Accurate Tank Monitoring with a Victron Cerbo GX in a Camper Van or RV

This post details how to use our unique tank monitoring kit that we developed to work seamlessly with the Victron Energy Cerbo GX. It will work with both  “standard” rectangular tanks as well as irregularly shaped tanks such as those that install over a wheel or our custom molded grey water tank for a Promaster.

The Cerbo GX is an awesome product from Victron Energy that deserves its own, dedicated post (yeah, we’re working on it!). In short, it allows you to monitor your entire electrical system as well as things like tanks (hence this post), add other things like a Ruuvitag temperature/humidity/air pressure sensor and even add GPS tracking to your rig. You monitor and interact with the Cerbo GX with either the GX Touch 50 touch screen (shown below) or you can use a virtual version through VictronConnect (via Bluetooth). If your van/rig has an internet connection, the Cerbo GX can push it’s data to the “free for life” cloud service from Victron called VRM for advanced reporting and troubleshooting.

About The Sensor

We sourced these sensors from Safiery in Australia. They are “external pressure sensors” that work by measuring the pressure inside the tank – accurate up to 2%. It installs into a 1/2″ female NPT port on your tanks – most fresh water tanks include this fitting so no additional holes or fittings are required. On grey water tanks, this fitting can be added or you can potentially use another inlet that might already be on the tank such as a 1-1/2″ inlet with a reducer bushing to 1/2″ size. When you order our grey water tank, you can request extra spin welding fittings be added in the location of your choice. This is a great option for those needing a 1/2″ connection for this tank sensor kit.

1. Installing The Sensor

The sensor should be installed as close to the bottom of the tank as possible and requires a 1/2″ female NPT connection to the tank. These are very common on fresh water tanks for the output/suction line connection. Below is an illustration on how to “split” such a connection for both the tank sensor and for a suction hose to your water pump using the parts in our kit.

And the following photo shows a fresh water tank with the sensor assembly attached. While not technically necessary, we prefer to seal our water tank fittings with a marine sealant such as this one.

If you don’t need to use the supplied tee, you can install the sensor as shown below. You may have a tank with more than one 1/2″ female NPT fitting on the bottom or you might be monitoring a grey water tank that doesn’t have a pump attached.


2. Wiring the Tank Sensor to the Victron Energy GX Tank 140 and the Tank 140 to the Cerbo GX

Next you’ll route the sensor’s wire to the location of your Victron Energy GX Tank 140. Typically this is installed near your primary electrical system components – near the Cerbo GX. The wire from the sensor is about 6.5 feet (2 meters) and then there is a short USB cable built into the GX Tank that needs to be connected to one of the USB ports on the Cerbo GX. If you need additional USB connections on your Cerbo GX we have tested this USB hub with the Cerbo GX and it works well.

The wire from the sensor has three conductors (wires): red, yellow and black. The GX Tank includes a few plugs/connectors that accept these three wires. Locate one of those and press it into one of the connections on the bottom of the GX Tank. You’ll want to use one of the connections on the right that are labeled “24V – In – GND”. Inserting an empty connector like this will provide you the correct orientation (top vs. bottom).

Then you’ll need to insert the three wires from the sensor into the connector. It uses tension to hold the wires so you’ll use a small, flathead screwdriver to press down on the orange lock, insert the wire, and then release the orange lock. Below is a photo of the sensor wires secured to the connector (red on the left, yellow in the middle and black on the right). The photo also shows an “empty” connector for reference on which ones to use from the GX Tank box.

If necessary, you can extend the wires from the sensor. We recommend maintaining the color coding and using WAGO, lever lock style connectors for this as shown in the photo below.

3. Configuring Sensor on the Cerbo GX

The last step is the most complex and very important. You need to configure the Cerbo GX with your particular sensor. Basically, the sensors will output varying voltage depending on how much fluid is in the tank. During the configuration process, you’ll tell the Cerbo GX how it should interpret these voltage readings.

To begin press the “menu” button on the Cerbo GX. Note that you can interact with the Cerbo GX with either the Touch 50 screen or the remote console accessed through either the VictronConnect app or VRM portal. Navigate to the following menu screen: Menu -> Settings -> I/O (near the bottom of the menu) -> Analog inputs. On this screen you should see 4x GX Tank “inputs” labeled one through four. There will be a unique alpha numeric name for each GX Tank you have – you can use multiple GX tanks if necessary. Each GX Tank will support two of the Safiery style sensors. Typically you’ll only have GX Tank one and see a display something like the following.

If you don’t see this, try disconnecting your GX Tank device from the Cerbo GX (remove USB cable) and then reconnect. If that does not work, try rebooting the Cerbo GX with the GX Tank connected.

If you have more than one GX Tank, you will see “inputs” one through four for each of them.

Once you see the GX Tank inputs on this menu page, turn “on” inputs 3 and 4. These are the two inputs that are on the far right bottom of the product that was described earlier.

Next, navigate back to the first page of the menu by either going “back” several times or leaving the menu and then reopening it. You should see a tank monitor with a somewhat random name appearing on the first menu screen. Select that and and then press into “device” menu. From here you can select the “name” row and type in an appropriate label for this tank monitor (i.e.: Fresh Tank).

