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Measuring liquid level in a tank using a dp sensor

Differential pressure sensors can be used to measure liquid level in a tank. Here we will explain how a DP sensor is used to measure liquid level.

  • Differential pressure sensors can be used to measure liquid level in tanks.
  • They measure the pressure exerted by the weight of the liquid at a point below the surface.
  • This pressure measurement needs to account for any pressure applied to the liquid surface.
    • Vented tanks: consider atmospheric pressure.
    • Sealed tanks: consider pressure of inert gas, vacuum, or emitted vapor.
  • To isolate the liquid pressure, subtract the pressure at the top of the tank.
  • Differential pressure sensors with the low side port connected to the top of a sealed tank, or open to atmosphere if a vented tank, and the high side port to the bottom of the tank can achieve this.
  • Pressure applied to the top is transmitted to both sides of the sensor, cancelling each other out.
  • The remaining pressure difference is due solely to the weight of the liquid.
  • Liquid transfer from the tank to the low side DP connection can occur causing a calibration shift which can be rectified by draining or zero tare techniques
  • Wet leg and remote seal techniques avoid condensate issues in the low side connection.
    • Wet leg: low side filled with reference liquid, compensate for its pressure in calculations.
    • Remote seal: uses liquid-filled capillaries to separate tank liquid from reference liquid.
  • When using a wet leg or remote seal, factor in the constant pressure from the reference liquid for accurate level measurement.
Differential pressure sensor products

There are many technologies that can be utilised to measure the liquid level in a tank, and one of them is to use a differential pressure sensor. Since a quantity of liquid exerts a force on to the bottom of a tank due to the weight of the fluid, it is possible to measure this force by way of the pressure generated in the fluid at a point below the surface of the fluid.

When measuring the pressure below the liquid surface, you are not only measuring the pressure due to the weight of the fluid, but also the pressure applied to the liquid surface as well. In a vented storage tank this would be atmospheric pressure. In a sealed tank it will depend on the liquid contents and its purpose, for what pressure is maintained at the surface. It could be a vacuum, inert gas at a fixed pressure, or a vapour emitted by the liquid contents with a varying pressure.

In order to measure the actual height of the fluid it is necessary to subtract the pressure of any air or gas at the top of the tank. Since a differential pressure sensor measures the difference in pressure between its two pressure connection ports, it is possible to measure the liquid level head pressure directly, if the negative side is connected to the top of tank, and the positive side is connected to the bottom of the tank. Any pressure applied to the top of the tank will pass directly to the negative side of the dp sensor, and will also pass to the positive side through the liquid, and therefore both will cancel each other out leaving only the pressure difference due to the weight of the liquid in the tank.

If the tank is always vented, it is not necessary to connect the low side of the dp sensor to the top of the tank, since atmospheric pressure difference between the top and bottom of the tank will be effectively zero compared to liquid level pressure, and therefore it is simpler to vent the dp sensors low side directly without any pipe connections.

Due to evaporation and condensation of the liquid inside the tank it is possible for liquid to be transferred to the connection from the top of the tank to the low side of the dp sensor. This will cause a calibration shift, which can either be removed using drain valve or compensated out using a zero tare on the output signal. To ensure accurate measurement, regular maintenance is required to address a potential calibration drift.

Alternatively, a calibration shift caused by condensate filling up in the low side connection pipework, can be avoided altogether by creating a so called ‘wet leg’ or adding a remote seal.

The ‘wet leg’ technique requires the whole length of the low side to be completely filled with a suitable reference liquid, so that the height is fixed.

If there are concerns about cross contamination of the liquids, then a remote seal can be used instead, which involves using liquid filled capillaries connected at one end to each side of the dp sensor, and the other ends are connected to an isolation diaphragm flange which is mounted directly to the tank.

In order to calculate the level of the fluid in the tank using this technique, it is necessary to compensate for the level of the ‘wet leg’, as well as the pressure on the surface of the tank contents. Since the pressure due to the wet leg is acting on the low side only, it reads as a negative pressure by the dp sensor. Since the ‘wet leg’ pressure is fixed it can simply be added as a positive value to the dp sensor reading to obtain the head pressure of the fluid in the tank.

Differential pressure sensor products

Here are the steps to follow to measuring liquid level with a differential pressure sensor:

  1. Select a Differential Pressure (DP) Sensor: Choose a DP sensor with a suitable pressure range for the expected liquid height and density in your tank.
  2. Install Pressure Ports:
    • High pressure port: Connect this port to the bottom of the tank. This will directly measure the total pressure exerted by the liquid.
    • Low pressure port: Connect this port to the top of a sealed tank, or leave open to surrounding air pressure if it is a vented tank. This will ensure the pressure acting on the liquid surface (atmospheric pressure for vented tanks, or specific gas pressure for sealed tanks) is ignored by the dp sensor measurement output.
  3. Self Corrects for Reference Pressure: The DP sensor will cancel out the pressure applied at the top port (low pressure side), since it is also transmitted through the fluid to the bottom port (high pressure side).
  4. Calibrate the System: Apply a known liquid level in the tank and configure the DP sensor’s output signal to correspond to the actual liquid height. This establishes a relationship between the measured differential pressure and the liquid level.
  5. Monitor Liquid Level: The DP sensor will continuously measure the pressure difference between the bottom and top of the tank, or between the bottom and vented atmosphere for vented tanks. With the established calibration, this differential pressure reading can be converted to the actual liquid level in the tank.
  6. Maintain Calibration: 
    • Due to potential liquid transfer from the tank to the low side DP connection, monitor for calibration drift. Implement routine maintenance procedures:
      • Drainage: Use a drain valve on the low side of the dp sensor to remove accumulated liquid.
      • Compensation: Utilise the zero tare function on the sensor to adjust for minor calibration drift.
    • Consider using a wet leg or remote seal technique (explained earlier) to eliminate the risk of condensate buildup in the low side connection, improving long-term measurement accuracy. When using these techniques, factor in the constant pressure from the reference liquid in your calculations.

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