Better tank monitoring

Tank monitoring on a sailboat is not the most glamorous of topics.  Recently, it became of great interest when the head holding tank overflowed at the very end of a long day with multiple people on the boat.  Not fun.

The old tank monitor was very hard to see (no audible alarm, only a light) and only went off when you were almost full – within an inch or two of the top.  Time to update this!

TLM100 NMEA 2000 tank monitorTLM100 NMEA 2000 tank monitor

I had added a WEMA tank sender to my diesel tank several years ago, and used a Maretron Tank Level Adapter (TLA100) to put it’s data on my NMEA 2000 network.  While this was great for the diesel tank, I was worried about having a tank sender in a waste tank.

Being a loyal Maretron customer, and having many of their existing sensors, I was happy to find the Maretron Tank Level Monitor (TLM100) – an ultrasonic tank monitor that required nothing being inserted into the tank!  On top of that, it can be very granularly calibrated for strange shape tanks.

I ordered two of them – one for the waste tank, and one for the water tank, which has never had any instrumentation, and is hard to get to.

TLA unitTLA unit

The unit itself is very simple, and works on tanks up to 40″ deep.  There are other models that work on deeper tanks, or on gasoline – the standard one is not for use in gas tanks.

Besides the standard NMEA 2000 connection and unit that does the processing, there’s a short cable to the sensor itself.  Included are warnings on blocking the sensor, the effective angles of the beam, instructions on the gasket and other details.  Essentially, you’ll need to use the provided gasket exactly, or risk blockages or inaccurate readings.

The sensor itself is very simple and looks to be very rugged and reliable.  There aren’t many places for stuff to get wedged or stuck, and looks very easy to clean.


Installation is a bit more complicated than a standard sender.  You need to consider a few things:

  • Go deep – I would recommend choosing the deepest part of the tank if you have an irregular one such as my head tank.  This will give the beam more overall liquid to measure and likely be more accurate.
  • Flat as can be – the bottom of the tank should be flat and not have any obstructions in the way of the beam path.

There are kits to focus the beam in the case of constantly listing vessels, to deal with obstructions, and much more.  In my case, I had a clear path to the bottom of the tank, and no other restrictions that would require any of those kits.

Standard tank settings including the #, type, and size.Standard tank settings including the #, type, and size.

My tank was very irregularly shaped – it is in the bow of the boat and molded to that shape.  It was clearly labeled with an overall capacity, so after installing the TLM, I opened N2KAnalyzer and started configuring things.

Granular tank percentage settings

You can set the overall tank size, as expected, but in addition, you can set various calibration points if you have a strange shaped tank, like mine.

I haven’t had the opportunity to fill and drain the tank yet and figure out the overall volume and percentages, but it’s super nice that you could set those things in a granular fashion.

To do so, you’d just enter the depth, and what the expected level (in percent) that the tank is at.


I’ve had the sensor installed for about a week, and have been on the boat several times in different conditions, and added water to the tank randomly in varying amounts to test things.  So far it’s reading the levels perfectly as I would expect.

I’m very happy at having an accurate tank monitor for this particular tank given our recent problems.  The next few steps include setting alarms on the DSM250 for when the tank gets too full, and then figuring out how to get the same sensor installed in the water tank.  Overall, I’m very happy at having a simpler solution to tank monitoring that I don’t have to worry as much about with moving parts.

13 thoughts on “Better tank monitoring

  1. After running out of fresh water several times this last year on Grace, I just installed one of these to help with that problem. Super quick to remove the useless, non functioning (and all in French) manual float gauge and drop this one in. Extended my NEMA 2000 bus to the v-berth in about 20 minutes, did some calibration, and I now have my fresh water level appearing at the Nav Station and on the MDF in the cockpit for easy review!

  2. I have them in my water and holding tank. Will add one more to my additional water tank. Calibrated and accurate enough that with a view hole, I can confirm levels if I had any doubt. Really impressed overall. I can display on my Garmin 740, GMI10 and Raymarine MFD and even on my Auto Pilot display. NMEA 2k is the only way to travel.

    Wish they were about half the price otherwise really like them. Sail in Seattle area as well on a Catalina 34.

    • Agreed on using N2K for these sorts of things rather than a single gauge somewhere static. Good to hear they display levels correctly on Garmin products and elsewhere. I wish there was some more competition here too to drive the prices down. There are several other companies that offer N2K gauges or senders, but they require a lot more work, specific tuning at the factory for your exact tank depth, or are just one part of the whole system. So far the ultrasonic ones have been my best solution both on Jammy for water and waste, and water on Grace.

      Hope to see you sailing by sometime!

    • Hi Ludovic,
      It actually goes into the tank. In my case I already had a hole from the old system, so I just re-used it. You can see in the third picture above the bottom of the unit which has a small protrusion that sticks into the tank. You can also get extensions or collars that help with deeper tanks which will reach much further in.

  3. Hi Steve,

    could you, please, if you have post the photos, especially for the holding tank. What software are you using to configure the tanks. Also do you use any monitoring software, especially mobile.

