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.
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.
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
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.
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:
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.
The other issue with such a small tank is, is the sensor sensitive enough?
I used one on my 15 gallon wing engine fuel tank which is only 10″ to 12″ and it worked quite well.
We are interested in this solution but we have a very small black water tank (only 15L). Would this solution still work accurately?
Yes, these pressure sensors are remarkably accurate. To maximize the accuracy, choose the smallest sensor range that is larger than your tank height. The sensor selection can be found here: https://www.maretron.com/products/fpm100.php.
Dear James,
it has been a while since we first and only met in Amsterdam. I am in the middle of the BlackWater Tank level decision. Thanks for all your time and effort you put into this article. Helps me a lot in my case.
James, it has been a while since you installed the FPM for the back water tank and i am looking for feedback on how you have found this solution. Did you get a year or 2 before needing maintenance or replacement. Could you give us any further advice on this solution.
Hi Duane. Yes, after 3 years experience, the Maretron FPM with the drop in submersible sensor is by far the best solution we have ever tried and we would do it again without a second thought. But it’s not been without issue. The first sensor went 2 1/2 years before failing which some might find annoying but that’s two years without every thinking of it. Never living with glitches. Never having to clean it. It just ran for 2 1/2 years without issue. Replacing it is a 10 min job and requires no calibration or anything else. Even having seen a failure, we’re super happy with the solution and recommend it over anything else we’ve ever used over the last 21 years.
Great update James and I with that recommendation I am pivoting a little on some thinking and design. Sounds like a robust solution, and with much less holding tank capacity, even more important to know and manage the tank state.
Your point is an excellent one that knowing current tank levels with precision, effectively makes the tank larger. You can run right close to the top if if you know that the level is accurate and doesn’t need further buffering. We used the same technique to increase our fuel tank effective capacity by knowing the level with precision and being able to safely run with less reserve capacity.
Having followed your adventure for a couple of years I have enjoyed your trip a lot while learning a lot. My wife and I are building a boat and want to use your generation 4 blackwater solution as our current boat blackwater level is always wrong. The blackwater tank will be 45 gallons with the correct SAE fitting on top for a drop in sensor. We are being told the tank is too shallow for the Maretron drop in sensor to work correctly. I believe the real problem is the wrong sensor as explained on the Maretron website. We intend to moniter fuel with the 1/4npt sensor, and freshwater using a drop in sensor. Your thoughts on the blackwater tank?
Pressure sensors are a good solution from measuring tank levels. With pressure sensors, you generally prefer external sensors to drop in sensors. External sensors are more widely available, are easier to service, and will likely last longer. If you have any external fittings near at the bottom of the tank, an external sensor is the better solution. If there are no external fittings at the bottom of the tank, then the only pressure sensing solution is the drop in sensor.
Regardless of the type of sensor used, they must be sized to the tank. When measuring level with pressure, there is little pressure change for a given level change so the accuracy of the pressure sensors is important. Getting a good quality sensor is important but you also want to size the sensor such that it is able to read the entire pressure range between tank empty (0 PSI) and tank full and want the tank full reading to be near to the pressure limit for the sensor. For freshwater, black water, and gray water look up the conversion of fluid level to pressure and ensure that the selected sensor will be able to read the tank when full and close to the full range of the sensor is being used. For example, if you do the math on pressure and find your tank produces 2.6 PSI when full, then a 0-3 PSI sensor would be the right choice.
In the case of your black water tank, I’m guessing the tank pressure range is far smaller than the drop-in sensor supplied by Maretron. It’ll still work but it’ll be less accurate than an appropriately sized sensor. If you can use an external sensor, getting an appropriately sized sensor will be easier. If you must use drop-in for this application, you may be able to get a drop in sensor from some other manufacturer. Any sensor with a 4-20mA output will work fine.
James, I’m just about to order one of the pressure sensors. Before I do, can you give a quick update? Still working as well as when you installed it? Thanks!
Never a hiccup. It seems like the submersible sensors have fairly long life and don’t plug up. I’m sure it’ll eventually fail but If I had to clean or even replace it every 2 to 3 years, I would probably accept that for having precise and always available data. Two thumbs up.
Excellent – thanks for the update. I get frustrated with the lack of accuracy of our water tank level sensor, too – maybe I’ll just buy two of them!
Yes, that’s exactly my approach. I have two. But, thus far, the second one hasn’t needed to come out of storage.
James, an update. I installed my sensor about a month ago. I don’t have any Maretron stuff, so the sensor output goes to a microcontroller with built-in wifi (Wemos D1 Mini, an ESP8266 implementation), which sends it to my RazPi, which broadcasts it over the boat’s wifi. I now have – FINALLY – an accurate blackwater level, represented in numbers of flushes. And now that I know about 4-20mA sensors, I’m going to add one for main engine oil pressure, and for the vacuum in the Racor fuel filter. Thanks, yet again, for such great information!
