It's related to the depth of the water above the orifice (valve) and unfortunately height off the ground doesn't come into play.
You could test to see if it's a significant effect using plain water, and count the drops when it's like 1/8 full and then when it's completely full.
If it is an issue, you could connect a piece of tubing to the interior valve port and use a float on the other end of the tube to cause it to always pick up water from say a inch or two below the surface. Then you would always be consistent with your drops.
You could test to see if it's a significant effect using plain water, and count the drops when it's like 1/8 full and then when it's completely full.
If it is an issue, you could connect a piece of tubing to the interior valve port and use a float on the other end of the tube to cause it to always pick up water from say a inch or two below the surface. Then you would always be consistent with your drops.