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How Do RV Holding Tank Sensors Work?

How Do RV Holding Tank Sensors Work?

There are some aspects of RVing that most of us don’t think about on a daily basis. They’re those things that, when they serve us well, are barely noticeable. But when they fail, well – let’s just say they can be noticed in a big way – case in point, RV holding tank sensors.

When your RV holding tank sensors are working well, you go about your daily business and rarely pay them a thought. Oh, you might check your tank levels by looking at your monitor panel to see if they need dumping sometime soon, but if they appear to be barely half full, you happily move along to the next thing on your list of (hopefully fun) things to do.

But if you check your holding tanks, you see a level of barely half full, and later you find yourself showering while standing in a sea of — well — “Disgusting Smelly Muck” is the technical term — then you’ve run into a screaming problem with some very important, hidden-away RV parts known as RV holding tank sensors.

You really don’t want to have to guess whether your gray, black, and freshwater tanks are nearing the full (or empty) mark. You certainly don’t want to accidentally run out of freshwater. And because we don’t want you to encounter that puddle in the shower, today’s post is all about – you guessed it – RV holding tank sensors.

This may not be the reading material you had in mind for today, but trust us when we tell you that if your RV has holding tanks and you use them without familiarizing yourself with this information, the day will likely come when you wish you had. So, let’s get this over with dive into this fascinating post!

How Do RV Holding Tank Sensors Work?

In general, RV holding tank sensors sit in or on the sidewall of the tank at incremental levels (¼, ½, ¾, and Full, or ⅓,⅔, and Full). When a tank’s contents hit each mark, theoretically – (we’ll repeat that loudly for emphasis) – THEORETICALLY, you’ll see the level of the contents of each tank proudly displayed with a light on the monitor panel on your RV wall, at the appropriate level.

Holding tank sensors showing empty tank

When you’ve just emptied your tanks, you want the display to indicate that the tank is empty, as shown in this photo. If you know the tank is empty and your display indicates otherwise, you’ve likely got an issue with your holding tank sensors.

So, sensors are placed in increments up the side of a holding tank (inside or outside – more on that later), and these sensors are connected to the coach monitor panel.

That’s how RV holding tank sensors typically work, but there are several different types of tank sensors, and some work better than others. Let’s take a look…

Types Of RV Holding Tank Sensors

There are essentially three types of holding tank sensors available on the market: through-wall/probe sensors, external electrical resistance sensors, and external acoustic sensors. Each works in a different manner to deliver the information you need – how full your fresh, gray, and black water tanks are at any given time. And as you might imagine, each type of sensor has its pros and cons.

Through-Wall/Probe Sensors

Probe sensors are usually stainless steel units that are mounted in a rubber bushing installed through the wall of the holding tank. Some probe sensors use screws that are welded to the tank during manufacturing. Either way, the sensors installed inside the tank conduct electricity when they become submerged in liquid. The sensors are wired to the monitor panel, which allows you to see the readings (E, ¼, ½, ¾, F or E, ⅓, ⅔, F) when the liquid in the tank completes the circuit with that sensor, illuminating the corresponding light on the panel.

These are the most common type of RV holding tank sensors used on the market. They’ve been around for a very long time and have a number of pros and cons:


The main advantages of probe sensors are that they’re inexpensive and, because there are no moving parts, are durable and last a long time. Since liquids are conductive, as long as there are contents in an RV holding tank, the process should work, right? Well – yes, with some exceptions, and that brings us to the main disadvantage of probe sensors.


The biggest disadvantage of probe sensors applies to their use in black and gray tanks, and that is gunk.

If the probe inside the tank gets gunked up with “stuff” (stop us if we’re getting too technical with our terminology here ????), then chances are good that the conductivity will be interrupted, and you won’t be seeing accurate readings on your monitor display. And frankly, this happens a lot. It can even happen when the walls of the tank get – well – gunky or caked with struvite, a crystal formed by chemical reactions between components of wastewater over time.

