Wednesday, August 13, 2014

Calibrating black and gray water tank gauges

I've spent the past three days struggling to calibrate the gauges on our black and gray water tanks.  The boats come with tank monitoring "gauges" as standard equipment, but they leave a lot to be desired.  We have similar gauges on our Grand Banks, and the problem is that there isn't enough granularity to the readings to know where things really stand.  The issue is that there are just 4 lights to tell you tank status.

Green - the tank is empty.  This is a very happy light to see
Yellow - Yellow comes on just above empty
Orange - Orange comes on at about the half full point.
Red - Red is the unhappy light and means you are in deep dodo.  On some systems, the toilets automatically disable when the red comes on.

Because Green only means the tank is completely empty, and Red only means the tank is completely full, in practice there really are only two useful indicators;  Yellow means you are somewhere below half full, and orange means you are somewhere above half full.  It would really be nice to know with a little more accuracy where things really are.  On the Grand Banks, I lift a hatch and back-light the translucent tank with a flashlight and you can see where the level is.  But this is what we have gauges for - at least we should.

Sooooo, when building the boat we installed ultrasonic level sensors in the black and gray tanks.  That seems simple enough, but when I hooked them up, I was getting erratic readings.  It would go up and go down all over the place, basically reading everything except the correct level.  Reverting to the installation manual, there are a number of cautionary notes warning against the use of silicon sealant or cork gaskets.  It says to only use the supplied gasket.  Now I can't for the life of me understand why the type of gasket would impact the sender, but for kicks I removed one and sure enough, it was gooped up with some sort of magic goo.  After scraping and peeling it all away, guess what?  The sender worked perfectly.  Wow, I'm dying to know why this is and have asked the manufacturer.  Stay tuned for the answer.  Or maybe one of our readers knows?

With the sender finally working, the process of calibration begins.  The process involves pumping the tank all the way down, then filling it in measured increments until full, and taking depth measurements at each increment.  These depth/fill quantities are then given to the sender which is then smart enough to report back the level in gallons or liters rather than the physical level.  This is really important with irregular shaped tanks, which mine are.  One side of each follows the curve of the hull, so 6" of water at the bottom of the tank where it is very narrow represents much less water than 6" at the top of the tank where it is very wide.  With these smart senders you just give them a bunch of measurement points (the more the better) and it translates every inch of level into gallons of whatever.

Now somewhere along the way here you must be wondering how unpleasant a job this must be, right?  Gray water sounds pretty gross, right?  And black water is down right disgusting.  Nope.  That's the beauty of a new boat.  There is nothing but water in any of these tanks and I can freely fill them and pump is back overboard without any problem.  Even my measuring stick comes out clean.  But you can see that there is a lot of incentive to do this now and be sure that I have it right, 'cause I sure don't want to have to do it again.

OK, with that out of the way, let's get on with calibrating.  In the picture below you can see the tank top.  The round black thing to the left partly under the black cable is the sender.  To the right you can see the blue water hose and the water meter with it's outlet aimed down a larger black opening.  The key here is the water meter.  It cost $80, but is a calibrated, commercial quality meter.  Starting with an empty tank, I filled it in 20 gal increments as measured by the meter.  After each 20 gal I used two conjoined yard sticks to measure the water level, carefully recording everything.  Then just keep adding another 20 gal until the tank is full.

Tank top, black sender on the left under black cable, water meter attached to blue hose.

What's full, you might ask?  Good question.  The tank is specified as 120 gal.  But the red panic light comes on when the fluid level is about 4" below the tank top.  Well, these tanks hold a lot more than 120 gal.  It took 170 gal to light the red light.   Another consideration is that the sounder has a 6" "deadband", which means that it is blind closer than 6".  So it will consider everything closer than 6" to the top as Full.  At the end of the day, I decided to calibrate for the rated 120 gal, which is about a foot below the tank top.  That will minimize how much sloshing hits the sender and hopefully limit fouling.  It also gives a significant margin in case I ever need to add a little more to the tank before pumping out.

