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.
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.
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.
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.
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.
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.
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.
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.
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:
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.