Transit-time ultrasonic flow meters provide a non-invasive method of measuring liquid flow through a full pipe. Clamp-on ultrasonic transducers are installed on the outside of the pipe, allowing flow to be measured without cutting the pipe or inserting a sensor into the liquid.
The following frequently asked questions explain how transit-time flow meters work, where they can be used, installation requirements, accuracy, pipe requirements, calibration, and how transit-time technology differs from other flow-measurement technologies.
What is a transit-time ultrasonic flow meter?
A transit-time ultrasonic flow meter is an instrument that measures the velocity and flow rate of liquid moving through a full pipe by measuring the travel time of ultrasonic signals.
Two ultrasonic transducers are typically mounted on the outside of the pipe. Because the transducers do not need to contact the liquid, transit-time technology can provide a non-invasive method of measuring flow.
Transit-time flow meters are commonly used for water, process liquids, chemicals, and other liquids that allow ultrasonic signals to travel effectively through the fluid.
How does a transit-time flow meter work?
A transit-time flow meter uses two ultrasonic transducers positioned along a pipe.
An ultrasonic signal is transmitted through the pipe and liquid in the direction of flow and another signal is transmitted against the direction of flow.
The signal traveling with the flow arrives slightly faster than the signal traveling against the flow. The flow meter precisely measures this difference in transit time, sometimes called Delta-T.
The meter uses the measured transit-time difference along with programmed pipe and liquid information to calculate liquid velocity and flow rate.
What is a clamp-on ultrasonic flow meter?
A clamp-on ultrasonic flow meter uses ultrasonic transducers mounted on the outside surface of a pipe.
Unlike an insertion or inline flow meter, the transducers do not penetrate the pipe wall and do not contact the liquid.
Clamp-on technology can significantly simplify installation because the pipe usually does not need to be cut and the process does not need to be opened to install the flow sensors.
What are the advantages of a clamp-on transit-time flow meter?
One of the primary advantages is non-invasive flow measurement.
Because the ultrasonic transducers are installed on the outside of the pipe, a clamp-on flow meter can provide several benefits, including reduced installation time, reduced material costs, minimal process interruption, and lower maintenance requirements.
There are no moving parts inserted into the liquid stream and no additional pressure drop created by an obstruction inside the pipe.
Clamp-on technology is particularly useful when cutting into an existing pipe would be expensive, inconvenient, or undesirable.
Does a clamp-on ultrasonic flow meter require cutting the pipe?
No. One of the major advantages of a clamp-on ultrasonic flow meter is that the transducers are installed on the exterior surface of the existing pipe.
The pipe generally does not need to be cut, drilled, or modified to install the ultrasonic sensors.
This can allow a transit-time meter to be installed without the extensive piping modifications associated with many conventional inline flow meters.
What liquids can a transit-time flow meter measure?
Transit-time ultrasonic flow meters are particularly well suited for relatively clean liquids that effectively transmit ultrasonic signals.
Applications can include potable water, well water, process water, deionized or demineralized water, cooling water, heating water, and many process liquids and chemicals.
Modern transit-time instruments may also tolerate limited amounts of suspended solids or entrained material.
The Spectra TTM-500 and other Spectra transit-time products are designed for applications ranging from clean liquids to liquids containing up to approximately 2% suspended solids.
Can a transit-time flow meter measure wastewater?
It depends on the characteristics of the wastewater.
Transit-time technology can work with wastewater containing relatively low concentrations of suspended solids as long as the ultrasonic signal can travel effectively through the liquid.
The Spectra TTM-500 is designed for liquids ranging from clean fluids to those containing up to approximately 2% suspended solids.
Applications containing higher concentrations of suspended solids, significant entrained gas, or heavy sludge may be better suited for another ultrasonic technology such as a Doppler flow meter.
What is the difference between a transit-time and Doppler ultrasonic flow meter?
