In 1842, Christian Doppler discovered that a stationary observer perceives a sound to have shorter wavelengths as its source approaches, longer wavelengths as its source recedes.
The Doppler effect explains why one hears rising pitch in the blowing horn of an approaching car. When the car zooms away, the pitch seems to drop. Ultrasonic Doppler flow meters put this frequency shift to work in so-called dirty liquids containing acoustical discontinuities; suspended particles, entrained gas bubbles or turbulence vortexes.
How does an ultrasonic flow meter work?
Ultrasonic flow meters are a non-contact means of measuring flow velocity. They are clamp-on devices that attach to the exterior of the pipe and enable measurement of corrosive liquids without damage to sensors.
The two types of ultrasonic flow meters, Doppler and transit time, each function by way of two different technologies. The Doppler ultrasonic flow meter must have particles or bubbles to reflect the ultrasonic signals. Consideration must be given to the lower limits for concentrations and sizes of solids or bubbles, and the liquid must flow at a rate high enough to keep the solids suspended.
When transmitted into a pipe that contains flowing liquid with such discontinuities, an ultrasonic pulse or beam reflects from them with a change in frequency that is directly proportional to the liquid’s flow rate. Thus, the ultrasonic Doppler flowmeter calculates flow rate from the velocity of the discontinuities, rather than from the velocity of the liquid.
Principle of operation
Questions which need to be answered before selecting an ultrasonic flow meter:
- Does the liquid have particulates of 100 ppm of 100 microns in size?
- Do you require a handheld or continuous process monitor?
- Do you require an analog output?
- What is the minimum and maximum flow rate for the flow meter?
- What is the minimum and maximum process temperature?
- What is the minimum and maximum process pressure?
- What is the size of the pipe?
- Is the pipe always full?
The Doppler ultrasonic flow meter operates on the principle of the Doppler Effect, which is the physical phenomenon of a sound wave changing frequency. In the case of ultrasonic flow meters, the frequency of an ultrasonic signal changes (Doppler Effect) in direct proportion to the rate of flow of the liquid when reflected by suspended particles or gas bubbles (discontinuities) in motion.
Typically, an ultrasonic Doppler flowmeter consists of a transmitter/indicator/totalizer and a transducer. The user selects a configuration appropriate to the application, taking into account the liquid, the size and concentration of solids or bubbles, the pipe dimensions and the pipe lining. The transmitter’s signal threshold usually adjusts to filter out mechanical and electrical noise.
A high-frequency oscillator in the transmitter drives the transducer, which, in the popular clamp-on design, mounts on the pipe exterior. The transducer generates an ultrasonic signal that passes through the pipe wall into the flowing liquid; the transmitter converts the difference between its output and input frequencies to electronic pulses. Processed, scaled and totalized, the pulses provide a measurement of flow.
Ultrasonic Doppler flowmeters that clamp onto the outside of a pipe operate non-invasively, without moving parts. They cause no pressure drop, risk no damage from the process liquid and entail little maintenance. If properly calibrated, they can have ±1% accuracy, however, the pipe wall and any air space between the wall and the liquid can generate signal interference. Moreover, a stainless steel pipe wall might conduct the transmitted signal to the extent that the reflected signal will seem to undergo a major shift.
Transit time ultrasonic flow meters measure the time difference between when an ultrasonic signal is transmitted from the first transducer until it is received by the second transducer. A comparison is made of upstream and downstream measurements. If there is no flow, the travel time will be the same in both directions. When flow is present, sound moves faster if traveling in the same direction and slower if moving against it.
A third ultrasonic flowmeter employs cross-correlation between upstream and downstream transducer pairs to compute flow. Some flowmeters of this design use microprocessors to switch automatically between “clean” and “dirty” modes based on correlation factors. A single cross-correlation hybrid flowmeter could, for example, monitor flow of either activated or digested sludge. Carefully engineered applications using such flowmeters have had reported installed accuracy within 0.5% of reading.
How do you select the right magnetic flow meter?
Ultrasonic flow meters are also ideal for applications where low-pressure drop is needed, and low maintenance is required. An ultrasonic Doppler flow meter is a volumetric flow meter that is ideal for aerated liquids such as wastewater or slurries. Transit time ultrasonic flow meters are ideal for clean liquid applications such as water and oil.
There are three main types of ultrasonic flow meters. Factors such as the type of output (analog or digital), size of the pipe, minimum and maximum process temperature, pressure, and flow rate can all impact which ultrasonic flow meter is right for your application.
Ultrasonic Flow Meter Styles
Ultrasonic flow meter with totalization
These ultrasonic flow meters are well-suited for full pipes of liquid that contain at least 100 ppm of 100 microns or larger suspended particles or gas bubbles for pipe sizes 0.25 to 120”. Ideal uses include flow measurement of wastewater sewage, reactivated sludge, waste activated sludge, primary sludge, slurries, crude oils, lime slurries, phosphates, and pulp stock.
Portable ultrasonic flow meter
This ultrasonic flow meter uses a non-invasive clamp-on transducer, which is placed on the outside of a metal or plastic pipe containing liquids with more than 100 ppm of 100 microns or larger suspended solids or entrained gases.
Clamp-on ultrasonic flow meters come in either single or dual-sensor versions. In the single-sensor version, the transmit and receive crystals are potted into the same sensor body, which is clamped onto a single point of the pipe surface. A coupling compound is used to ultrasonically connect the sensor to the pipe. In the dual sensor version, the transmit crystal is in one sensor body, while the receive crystal is in another. Clamp-on Doppler flow meters are subject to interference from the pipe wall itself, as well as from any air space between the sensor and the wall. If the pipe wall is made of stainless steel, it might conduct the transmit signal far enough so that the returning echo will be shifted enough to interfere with the reading. There are also built-in acoustic discontinuities in copper, concrete-lined, plastic-lined, and fiberglass-reinforced pipes. These are significant enough to either completely scatter the transmitted signal or attenuate the return signal. This dramatically decreases flow meter accuracy (to within only ±20%), and, in most cases, clamp-on meters will not work at all if the pipe is lined.
How to install an ultrasonic flow meter
Both Doppler and transit time flow meters are designed to clamp onto the outside of a pipe without breaking the line or interrupting the flow. This also eliminates pressure losses and prevents leaking, which is common with an inline flow meter.
The accuracy of an ultrasonic flow meter relies on proper mounting. Large temperature changes in the pipe or a significant amount of vibration may affect the alignment of the transducers and acoustic coupling to the pipe. In most cases, ultrasonic flow meters won’t work at all if the pipe is lined with materials such as copper, concrete, plastic or fiberglass. These factors must be accounted for during installation. In addition, to provide an accurate volumetric flow rate, all ultrasonic flow meters require the pipe to be full.
Popular industry applications
Ultrasonic flow meters are mostly used in oil and gas industries. They are also used in chemical, pharmaceutical, food and beverage, metal, mining, pulp and paper and wastewater industries. Some examples of popular applications are explained in these articles:
- Flow Measurement in a Return Activated Sludge Line
- Monitoring Water Flow in an Egg Farm Application
- Hybrid Ultrasonic Flow Meter Application Helps Ensure Accurate Enzyme
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