Black Water Level Sensor Take Four

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Reliable black water levels make the boat easier to operate and lower costs by reducing the number of pump-outs required.  We have sufficient black water tankage to go for two weeks in normal use and, with some care, we can go much more than a month between pump outs. However, when using inaccurate level sensors, you have to play it conservative. You don’t ever want to have an over-filled black water tank and, with level uncertainty, you need to pump the tank far earlier than actually needed.

We wintered last year in London and, while there, needed to pump-out several times. The black water tank on Dirona is actually fairly big at 120 gallons (454 liters). With precise level sensors, we likely would not have required a pump-out while in London and certainly wouldn’t have required two.  Uncertainty in level indication forces worst-case thinking.

Years back we learned the same lesson with fuel levels.  With precise level indications, our boat’s range was effectively lengthened.  Again, if you aren’t sure about fuel levels, you have to be conservative.  More precise level sensing allows the tankage to used more completely without risk of overflow for black water or underflow (empty at sea) for fuel.

We believe we now have the final solution for black water level sensing where the level reads both very precisely and the reading is always available. For sure we now have very precise readings and that’s a first. What we don’t yet know is how long the sensor will last in a black water environment but reports from others have been good, the sensors appear to be well made, and the manufacture reports they have tested well.

Let’s look at the four different attempts we have made to get the accurate and reliable black water sensor system we are currently using.

Generation 1

Our first attempt was to use the Maretron TLM100 ultrasonic sensor system. We use the TLM100 on freshwater and gray water—it’s low cost, accurate, and super reliable. So we felt good about using this system in the black water tank.

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Maretron TLM100

When the TLM100 failed on the black water tank, we changed sensors believing we had a fault. But the sensor, once cleaned and tested out of the tank appeared to work properly. We could always get this system working by doing one or both of 1) cleaning out the black water tank or 2) cleaning the sensor. The first failure mode is driven by floating debris on the top of the tank and can be solved for a short period of time by cleaning out the tank. Another solution is to reduce floating debris through toilet paper selection. Some toilet papers seem to produce less floating debris and yield more reliable sensing. But travelling the world, we get different toilet paper in different regions and most cause problems. The second failure mode, solved by cleaning the sensor, is caused by fluid splashing on the sensor forming bubbles and eventually dried-out material.

Generation 2

Understanding the two failure modes of the TLM100 on black water tanks, we talked to the manufacturer and they recommended a short aluminum focus tube. The tube prevents random ultrasonic reflections from reaching the sensor which means it’s only reading the reflections directly off the surface of the fluid. In addition, the focus tube makes it very difficult for the fluid to splash the sensor causing build up on it unless the tank is quite full.

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Short Focus Tube

The generation 2 design was notably better and we actually got readings more commonly, likely due to the sensor not being splashed. And most of the readings we got were accurate. The system probably worked 30% to 40% of the time which was a major step forward, but we really wanted something that worked all the time. The second problem was the aluminum focus tube would corrode rapidly in the corrosive environment of a black water tank.  That would force replacement every 12 to 18 months but, if it actually had worked reliably, that would be fine with us. Unfortunately this design just didn’t work frequently enough.

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Short Focus Tube Corroded Away

Generation 3

Generation 3 takes what the manufacturer learned with the short focus tube (Generation 2) and refines it in some very sensible ways.  In this new design, the focus tube is now the entire depth of the tank. It runs all the way to just barely above the bottom. The bottom of the tube is capped and there are small holes both in the side of the focus tube and the cap to allow fluid in to be able to measure level inside the tube. This tube with the small holes doesn’t allow floating debris into the tube and the bottom cap gives reliable readings off the bottom.

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Long Focus Tube Experiment

We liked the thinking behind the full-length focus tube design and were optimistic when we deployed it. It worked super-well for a couple of weeks before failing to get readings at the bottom of the tank. We took it apart and found that dried debris at the bottom of the tube had formed a spongy deposit that prevented reliable reflections when the tank was empty. We hadn’t thought of this failure mode and it appears to be repeatable in that, as fast as it’s cleaned it reforms fairly quickly. But, on the other hand, when the tank is near empty, the level is not nearly as important to know as when it’s near full so we made the decision to continue to use it.

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Debris at Bottom of Long Focus Tube

If this were the only failure mode, we would be fine. Unfortunately, we learned the small holes in the focus tube still allows debris into the tube and it still ends up unable to read. It just takes longer to fail. This level sensing solution worked for us about 60% of the time so still is notably better than the short focus tube. But it still has the same failure modes: 1) needs frequently cleaning, and 2) the aluminum tube will corrode away in 12 to 18 months if used continuously.