Both technologies can use ultrasonic sensors mounted on the outside of a pipe, but they measure flow differently.
A transit-time flow meter measures the difference in travel time between ultrasonic signals traveling with and against the direction of flow. Transit-time technology is particularly well suited for clean liquids and liquids containing relatively low concentrations of suspended solids.
A Doppler flow meter sends an ultrasonic signal into the liquid and measures the frequency shift of signals reflected from suspended particles or entrained gas bubbles. Doppler technology therefore requires sufficient reflectors within the liquid.
Transit-time technology is generally preferred when higher accuracy is required and the liquid is suitable for transit-time measurement. Doppler technology can be advantageous for wastewater, sludge, and other liquids containing enough suspended material to produce a reliable reflected signal.
Learn more about the difference between Doppler and Transit-Time ultrasonic flow meters.
How accurate is a transit-time ultrasonic flow meter?
Accuracy depends on the meter, installation, pipe information, transducer placement, liquid characteristics, and flow profile.
When properly installed, the Spectra TTM-500 can provide accuracy of approximately 1% of rate or better.
Accurate pipe dimensions and material information, correct transducer spacing and placement, adequate straight pipe, good acoustic coupling, and a fully developed flow profile are important for achieving the best measurement performance.
What pipe sizes can a clamp-on transit-time flow meter measure?
The available pipe-size range depends on the meter and transducers being used.
For example, the Spectra TTM-550-P portable transit-time system is normally supplied with M-2 sensors designed for pipe sizes from approximately 2 inches to 36 inches.
Optional sensors are available for smaller pipes from approximately 0.5 inch to 2 inches and for large pipes from approximately 36 inches to 210 inches.
The appropriate transducer should be selected for the pipe diameter and application.
What pipe materials can be used with a clamp-on ultrasonic flow meter?
Clamp-on transit-time meters can be used with many common pipe materials that effectively transmit ultrasonic energy.
Examples may include carbon steel, stainless steel, ductile iron, cast iron, copper, and many plastic pipes.
Pipe material, outside diameter, wall thickness, liner information, and pipe condition are important because these factors can affect ultrasonic signal transmission and proper transducer spacing.
Heavy corrosion, scale, coatings, liners, or irregular pipe surfaces may interfere with ultrasonic signal transmission.
Does pipe wall thickness affect ultrasonic flow measurement?
Yes. The ultrasonic signal must travel through the pipe wall before entering the liquid, so accurate pipe-wall thickness information is important.
The meter uses pipe dimensions, pipe material, and other parameters to calculate the ultrasonic signal path and determine the proper distance between the transducers.
Incorrect pipe-wall thickness or pipe diameter information can cause incorrect transducer spacing and reduce measurement accuracy.
How much straight pipe is required for a transit-time ultrasonic flow meter?
Transit-time meters perform best where the liquid has a stable and well-developed flow profile.
Elbows, valves, pumps, tees, reducers, and other piping components can create turbulence or distorted flow profiles.
For this reason, the transducers should be installed on a suitable straight section of pipe with adequate upstream and downstream distance from major flow disturbances.
The required straight-run distance depends on the piping configuration and the meter manufacturer's installation recommendations.
Where should clamp-on ultrasonic transducers be installed on a pipe?
Transducers should be installed on a clean, suitable section of pipe with a stable flow profile and adequate straight pipe whenever possible.
The installation location should avoid areas with heavy corrosion, loose scale, damaged pipe surfaces, or other conditions that could interfere with ultrasonic transmission.
On horizontal pipes, sensor positioning may also be selected to reduce the possibility of measuring through accumulated sediment at the bottom of the pipe or air pockets near the top.
Correct transducer orientation and spacing are essential for reliable measurement.
Do ultrasonic flow meter transducers require coupling gel?
Clamp-on ultrasonic transducers generally require an acoustic coupling material between the transducer and the pipe.