After naming the sensor, click back to the previous menu and then select the “setup” menu and scroll down in this menu until you see the “sensor value” row that we’ve highlighted in the image below.

The tank should be empty at this stage of the configuration. Note the sensor value when the tank is empty. In our screenshot, the sensor is sending 2.6 volts.

Next you should go through each of the settings shown in the screenshot below. Most of them should be fairly clear/obvious. Use voltage reading you noted in the last step for the “Sensor value when empty” and, for now, you can set the “Sensor value when full” to the same number – we’ll come back to that later.

You can probably guess the next step! Fill up the tank. Once it’s full, let the air pressure settle for a minute. Note, it’s important that your tank is well vented for this pressure sensor to work correctly. Open the menu on the Cerbo GX, navigate to the tanks sensor you’re configuring, open it’s “setup” screen and note the “Sensor voltage” now that the tank is full. Finally, enter this reading into the “Sensor value when full” setting.

For a “standard” (not irregularly shaped tank), this completes the configuration.

Now that you’ve configured the tank attributes and how the sensor “reads” when the tank is empty and full, the Cerbo GX can interpret how full your tank is using a built-in algorithm. You may wish to experiment with the “Averaging time” setting as you use your monitor. We find setting this to 1 to 3 seconds works well but you can adjust this to your preference.

With a configured tank monitor sensor, you can open the menu on your Cerbo GX at any time and see the “full” percentage and, if you select that row in the menu it will open another screen with more information on the tank including the estimated “remaining” value. That label – “remaining” – makes a lot of sense when applied to a fresh water tank – how much water does the algorithm in the Cerbo GX think is remaining in the tank. However, when you change the context to a grey water tank you have to switch your thinking a bit. The label is still called “remaining” but it really means how much is in that tank – how “full” is it.

You may wish to enable the following other settings/screens/pages on the Cerbo GX.

1. Menu -> Settings -> Display & language -> Show boat & motorhome overview. That “page” looks something like the following image. This is our test system and we’ve customized the logo.

2. Menu -> Settings -> Display & language -> Show tanks overview

Note: As a tank is filling up or emptying, the pressure in the tank can swing fairly dramatically. The better your tank venting is, the less these swings will be. But, in all cases, you will temporarily see inaccurate readings at times when you’re quickly filling or emptying  a tank. Typically this instability is caused by pressure in the tank building during filling/emptying and will only last for 30-60 seconds. So, if you see a reading that goes up or down quickly or seems “off”, simply check again in a few minutes. If your sensor sends a voltage that is much lower or higher than you’ve configured “Sensor voltage when tank is empty” or “Sensor voltage when tank is full”, the tank sensor will read as an “error”. Again, this is likely temporary swings in pressure caused by filling or emptying rapidly.

If you’re configuring the sensor with an irregularly shaped tank, you need to perform some extra configuration to account for the unique shape.

Basically, you have to fill up the tank in measured/known amounts and then add some additional configuration to the tank sensor “custom shape” menu.

To get started add two gallons of water into a 5 gallon bucket and mark that level on the bucket with a marker.

Now fill up your tank 2 gallons at a time. Each time you add two gallons pause and note the following: 1) the sensor voltage (shown in the tank monitor setup screen), 2) the percentage “level” (shown on the main menu page) and 3) what percentage full it should be.

To calculate that last one – the percentage full it should be – you divide the number of gallons currently in the tank by the total capacity of the tank. For example, if you have a 24 gallon tank capacity and you have poured in 3x, 2 gallon buckets of water (6 gallons total in the tank), the math would be 6 / 24 which equals .25 which means that the sensor should read 25% at that stage of the test. We recommend using this sample Google Sheet (download a copy for yourself).

Once you’ve gone through this exercise return to the Cerbo GX and select the tank sensor you’re configuring, choose “Setup” and then open “Custom shape” page. This is where you can bridge between what the sensor “thinks” about the percentage full (based on the sensor voltage) and what you learned by filling up the tank. This process can take a bit of trial and error. Below are the settings we used with our custom molded greywater tank for a Promaster vans so you can see an example. Each custom shaped tank will be different of course.

Some Other Important Notes

The Safiery sensors are made of food/water safe 304 stainless steel housing. The sensor can be installed outside the vehicle it is rated to IP 64 which is splash proof and dust proof but can’t be submerged. It can momentarily pass through water.

Every sensor is a little different in the voltages it will read. In other words, if you configure a sensor you received in your kit and replaced it some day with another sensor – from the same company (Safiery) and even from the same batch – they are going to produce different voltages under the same conditions. Your first sensor might read 1.4 volts when your tank is empty and the new one may very well read 2.1 volts on the same tank. This was alarming to us at first but the folks at Safiery assure us it’s normal with a sensor of this type. They wrote: “these are a (USA sourced) MEMs Chip bonded into the stainless housing. The bonding of the circular chip is never the same. A little more adhesive in certain places. This is the reason why. Once calibrated, they will behave with a very similar repeatability.” That last part is really the key. Once you configure your particular sensor it should be consistent.

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