After filling the tank, noting all the dip stick measurements, and finally feeding the calibration info into the sender, I tested it by running the pump-out to empty the tank and watch the level drop as reported by the sender.  Everything worked correctly.

Between trying to get the sender to work, including one complete tank fill end empty cycle that was wasted because the sender was not working, and the final successful calibration, it took two full days to get the gray water tank done.  The black water then only took a half day, including chasing out one of the screw holes which was galling a bit.  If you have used stainless nuts and bolts much, I'm sure you have experience galling which is the spontaneous self-welding of threaded stainless parts.  If you have burs or other binding of threads, watch out.  One of the sender screws nearly seized coming out, so I had to get a tap and clear the threads in the tank top, and get a new screw before putting the sender back in.  Nothing that can't be solved by the 5,352nd trip to the hardware store.

That's two tanks down and 5 more to go (4 fuel and 1 water).


  1. Great effort. And since I am not a boat owner, it amazes me that for the sophistication and money that you and others spend on these boats, something like this isn't standard issue? Nordhavn could do it once, and replicate for everyone. Am I missing something? I am enjoying your blog, and living vicariously through you! Thanks for sharing your blog and being so candid.

  2. Nordhavn would LOVE to have these boats be standard issue and replicated for every owner. The problem is us, the owners. We all want something different. Sometimes a little bit different, and sometimes a lot different.

    It's much like houses. Every one is different. And it's especially true as houses become bigger and fancier. Boats are no different, at least for larger ones. Smaller boats are more like cars. One basic design and a list of options, and that's it. It's all about how many you build, and how expensive they are.

  3. I get that. Especially with accommodations, navigation, types of engines, etc. But some of the stuff that you and others do is or should be common. I doubt many people would opt to not have better guages on the tanks. Maybe they would, but the more they can standardize and set themselves apart, they differentiate themselves, become the standard, and just keep raising the bar. And for something so basic, it shouldn't be an option. Just as their fuel system is known for its reliability, etc., it isn't an option - it is just what they do that contributes to it being a Nordhavn. And I suspect that if I were a new N60 owner, I could think of lots of other stuff to do other than spend several days figuring out how to calibrate the level of a water tank! If nothing else, it would get you on the water 3 days sooner! It wasn't a 3 hour project. JMHO. Always looking for ways to improve a brand.

  4. p.s. Don't take this as a criticism. I admire you for figuring this out and doing it. Love your sharing of the process.

  5. I hear ya. On the one hand, the tank monitoring system that comes with the boat as standard equipment is the same thing that is used in pretty much every other boat. I'm the only person I know who us adding more precise gauges, so from that I conclude that everyone else is happy with the standard Dometic tank monitoring system, and that I'm a bit OCD.

    Then, the challenge for those of us who are OCD is that there are a variety of different types of gauges with different interfaces, different displays, and different methods of calibration. And we all want something different.

    On this particular topic I think I need to accept that I'm running well off the beaten path. I obviously think that what I am doing is an improvement, and one worth the trouble and expense, but I accept that I'm in the minority.

  6. Compelling read as always Peter. The BTU load generated by light bulbs is the elephant in the room rarely discussed. On larger boats it alone can be a compelling reason to go LED even on boats with 24 hour generator use.

    The use it for a year approach has been ours too. Actually more like 1 1/2 years for us and a very good thing too. Most of what we thought we wanted at purchase time has changed completely. Had we ran up a large yard bill even a year ago upgrading I am sure we'd have regretted nearly every change and be re fitting now.

  7. It's been nearly one month since the last update. Hope that all is well with what is certainly an exciting chapter in the lives of this family.

  8. Waste tank gauges are a bugaboo for everyone. I know it is one of the most common sore spots in the "land yacht" world. And thanks for offering a solution to the issue

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