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Measuring flow rate of a liquid or gas using a dp sensor

An introduction to the dp flow method of using a differential pressure sensor for measuring the flow of liquids and gases.

  • Components of a dp Flow System:
    • Primary Element: A mechanical part within the pipework, chosen based on specific flow characteristics. Options include orifice plates, venturi tubes, wedges, cones, and pitot probes. Their shared function is to induce a pressure drop.
    • Secondary Element: A differential pressure sensor connected to the primary element that measures the pressure differential induced by the flow.
  • Bernoulli’s Principle and Flow Calculation:
    • Flow rate is proportional to the square root of the pressure difference (Bernoulli’s principle).
  • Sensor Enhancements for Flow Measurement:
    • Differential pressure sensors with square root extraction output ease calculations, removing the need to process non-linear signals.
  • Calibration:
    • Calibrate flow-measuring instrumentation using either direct minimum/maximum flow measurements or a formula with coefficients designed for the specific primary element.
  • Multivariable Transmitters
    • Some advanced differential pressure sensors measure static pressure and temperature, enabling compensation for density variations, leading to higher flow calculation accuracy.

Although flow measurement is an indirect parameter for a differential pressure sensor to measure, it is actually a very common use case due to the adaptability of the dp flow method to many different flow metering applications.

There are two main parts to a dp flow method installation, the primary element and the secondary element.

The primary element is a mechanical component which is designed for the pipework and flow characteristics specified for a particular application. The primary element can be one of a few distinct types which include orifice plate, venturi, wedge, cone and pitot probe. Each of these primary elements have their own particular advantages for different applications, but they all have one common purpose, and that is to create a pressure drop in the flow.

The secondary element is the measurement component, which is fitted to the primary element to measure the pressure difference caused by the gas or liquid passing through it. The ideal device to accomplish this is a differential pressure sensor, since it has a high and low pressure connection to allow it to measure the pressure difference directly, independently of static pressure.

Now that you have a measurement of the differential pressure generated by a restriction in the flow, you can use the equation from Bernoulli’s principle to calculate the volumetric flow rate. The main rule that is used from this equation, is that the flow rate is proportional to square root of the pressure difference.

To simplify the processing and improve the performance of dp flow measurements, many dp sensor provides a square root extraction output, saving you having to use instrumentation which can handle a non-linear signal.

If you have a linear dp flow signal from the dp sensor you can now calibrate your instrumentation to measure flow either by direct measurement of the minimum and maximum flow, or using a provided flow calculation formula which includes the correct coefficients to use for the particular characteristics of the primary element installed.

Some types of dp sensors now include the ability to measure static pressure and temperature. These so called multivariable transmitters, enable the user to incorporate variations in density into their flow calculations where more accuracy is required.

Here is a breakdown of the steps required to using a differential pressure sensor to measure the flow of liquids or gases:

  1. Select a Primary Element: Choose an appropriate primary element (orifice plate, venturi, etc.) based on your pipework, media type, flow characteristics, and desired accuracy.
  2. Install the Primary Element: Physically install the chosen primary element into the pipe system.
  3. Connect the Differential Pressure Sensor: Connect the differential pressure (dp) sensor to the primary element, ensuring the high and low pressure connections are correctly oriented.
  4. Calculate Volumetric Flow Rate: Utilize Bernoulli’s principle to calculate the volumetric flow rate. Remember, the flow rate is proportional to the square root of the measured differential pressure.
  5. Utilize Sensor Enhancements (Optional): If the dp sensor features square root extraction, directly use the output value for simplified calculations.
  6. Calibration: Calibrate any connected instrumentation to accurately measure flow. This can be done either through direct flow measurements or by using a provided formula tailored to the specific primary element.
  7. Incorporate Density Compensation (Optional): For increased accuracy, utilize multivariable transmitters that measure static pressure and temperature to adjust the flow calculation for density variations.

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