When I pump the water until it can pump no more there is still a few gallons of water left in the tank that must be drained. I don't know if there is a uniform level for the bottom of the dip tube. It needs to be off of the bottom so sediment on the tank bottom is not drawn into the system.
I don't put much stock into the gauges either. I believe they operate by electrical conduction through the water between the sensor contacts so anything that can hold moister that may be in contact with sensors will give false readings. Of course, the black tank is the most troublesome of the three tanks because of the things that hang out in there. There is an improved version of the sensors that is for sale that supposedly will reduce false readings, they have a plastic/nylon shroud that keeps things from clinging to them. This may have already been discussed here on the Forum or I saw it on the RV Forum.
PS: I found it: http://www.hilotrailerforum.com/f27/hoist-miracle-probes-64/
I don't put much stock into the gauges either. I believe they operate by electrical conduction through the water between the sensor contacts so anything that can hold moister that may be in contact with sensors will give false readings. Of course, the black tank is the most troublesome of the three tanks because of the things that hang out in there. There is an improved version of the sensors that is for sale that supposedly will reduce false readings, they have a plastic/nylon shroud that keeps things from clinging to them. This may have already been discussed here on the Forum or I saw it on the RV Forum.
PS: I found it: http://www.hilotrailerforum.com/f27/hoist-miracle-probes-64/
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