    Best regards


    • Hi Greg,
      I don’t have this particular boat anymore, but I am adding the same sensor to the holding tank on Grace. I already have one for the water tank installed on Grace as well. What specifically would you like pictures of for the tank? It is not that exciting – just a big blue tank with a black sensor in the top 🙂

      I am using Maretron’s N2K Analyzer software ( which is free, but you have to have one of their USB gateways to use it, which are a bit expensive. You can also use a Maretron DSM display to configure the tank sender as well, which I also have.

      For monitoring, I use a bunch of software, but my primary mobile software is WilhelmSK on my phone, which then connects to a SignalK server running on the boat to get all sorts of data, including tank levels.

      • Hi Steve,

        I have PVC holding tank without any additional holes but just for 2 hoses – in and out. So i thought may be learn from your installations and how to better attach monitor to this particular tank.

        I really enjoy your blog. I am planning to do many upgrades on my boat similar what you already did. I hope I am not annoying you with my questions.

        I would like to put N2K network, with iKomminicator and d-link router (for now, to save the cost) and implement local and cloud monitoring system.

        You review of iKomminicator really makes the difference. Initially I thought to use Raspberry PI or Dogbone, but after your review I think iKomminicator definitely the way to go.

        As for cellular, i think i will use buster you recommended and Nexus 4/or iPhone as a hotspot.

        For monitoring, i guess you are using VPN tunnel, and collect data from iKommunicator. Right?

        I am still battling with navigation computer – iPad or Android-based. I try to stay away from windows-based systems.

        • For most tanks, you would just cut a hole per their specs and use self tapping stainless screws to hold the sensor in the hole. My tank had a hole already for an older float-style sensor, and I was able to use the hole, but had to make new holes for the screws.

          You are not annoying me with your questions at all! Perhaps some of these belong on specific pages/posts about the technology, or in email, which I am happy to do.

          I actually prefer running SignalK on a dedicated computer instead of on iKommunicate. I find that being able to update it more frequently than Digital Yachts has done with the iKommunicate is very important to get the newer features and functionality I need.

          I am using a VPN – you’ll find details on that in my post here:

          Depending on what you mean by navigation computer, that will vary the choices widely. If you’re just looking for something to plan voyages, I highly recommend an iPad if you are Windows averse. There are a ton of great apps for the iPad including Navioncs and Timezero that are fantastic at planning.

          If you are also considering using it as the navigation controller when you are underway, I would still consider a purpose-built chart plotter over an iPad. Any sort of tablet is going to heat up in the sun and shut down, or have issues being water-protected in inclement weather. If you are on a motor boat and have everything enclosed that might not be that big of a deal, but keep in mind that none of that hardware is designed to run all day long doing those sorts of things. They will wear out faster, and have issues not normally seen. I would rather not have my navigation computer stop working at a critical point.

          You can get much smaller and cheaper chart plotters like the B&G Vulcan 5″ for under $500 which is a pretty good deal.

  4. Good Morning Steve,

    While your original article is several years old, it appears that you still respond to comments. Your article appears to indicate that you did not use a focus tube, and that the unit still works correctly. I would like to use TLM 100 but cannot put in a focus tube as I do not have the clearance above the tank. To get the current tube out I would have to pull it halfway out, secure it and cut it in half. Does the unit work without a focus tube? I posted this same question to Maretron, and could not get a straight answer.
    Ken Pfaff
    s/v Wanderlust

    • Hi Kenneth,
      I do very much so respond to comments! From my use of the TLM100 now on two different boats, I have never used a focus tube. From what I have seen with other people’s installs, and in talking with Maretron originally, the focus tube is used in a few situations. If you are on a planing hull, and the boat spends a decent amount of time at an angle, a focus tube could be useful, although not required. Also, if your tank bottom is irregular, and you get odd readings, a focus tube would also be a way to solve that particular problem.

      I have used the TLM100 in 7 different tanks myself now, and several have had odd shapes, but I have not had a significant issue with them. Sometimes when the liquid is splashing around when the level is low, I get odd readings, but that would happen with a normal float type tank reader as well.

      I would say as long as you don’t have a crazy bottom to your tank, you should be OK without the focus tube. Worst case, you could order one and try it in an existing hole, and if it doesn’t work, return it.

      Hope that helps!

  5. Hi Steve,
    Thanks for the very quick response it was very helpful, now I just have to determine if I want to splurge for the expense. I thought of going with a nema 2000 connection as I already have a network and I have the backbone cable close to both the Holding tank and my Nav Station. I currently don’t have a DSM410 display but all of my Ray Marine (actually Raython) are very old and if any of then go I would have a new one to replace it.
    Once again thank you very much.

    PS. I started reading you wifi antenna article so that may be my next project.
    Ken Pfaff
    s/v Wanderlust

    • Hi Ken,
      I assume you have something on your NMEA 2000 network that could show tank levels? If not, then it wouldn’t really be useful of course. Also, keep in mind you need Maretron’s USB connector or a DSM to configure the tank sender, so make sure you factor that into your cost. It can be expensive, but I prefer an ultrasonic sensor that doesn’t get stuck, junk on it, or stop working – never have had issues with them once installed. Worth the extra $ for me when I know I can read tank levels right, and don’t run out of water/diesel at an inopportune time!

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