RV Holding Tank Sensors & False Tank Level Readings

The other day I received an email from a colleague at Rollin On TV (ROTV). RV Education 101 produces RV how-to segments for the ROTV show. The question was about RV holding tank sensors on the RV’s gray and black water holding tanks. More to the point, it was about why people sometimes get false tank level readings at the monitor panel in the RV. She thought this would make a good topic for the ROTV Facebook page. I agreed, this is a great topic, and I was surprised when I realized I never wrote an article on RV holding tank sensors article prior to this.

So today is the day I write about RV holding tank sensors and false tank level readings. In this post I will explain what the RV holding tank sensors are, how they work, why you sometimes get false tank sensor readings, and what you can do to correct the problem.

RV gray and black water holding tanks

For readers not familiar with the inner working of an RV let’s start with RV holding tanks in general. RVs come equipped with a gray water holding tank, a black water holding tank, and a fresh water holding tank. Our concern today is with the RV’s gray and black water holding tanks.

Basic RV Water System Chart

When you travel and camp in an RV the water that goes down the kitchen and bathroom drains, and the waste flushed down the toilet needs to be stored until it can be properly disposed of. You guessed it, it is stored in the gray and black water holding tanks. The gray tank is used for dirty water from the sinks and shower /tub, and the black tank is for waste from the toilet.

Monitor Panel

While you are camping you need to know how much content is in the holding tanks so you know when it is time to empty them. To accomplish this, the RV has a monitor panel. It displays the gray and black water tank levels in ¼, ½, ¾, and full tank increments. Note: Some tank sensors are installed at the 1/3, 2/3 and full levels.

RV 101 Tip: For the best results, you want the black water tank nearly full when you empty it, followed by a full gray water tank to help flush and rinse the RV sewer hose.

RV holding tank sensors

So the question is, how does the monitor panel in the RV know how full the fresh water, gray water and black water holding tanks are at any given time? The answer is, by way of RV tank sensors or tank probes installed in the fresh water, gray water, and black water holding tanks.

Most tank sensors are permanently installed through the side of the tanks. A sensor is installed at the ¼ tank, ½ tank, ¾ tank and full tank position on the side of each tank. Note: some tank sensors are installed at 1/3, 2/3 and full levels. Then the probes are wired to their respective positions at the monitor panel.

How do Holding Tank sensors Work?

There is a negative tank probe installed at the bottom of the tank in the empty position. The remaining tank sensors are positive. As the content in the tank rises and makes contact with one of the positive tank sensors it completes the circuit which in turn illuminates that tank level reading at the monitor panel.

In a perfect world, this is all we need to monitor and know when to empty our RV holding tanks, right? That is wishful thinking on our part. When I explained what the tank sensors are you may have noticed I substituted the word sensors with probes at various times. The reason is, when you install a tank sensor through the side of a holding tank a probe sticks out on the inside of the tank to register the tank level when the water reaches the probe inside the tank.

This is where all the “faulty reading” problems begin. For one minute think about what goes down the kitchen sink, bathroom sink, and shower drains, directly into the gray water holding tank. You have soap scum, grease, food particles, and hair just to name a few. The black water holding tank is even worse!

When a sensor inside the tank gets covered by food particles or toilet tissue it completes the circuit and relays the information to the monitor panel. It is possible for the holding tank to be empty but still register as 1/3, 2/3 or full because the tank sensor for a certain level in the tank is covered by debris. If the tank sensors are not displaying the proper amount of content in the tank you don’t know how much is in the tank until it is completely full. When the gray water tank is full, the the water backs up through the shower/tub drain. When the black water tank is full the wastewater come back up into the toilet. These are two problems all RV owners can do without.

black tank flush

False Tank Sensor Reading Solutions

There are a couple things we as owners can do to correct false tank sensor readings. We are fortunate to have a built-in black tank flush on our RV. I try to flush and rinse the black tank every time I empty it. The tank flush has a nozzle that sprays high pressure streams of water in every direction inside the tank. After I empty the tank, I flush the tank for several minutes. This cleans the tank sensors and helps prevent build-up on the sensors that can result in false tank readings. If your RV does not have a built-in tank flush you can have one installed, or you can use what is referred to as a tank wand to rinse and clean the inside of the black water tank.

tank cleaning wand

Tank wands are easy to use:

  • Connect your maintenance hose to the end of the tank wand.
  • Place the tank wand as far down the toilet as you can.
  • Turn the water valve on at the handle and move the wand all around as the high pressure nozzle rinses and cleans the tank and the tank sensors.
tank sensor cleaner

Another method to clean tank sensors is to add a tank sensor cleaner to the holding tank after the tank is emptied. Add enough water to completely cover the bottom of the holding tank and add the sensor cleaning treatment according to the instructions. As you drive or tow the RV, the cleaner moves around and cleans the sides of the tank and the tank sensors. The sensor cleaning treatment will continue to work as the tank levels rise too.

Every time I empty the gray water holding tank I add some dishwashing liquid down a sink drain and run enough water to cover the bottom of the tank. It suds up and helps clean the tank sensors, especially as the RV is moving and the water splashes on the sides of the tank. There are also gray water holding tank treatments with tank sensor cleaning additives that you can use.

RV 101 Tip: Every time you empty the holding tanks add enough water to completely cover the bottom of the holding tank and then add the holding tank treatments. This will keep the holding tank cleaner and help prevent clogs and other holding tank related problems.

Horst Miracle Probes

Horst Miracle Probes

If you want to solve the “faulty tank reading” problem once and for all there are replacement tank probes for the gray and black water tanks that will do the job. I am not affiliated with the product, but I have installed these replacement probes on holding tanks and they worked well. The product is called Horst Miracle Probes and they are available on Amazon through Valterra Products. They replace the existing tank sensors, but make sure you can access the existing probes on your holding tanks prior to starting a project like this. All you do is remove the old probes and install the new Horst Miracle Probes in their place. If the sensors on your holding tanks are the type that cannot be removed, you need to drill new 3/8 inch holes next to the existing sensors to install the new probes. Connect the original wiring to each new probe as you replace them. The Horst Miracle Probes are covered by a Teflon shield to prevent any debris from getting caught on the sensor itself. This solves the false tank sensor readings for good.

There you have it, a quick primer on RV holding tank sensors and false readings. If you want to learn more about using and maintaining your RV visit RV Online Training to see all of our RV training products we offer.

Mark J. Polk

RV Education 101

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