Gas Flow Meter Classification – Part 2

Pitot Tube / Prandtl Tube

Usually, a Pitot tube is a rod bent at a right angle, lowered into the flow so that the air passing through the pipeline flows onto the hole at the tip of the tube. This air creates the total pressure. If there are holes in the walls of the tube at some distance from the tip, where the air creates static pressure, such a device is called a Prandtl tube. The Prandtl tube allows subtracting the static pressure value from the total pressure value, thus obtaining the dynamic pressure value, which is then converted into velocity and then into flow rate.

Besides this device design, there is also a version where the tube is not bent at an angle but is completely straight. It is immersed in the pipeline along its entire length and has holes on the side facing the incoming flow along the entire length of the tube. This allows measuring velocity not only at the center of the pipeline but across the entire diameter, which reduces calculation errors since the flow velocity differs at the center of the pipeline and near the edges.

An even more advanced Pitot tube is similar to the previous design but has a series of holes also on the side opposite to the pressure side, where only static pressure is measured. By measuring the pressure difference on both sides of the device, the flow velocity is determined. This version of the device can also be classified as a differential pressure flow sensor.

The advantages of these devices include relatively low cost and low pressure loss. A disadvantage is that the sensor is extremely sensitive to deviations of the flow from the measurement axis (for example, due to turbulence or movement of the device itself) and that to determine the flow rate, it is also necessary to know all parameters affecting the gas density (pressure, temperature, humidity). Additionally, one of the drawbacks is that the accuracy of these flowmeters significantly decreases at low flow velocities (see diagram). Moreover, the devices are highly susceptible to clogging.

Turbine Flowmeters

Turbine flowmeters consist of a pipe section with a rotor mounted inside, supported by bearings located in the pipeline section. The rotor height is usually 25-30% of the pipeline diameter. In some designs, a flow straightener is placed at the sensor inlet. The rotor blades rotate as gas passes through the pipeline so that the rotational speed is proportional to the volume of gas passing through. Outside the measuring chamber, there is a magnetic coil generating an electrical signal each time the rotor blades cross the magnetic field. Each pulse corresponds to a certain volume of gas passing through the pipeline.

The advantages of turbine flowmeters include that the flow profile and vortices have little effect on measurement results. These devices are stable and have high reproducibility. Disadvantages include the presence of moving parts and sensitivity to contamination, which makes the device less reliable and requires regular maintenance. Additionally, as in most cases, to calculate flow, information about the pressure and temperature of the medium is needed.

Ultrasonic Flowmeters

The operation of an ultrasonic flowmeter is based on the fact that the speed of ultrasonic waves traveling with the flow differs from the speed of ultrasonic waves traveling against the flow.

The flowmeter can be equipped with a pair of ultrasonic transmitters located on opposite walls so that the direction of the transmitted waves is at a 45° angle to the flow. The signal sent in one direction moves faster because the flow velocity adds to its speed, while the signal traveling in the opposite direction is correspondingly slowed down. The device measures the time required to transmit the ultrasonic signal and compares the values for the signal traveling along the flow and against it. Based on these values, the flow velocity is calculated, which is then converted into flow rate.

Another type of ultrasonic flowmeters uses the Doppler effect for measurement. In this case, two sensors are installed on the same wall of the pipeline. The ultrasonic wave from one transmitter passes through the flow, reflects, and upon returning, reaches the second sensor. According to the Doppler effect, when there is movement between the transmitter and receiver, the frequency and wavelength change proportionally to the speed of movement. By calculating the difference in frequencies between the transmitter and receiver, the flow velocity and, consequently, the flow rate can be determined.

More complex ultrasonic flowmeters may use several pairs of transmitters arranged around the entire circumference of the pipeline.

Ultrasonic flowmeters have many advantages (ease of installation, accuracy, reliability, wide measurement range, possible high medium pressure), but they are extremely expensive. The significantly higher cost compared to other types of flow meters is the main drawback of ultrasonic flowmeters. Additionally, accuracy readings strongly depend on the specific flow conditions.

Gas Flowmeter Classification – Part 1
Gas Flowmeter Classification – Part 3

You can select a flowmeter suitable for your task in the product catalog or by contacting our technical specialists.