* Waste Tank Monitors *


The accuracy of RV waste-water tank sensors is notoriously bad. After a few months of use, either they show full when the tanks are empty, or they show erratic readings all the time. TypicalTankProbeA typical sensor is on the right ...the tip of the screw protrudes a very short distance into the inside of the tank, and a wire is attached to the head of the screw to show a reading to the monitor system inside the RV. There are normally four sensor probes per tank ... a ground probe plus one at 1/3, one at 2/3, and one at full levels. The erratic readings happen because the walls of both the gray and black water tanks become coated with slime and crud, and "stuff" snags on the protruding screw sensors. This causes conductivity between the sensors even when there is little or no liquid in the tank, and you have erroneous sensor readings. The end result is that you cannot trust what your sensor panel tells you ...you don't know how full or empty your waste water tanks actually are. While experience will teach you about how long you can go between dumps, it would be nice to have a monitor system that would give you somewhat accurate reminders!

We've read many suggested cures over the years, most very iffy and some pretty costly. They range from back flushing (which we do), to ice cubes in the black tank, to chemicals, to water softener, to totally HorstProbesBlackTankHorstProbesGrayTankthe complete tank monitoring systems, wiring and all. Most reports from people who have tried one "cure" or another, even spending hundreds of dollars to replace the entire system, indicate there is nothing really works. We did read about one system of external tank monitors that did have some good reviews, but that is one that requires replacement of inside monitor panels and wiring along with the tank sensors at a cost of several hundred dollars PLUS labor. Then I read about Horst Miracle Probes. Based on their design (black tank right, gray tank left) and several reviews I have read, the Horst probes hold great promise for more accurate readings. The probe protrudes much further into the tank, and all except the tip is enclosed in teflon, which is non-conductive and self-cleaning. In addition, the black tank probe has a teflon "roof" over the actual probe to prevent paper, etc from catching on the probe and causing erroneous readings. The Horst probes use the original wiring and monitor panels, and the price is quite reasonable -- apx $70 for a set of both gray and black tank probes. So we decided they are worth a try.

The hardest part of the installation is getting access to the original probes. Our water tanks are totally enclosed in the belly of the motorhome. GrayTankThe first task is to find out where the black and gray tanks are, and then to find out where the OEM sensor probes are installed on each tank. On our 2003 Winnebago Ultimate BlackTankProbes Advantage motorhome, the gray tank probes are behind a metal panel above the water pump in a basement storage compartment on the passenger side of the motorhome, pictured on the left. I found them about a year earlier when we replaced the water heater, which is located immediately behind the water pump compartment. The black tank probes, pictured on the right, are located behind a somewhat more difficult to remove metal panel in the water service compartment on the driver's side. I found them a couple of years earlier when I replaced the gray tank dump valve, located behind a lower metal panel in the same area.

Since I knew where they were, getting to the sensors wasn't too bad, and removal of the OEM well-nut probes and installation of the Horst probes was fairly easy. With my original well-nut probes, I could simply remove them and insert the new Horst Miracle probe into the same hole, then attach the original wiring to each probe. Some RVs have molded in probes. In that case, you would simply drill a new CountersunkProbe3/8" hole apx 1" or more to the side of the original probe, install the new Horst Miracle probe, and change the wiring from the old probes to the new probes. I encountered a possible problem with my gray tank when I found that the gray tank walls are apx 3/8" thick rather than the standard 1/4" thickness. The extra thickness of the gray tank walls made the well-nut type mount on new the Horst probes want to pop back out of the hole when I tightened them down. After some mental processing, I used a Dremel tool and small grinding wheel to cut a countersink area for the rubber head of the probes apx 1/8" into the tank wall. That left 1/4" thickness for the well nut to lock into, and worked very well! The picture to the right shows the countersunk Horst probe.

If you already know where your tank sensors are and how to get to them, complete installation should take less than an hour depending on what you have to remove to get to the probes. I left the metal panels off after installing the new probes so I could wait until the tanks are filled to verify there were leaks around any of the probes.

In the first few days of use, the Horst probes showed both black and gray tanks as empty. This continued for several weeks regardless of the true tank levels. Knowing the circuit board on the display panel had had adjustments made to the readings in the past (there's a tiny "pot" on the board for each set of lights), I tinkered with the adjustments on the circuit board off and on when I knew the tanks were not empty. I did find that it helped to disconnect the second monitoring station in the water service bay ...when that board was connected, the inside panel lights went to full regardless of actual levels. Then one day about five weeks after the sensors were installed I noted the gray tank panel was giving me a 1/3 filled reading. Several days later it went to 2/3. Success ...the gray tank was reading accurately! A few weeks later, after more tinkering with the circuit board adjustment for the black tank I started getting readings there too, though very slow to register and then very quick to go to full. After apx 3 months of use, both black tank and gray tank monitors were reading accurately. After the "break-in"/settling-in time, the Horst Miracle Probes are a great improvement over the original probes.

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