Is That Holding Tank Full Already?

Monitoring tank usage is important on LUX. We often are traveling in no discharge areas and have to get a holding tank pumpout done before it is full. The float sensors on the holding tanks aren't great. Calcium buildup quickly makes them ineffective. We needed a better way to monitor holding tank capacity and decided that it would also nice to be able to monitor water tank usage, though it is relatively easy to just open the forward lockers and look at the water levels.

There are a number of tank monitoring systems around. Getting one that works for a combination of water, waste, and fuel is rather interesting. It needs to work with potable water, corrosive waste, and flammable liquids. Two systems seem to get the best marks: capacitive sensors or pressure sensors. 

The capacitive sensors work only on non-metalic tanks. Two strips of conductive tape are applied to the side of the tank and the sensing electronics is calibrated to determine the range of values that correspond to the volume of liquid in the tank.

Pressure sensors work by using a dip tube that extends from the top to nearly the bottom of the tank. Either a manual or automatic pump is used to pressurize the tube until bubbles come out the bottom. The pressure is then read as an indication of the amount of liquid that is covering the tube.

Float switch-based systems typically don't work well, particularly in waste systems. Bits of gunk (that's a technical term ;-) ) and calcium buildup will quickly inhibit the float switch operation.

After looking around at some systems, we decided on the Profile Series 8-tank monitoring system from Ferriello Sales (now New Providence Marine Tank Monitor). <http://www.newprovidencemarine.com/> It can monitor up to 8 tanks and can accept input from a variety of sensors, including our existing fuel sensors, a propane tank sensor (for tanks equipped with a sensor), as well as capacitive sensors. The system can handle weird sized tanks, which is a plus on boats. The display can show a summary of all 8 tanks or a higher-resolution display of each tank's capacity. Alarms can be set for full or empty levels. Alarms can be disabled at night, as determined by a photo sensor. Dennis, the owner, has received a number of accolades from people regarding the superior level of service that he provides. Our experience was similar. The system cost, including wire from West Marine, was about $450.

We intend to monitor six tanks: two each of water, waste, and fuel, even though the fuel monitoring is redundant with the fuel gauges at the helm. We don't have propane tanks with sensors, but that's a possibility for the future.

The display is mounted at the top of the navigation station and provides good visibility. The display is in the process of being mounted and is sitting on the counter in front of the open DC electrical panel. The blue tape at the top of the nav station is used to mark the cutout and to protect the gelcoat during the cutting operation. We neglected to get a photo of the completed installation - something to do on the next trip to LUX.

We have reserved space above the Shore Power panel for the genset control panel, which will be installed this summer, 2014. This will put all the AC equipment panels together for ease of monitoring.

The holding tanks were the first to get sensors. The process is easy to follow and works well. We used nearly 100ft of wire to connect both holding tanks and expect to use that amount for the water tanks. The sensors require three connections: Ground, Power, and Sense. The Power and Sense connections have to come from the display. We were able to use the ground wire from the old tank "full" sensors at each tank, so we only needed to run two wires (Red=Power, Blue=Sensor) from the tanks to the display. We used quick-disconnect electrical connectors so that the tanks can be easily removed in the future.

Calibration required emptying the tanks, then filling them, which we did with water at the next pumpout. The system works well, allowing us to monitor all 8 tanks with one glance as well as more detailed views if we're filling water and fuel tanks.

We ran out of wire after doing the holding tank sensors, so we'll add water and fuel monitoring when we return to LUX.

Update 8/5/2021: Yes, we added monitors to the two fresh water tanks and all four have been valuable tools for knowing when to pump out or add water or switch tanks.

-Terry

Water Tank Leaks and Welding - Again

When we picked up LUX in Marsh Harbor for the spring trip back to Annapolis, the local charter maintenance guy told us that the stbd water tank was leaking. He had talked with the local Moorings folks, who told him that they repaired the tanks with fiberglass. To our knowledge, that wouldn't work because the fiberglass won't bond to the High-Density PolyEthylene (HDPE) of the tank.

Sure enough, when we filled the tank, the water slowly leaked out. When we removed the tank, the fiberglass patch popped right off, as shown in the first picture. We noted that our old weld job had held - we are dealing with a new fracture.

As usual, the first step of a repair is doing the preparation. After removing the old fiberglass repair, we used a rotary tool to open the crack to a U or V shape and to remove the old, damaged material, which had a brown color. We used a heavy duty cutoff disc held at an angle to the fracture. The finished prep is in the next picture. You can see the marks left from the rotary tool cutoff disc.

We have a set of HDPE plastic strips from an old milk jug. The jug must have the recycle designation HDPE molded into the bottom. HDPE is also used in potable antifreeze jugs and a variety of other gallon jugs. It is good to only use material from a food-grade source. The white material we have is a nice contrast to the green tank material - we can easily see what work we've done.

Now we are ready to start welding. We use a heavy duty heat gun ($20 from Home Depot) and a Weller hot knife (Sailrite). The heat gun is used to heat the tank and the strip of new material so that they start to turn translucent. Move the heat gun further away at this time, applying just enough heat to keep the material warm but not enough to cause it to sag. Then use the hot knife to press and weld the new strip to the tank material. The localized heating from the hot knife will cause the new strip of HDPE to melt as well as the layer of tank material just below it. There are several videos on the Internet that show plastic welding and watching a few of them is helpful before starting on something important.

