Flow measurement is an essential component of countless industrial processes, from monitoring water distribution systems to optimizing the performance of oil pipelines. In this intricate world of fluid dynamics, two types of ultrasonic flow meters have emerged as prominent players: Transit Time and Doppler Ultrasonic Flow Meters. Both serve the same fundamental purpose – measuring the flow of liquids through pipes – yet they do so in strikingly different ways.
Doppler Flow Meters vs. Transit Time Ultrasonic Flow Meters
Ultrasonic flow meters are non-intrusive devices that measure the flow rate of liquids using acoustic vibrations (ultrasonic waves). Doppler and transit time ultrasonic flow meters are the two types. Both are intended to clamp onto the pipe wall without breaking the line or interfering with the flow.
The Doppler and transit time flow meters work on the same concept, although the technology is vastly different. It is critical to understand the flow meter to use for your application in order to achieve reliable measurements.
What are Transit Time Flow Meters?
Transit time flow meters calculate fluid velocity by measuring the time difference between upstream and downstream ultrasonic signals. The time-of-flight principle is used in this form of flow meter, in which an ultrasonic signal is delivered through the liquid and received by a downstream receiver.
The velocity of the fluid influences the travel time of the ultrasonic signal. The transit time is shorter when the liquid flows in the same direction as the ultrasonic signal, and it is longer when the fluid flows in the opposite way.
The flow meter calculates the velocity of the liquid by comparing the transit times of the upstream and downstream signals. One of the key benefits of transit time flow meters is their ability to monitor the flow rate of both liquids and gases. They are also reasonably priced and simple to install, making them popular for a wide range of industrial applications. However, they may not be suitable for liquids containing bubbles or solids as these can affect the accuracy of the measurements.
Principle of Measurement
The transit time flow meter employs two ultrasonic transducers that serve as both transmitters and receivers. The transducers are clamped at a certain distance apart on the outside of a closed pipe.
The transducers can be mounted in the V-method, which causes the ultra sound to transverse the pipe twice, the W-method, which causes the ultra sound to transverse the pipe four times, or the Z-method, which causes the ultra sound to transverse the pipe only once.
The mounting method chosen is determined on the pipe and liquid properties. When a flow meter is operational, two transducers broadcast and receive an ultrasonic signal that goes downstream first and then upstream. Because ultrasound travels quicker downstream than upstream, there will be a temporal difference (t).
When the flow is still, the time difference(Δt) is zero. As a result, if we know the time of flight downstream and upstream, we can use the following formula to calculate the time difference, and then the flow velocity (V) and flow volume (Q).
Where:
V: Liquid velocity
K: Â Constant
D: Distance between the two transducers
dt: difference in time of flight
Advantages of Transit Time Flow Meters
- Wide range of fluid compatibility.
- High accuracy in clean, homogeneous fluids.
- No moving parts, reducing maintenance.
- Suitable for both liquid and gas flow measurement.
- Non-invasive and non-contact measurement.
- Minimal pressure drop in the pipeline.
- Can measure bidirectional flow.
- Excellent for applications with varying flow conditions.
- Good performance in low-velocity flows.
- Long-term stability and reliability.
Application of Transit Time Flow Meters
- Water (hot water, cooling water, potable water, sea water etc.)
- Petroleum products.
- Chemicals, including alcohol, acids, etc.
- HVAC, energy measurement system.
- Beverage, food and pharmaceutical processors.
- Secondary sewage, waste treatment, etc.
- Heat energy boiler feed water in power plants (nuclear power plants, thermal power plants, and hydropower stations).
- Metallurgy and mining applications.
- Detection, inspection, tracking, and collection of pipeline leaks.
What are Doppler Flow Meters?
Doppler flow meters operate differently from transit time flow meters. They measure the velocity of the fluid using the Doppler effect. The Doppler effect is the shift in frequency of a wave caused by the movement of the source and receiver relative to each other.
An ultrasonic signal is sent into the fluid at an angle in Doppler flow meters. When the signal encounters moving particles or bubbles in the liquid, its frequency changes, creating a shift in the signal frequency. The flow meter calculates the fluid’s velocity by measuring the signal’s frequency shift.
Doppler flow meters are ideal for fluids containing bubbles or solids since they rely on the movement of these particles to determine the fluid’s velocity. However, when measuring the flow rate of liquids, they may not be as accurate as transit time flow meters, and they require a particular level of particle concentration to function well.
Principle of Measurement
The flow meter works by sending an ultrasonic sound from its transmitting transducer, which is reflected by helpful sonic reflectors suspended in the liquid and recorded by the receiving transducer. If the sonic reflectors move within the sound transmission path, sound waves will be reflected at a frequency shifted (Doppler frequency) from the transmitted frequency.
The frequency shift will be directly related to the speed of the moving particle or bubble. This frequency shift is interpreted by the instrument and converted to various user-defined units of measurement. Some particles must be large enough to cause longitudinal reflection – particles larger than 100 microns. The installation location must have enough straight pipe length upstream and downstream when installing the transducers. The upstream requires 10D and the downstream requires 5D straight pipe length, where D is the pipe diameter.
The Doppler ultrasonic flow meter is designed to measure the volumetric flow of liquid within the closed conduit, the pipe line must be full of liquids, and there must be a certain amount of air bubbles or suspended solids in the liquid.
Transducers are clamp-on or hot-tapped insertion types; users don’t need to shut down the pipe flow when installing transducers.
Advantages of Doppler Flow Meters
- Suitable for measuring dirty or aerated liquids.
- Effective for applications with suspended solids or bubbles.
- Non-invasive and non-contact measurement.
- Minimal pressure drop in the pipeline.
- Can measure bidirectional flow.
- Works well in open channels and partially filled pipes.
- Ideal for applications with large pipe diameters.
- Good performance in high-velocity flows.
- Long-term stability and reliability.
- Can detect flow in opaque or corrosive fluids.
Applications of Doppler Flow Meters
- Raw sewage
- Activated sludge
- Ground water
- Pulp and paper slurries
- Chemical slurries
- Drainage
- Mining recirculation
What Is the Distinction Between a Doppler and a Transit Time Ultrasonic Flow Meter?
Doppler flow meters are not suggested for use with clean fluids. On the other hand, transit time flow meters are often used to measure the flow of crude oils and simple fractions in the petroleum industry. They also work well with viscous liquids, provided the Reynolds number at minimum flow is less than 4,000 (laminar flow) or above 10,000 (turbulent flow).
Transit time flow meters are the industry standard for measuring cryogenic liquids at temperatures as low as -300°C, and they are also used in molten metal flow metering. Measurements of liquid argon, liquid nitrogen, liquid helium, and molten sulfur have often been reported.
Doppler flow meters are typically ineffective in raw wastewater applications due to a lack of acoustic discontinuities. However, untreated wastewater is not typically clean enough to measure transit time. Other wastewater-related applications can be just as difficult.
Doppler flow meters are commonly used to measure activated sludge, digested sludge, and mining slurries. However, acoustically absorbent slurries like lime or kaolin can be problematic.