[ReelMagic]

I own a 2003 38 Open as well and had a similar question when I purchased the boat more than 15 years ago. Here's what I did.. I emptied/cleaned the holding tank and then calibrated the holding tank gauge/sensor. I wanted to know what 1/4, 1/2, 3/4, & full meant...I removed the tank sensor and took measurements at various gauge levels.... I've attached a copy of that calibration.. What should be noted it that the tank sensor is only 8" long and the tank is 12" high. Therefore the sensor doesn't go to the bottom of the tank so 1/4 doesn't mean its got 10 gal in it (tank capacity is 40 gal)... Take a look at my attachment as that is what my tank meter reads at 1/4, 1/2, 3/4 and full... I generally empty the holding tank after each weekend, or 4-5 days when on a trip.. M2C