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Tank gauges

I don't know how it is with other RVs, but my Lazy Daze's tank gauges are lousy. They're imprecise and inaccurate, and the complaints from owners are legion. (I know—I used to oversee an internet forum for LD owners.)

Let me give you a typical example. I dumped my holding tanks yesterday, and back-flushed them thoroughly with my HydroFlush attachment. Believe me, they were clean when I got through! Yet afterward, the "idiot light" tank gauges showed my black tank as 2/3 full and my gray tank 1/3 full. The fresh water gauge is usually a little better, but this time—after filling the tank to the brim—it showed only 2/3 full. That's what I mean by "inaccurate."

Idiot lights

And when your gauge has only four possible readings—E, 1/3, 2/3 and F—you don't really know much about the state of your tank. With the freshwater tank, for example, the difference between any two of those lights is sixteen gallons... in my case, enough for about a week of dry camping. So if my freshwater gauge reads "1/3" (assuming it's correct), that means I have somewhere between seven days and one day to go. Not very helpful, is it? That's what I mean by "imprecise."

How come?

Bolt heads

Why are the gauges so inaccurate? Because they consist of four bolts sticking into each tank at varying levels, with four wires leading to four LEDs on the gauge panel. The liquid in the tank conducts electricity between the bolt heads, and that makes the LEDs light up. Simple, right? Except... if a shred of toilet paper or other debris gets caught on a bolt, it creates an electrical path, and hence a false reading. Even the soap film that builds up inside a gray water tank can be enough to short-circuit the bolts and give bogus readings.

I and my friends like to boondock in remote, scenic places, and the false readings and vagueness of our tank gauges were a constant annoyance. So of course we went looking for solutions. Well, there are plenty of alternatives. The most popular are based on "capacitive sensing," which can be done with sensors outside the tank, so they are not vulnerable to short-circuiting and false reading problems due to impurities in the tank. How can they sense fluid levels from outside the tank? Well, sort of like an electronic stud sensor. Just as it can detect a thick piece of wood behind a thin wall, capacitive sensors can detect fluid behind a plastic tank wall.

A typical capacitive tank sensor such as Garnet's SeeLevel II uses thin conductive strips glued vertically to the outside of the tank. Wires lead to a display panel that measures the capacitance between the strips and translates that into a tank level reading.

First attempt

My friend Kate and I looked at a number of such gauges, and decided that the Vena system had by far the nicest display... a beautifully designed LCD panel that laid out all the information for all your tanks so you could see it at a glance. Kate bought the last one in stock and had it installed by an excellent RV technican who happens to be a friend of ours.

Unfortunately, it didn't work. We calibrated and recalibrated it many times, a tedious business that involved repeatedly filling and dumping all the tanks, but the readings were inconsistent and—worse—jumped all over the place. This was very disappointing in a system that had cost over $300 to purchase and another $200 to install! Vena's techs tried hard to help us troubleshoot the system over the phone, but they admitted that it worked in some rigs and not in others, and that they didn't really know why. I suspected that the problem lay in the sensors and their wiring. Sending very weak analog signals through twenty feet of wire is a dicey business. In any case, after weeks of struggling, Kate gave up and returned the system to Vena for a refund. Too bad—I really loved their display.

Back to the drawing board

We considered Acu-Gage, which seems to be the best-established capacitive tank gauge system, but the displays were unimpressive and the sensors were very similar to the Vena sensors: simple foil strips with long wires attached. After the Vena experience, I was wary of that design.

So when I read in a description of Garnet Industries' SeeLevel II tank sensors that "When the sender transmits the water level information to the display, it sends a digital code that has built-in error detection, making it impossible for the display to read an incorrect level, even if the wiring is bad," I thought "These folks are on the right track!" Kate and I both ordered SeeLevel II model 709 systems from the RV Upgrade Store, where they cost less than $200 apiece for the display plus three sensor strips.

The SeeLevel II sensor is a thin, flexible circuit board about 3" wide, adhesive-backed, with surface-mount chips on it. As you can see in the photo, there are two broad strips of copper, one on each edge of the sensor board. One is solid, the other segmented. Thanks to an exceptionally clever layout, the board can be cut with scissors to match the height of your tank. When connected, it self-calibrates—no filling and dumping needed! SeeLevel II sensor Dedicated logic on the board measures the capacitance between these two strips, which is of course influenced by the liquid inside the tank. It determines the liquid level, converts to digital format and sends an error-checked digital signal up the two-wire cable that links all three tank sensors to the display panel. (That's right: all three tanks use the same two wires.) It's a pretty much foolproof system, because if the signal is corrupted, the display knows it and shows 'Err', while if a wire is broken it shows 'OPn' for that tank.

Installation, and a digression on tank insulation

Kate's sensor installation was easy... all we had to do was clean a section of each tank with alcohol and stick on the sensor strip after cutting it to length. I had a tougher challenge: my tanks have Lazy Daze's optional sprayed-on foam insulation. This is one of those things that sounds like a good idea—after all, who wouldn't want to prevent their tanks from freezing?—but isn't.