That’s AWESOME. Now that you can process 4-20msec sensors, you have pretty much every sensor possible now available to you. It’s an extremely broadly used standard. Nice work Brian.
Hello James, Jennifer, and Spitfire! Long time, no contact. Quick question about the PTS (sender): do you know what its output is? Variable voltage, I’m guessing, but every reference I can find says that you plug it into the FPM100, and the FPM100 takes it from there. I would like to read the output on an Arduino. I’m wondering if any of the documentation that came with the sensor gives that kind of detail? If no, do you have a contact person at Maretron Tech Support that might be able to answer the question? Thanks!
What’s being used in this application is the 4-20 milliamp loop that is dominant in the process control world. In this model, 4 milliamps is the low end of the measured range and 20 milliamps is the high end. This signalling standard is popular in industrial process control because it’s stable over long distances (voltage will fall over distance but current doesn’t and these can work at distances approaching a kilometer), it’s simple, and it’s cheap.
Here’s a pointer to more data on the interface and it’ll be easy to read from an Arduino: https://www.dataq.com/blog/data-acquisition/4-20-ma-current-loop-measurements/
For much more data, search for 4-20 milliamp arduino and you’ll see lots of examples. I used one of these to directly read using a Raspberry Pi: https://www.amazon.com/DROK-Current-Voltage-Converter-Transmitter/dp/B073PRCDY2/. This converts current to voltage and then you can use any analog input system. Arduino can do analog input directly but on a Raspberry Pi, I use a separate i2c to analog input device like this one: https://www.amazon.com/gp/product/B01MTU9LJV/.
James – WOW! I wasn’t expecting that thorough of an answer – thank you so much!
Hey James – Love the posts as you guys travel. Its amazing to see the travel, but also the real world use of some of these things.
My question – what is your judgement on tank levels. I know that you have said the immersion sensor seems to work best for black level (as long as things dont dry up which I doubt they will for liveaboard use).
For Diesel – do you prefer the pressure sensors ? You mention Setra – is this the 209 sensor series with 0-2psi ? What is your take on a submersible for diesel water tanks as well ? As I think it’ll be easier to drop a submersible pressure transducer than to install a new 1/4″ NPT.
For fresh water – submersible pressure or bottom of tank pressure sender ?
Thanks for the clartifiucations.
Hi Ed. For black, gray, and fresh, there are no openings in the bottom of the tanks so an imersion sensor is the right way to go. I’ll likely stay with the TLM100 on the freshwater tank since it works fine but, if I were to do it again, I would go with pressure sensors across the board. On the diesel tanks, they mostly have multiple connections at the bottom of the tank. I put a tee fitting in the bottom and use the Setra 209 sensor of the appropriate pressure. As I recall I use both 0-2PSI and 0-3PSI. The Maretron sensors have a good reputation as well. I think I used a Setra only because I like to get exactly the right pressure range and Setra has a wide range of offerings.
The only downside of putting it in a fitting is if there is high flow through the fitting, it’ll lower the pressure reading through the venturi effect. So, I use an unused fitting that isn’t normally flowing fuel and it works well. You could use an immersion sensor as well but that requires you put an opening in the top of the tank which is usually more hassle than installing a tee.
Thanks for the response. Yeah, ironically I have really access to the top of my diesel tank and not necessarily the bottom. And for whatever reason there are like 4 NPT fittings up top which wont help with an immersion sensor either. Its currently a Hart tank tender which works amaizngly well, just cant feed that data via nmea 2k… .
In terms of water tanks – yes, access is almost always easier from up above to drop an immersion sensor (or drill for SAE 5 hole). But since I’m putting new sensors for tank levels, I can choose what type of sensor I need/want.
Once again, thanks for your reply.
another question on the submersible – do you use the hull focus tubes to kind of keep them in place, or do you just let them kind of move around with the pseudo slightly stiff cable along the bottom of the tank ? My guess is having it hang slightly off the bottom isnt the ideal solution, but then again – having it sloshing or moving around along the bottom just sounds so – inaccurate (even though the pressure reading shouldnt/wouldnt care).
I gave it thought for a while but ended up concluding that just barely resting on the bottom would work fairly well. In this model they can only move an inch or so in any direction but they are on the bottom. As you said, Pascals laws says anywhere along the bottom of the tank is fine so an inch or so one way or the other seems fine to me.
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.
James,
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.
Alternative to reams of paper, perhaps? Here is link to similar discussion on sailboats: http://www.cruisersforum.com/forums/f74/bidet-toilet-195535.html
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.
Pressure sensors with the FPM100 are both impressively accurate and very reliable. You’ll be happy with your planned new deployment.
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 (www.xanthiona.com) 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.
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.
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.
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.
For those travelling through salty waters, one interesting device I’ve run across is the Electroscan:
* http://www.canadianyachting.ca/products/plumbing/1274-electro-scan-waste-treatment-system
* https://www.boatus.com/magazine/2012/april/taking-care-of-business.asp
* https://raritaneng.com/raritan-product-line/waste-treatment/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.