Toilet paper, grease, food particles, or even hardened…umm…other stuff can eventually prohibit internal probe sensors from giving accurate readings. They need to be cleaned regularly and thoroughly in order to continue to work properly.

Sensors can be subject to corrosion over time as well, and this can also cause false readings. In addition to not reading out when the level in the tank reaches the appropriate sensor, gunk built up on the sensor and tank walls can complete the electrical circuit and cause a single light to light up on your monitor panel. That means you may push the button to check your tank level and see both the “E” (empty) and ½ or ¾ lights lit. Which clearly doesn’t make sense.

To help prevent material in your holding tanks from getting stuck to the sensors, some through-wall probes include a Teflon “hood” or “shield.” This “shield” prevents toilet paper and other tank contents from sticking to the probe itself, helping it to properly sense tank contents and not show false level readings.

A "hooded" rv tank sensor probe can help prevent problems with gunk

A “hooded” tank sensor probe like this Horst Miracle Probe can help prevent tank contents from clinging and causing false readings.

Through-wall sensors also mean that there are now holes in the side of your holding tanks which could, over time, develop leaks.

Electrical Resistance Sensors

Electrical resistance sensors are different from probes because they are mounted on the outside of the holding tanks and determine the amount of liquid/contents in the tank by detecting a change in electrical properties. They’re able to do this through the wall of the tank itself, which means there’s nothing that sticks into the tank… and thus nothing to get gummed up.

In most common installations, several small sensor pads are mounted at the same ¼, ½, ¾, and Full (or ⅓, ⅔, and Full) levels. When the contents of the tank reach the height where each sensor is mounted, it detects the change in electrical properties caused by the presence of the liquid on the other side… and signals the monitor panel that that level light should be lit.


The advantage of electrical resistance sensors is pretty obvious: there’s no part of the system that penetrates through the tank, so not only is there no internal probe or sensor of any type to get gunked up, but there’s also no risk of a leak.


On the downside, electrical resistance sensors are more expensive. With most versions, they aren’t providing any more granular detail about the tank level since they’re mounted at the same incremental spots. And badly-maintained tanks can still cause problems: “gunk” left on the walls from lack of tank cleaning or from being left without dumping for a long time could leave a coating that wicks water up the wall, causing the sensors to think the tank is more full than it is. The build-up of struvite can do the same thing.

Struvite can interfere with holding tank sensors

Struvite crystals build up on the walls of RV tanks over time, no matter how meticulously they may be cleaned. Struvite can interfere with the ability of holding tank sensors to properly sense tank levels.

Acoustic/Ultrasonic Holding Tank Sensors

Though we don’t know of any North American RV manufacturer using this type of system, acoustic/ultrasonic sensors offer many advantages. This type of sensor is often used in industrial applications where tank contents may cause problems with other systems and/or where greater accuracy is important.

Acoustic/ultrasonic holding tank sensors are typically installed inside the top of the tank, aiming an acoustic emitter down at the surface of any liquid/material contained there. Once calibrated to know what “empty” is, these units can continuously monitor the level of a tank’s contents, regardless of what it is (fresh, black, gray, liquid propane).


The primary advantage these sensors have over the other options is that they can report an exact percentage for any current level of tank contents. This eliminates any guesswork related to exactly how full (or empty) your tank currently is.


Acoustic/ultrasonic holding tank sensors are more expensive than other options. They also require penetration through the tank for the emitter to be able to “see” the surface of the tank contents (though being at the top of the tank means it’s less likely to get gummed up or blocked).

How Do RV Manufacturers Choose a Holding Tank Sensor?

Honestly, the answer to this question is usually cost.  Just as a $20K car doesn’t have all of the same features/options that a $200K car does, the same is true for RVs. At lower price points, there just isn’t the same profit margin available for the use of more expensive components, without affecting the end sale price of the RV.

