calibrating with 4 liter jugs with different results. I believe the tube gets an air lock which then confuses the issue. In use the level seems to vary quite a bit at the dock. I suspect any fwd/aft up down motion would be amplified in the tubing and since I only have 11" of depth it doesn't take much to give doubtful results. The tubes are not easy to read. Perhaps a rearrangement of the tubing is worth another try Thanks John