Quck answer
To calculate tank level using pressure, you need to know the specific gravity of the liquid in the tank and the height of the liquid column. First, measure the pressure at the bottom of the tank using a pressure sensor. Then, convert the pressure reading to inches or millimeters of water column using a conversion factor. Next, divide the pressure reading by the specific gravity of the liquid to get the height of the liquid column in inches or millimeters. Finally, subtract the height of the liquid column from the total tank height to get the tank level.
Measuring hydrostatic or liquid leg head pressure is a precise and dependable way to indicate the liquid level within a tank. The method is based on the principle that water or any other liquid with a known specific gravity exerts a given pressure at the bottom that increases as the level goes up. Although this method is typically used for water levels, it can be adjusted to measure levels of other liquids, as long as their specific gravity is known, and in pressurized tanks where static or blanket pressure is known.
Vented Water Tank
Step 1
Define the purpose. In this example, a plastic water storage tank is being used to store tap water for emergency purposes. The tank is 96 inches tall and elevated 6 inches off the ground on a pad to allow a plastic pipe to emerge from its bottom. An accurate zero-to-10-pound-per-square-inch (psi) pressure gauge reads exactly 3 psi. Using this information, you can calculate the level.
Step 2
Convert the 3-psi pressure gauge reading to inches-of-water (inH2O) pressure. Since 27.71 inches of water height equals 1 psi, then the pressure is 3 psi x 27.71-inH2O/psi = 83.13 inH2O.
Step 3
Calculate the exact water level based on its specific gravity of 1.0. Take the converted pressure term, 83.13 inH2O/ 1.0-S.G. = 83.13 inches.
Higher Specific Gravity (1.1 S.G.)
Step 1
Determine the calculation for higher specific gravity levels. In this case, the same tank still reads 3 psi, but it contains a brine solution with a specific gravity of 1.10. You can calculate the level from the pressure value and additional information.
Step 2
Convert the 3-psi pressure gauge reading to inches-of-water (inH2O) pressure. Since 27.71 inches of water height equals 1 psi, then the pressure is 3 psi X 27.71 inH2O/psi = 83.13 inH2O.
Step 3
Adjust the pressure reading to account for the higher specific gravity. Dividing the 83.13 inH2O static pressure reading by the specific gravity of 1.1 results in a tank level of 75.57 inches. The lower calculated level is due to the greater weight of the liquid per inch of height exerting the same pressure as a greater level of lighter liquid.
Pressurized Tank
Step 1
Determine the level application. In this example, a pressurized water tank with a specific gravity of 1.02 has two accurate zero-to-10-psi pressure gauges, one at the top and one at the bottom. If the top gauge reads 4 psi, and the bottom gauge reads 7.5 psi, you can calculate the level from the pressure difference.
Step 2
Calculate the pressure on the bottom gauge solely due to the water level. Since the top gauge has no water above it, its reading is from static tank pressure only. Subtracting the top gauge reading of 4 psi from the bottom reading of 7.5 psi results in a net pressure due to level only of 3.5 psi.
Step 3
Convert the 3.5-psi pressure gauge reading to inches-of-water (inH2O) pressure. Since 27.71 inches of water height equals 1 psi, then the pressure is 3.5 psi x 27.71-inH2O/psi = 96.985 inH2O.
Fourth Step
To determine the real level of the tank, divide the net head pressure by the specific gravity. By plugging in the given values, 96.985 inH2O/1.02 SG = 95.08 inches.
FAQ
1. How does pressure relate to tank level?
Pressure is directly related to tank level. As the tank level increases, the pressure at the bottom of the tank also increases. This is because the weight of the liquid in the tank creates a force that exerts pressure on the bottom of the tank.
2. What is the formula to calculate tank level using pressure?
The formula to calculate tank level using pressure is: Tank Level = (Pressure – Atmospheric Pressure) / (Liquid Density * Acceleration due to Gravity * Height of Tank). This formula takes into account the pressure difference between the bottom of the tank and the atmospheric pressure, as well as the liquid density, acceleration due to gravity, and the height of the tank.
3. How can atmospheric pressure affect the calculation of tank level?
Atmospheric pressure plays a role in the calculation of tank level because it affects the pressure at the bottom of the tank. If the atmospheric pressure is high, the pressure at the bottom of the tank will also be high, resulting in a higher calculated tank level. Conversely, if the atmospheric pressure is low, the calculated tank level will be lower.
4. What is the significance of liquid density in tank level calculation?
Liquid density is an important factor in tank level calculation because it determines how much pressure the liquid exerts on the bottom of the tank. Liquids with higher density will exert more pressure, resulting in a higher calculated tank level. Conversely, liquids with lower density will exert less pressure, leading to a lower calculated tank level.
5. How does the height of the tank affect the tank level calculation?
The height of the tank is a crucial parameter in tank level calculation because it represents the distance from the bottom of the tank to the liquid surface. The greater the height of the tank, the higher the calculated tank level will be. Conversely, a shorter tank height will result in a lower calculated tank level.
6. What is the role of acceleration due to gravity in tank level calculation?
Acceleration due to gravity is a factor in tank level calculation because it affects the weight of the liquid in the tank. The higher the acceleration due to gravity, the greater the weight of the liquid, resulting in a higher calculated tank level. On the other hand, a lower acceleration due to gravity will lead to a lower calculated tank level.
7. Are there any limitations to using pressure to calculate tank level?
Yes, there are some limitations to using pressure to calculate tank level. The accuracy of the calculation depends on various factors such as the accuracy of pressure measurement, the uniformity of liquid density, and the absence of any other forces affecting the pressure in the tank. Additionally, the calculation assumes a constant liquid density, which may not always be the case. It is important to consider these limitations and validate the results using other methods if needed.