As with any welding (plastic or metal), the first layer can be challenging, because there isn't much of the old material left and it may be necessary to bridge a gap. Only heat the materials enough to soften them and use the hot knife to weld them together. We started with a narrow strip at the bottom of the repair and worked up to larger width strips. Here is a sequence of photos that show the process of welding a strip of material to the tank.

Cut off the desired strip length with the hot knife and tack down an edge.

Use the heat gun to heat the strip and the tank until the strip is translucent. Press the new strip into the pliable material under neath. Move the hot knife slowly to allow the heat to penetrate the thin strip, welding it to the material underneath.

Use the hot knife to press the new material down onto the softened tank material. Make sure you don't get bubbles trapped under the new strip. If that happens, cut through the strip with the hot knife to open the void, then slowly work material back into the cut using the hot knife. I like to work from the center of the new material out to the edge to help prevent voids.

We're nearly finished at this point, having added multiple layers of HDPE to build up to the original tank thickness. It sometimes helps to hold a board or other backing plate behind the weld to keep it from sagging. If the tank is sagging around the weld, you're using too much heat gun. Let it cool until the HDPE cools enough to become opaque, then start over. Working with small strips allows easy heating of the strip without over heating the tank itself. This is a prime example of many small steps being best.

Practice helps a lot. Food-grade 5-gallon buckets used in restaurant supply are a good source of HDPE as well as a container on which to practice welding.

We thought about adding a big patch over the entire area, but decided that working out the voids that would undoubtably occur was not worth it. That doesn't mean that it isn't a reasonable approach - just that we thought that multiple narrower strips were easier to add and would result in the same strength.

Finally, we taped the hot knife to a length of PVC pipe and smoothed out the weld on the inside of the tank.

Here's a picture of the procedure in progress.

And the final result. Practice has definitely helped, because this is is the best looking weld so far. We used a strip of HDPE that was about 12 inches long and 3/8 to 1/2 inch wide to do this weld.

After a day-long offshore trip from Wrightsville Beach to Beaufort, NC, the weld is holding. So the short-term prognosis is good.

  -Terry

More Drips and Plastic Welding

On our trip to the Bahamas, we encountered some sloppy seas, which helped us diagnose the remaining port engine fuel problems, but also generated new problems with the water system. This time it was with the water tanks. We filled the water tanks in St. Augustine, FL, and noticed the sound of running water after we were through. We checked the tank and found that the weld that we had made to the starboard tank had split. (See my post Drip, Drip, Drip for the description of the welding.) We hadn't brought any of the tank welding materials with us. We have a low-powered heat gun and a SailRite Weller soldering-iron-hot-knife, so we have the basic tools. We lacked the proper plastic to use to effect a new weld. So I assembled one of our folding bicycles and headed off to find a store to buy something that was made with the right material.
I had noticed in Annapolis that when doing the welding with the Low Density Polyethylene (LDPE) that it seemed much more flexible and less dense than the original material of the tanks. I thought about this on my bike ride to Target. I wandered around the store, looking at all sorts of containers. The recycle label on the bottom has a set of letters under it, indicating the type of plastic. Most of the storage containers in the store are PP (Polypropylene). On a chance, I picked up a bottle of carpet washing soap and its container was labeled HDPE (High Density Polyethylene). I called Mike and we discussed the material that we previously used versus what is used to make the tanks and decided that HDPE might actually be the right material. The bottles I found seem to have the same consistency and stiffness as our water tanks.
While on the phone, Mike found a gallon jug that had previously contained pink (potable) antifreeze. So I head back to LUX without buying anything.

We cut the bottle into strips about 1/8 to 1/4 inch wide to use as our welding rod. We added an aluminum shield to our light-duty heat gun to focus the heat and the Weller hot-knife was heated up.

The old LDPE was removed with the Dremel tool using a sanding drum. (The Dremel tool is an essential boat working tool - don't leave home without it and a good supply of bits, cutoff wheels, and sanding drums.) The HDPE was added to the seam and it bonded well. The new material is white, but when heated it all turns translucent. That's how you know that the material is bonding to the tank. Try to minimize the area being heated. The tank tends to buckle when it is heated. Also don't overheat the tank or it may sag and open a really big hole that will be very difficult to close. The hot knife was used to melt and shape the plastic while the heat gun is used to keep the plastic in the area hot. The combination tends to work suitably well. We used a wood board on the hot plastic to force it flat again while it cooled.

We later found that the repair that we made in Annapolis had a small leak. Doing the same repair process, we found that there was a small path that water could follow through the weld, which hadn't been done very well, due to our inexperience in plastic welding.
The result is a reasonable weld. The white is the new material and the black marks are where material that adhered to the hot knife burned and then came off as I was working the plastic. We'll have to see how well it holds over time. It allows us to continue using both water tanks and we don't have to get new tanks made and shipped to the Bahamas - certainly an expensive proposition.


We have a couple of thoughts for the future. We want to add a generator to LUX and know that if we have to remove the water tank after the generator is installed, we'll need to cut the tank in half. We will then need to replace it with two smaller tanks that can be installed separately.

  -Terry