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How to monitor a fuel tank level sensor with your RMS unit.

Having a remote start generator to keep your battery bank topped up is vital for any remote communications site that falls prey to extreme winter conditions. Knowing how much fuel is in your tank is also vital so that plans can be made to visit the site before a no fuel situation occurs.

The aim of this project is to show you how to monitor an after market fuel sending unit with your RMS-200 board and to display the fuel level with a nice graphical gas gauge. Please read the disclaimer at the bottom of this web page before proceeding.

The Fuel Tank

The fuel tank for this project is from Tidy Tanks. These portable tanks are made of steel and are certified for diesel or gasoline. They come in various sizes.

The Fuel Tank Level Sensor or Sending Unit

The fuel tank sending unit for this project is from Wema USA. These fuel tank sending units are made of stainless steel and are very accurate. A 2 inch to 1.5 inch Iron Pipe reducer was needed to allow the sending unit to screw into the fuel tank.

What you will need

One 10K resistor 1/4 watt. One Wema fuel sending unit that is the proper length to match your fuel tank. One 2 inch to 1.5 inch Iron Pipe reducer (if you are using a Tidy Tank). One RMS-200 board. Some wire (pink and black if possible).

Wiring Diagram

The RMS-200 comes with a 5 volt power output (blue connector). This is a good low power source for use with a fuel sending unit. Using the diagram below as a reference. Connect ground (pink) wire from the WEMA sending unit to the negative output of the power terminal and to the negative input of one of the RMS-200 voltmeters. Connect the sense (black) wire from the WEMA sending unit to the 10k resistor. Connect the resistor to the positive output of the power terminal. Connect a wire from the outside resistor lead to the positive input terminal of one of the RMS-200 voltmeters as shown below.

As a safety precaution, make sure the resistor is covered by some heat shrink tube or electrical tape as shown below.

Theory of Operation.

The fuel sending unit is like a variable resistor that measures from 30 to 240 Ohms depending on where the float is. Adding another resistor and measuring between the two resistors creates a voltage divider. When the sliding float is at the top of the fuel sender shaft, the RMS-200 voltmeter reads approximately .0200 of a volt. When the sliding float is at the bottom of the fuel sender shaft, the RMS-200 voltmeter reads approximately .1000 of a volt for a total range of .0800 of a volt. The voltage reading is not very intuitive as to the level of fuel in the tank. Fortunately, there is an easy way to display the voltage representation of fuel in the tank with a nice graphical gauge.

The graphical gauge above has 100 steps from empty to full. To get 100 steps in our voltage range we must divide our total range(.0800) by 100. Therefore, each step on the gauge = 0.000800 of a volt. So to translate the voltage reading to a fuel level on the gauge use the formula below.

PERCENT_FULL = (0.1000 - VOLTS) / 0.000800

To display the fuel reading on your RMS-200 board download the gas gauge files. Extract the zip file to your computer. Using FTP, upload the gas_gauge.html and the gas_gauge.xml files to your RMS-200 board at /data/custom/

In the gas_gauge.html file, edit the line that reads var VOLTS = (data.vms.vm1); to the voltmeter chosen to read the gas gauge (data.vms.vm1 = voltmeter 1, data.vms.vm2 = voltmeter 2) etc. You can also edit the line below that to change the formula to suit your fuel tank and sending unit.

In the RMS-200 web interface, navigate to the Setup page, then click on the Device Manager icon. Add a custom device. In the Name box put Gas Gauge, in the Path to CUSTOM file box put custom/gasgauge.html and click OK. A gas gauge icon will appear in the left navigation bar. Click on the new icon to see your gas gauge showing the fuel level of your gas tank.

Things of Note:

Due to the rounded top of the Tidy tanks the gas gauge will drop off quicker in the top quarter of the tank reading. You may have to experiment with the values to closer match your tank. The current going through the sending unit is less than 500 micro amps.

This concludes the remote fuel tank level sensor project. We can now remotely monitor our fuel levels and make sure our generators never run out of fuel. We have our RMS-200 board email us whenever the tank is below 1/4 full.


WARNING: Electrical projects are inherently dangerous, and even the most benign task can cause serious injury or death if not done properly.


You must be particularly careful when dealing with electricity in the vicinity of petroleum products such as gasoline – always use common sense. A spark near gas fumes can kill you! Any advice, guidance or other information provided on this website or within any of our publications cannot completely anticipate your situation. If you are at all unsure about completing any aspect of this or other project, consult a qualified electrical contractor to perform the service(s) for you.

ALWAYS follow electrical code requirements specific to your area, and before undertaking any electrical project, contact your local electrical authority and your insurance company to ensure that you comply with all policies, warranties, regulations and authorities concerning this work.


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