"Sounding" a tank is how oil levels were gauged before meters and instruments took over the task of determining the level of the tank's contents. You can use the same method for any tank lacking a gauge--for example, your home heating system--if you have a measuring tape and a calculator. First, you need figure out how many gallons of oil are represented by each inch of the tank's height. Then, by measuring how many inches of oil are in the tank, you can determine how many gallons of oil remain in the tank.
Calculate the size of the oil tank by measuring the dimensions of the tank with a measuring tape, then multiplying the length of the tank by the height of the tank. Multiply the result by the width of the tank.
For example, if your tank is 36 inches by 22 inches and 18 inches deep, multiply 36 by 22, then multiply the result by 18. The result is 14,256, the number of cubic inches in your tank. Divide by 1,728, the number of cubic inches in a cubic foot: 14,256 / 1,728 = 8.25. This (8.25) is the number of cubic feet in the tank.
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Divide the capacity of the tank in cubic inches by the height of the tank. Your goal is to determine how many gallons of oil are represented by the depth of the oil in inches. By dividing the tank's capacity by its height, you determine how many cubic inches of oil are represented by each inch of the tank's height: 14,256 / 18 = 792 cubic inches of capacity per inch of height.
Divide the capacity (in cubic inches) by 231 (the number of cubic inches in a gallon) to determine the total capacity of the tank, in this case 14,256 / 231 = 61.7143 gallons. Divide that by the height of the tank, 18 inches. In this case: 61.7143 / 18 = 3.4286, the number of gallons each inch of the tank’s height represents.
To measure the amount of oil in the tank, extend a measuring tape down into the tank. As soon as the end of the tape touches the bottom of the tank, bring the tape up out of the tank by lifting the tape measure, not by retracting it. Look at the end of the tape measure and find where the oil stops on the face of the tape. This is the depth of the oil.
While measuring the amount of oil in the tank, the tank must be open. This means that flammable vapors will be present in the area. Open flames, smoking and spark-making equipment should be excluded from the area while measurements are being made.