Better waste tank monitoring with Maretron

Have Another Day’s waste tank monitors have never been very accurate and that’s only gotten worse as time has gone by. In well over 10,000 nautical miles of cruising we’ve only overflowed a waste tank once, but that’s once too many. So, when the gauges on both tanks recently seemed to completely stop working, I decided it was time to find a new solution.

When Carver built Have Another Day she was equipped with TankWatch 4 monitors on both 50 gallon waste tanks. The four lights on the display show you if the tanks are empty, low, half-full or full. Over time the gauges have malfunctioned in several ways: multiple lights lit at the same time, one or more steps skipped, or not resetting to empty after a pump-out.

All the products in Dometic’s TankWatch line rely upon small float switches suspended in the waste tank, and they’re prone to buildup and fouling from the contents of the tank. I’ve had mine out a few times to clean them — no fun! The most recent attempt at cleaning netted only marginal improvement, further indicating this system was in need of replacement.

I could have replaced just the TankWatch senders, which are still easily available from Dometic. But, I wanted something with fewer moving parts and hopefully more granular information than the four lights provided by my TankWatch monitor.

In consultation with Maretron, we decided that a FPM100 paired with submersible pressure transducers would be the best fit for my boat. With a total of six possible tanks to monitor — two black water, two fresh water, and two fuel — the six channel FPM100 was a pretty obvious fit. The FPM100 can be used to monitor any combination of six tank sensors or pressure transducers.

Maretron has a helpful knowledgebase article that guides you through selecting the tank monitoring product that best fits your boat. The FPM100 has a list price of $395, but I think the logical way to calculate the price is per tank monitored and at roughly $65 per tank this seems like a very solid value. Because the FPM can be used for either tank level or pressure measurement you could use additional channels for other purposes if you don’t have six tanks to monitor.

The submersible pressure transducers have a list price of $250, but if your tank has bottom access — like for a site tube — you can use their screw in pressure transducers that list for $150. Maretron says they’ve found the submersible pressure transducer the best fit for waste tanks versus ultrasonic, floats or other technologies.

My waste tanks are about 32 inches tall so the 0 to 1.5 PSI sensor was an easy call. It will support up to 41.5 inches of water so it would comfortably measure my tanks. My fuel tanks are over 40 inches tall and I think I would move up to the next pressure range to ensure measurement over the entire range of the tank.

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The sensor comes with a plate designed to fit a standard 5 bolt sending unit cut out. However, the TankWatch senders use a 3 inch threaded fitting to mount into the tank and my tanks didn’t have a 5 bolt cut-out in them. I really didn’t want to put any new holes in my waste tanks, so fortunately I found inspection port caps that mate to the fitting in the top of the tank. I used a 1.5-inch hole-saw to drill into the cap and then secured the plate and cable gland to the cap.

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The rest of the physical installation consisted of simply removing the old (unpleasant) senders and cleaning up the area. Once that was complete all I had to do was make sure there was sufficient cable below the gland to allow the sensor to lay on the bottom like Maretron’s diagram shows above.

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With the physical install out of the way — and my hands washed several times, even though gloves were used — it was time to configure my system, which requires a Maretron DSM display or gateway. I used a USB100 USB to NMEA 2000 gateway connected to my Windows 10 PC running Maretron’s N2KAnalyzer. In the device’s configuration screen, you set up the FPM100 on a channel-by-channel basis, each of which can monitor pressure or tank level. The channel then needs to be configured with the pressure range of the transducer connected. Lastly, fluid type (water, diesel, etc. because they different densities and hence different pressures at depth), tank type (fuel, fresh water, gray water, black water, etc) and tank instance number.

Also, each channel can be labeled with a more detailed description of what’s being monitored. After specifying a label, it will be displayed by all Maretron devices and software on the network.

Once that was done I filled each holding tank to the highest point I would ever want them to reach. That’s just below the spot where the Dometic (formerly Sealand) vacuflush VG-2 vacuum generator dumps into the tank and the vent filter comes out; so if the level is any higher, things get messy. I suspect that the 50 gallon rating of the tank is based on its total dimensions and the reality is less with the way the hoses are mounted. I don’t think this few gallon difference will change much and that using the more precise number that could be calculated would just cause me confusion.

I mentioned earlier that Maretron’s sensors list for $250 each. I wondered if similar sensors could be purchased for less elsewhere. My quick research initially indicated that quality sensors from reputable companies — mostly designed for industrial applications like waste-treatment facilities — were generally more expensive than what Maretron was offering. But, some more digging on Amazon and AliExpress did turn up some very inexpensive options.

All the sensors available on Amazon seemed to be designed for deeper water. The best I could find was a sensor listed as covering 0-3 meters. Since accuracy on all these transducers is listed as a percent of range I figured I would be giving up accuracy by using a sensor for a larger depth range than I needed. But, that’s what I could find on Amazon so that’s what I bought. The purchased sensor was listed as covering 0-3 meters but the one that arrived is labeled for 0-5 meters, likely making it even less accurate.

But, I had it in hand and the price was right, so I gave it a try. I selected my port fresh water tank for experimentation. Conveniently that tank is currently unmonitored and has the same 3″ access hatch as my waste tanks.

Because I didn’t know the exact pressure ranges for this sensor I had to do some more experimenting to calibrate. That experimentation primarily consisted of measuring outputs from the sensor when empty and full and then trying values in the calibration table. I estimated the pressure range of this sensor must be about 0-7.1 PSI based on this handy calculator that suggests five meters of water carries a pressure of 7.112 PSI. So far I’ve been perfectly happy with the results I’m getting. A few doses of water run through the tanks thus far suggests this system is far more reliable than the float based sender in my other water tank.

I also have a 0-1 meter sensor on the way from China via AliExpress that I’m hoping might be a little more accurate than the 0-5 I got from Amazon. But, I’m not expecting either the accuracy, reliability, or longevity from these sensors that Maretron’s own sensors deliver. However, at least for me, I’m happy with fresh water measurement being roughly accurate.

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The last piece of the puzzle is displaying the data. The TankWatch monitors in the heads no longer work — and I’ll need to figure out something to do with that space, even if it’s just a blanking plate, though I don’t find that too troubling. Perhaps the most obvious way to display the data would be on one of Maretron’s DSM line of displays (available in 4.1 and 5.7 inch sizes) or on my MFD. While both of these work I’m looking for the smallest and most cost-effective way to show this information. Ideally I’d love to find an inexpensive 2″ gauge with a single push-button on the front that cycles through the available tanks. The closest I’ve found is from Oceanic Systems, but their gauges are for a single tank and don’t allow cycling through.

If anyone knows of a NMEA 2000 gauge that could easily cycle through the available tanks and display their levels for less than $200, I’m probably in for two. Until then, I have a wealth of ways to display what seems like accurate and reliable tank levels though none of the displays are as easy as I might like.