We don’t mind having to replace the focus tube every 12 to 18 months if the system is reliable and works.  But, having to clean it every month or two really doesn’t work. It’s not that pleasant to work with black water systems. But what really makes this black water level sensor unacceptable for our uses is it only reads roughly 60% of the time. It’s a great set of ideas that is considerably better than past designs, but was still just not reliably giving us black water levels.

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Removing Long Focus Tube

Generation 4

Taking a page out of our fuel tank level sensing, we decided to move from ultrasonic sensing to pressure detecting. In the fuel system we have pressure sensors in the bottom of each tank and use the Maretron FPM100 to detect levels. What the FPM100 is doing is measuring the height of the fuel column based upon knowing the specific gravity of diesel and the pressure in the bottom of the tank. This system produces amazingly precise measurements. A frequent question when filling the fuel tanks, especially when a fuel truck is delivering fuel directly to the boat, is “how much do you need?” Sometimes we need to pre-purchase the exact amount of fuel needed, for example in in Rodrigues, Mauritus, so a reliable fuel measurement is particularly important to avoid over- or under-purchasing by a substantial margin. Over the years, the number we give is never more than 3% from what we actually take and sometimes within 10 gallons. When you are taking on well over 1,000 gallons of fuel, numbers this accurate are impressive.

Given the reliability of pressure the Maretron FPM100 pressure sensing solution, why wouldn’t we have applied it to the black water systems rather than continuing to work hard to try to get the TLM100 working reliably with the black water tank? The only reason is there is no opening in the bottom of the black water tank in which to install the pressure sensor. The only holes in the black water tank are in the top and you can’t measure pressure at the top of the tank. All the fuel tanks have fittings in the bottom so sensing at the bottom isn’t challenging but the black water tank only has openings in the top so there really is no way to install a black water sensor.

We use and love the Maretron FPM100 and use it to measure: 1) levels on our 4 fuel tanks, 2) hydraulic system pressure, 3) vacuum on the primary fuel filter, and 4) transmission oil pressure. It’s extremely accurate and very reliable. We have never had a failure of any kind in FPM100s (we use three of them on Dirona) or in the pressure sensors (we now use 8 sensors on Dirona). It’s also versatile and allows us to measure vacuum, pressure, fuel levels, and water levels. So with our positive experience with the FPM100, we were excited when Maretron released the PTS-0-1.5PSI-01 (see the accessories tab on FPM100) in tank pressure sensor:

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Maretron PTS-0-1.5PSI-01 Drop-In Sensor

This allows us to use the FPM100 to measure pressure at the bottom of the black water tank without having a fitting or opening in the bottom of the tank. Effectively the sensor just drops in from above and sits on the bottom of the tank.

Just like the fuel tank level sensors, this is a highly accurate solution that is always reading. We get levels 100% of the time rather than 60%. What we don’t know yet is if it will last more than a year, but we think this is likely. Reports from others have been good, the sensors appear to be well made, and the manufacture reports they have tested well. But time is the only true test. The reason we hope for a year is we can live with replacing it annually given that it’s both accurate and reliable.

We think the version four black water level sensor is going to be the one. We’ve never had results this precise nor have we ever had sensing that was always available and never just blinks out for a while. This solution looks like a winner but, as always, we’ll post all we learn.

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18 comments on “Black Water Level Sensor Take Four
  1. Tom says:

    Hi James,

    I am in the process of installing a Maretron system in my boat and am not sure how best to fit the pressure sensors on the fuel/water tanks. If you get a chance, I would be very grateful for some insight into how exactly you did this, perhaps including a photo or two of the sensor in place.

    Many thanks,
    Tom Reed

    • Sure and you’ll like the combination of the Maretron FPM100 and pressure sensors. On the black water tank I used a Maretron PTS-0-1.5PSI-01 sensor. This drops in through the top and sits reading the fluid pressure on the bottom. I just love having glitch free black water readings. I don’t yet know how long it’ll last but it looks very well made. I’m hoping for 1 to 2 years between needing to clean it and at least 2 years in operation. For the fuel tanks I used Setra Sensors. Any 4-20mA sensor will work well. I use Setra because they make a vast variety of very high quality and accurate sensors but Maretron sells a wide variety and they also work well and they are easy to get. I use FPMs to read fuel filter vacuum (0..-14.7 PSI), hydraulic pressure (0..5000 PSI), transmission pressure (0..500PSI), fuel tank levels, and black water levels.

