If done while using the pumping hose, you can see the clear water coming up- if there's a clear section of pipe in the pump hose.
Before I installed the larger vent we would pump out, add several gallons of with water, pump out, rinse and repeat... I was thinking the vent would do that, but I see what you're saying. As an analytical chemist I do tend to overthink most things!!
And I thought only engineers did that!
Ok...NOW I see what the problem is: Both y'all are confusing rinsing out the TANK with just rinsing out the vent line.
Rinsing out the tank is a completely different operation that nothing to do with the size of the vent or type of vent thru-hull...in fact, nothing at all to do with the vent as long as the vent isn't blocked:
Immediately AFTER pumpout, put 2-3 gallons of water into the tank
via the deck pumpout fitting--'cuz that sends the water into the tank at the bottom to stir up any sludge and hold it in suspension so it can be pumped out. Repeat 2-3 x. Then, if you have a macerator or other overboard discharge pump, put another 2-3 gals of water into the tank and turn it on to rinse out the pump and its plumbing (might be best to wait till you're away from the dock to do this last step 'cuz even though you're only discharging clean water at this point, someone seeing it is likely to have a conniption!)
All this is in my book, btw......
Peggie
"
If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein