Whatever Floats Your Sensor

Knowing Whats on the Inside of Your Holding Tank is What Counts.


For those of us who sail lakes and coastal waters, knowing the status of ourstorage tanks with some degree of accuracy is the mark of a responsible captain.

When a holding tank gets full, pumping the head can push effluent out the vent and down the side of your boat. If the vent ever becomes clogged, more pumping could have disastrous consequences inside. And then theres the freshwater tank. Many of us have had the joy of using a dipstick or sight glass (a clear tube on the outside of the tank that runs from bottom to top) to check. But today, there are better and easier ways to keep tabs on how full or empty your tanks are.

In the February issue, we discussed different ways to measure tank levels in our evaluation of five externally mounted sensors suitable for composite plastic or fiberglass tanks appropriate for marine use. Testers noted that having no floats or probes inside the tank was a distinct advantage as they are likely to be fouled or require periodic cleaning. On many boats, mounting external sensors on the side of a tank is too much of a hassle or is not an option because of where the tank is mounted. If the top of the tank is accessible, then it may be possible to install an internally mounted liquid-level sensor in the tank. And so it is this lineup of internal tank sensors for marine use that were focusing on in this review.

Internally Mounted Tank-Level Sensors

What We Tested

For this comparison, Practical Sailor tested seven pre-packaged kits containing internally mounted tank-level sensors and remote monitor panels. Those that used some form of float sensor were the SeaLand TankWatch 1, Dometic DTM4, Groco TLM Series, and Wema SHS-8. Two test products used air pressure for reading: Fireboy-Xintex PTS and Hart Systems Tank Tender. And one product-the BEP Marines TS1 sender-used ultrasonic technology to measure tank levels. All should work on standard composite, polyethylene, or aluminum tanks.
Most of the products tested are rated for wastewater holding tanks, fresh, or “sweet” water tanks, and aluminum or metal fuel tanks for diesel. Gasoline tanks require different sensors.

The quality and workmanship of all units tested appeared to be Good to Excellent. The sender units were easily mounted on top of our tank, and the monitor panels were small enough to be mounted anywhere. All drew minimal amperage. All instructions were adequate given the complexity of the products installation.

Whatever Floats Your Sensor

The accuracy of any tank level reading is dependent on the accuracy of the length of probe or calibration needed when ordering the probe senders. Float-type level sensors in holding tanks have a bad reputation for fouling and requiring periodic cleaning, and we kept this in mind during testing. Some manufacturers offer different size sensor units, factory adjustments, or programming for different size tanks. Some also offer other gauges or monitor panels for multiple tanks.

If you are considering adding an internal tank sensor to your boat, remember, you must have access to the top of the tank to mount the sensor.

BEP Marine TS1

BEP Marine takes a different approach to measuring liquid levels with its acoustic “sonar” technology. Its TS1 ultrasonic tank sender can be used on composite, fiberglass, and metal tanks for fresh water, grey, or black water holding tanks, and diesel fuel tanks.

Internally Mounted Tank-Level Sensors

Just released in October 2007, the sensor will operate in any tank up to 6 feet deep and is programmable for odd-shaped (non-rectangular) tanks. It can be programmed to work with a wide variety of gauges and monitoring meters with different output voltages.

Our test sensor was factory programmed for our tank, and we used the corresponding BEP 600-TLM monitor unit for display. Users can also order software and an interface box to connect the sensor to their computer.

The advanced ultrasonic technology is extremely accurate, and the digital screen shows liquid levels three ways: in U.S. gallons, as a percentage, and with a bar graph. This multi-method data delivery makes reading the display nearly foolproof.

Testers found the instructions to be clear and well written, and the installation was fast and simple. We drilled a 1-inch opening plus five screw holes on the tank, inserted the sensor and gasket, and tightened the screws. We connected three wires to the monitor and two more for power (ring-eye terminals provided), and we were done.

Internally Mounted Tank-Level Sensors

This is a very sophisticated tank sensor. It can monitor up to three tanks with selectable programming for 14 types of tanks and independently adjustable high-low alarms with visual and audible functions. The display is backlit for easy night viewing.

The BEP device comes with a 15-page instruction manual, and testers found that it pays to read the instructions carefully before starting.

Testers were most impressed with the sophisticated electronics of BEP Marines ultrasonic tank sensor and digital read-out monitor. However, because it is a new product and U.S. distribution is just getting started, we hesitate to recommend it. Were optimistic that our longterm testing will prove the BEP to be a favorite.

Bottom Line:

It is moderately priced, well made, easy to install, and gives very accurate continuous readings. If you are into advanced electronics, and want the latest on the market, its worth your consideration.

Dometic DTM4
Dometics single-tank monitoring kit (an update to its previous SeaLand TankWatch 4) comes with three probe float-switch assemblies, an indicator panel, and the necessary wire harnesses.

The probe cap switch assembly is designed to screw into a standard 3-inch female pipe thread tank opening-like the observation port on many tanks. If your tank doesn’t have a 3-inch threaded opening, an optional universal flange kit is available. The cap comes with three float-switch probes that must be cut to appropriate lengths. One operates when liquid is about 2 inches from the tank top to indicate tank full, one in the middle to indicate half-full, and a third to indicate empty.

The kit includes an eight-page instruction manual with detailed assembly and wiring diagrams. Testers found installation a little time-consuming: Our probe kit came with two 22-inch-long probes, inside of which were wires to the pre-assembled wire harness. We needed lengths of 10 inches and 6 inches for our tank, and so followed the instructions to trim the probes to fit.

The plastic indicator panel has symbols indicating the type of tank and four vertical LEDs. There are no switches or openings on the panel. The tank-empty LED stays on, and as the tank fills, the other LEDs illuminate and remain on.

Dometic also offer two- and three-tank monitors that use the same probe float-switch assembly.
Bottom Line:
The Dometic DTM4 took the most time to install and is moderately priced.

The Fireboy-Xintex monitoring system came with a new series PTS-10 tank sender and an LLM-1 single-tank display gauge. The PTS uses pneumatic (air pressure) reading technology with no moving parts, encapsulated electronics, and a simple

Internally Mounted Tank-Level Sensors

two-wire hook-up. It can be purchased with either a 1-inch threaded or gasketed screw-mount flange and the plastic tube probe is factory cut and calibrated to the depth of your tank.

Since we had a threaded opening, installation was fast and easy. The LLM gauges come in either one- or two-tank versions and are labeled either for “fresh water” or “holding tanks.” The round gauge has five LEDs in a semicircle (to indicate tank empty, , , , and full). Pressing a button illuminates the appropriate liquid-level LED for three seconds.

Bottom Line:
 The Fireboy has no moving parts to break and an easy-to-read gauge. We feel the PTS-10 is an improvement over the companys older model and a good functioning system for the money. It gets the PS Best Choice.

Groco TLM

Grocos TLM series of tank level monitors uses an internal probe with two free-floating mercury micro switches attached to the custom length probe with 2-inch wires. The length of the plastic TLM sender probe must be custom cut for your tank and is on a 1-inch threaded flange with a female threaded 1-inch adaptor included.

The small 3- by 2-inch monitor panel has two corresponding LEDs marked ” and “.” The float switches work so that the bottom one floats up to indicate one-

Internally Mounted Tank-Level Sensors

quarter full and the top one floats up to indicate three-quarters full. One or both will illuminate as the liquid level rises, when the “Show level” switch is pressed.

The kit included four screws and 25 feet of cable with the necessary connectors. The monitor panel comes with a rubber gasket and the switches and LEDs are sealed so that it can be mounted in the head, where it might face occasional splashes from a shower.

The TLM appears rugged and well made and comes with a one-page instruction sheet that is simple and well written.

Bottom Line:
Grocos TLM-10, with its two free-floating micro-switches, should get around the fouling problem, but the gauge has only two LEDs marked for level indicating, and it was not very accurate during testing.

Hart Tank Tender

The Tank Tender by Hart Systems takes a different approach to monitoring tank levels. It uses air pressure direct to an analog gauge, which means theres no need for electrical connections.

Internally Mounted Tank-Level Sensors

The pneumatic gauge displays the level of the tank in inches, either water or diesel fuel (not gasoline). We tested Harts No. 30-2 PV, which will monitor two tanks up to 30 inches deep and comes with a “purge” switch for a holding tank.

Hart offers several models (that can monitor up to 10 tanks) and has been selling these units to the marine and industrial-commercial markets for more than 25 years.

The kit included a 6-by-4-inch panel with gauge, a tank penetration fitting with 30 inches of rigid 1/8-inch tubing and about 30 feet of 1/8-inch flexible nylon tubing.

Installation was simple: Cut the rigid tubing to fit the tank, drill a hole in the tank top, screw in the top fitting, and run the nylon tubing to the monitor panel. To operate, press the button for the selected tank and slowly pump one or two strokes. The gauge needle show the level of the tank in inches. Accuracy should be within one-quarter inch, or 1 percent, at mid scale.

For holding tanks, first place the switch in the purge position, pump a couple times to blow out any debris at the bottom of the rigid tube, then switch back to get a reading. Since it just reads inches, you need to know how deep your tank is and what constitutes almost full. Testers particularly liked that there is not much that can go wrong with this unit, and it works without power.

Bottom Line:
Easy to install and use, the Tank Tender is a quality product with proven technology. It is a little pricey, but it will monitor multiple tanks with accuracy and without electricity. We recommend it.

Sealand TankWatch 1

Made by Sealand (Dometic Inc.), the TankWatch 1 is an extremely simple full-tank warning device. The kit includes one short float-type switch assembly, a rubber gasket, an indicator panel, and all necessary screws. The one-page, seven-step instructions are well written with clear diagrams.

Internally Mounted Tank-Level Sensors

To install the sensor, testers drilled a 1-inch hole and four screw holes in the top of the tank, inserted the float assembly, and screwed it down. We then hooked up two wires to the panel and two to 12-volts DC. And that was it.

The 1 5/8-inch by 2 1/2-inch indicator panel is simply a red LED labeled “Tank Full” that will illuminate when liquid is within 2 inches of the top of the tank. The TankWatch 1 draws only .016 amps when on, and it worked as intended during testing.

Bottom Line:

If you are on a budget, the no-frills Sealand TankWatch 1 is the cheapest and easy to install, but it offers only a tank-full warning.


The Wema system we evaluated came with an SHS-8 sensor and an HTG monitor. The sensor consists of a float mounted on an 8-inch vertical metal tube encased in a 1-inch protective metal pipe with a sender and threaded cap. This screws into a 2-inch threaded collar that attaches to the tank with a gasket and five screws. All construction is heavy-duty 316 stainless steel. The Wema is specified for only holding tanks.

The HTG is a 2 1/16-inch-wide round gauge with an analog needle that moves from empty to full as the tank is filled. Each device has a concise one-page instruction sheet with good diagrams. Installation and hook-up were easy, and testers found the readings to be accurate.

Bottom Line:
Of the float- switch type monitors, PS preferred the economical Wema. Its not much more than the bargain-basement TankWatch but offers more data. It gets the nod for PS Budget Buy.


Our Best Choice goes to Fireboy-Xintex for its new pneumatic probe with encapsulated micro-processor sender and a simple electronic level gauge.

With good quality, excellent accuracy, and a no-frills analog needle gauge, the Wema SHS-8 gets the Budget Buy.

For those needing a dual-tank monitor, the Hart Tank Tender is Recommended. It is pricey, but its accuracy and no-power appetite lead us to believe its worth the money.

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at darrellnicholson.com.