Flow Measurement 101

Used to monitor or control flow rate — the amount of a gas or liquid moving through a pipe — flow meters are used in a wide range of applications. A critical part of many industrial processes, flow measurement ensures that fluid is at the right place within a given process, at the right time. Therefore, successful product quality, process optimization, overall process safety, environmental protection, and, ultimately, profits, all depend on proper flow measurement.

Because different industries require different levels of precision and accuracy, these instruments are essential for ensuring all requirements are properly met. Depending on the specific application, several different flow measurement systems and methods are available for reliably quantifying bulk fluid movement.

Below are some basic flow meter terminologies to familiarize yourself with before beginning a project.

Flow Meter Terminology

  • Repeatability — The capacity of a flow meter to determine the same value for a flow rate more than once.
  • Turn down (turn down ratio) — The range of flow rates between maximum and minimum flow over which a flow meter will work successfully (depending on the accuracy limits and tolerance specifications)
  • Flow meter accuracy — The margin of error that can occur within a flow meter’s measurements
  • Percent of flow rate (%R): The error in the measurement of flow rate when a meter’s accuracy is given as a percentage of flow rate; error = % flow rate x measurement
  • Percent of full scale (%FS) — The error in the measurement of flow rate when a meter’s accuracy is given as a percentage of full scale; error = % full scale x full scale flow
  • Percent of calibrated span (% CS) — The error in the measurement of flow rate when a meter’s accuracy is given as a percentage of calibrated span; error = % calibrated span x calibrated span
  • Percent of upper range limit (%URL) — The error in the measurement of flow rate when a meter’s accuracy is given as a percentage of the upper range limit; error = % URL x upper range limit

Types of Flow Measurement

  • Coriolis — Coriolis mass flow meters measure the force resulting from the acceleration caused by mass moving toward — or away from — a center of rotation. These meters contain a constantly oscillating tube that runs the length of the meter, and two sensors at either end of the tube measure oscillation. As liquid flows through the tube, it oscillates in a snake-like motion, causing the sensors to measure the oscillations in different directions.
  • UltrasonicUltrasonic flow meters use ultrasound and differential transit time in their measurements. Pairs of sensors are fitted across from each other in the measuring tube, each of which can emit and receive ultrasonic signals. The transit time between these sensors is then measured to obtain a reading.
  • Electromagnetic — Electromagnetic coils inside electromagnetic flow meters generate a magnetic field and electrodes that capture voltage. If there is no flow, the electrodes do not measure voltage. In the meantime, the electromagnetic field charges the particles in the liquid. As the liquid moves, the positively and negatively charged particles separate and collect on the opposite sides of the tube. The resulting electrical voltage is detected by the electrodes and measured.
  • Thermal — Thermal mass flow meters comprise a family of instruments used to measure the total mass flow rate of a fluid, primarily gases, flowing through closed conduits. Meters have two sensors on the inside of the tube; one of the sensors measures the temperature of the gas regardless of flow velocity, and the other sensor is heated in order to constantly maintain a temperature difference between the sensors and the material.
  • Vortex — Vortex flow meters are constructed based on a principle called the von Kármán effect, which states that flow will alternately generate vortices when passing by a bluff body. In vortex flow meters, the bluff body extends vertically into the flow stream and is used to disturb the flow of a liquid or gas. Flow velocity is proportional to the frequency of the vortices. Flow rate is then calculated by multiplying the area of the pipe times the velocity of the flow.

Learn More

Endress+Hauser, which established U.S. operations in 1970, is among the largest instrument manufacturers in the nation. More than 80% of our instruments sold in the United States are made domestically, and we have several sales, service, and training centers spread throughout the country, allowing for quick customer support, no matter where our clients are located.

To learn more about flow measurement, download our free whitepaper, “Measurement 101: Temperature, Pressure, Level and Flow,” in which we cover more terminology, outline the best types of measurement methods for specific applications, and delve into several lesser-known types of measurement systems.

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