If you have a boat, motor home, truck or tractor that burns diesel fuel, you must know the level of diesel in your tank so you don't run out of fuel unexpectedly. You can check the tank level in several ways, including by using a fuel gauge that monitors levels with a float, a gauge that operates on air pressure (such as a Tank Tender) or a simple measuring stick.
Locate the diesel fuel gauge on your vehicle. The gauge is usually near the diesel tank or near the driving console.
Read the diesel fuel gauge, which will show the fuel marker somewhere between full, three-quarters full, half full, one-quarter full and empty.
Calculate the exact level of diesel in the tank using the reading on the gauge and the tank's capacity. For example, if the gauge shows that the diesel tank is one-quarter full and the diesel tank capacity is 30 gallons, multiply 0.25 times 30. The tank is holding approximately 7 1/2 gallons when it is one-quarter full.
Select the tank for in which you wish to take a reading with a monitor called a "Tank Tender." Tank tenders usually have a monitor that will show the reading for multiple tanks, including diesel or water tanks.
Pump air into the hose leading from the tender monitor to the diesel tank by slowly pushing the plastic primer button leading to the appropriate tank up and down three to four times. Do not over-pump the primer button.
Wait and observe the reading on the tender's gauge when the needle settles. The needle will show you the number of inches of diesel in the tank.
Calculate the exact level of diesel in the tank using the reading on the tender and the tank's height in inches. For example, if the tender reading shows 10 inches of diesel and the tank is 20 inches tall, then your tank is about half full. But note that not all tanks are a symmetrical, rectangular shape.
Make sure your diesel tank is on a level or stable surface.
Open the cap on the diesel tank. If the cap does not have an attachment to the tank, place it beside the tank so that you will not lose it.
Pull out the metallic ruler on a measuring tape several feet, depending how tall the diesel tank is. Use a stiff white or yellow metal measuring tape.
Clean the metallic ruler with a clean paper towel or rag.
Place the measuring tape into the tank with the foot of the metallic ruler facing downwards, until it hits the bottom of the diesel tank.
Pull the measuring tape out of the tank. Find the place on the metallic ruler where the diesel line ends. The metallic ruler will be wet with fuel from its tip to a certain measurement, such as 3 feet. This measurement is not meaningful, however, until you compare it to the tank's capacity.
Calculate the level of fuel in the tank by dividing the measurement taken on the metallic ruler by the total height of the tank. For example, if the tank is 12 feet tall, and you have measured 3 feet of diesel in the tank with the measuring tape, divide 3 by 12. You'll get .25, or one quarter. The tank is one-quarter full.
Tenders for diesel tanks tend to be installed and used in boats or RVs. If your tank's shape is not symmetrical, you should make an accompanying chart to help you monitor your tank levels. You will need to run through at least two tanks of diesel, monitoring the amount of diesel that you put in the tank and taking periodic readings to make this chart.