      For the fuel pressure sensors I install a Tee fitting in the bottom of the tank and install the fuel pressure there. You can chose any connection at the bottom of the tank to get an accurate reading on the level of the tank but you need to chose a connection that isn’t normally flowing. The Venturi effect will seriously change the readings if there is any flow at the Tee fitting. You just need to chose fittings that aren’t flowing in the normal case and that’s easy on fuel tanks. The reason we use a drop-in sensor on the black water tanks is there are no fittings at the bottom of the black water (or grey or freshwater tanks). So, for these, the drop in, fully immersed fittings are the best choice.

      I’ll send you a picture of one of the fuel pressure sensors.

  2. Don Kovacik says:

    I’ve been following the adventures of Dirona for some time now, can’t thank you and Jennifer enough for sharing your journey with us all. Funny that the first time I stick my nose into a subject and it’s on black water.
    Since my admiral (aka wife) is convinced that this system is the most mission critical on board our vessel I have to keep it running flawlessly. I’m definitely one of those many who have struggled to find a dependable and accurate black water monitoring system. I went through a couple of different systems, each proving either inaccurate, undependable or both. Although I still use my current system for quick reference I implemented an old school, simple solution to get extremely accurate level readings.
    My black water system consist of two 40 gallon rectangular plastic tanks, one for each head, mounted port and starboard in the engine room. Each holding tank has a single 1-1/2 inch PVC pipe tapped into its side at the bottom. This white PVC rigid schedule 40 pipe runs up vertically parallel to the tank and is eventually plumbed via a flex line to the topside deck pump out accesses.
    I cut out this opaque white vertical section of PVC pipe and replaced it with a clear 1-1/2 inch schedule 40 PVC pipe. Although this requires accessing my engine room to view, it seems a small price to pay for the reassurance of knowing exactly what the level is.

    • You can’t beat sight tubes for simplicity and and reliability. We have sight tubes on all four of our fuel tanks in the engine room. The primary data comes from the pressure sensor to get us remote data but, when on passage and fuel load really matters, we check against the sight gauges. The black water tank is forward against the hull of the boat and it’s sides are not accessible for sight gauge reading so that’s not an option on this boat.

  3. Gary Weiner says:

    Alternative to reams of paper, perhaps? Here is link to similar discussion on sailboats:

    • It’s a functional solution that would work but it wouldn’t be our first choice. We want a boat that is, as much as possible, the same as a very nice apartment in a world class city (except that it can cross oceans visit exciting locations all over the world). We don’t want to have to manage power loads, remember to start the generator, not have access to the internet, or have to manage without toilet paper. But, I agree that solution would work.

      • Hi James, Jennifer et al and let me add my thanks for your thoroughly enjoyable and most helpful blog posts. I know first hand how time consuming this is and am most appreciative of your time and efforts.

        Hoping to add some value from my experiences with black water systems on boats, I’d like to strongly second Gary’s suggestion to solve not only level sensing but most other problems with the black water system by swapping out your toilet seats for a bidet model and eliminating TP from the picture all together. Having lived outside North America for much of my life and full time live aboard sailor since 2007 I can’t imagine living on sea or land without some form of a bidet and frankly I am at a bit of a loss as to why North America seems to be one of the few regions of the world that doesn’t see the benefits of “washing rather than wiping”. Having made a similar suggestion to many land based friends and boaters, all who try a bidet say they can never go back. Benefits in any situation are cleaner, healthier, safer, more comfortable and environmentally friendly.

        In the case of those of us living on boats and being our own sanitation department I suspect that of all the maintenance items we have to look after anything with “black water” in it is usually our least favorite items on the list. Since switching to the combination of fresh water only VacuFlush toilets with bidet seats has all but eliminated black water from showing up on any of my To Do lists and the few times it has appeared it was for a relatively quick and easy replacement of duck bill valves and once a failing electrical motor on the Sealand diaphragm pump.

        The trend towards using bidets, both on land and at sea, seems to be increasing and there is now a huge range of companies, styles and models to choose from in these toilet seat replacement type bidets. It can be as basic as the addition of a simple hand sprayer as noted in many of the comments in the link Gary provided above up to much more full featured models with hot/cold water, multiple spray heads and for different men/women washing and blow dryer. I know, I know, but don’t knock it till you’ve tried it! 🙂

        My wife and I are currently in the process of building our new boat, a 24m all aluminium eXtreme eXploration Passage Maker or XPM boat with Naval Yachts here in Antalya Turkey and while we are just finishing Stage 1 of all the hotworks of the hull and most other system components await ordering, our complete VacuFlush system is already here awaiting installation and we are down to a short list of the bidet seats to install on them.

        Even above and beyond the many benefits, from my perspective I don’t see “having to manage without toilet paper” as a bug but as a feature. IMHO it is the most “civilised” way of looking after this aspect of our personal hygiene and comes with the very significant bonus of banishing the most disliked items from my boat maintenance list.