The couplant helps eliminate air gaps between the transducer face and pipe surface. Air gaps can significantly weaken ultrasonic signal transmission.
The pipe surface should be properly prepared and the appropriate acoustic couplant should be applied according to the meter manufacturer's installation instructions.
The Spectra TTM-550-P portable system includes acoustic couplant as part of its standard package.
Can a transit-time flow meter measure flow in both directions?
Many transit-time ultrasonic flow meters can detect flow direction because the measurement is based on the difference between upstream and downstream ultrasonic transit times.
Whether bidirectional flow measurement, totalization, and outputs are available depends on the specific meter configuration and programming.
Application requirements should be reviewed when bidirectional flow measurement is required.
Can an ultrasonic flow meter measure flow without contacting the liquid?
Yes. This is one of the principal advantages of a clamp-on ultrasonic flow meter.
The ultrasonic transducers are mounted on the outside of the pipe, so neither the sensors nor the meter need to contact the liquid being measured.
This makes clamp-on ultrasonic flow measurement useful for applications where process interruption, contamination, pressure loss, corrosion, or pipe modification are concerns.
Does a transit-time ultrasonic flow meter need to be calibrated?
Flow meters should be verified or calibrated according to the accuracy requirements of the application, quality program, or regulatory requirements.
Although clamp-on ultrasonic flow meters have no moving parts in the liquid stream, calibration or performance verification can still be important when measurement accuracy must be documented.
Correct installation and programming are also critical. Pipe dimensions, pipe material, liquid properties, and transducer placement must be entered or configured correctly for reliable measurement.
Vortex Technologies provides flow-meter calibration and third-party certification services for a variety of flow-measurement technologies.
What causes inaccurate readings from a clamp-on ultrasonic flow meter?
Several installation and application conditions can affect measurement performance.
Common causes include incorrect pipe diameter or wall-thickness information, incorrect pipe material, improper transducer spacing, poor acoustic coupling, incorrect transducer alignment, corrosion or scale on the pipe, inadequate straight pipe, turbulence, partially filled pipes, excessive suspended solids or gas bubbles, and selecting the wrong transducer for the pipe size.
Signal strength and signal quality should be checked during installation. If the meter does not receive a strong and stable ultrasonic signal, the installation and programmed parameters should be reviewed.
How do I choose the right ultrasonic flow meter for my application?
Begin with the characteristics of the liquid and piping system.
Important considerations include liquid type, suspended-solids concentration, entrained air or gas, pipe material, pipe diameter, wall thickness, flow range, required accuracy, available straight pipe, operating temperature, permanent versus temporary installation, required outputs, and whether data logging is needed.
For relatively clean liquids where high accuracy is important, a transit-time ultrasonic flow meter may be an excellent choice.
For liquids containing higher concentrations of suspended solids or entrained gas, a Doppler ultrasonic flow meter may be more appropriate.
Vortex Technologies can evaluate an application and help determine which ultrasonic flow-measurement technology and transducer configuration are appropriate.
Spectra TTM-500 Dedicated Ultrasonic Transit-Time Flow Meter
The Spectra TTM-500 is a dedicated, non-invasive ultrasonic transit-time flow meter designed for permanent flow-monitoring and process-control applications.
The system uses clamp-on ultrasonic sensors, eliminating the need for sensors or moving components to be inserted into the liquid stream.
When properly installed, the TTM-500 can provide accuracy of approximately 1% of rate or better. The system is fully programmable and is designed for applications ranging from clean liquids to liquids containing up to approximately 2% suspended solids.
View the Spectra TTM-500 Dedicated Ultrasonic Transit-Time Flow Meter »
Portable Transit-Time Flow Measurement
For temporary flow measurements, flow surveys, troubleshooting, or applications requiring a portable instrument, Vortex Technologies also offers the Spectra TTM-550-P Portable Transit-Time Flow Meter.
View the Spectra TTM-550-P Portable Transit-Time Flow Meter »