A tale of two flowmeters

Instrumentation firm Pulsar Measurement offers a few thoughts on Doppler vs transit-time flowmeters, and the situations to which each is best suited.

Pulsar Measurement Instrumentation

Doppler and transit-time flowmeters are both popular for the non-invasive measurement of flow in full pipes. We tend to confuse these technologies because they are both ultrasonic and both measure flow by using sensors clamped onto the outside of a pipe.

In the real world, they each work best in quite different types of application. Success in your installation depends on understanding the differences and making the right choice.

Ultrasound is sound generated above the human hearing rate – above 20 kHz. Both Doppler and Transit-Time flowmeter technologies are called “ultrasonic” because they operate far above the frequencies or sound range that we can hear.

At the heart of each ultrasonic transducer is a piezo-electric crystal. These (roughly coin-sized) glass disks are polarized and expand or vibrate by a miniscule amount when an electrical current is applied to the surface electrodes.

As it pulses the transducer emits an ultrasonic beam approximately 5° wide at an angle designed to efficiently pass through a pipe wall. The returning echo (the “pressure pulse”) is registered by a second passive crystal, which creates electricity in response. This is the received signal in a Doppler or transit-time transducer.

Doppler flowmeters work best in dirty or aerated liquids like wastewater and slurries. Transit-time flowmeters, on the other hand, work best with clean liquids like water, oils, and chemicals.

Contact Pulsar Measurement for specific advice and information on selecting and applying these technologies.