        So my Canadian 2 cents worh at least is that bidets are well worth considering as a future upgrade to boats and life!

        • I forgot to add my thanks James for your confirmation that the Maretron FPM100 pressure sensing method for BW tank levels is the one that seems to work out best. We have already specified and standardized on FPM100 sensors for all our tanks aboard the new boat we are building so this is great confirmation to get from your first hand experience. Between fuel, potable, black and gray water we have 18 different tanks we need to have accurate levels for so standardizing on the same FPM100 system for all of them as well as using this for fuel system vacuum and engine pressures helps reduce costs and spares onboard.

          Great to know that we are already starting with Generation 4 thanks to all your evolutionary testing.

  4. Hello James, Rob Westermann from Artnautica LRC 58 in Harlingen, pointed out your publication on the black water tank sensor issues. On our yacht Xanthiona ( I also use the Maretron TLM100 for the black water tank and experienced similar issues. The short focus tube was installed, but after more than 25 % full the readings still seize. As I also had issues with the pump, we recently decided to not flush the toiletpaper any more. And surprisingly I am now getting accurate readings on the TLM100 sensor. We only cruise inland (so far) waters and have no experience how it will behave in rougher waters. But so far so good. I enjoy reading your very interesting weblog.

    • Hey Rob. Good hearing from you. Yes, no toilet paper or even very carefully selected toilet paper seems to work well with the TLM100. When in Australia using whatever toilet paper we got there, the TLM100 was doing super well. If I flush the tank, then it’s back to measuring correctly. The issue is clearly floating bits of toilet paper and, if those are eliminated, the system works far better. Since we live the boat, we do use the toilet, want to flush toilet paper, and want readings all the time so we moved to the submersible.

  5. Eric Patterson says:

    James I’ve used a similar type on sludge systems. You will expierence issues with possible interference in this application with solids. You may want to consider encasing the device in a screened enclosure. We used 2” pvc capped on both ends with a whole tapped for a grommet on one end. Wrap with 40 mesh 316ss screen and secure with non nylon zip ties. Some of these have breathers that allow a zero reference to atmospheric pressure although I doubt your boat will ever be above sea level just realize atmospheric pressure is a factor. Otherwise I love this and already planned it on my boat or perhaps continuous level float as I mentioned earlier which I am actually from my expierence in the waste water industry feeling better about. It’s funny as I read your notes you have went though all of the problems that we have in the industry throughout the years and came to the same conclusions.

    • Eric Patterson says:

      Forgot to mention drill holes in the pvc pipe. Duhhh.

      • Yes, you are right that the sensor does have a breather to sense atmospheric pressure. It’s a small tube that runs in the electrical cable the length of the cable. I’ll be prepared to occasionally clean the probe or to build a PVC and screen assembly along the lines you have described. Thanks for the benefit of your experience.

  6. James – Timely post! One of my winter projects was to look at the probes of the black/grey water tanks (using the TankWatch 4 system) as they are ready to be cleaned/replaced again. But as we are slowing expanding the maretron system on Red Rover, this looks like it would be a much better approach than to stay with what I have currently. My question for you, how do you dictate to the system when the tank is full? Empty seems straight forward, but do you need to fill to where you want full to be with water and capture that pressure setting? Or is it just noting the depth/height of tank?

    • Yes, the key to accurate results from sensors is to calibrate them for your application. We did our calibration using by measuring the flow rate of a hose at a fixed opening by repeatedly filling 5 gallon utility containers. We got consistent results. We know the size of the tank so we device that by 16 and figure out the number of second between each calibration point. It worked fairly well with the top of the tank arriving slightly early (the tanks are never exactly what is claimed and some of our fuel tanks were very different). This approach produces a well calibrated system that works well and it doesn’t take long to complete.

  7. DM says:

    For those travelling through salty waters, one interesting device I’ve run across is the Electroscan:


    It takes it salt (NaCl) from the salt water, breaks it into base components sodium (Na) and chloriee (Cl), and then uses the latter to kill bacteria in waste just as you would use bleach to disinfect something (chloride being the active component).

    Per some of the reviews, the results were found to be only 2.43FCU/100mL; by comparison, 14FCU/100mL is the shell-fishing-approved standard.

    Not for every boat, but could be handy for some folks.

    • Treatment is an option and it should be a good option but, in many jurisdictions, you can’t dump sewage from a boat whether it’s been treated or not. If the treatment is effective, one would hope the laws will change and the technique will be permitted. The only other issue of which I’m aware is treatment systems take space and I sometimes quite a bit. I suppose you could replace a large black water storage tank with a small one and use the recovered space for the treatment plant.

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