To begin with, after a few days of subzero temperatures your tanks will freeze anyway; the insulation merely slows the process. But even before that, your valves will freeze, because they're out in the open (we're talking about a Lazy Daze here; some rigs have valves in a compartment)... so you won't be able to dump until they thaw, regardless of whether your tanks are still liquid. Worse, the insulation doesn't just slow down freezing... it slows down thawing as well! The old hair dryer trick won't work, because the insulation will keep the hot air away from the tanks. In short, once they're frozen, they'll stay that way longer thanks to the insulation.

If you really expect to be camping in severe temperatures for extended periods (as opposed to a cold snap that lasts a few days, which is no big deal for an uninsulated tank), here's what you need to do: first, put electric heater pads (available from RV dealers) on the undersides of your tanks, and wrap electric heating tape (available from Wal-Mart or any hardware store) around your sewer pipes right up to the backs of your dump valves. Then add sheets of rigid (not sprayed on!) foam insulation to the tanks, and wrap the pipes with flexible foam insulation tape. That'll keep the heat from the electric heaters where it belongs.

Bcause my tanks and pipes are already covered with great big globs of sprayed-on foam, I don't have these options. Working with a utility knife and a putty knife, I was eventually able to scrape off enough foam to place the SeeLevel sensor strips on the tanks, but it was a pretty nasty job. If you have to tackle this, be sure you're wearing a dust mask and safety glasses!

Insulated tank

Note: Garnet recommends spraying the panels with automotive undercoating after installation and testing, and we did that after these photos were taken. The rubber/asphalt spray cost only a few bucks at Wal-Mart.

Running the wires is the hardest part of any installation of this type. I could have disconnected two of the existing sensor wires and used them, but I'm cautious—I wanted to keep my options open, and specifically to keep the existing tank gauges intact just in case anything went wrong with the new system. So I pulled a two-wire cable up through the floor and into the kitchen cabinet where the display was mounted.

I should probably go into a little more detail about that operation. I toyed with a number of possible ways of getting the wire from underneath the coach into the under-sink cabinet, but in the end the simplest method was the best. I located the hole in the floor that Lazy Daze had used to bring in the wires from the existing sensors. Of course it was well plugged with sealant, which would have been very difficult to remove. But I was able to penetrate the sealant with a coathanger wire pushed down from above... and then to push a #2 Phillips screwdriver back up from underneath the coach. That made just enough of an opening that I was able to force through the cable from the SeeLevel sensors. The fit was so tight that I didn't even bother to recaulk—it isn't going to leak!

Since the display panel was to be placed in an upper cabinet, I then had to figure out a way to get the cable from the lower cabinet to the upper one... preferably without visible wires, or drilling a hole in my kitchen counter. At the suggestion of Kate's husband Terry, I ran the wires inside the black tank's vent pipe, which in my rig runs vertically through both lower and upper cabinets in the corner behind the sink. It was a simple matter to drill 3/8" holes in the pipe inside the two cabinets, and then shove the cable up from the bottom hole until I could snag it with a bent coathanger in the upper hole and pull it through. Of course I plugged them with Parlastic-type sealant immediately afterward.

Note: this turned out to be a really bad idea. Years later, I began to notice sewer smells in the bathroom and kitchen. The smell gradually got worse and worse, until eventually a friend tracked it down to failed sealant on those holes in the vent pipe. I had forgotten all about them by that time. Moral: don't drill holes in a holding tank vent pipe!

By the way, the SeeLevel II unit also displays propane levels. It doesn't use capacitive sensor strips to do this—they won't work through a metal tank—but a one-wire connection from the existing propane tank sensor does the trick. (LD owners: remove the stove-hood display panel and tap into the white/green wire you'll find back there, using a crimp-on "tap splice" so you don't have to cut any wires. You can pick up the needed 12V and ground connections in the same way.)

The SeeLevel II display is nothing fancy—just three LED digits and a few pushbuttons to choose which tank to monitor—but the system works flawlessly, unlike the Vena capacitive gauges we tried on Kate's rig. Here's what the display looks like mounted in the end of my kitchen cupboard, along with the monitor panels for my HPV-30 solar charge controller and my PROsine 2.0 inverter/charger.

Monitor panels

Remember those bogus readings I got from my "bolt head and idiot light" gauges after dumping and filling yesterday? Here's what the SeeLevel II gauges told me this morning after I installed them. Remember, this is with black and gray tanks nearly empty and freshwater tank nearly full:

 Idiot lightsSeeLevel II
Fresh water2/397%

MUCH better! The new gauge readings are both more accurate and more informative.


Followup, ten-plus years later: The SeaLevel gauges are good, but not perfect. The fresh water and gray tank sensors have always given accurate readings, but the black tank sensor can be fooled by crud that sticks to the inner tank wall. When this happens, the symptom is a non-zero reading after dumping. It's mildly annoying, but not a problem. It would be a problem if the gauge told you that the black tank wasn't full when it actually was—but that never happens. In any case, thoroughly flushing the black tank usually cures this problem.

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