RV holding tanks have sensors installed on them in order to monitor the level of water or waste in the tank. The two most prevalent sensor styles are probe and capacitive. The probe style sensors are the most common and are most often installed at the factory as original equipment. Capacitive sensors can also be included as original equipment  they tend to be on only high-end RVs. However, this style of sensor can be installed on nearly any RV with an aftermarket kit.

Probe Sensors

The probe sensors are essentially a piece of metal that is exposed on the inside of fresh, gray and black water holding tanks. They usually are installed in a set of either four or five sensors with wiring that runs directly to a control panel on the inside of the RV.

RV control panel with a four sensor setup. Hand pressing the button shows the tank is empty
RV control panel with a four sensor setup

In the set of four sensors, the controller will show empty, one-third full, two-thirds full and full. In the set of five sensors the indicators will be empty, one-fourth full, one-half full, three-fourths full and full. Both configurations require one sensor to be used as a ground that must be at least ten inches from any of the sensors used for measuring the liquid in the tank. 

How Probe Sensors Work

Finger pressing button to show RV gray water holding tank level
RV control panel showing gray water holding tank level

How does this probe sensor work? A low voltage signal that is measuring a specific range of resistance is being sent to the sensor by the control board. As the liquid level rises and touches the metal sensor inside the tank along with the ground sensor, the circuit is closed.

This causes the control panel to illuminate the light bulb corresponding to the level of the wire the liquid is in contact with. 

Different Types of Probe Sensors

Different styles of RV probe sensors
Different styles of RV probe sensors

There are two styles of probes: one is a flat piece of metal that makes contact with the liquid, the other is a small metal rod. The flat style looks similar to a metal button and extends roughly a quarter of an inch into the tank, with  a rubber grommet behind it. The rod style is about an inch to an inch and a half long and are either exposed metal or has a plastic hood covering half of the sensor. These hood-style sensors must be oriented with the hood covering the top half of the sensor. If the sensor is installed incorrectly, it will most likely misread. One caveat: it can be difficult to make sure that the sensor is oriented correctly since you cannot see through the tank wall. 

Capacitive Sensors 

Capacitive style RV holding tank sensor
Capacitive style RV holding tank sensor

Capacitive sensors are more expensive than probe sensors–this is apparent when you notice which RVs they are installed in at the factory and when you see the cost of an aftermarket kit. Granted, capacitive sensors show a far more accurate liquid level than probe sensors do. Also, they are installed  on the outside of the holding tank with adhesive, so there are no holes in the holding tank that may potentially leak. Unfortunately, this style of sensor cannot be installed on a metal holding tank as the sensor will become grounded and will not show the liquid level.

How Capacitive Sensors Work

Capacitive RV sensor operation
Capacitive RV sensor operation

How does a capacitive sensor work?  As the liquid level rises, the sensor uses variable capacitors along the adhesive backed sensing strip that send a signal to a microchip which then relays the message through two wires to the controller in the RV. The controller in the RV will show a percentage of how much liquid is in the tank from 0 to 100 percent. Because they are on the outside of the tank, these sensors will be less likely to have mis readings. If the sensor is misreading, the only way to correct the misreading sensor is to deep clean the holding tank. 

Additional Types of Holding Tank Sensors

There are other aftermarket solutions for reading the level of liquid in RV holding tanks; these are typically pulled from boat holding tanks. They include but are not limited to: float style, sonar style and magnet reed switch style. As they are not specifically for RVs we will not go into detail about them in this article. 

If you are curious about how to remove and replace holding tank sensors, check out this article!

Got a question you’d like answered by our certified RV technicians? Submit your question here!