Can RV Holding Tank Sensors Give False Readings?

Yes! It’s not unusual for RV holding tank sensors to give false readings under certain circumstances. In fact, it’s a very common complaint among RV owners.

The most common complaint is that immediately after the tanks are dumped the RV owner finds that the tank monitor continues to read as “full” (or ½ or ¾ full). Or that a single light in the series of tank level indicators stays lit, regardless of what level the tank is at. Knowing that the tanks were just emptied and not refilled, this is a clear indication that the tank sensors are giving false readings.

As we mentioned earlier, this is typically due to sludge that has built up inside the tank, “gunk” or toilet paper caught up around the sensors, or the accumulation of struvite crystals on the walls over time. If you’re interested in more information about struvite and how to deal with it, we explain it and share our experience with struvite in this video:

The long and short of it is that yes, RV holding tank sensors can give false readings, and it happens all the time.

Keeping the sensors clean is one way to deal with this issue, of course, and upgrading to a different type of sensor is another option. However, it’s important to note that no RV holding tank sensor is 100% flawless, especially if tank maintenance is not consistent.

So is there another option?

A Better RV Holding Tank Sensor: SeeLevel Sensors

SeeLevel sensors work differently, using electronics to measure the capacitance difference that’s caused by the change in the level of the contents of your tank. They are the most like the electrical resistance sensors because the entire system is mounted on the outside of the tank, which better addresses the problem of sludge build-up causing false readings (but doesn’t eliminate it completely – more on that in a minute).

SeeLevel 709P3W1003 Tank Monitor System
  • Package Dimensions: 46.228 H x 9.906 L x 10.16 W (centimeters)
  • Package Weight: 1.0 pounds

How They Work

SeeLevel sensors use adhesive sensor strips with circuits embedded in repeating sections, allowing them to read the liquid level inside the tank and send the report to a display. Tank levels are reported in roughly +/- 5-10% increments (resolution is limited by tank shape and height), offering more accuracy and more detail than other typical types of RV holding tank sensors. Because of the repeating sections, the sensors can be trimmed to fit various tank heights, making them, useable on a wide range of tanks.


SeeLevel sensors are easy to install since the entire process takes place on the outside of the tank. The system uses wiring (2 wires), making this a relatively simple DIY project. Readings are more detailed, as the externally-mounted sensors have multiple sections that read tank content levels and provide greater accuracy.


One of the downsides of the SeeLevel holding tank monitoring system is the expense. The system costs more, but you’re paying for greater accuracy and detail, and that’s worth something.

Personally, we don’t really see this as a con, but it’s important to note that while the SeeLevel system doesn’t allow for false readings as readily as other systems do because the sensors aren’t susceptible to the effects of tank sludge, SeeLevel’s manufacturer, Garnet Instruments, notes that extreme sludge build-up will indeed produce inaccurate tank readings (the tank will read full even when it’s empty), so it remains important to tend to tank maintenance to prevent inaccurate readings.

Our original equipment, resistance-type sensors that came installed on our ’05 Newmar began acting up a few years ago, which is why we upgraded our system to use SeeLevel sensors. The increased accuracy of the system has been a real help, especially when spending significant time without hookups. We now know that we can rely on our tank level readings and plan our length of stay accordingly.

How Do You Clean Your RV Holding Tank Sensors?

There are essentially four ways to clean your RV holding tank sensors.

You can regularly use a tank cleaning wand like this one when you dump your tanks.

Valterra A01-0184VP Master Blaster RV Tank Cleaning Wand with Power Nozzle , White
  • Package Dimensions: 27 L x 2 H x 3.75 W (inches).Fit Type: Universal Fit
  • Package Weight: 0.5 pounds

This is generally something you’d use to clean your black tank (since there’s not usually a way to insert a wand like this into your gray tank). The wand attaches to your hose and sprays high-pressure water in a circular motion to clean the sidewalls of your tanks.

