In article ,
"Tom Miller" wrote:

Just a drop of dish washing detergent in the water reduces the surface
tension and makes the "air/water interface" more sensitive.

One trick I've used on hygrometers is to treat the inside of the tube
with Rain-X, so the water meets the wall perpendicular to the wall,
making the location of the water level unambiguous. This also makes the
interface more sensitive because the water doesn't stick to the treated
wall. Likewise battery acid.

Joe Gwinn