Another way to keep both your black and gray tanks clean is to add about a cup of dishwasher detergent to the tanks. Add water to the tanks and allow the detergent mixture to slosh around while you drive your RV.

A third way to clean your RV holding tank sensors is to use an enzymatic tank cleaner like Happy Campers.

Happy Campers Extreme RV Holding Tank Cleaner
  • Super cleans RV holding tanks
  • Sensors: Restore poor working sensors

And finally, you can have your RV holding tanks power washed by a professional. Even though we keep our holding tanks clean and well maintained, we recently had to have ours power washed due to the build-up of struvite over the past couple of decades.

If you’d like more details on any or all of these methods of cleaning your RV holding tank sensors, check out our post, 4 Ways to Clean Your RV Tank Sensors.

How Do You Reset Your RV Holding Tank Sensors?

Calibrating your holding tank sensors is not something that needs to be done often, but if you haven’t done it in a very long time, or if you’ve just purchased a new (or new-to-you RV), you may want to undertake this project. It’s not complicated, but it will require a little time and a small Phillips head screwdriver.

The best way for us to explain the process of resetting or recalibrating your RV holding tank sensors is to post this video of us calibrating our sensors, which you can follow step-by-step:

Just be aware, not all monitor panels can be calibrated this way, and some can’t be calibrated at all. Typically, systems that use through-wall sensors don’t have any means of calibrating the monitor, and issues with inaccurate displays are most commonly a result of clogged/gunked-up sensors.


RV holding tank sensors are pretty important, despite being silent and hidden. And while they may not be a part of your RV that you think about regularly, they need to be working properly or you could find yourself showering one day, up to your ankles in nasty gray water!

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Tuesday 4th of April 2023

Good article. I am using the "hooded" internal sensor that you have a photo of in a urine collection tank I built for my disabled wife (night time collection). Those worked perfectly for a couple weeks, then started to fail and now they work randomly. The research I did revealed that in an acidic atmosphere (not acidic liquid), the copper on those sensors, as well as other metals, will corrode, in the case of copper, copper acetate is formed. This corrosion prevents the sensors from working properly. Note that this tank collects urine only, there is no "gunk" and I clean the tank daily. I have given up on the internal sensors and am moving to the external, capacitance type, which are around $15 each. When dealing with RVs, the extra cost of external sensors compared to their accuracy and disaster prevention are well worth the expense. As a software engineer, I plan to build my own tank level system for all of my tanks using these sensors and an SBC - the parts will substantially less than a SeeLevel system and I really don't need 65% v 68% accuracy. The sonar based sensor would be the best, if I could get to the top of the tank and the sensor was water proof.


Thursday 12th of August 2021

How do you add the SeeLevel system to tanks hidden under the floor in a motorhome?


Thursday 12th of August 2021

Good question, Scott. It can be a bit of a challenge! In our case, we have a basement compartment with a pegboard for hanging tools, etc... and right behind it are the sides of all three tanks. So it was a (relatively) simple matter to remove the pegboard for access. But every RV is different. Sometimes you can get access from behind the water compartment. Sometimes you can access them from below. When in doubt, you can post in your brand's forum over on and ask if anyone's found how to access them for your year and model. There's usually someone who's had to do it for any number of reasons.

Mark Bodette

Thursday 12th of August 2021

Another great article, we will have to look this guy up when we’re in the Lake Havasu area (sometime this winter). Question - did you ever do a video on installing the SeeLevel system?


Thursday 12th of August 2021

Hi Mark. Thanks! Glad this was helpful. Hydro Clean did a great job blasting out the tanks, we know you'll be happy with the results. Mike's a great guy! And good question about the SeeLevel system... we have it "in the can" and it's on the queue to get edited, so hopefully we'll get our butts in gear and get it out. ???? It's a great system and we're loving it! So nice to actually know how full/empty your tanks are.

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