| Ultrasonic flow meter technology has much improved | | | | bottom of the pipe, the second transducer receives |
| over the last few years and now forms a viable flow | | | | the signal. The ultrasonic flow meter is then able to |
| measurement technique compared to other methods | | | | determine flow by calculating the time it takes to |
| such as turbine and electromagnetic type systems. | | | | receive the signal. |
| The first Doppler ultrasonic flow meter units became | | | | Portable ultrasonic flow meter kits are now available |
| available in the early 70’s. To begin with, as with | | | | for plant wide surveys with minimal fuss and no |
| all new technologies they were not very accurate, | | | | down time. The first portable ultrasonic flow meter |
| difficult to install and costly. However, the newest | | | | units were heavy and quite large, basically a luggable |
| Time of Flight ultrasonic flow meter and Doppler | | | | design. As ultrasonic flow meter design has improved |
| types are proving to be both reliable and accurate. | | | | over time, the instruments have become much |
| Accuracy has improved significantly with most | | | | smaller, lighter and more power efficient and are truly |
| ultrasonic flow meter manufacturers claiming 0.5% | | | | hand portable. Also many ultrasonic flow meter |
| accuracy if the meters are correctly installed. | | | | models now have integral data loggers, allowing the |
| With these two technologies, which ultrasonic flow | | | | user to leave the meter on site so it can record |
| meter should you use? | | | | average flow rates and flow totals. |
| Although Doppler ultrasonic flow meters use an older | | | | An ultrasonic flow meter will be most accurate when |
| technology, they are ideal for liquids with air bubbles | | | | fitted on a long, straight piece of pipe, free from |
| or slurries with significant solid particles. They | | | | obstructions such as elbows and valves. Also, any |
| determine the flow using the Doppler shift method, | | | | rusty pipes should be cleaned before using an |
| by measuring the change in frequency created by an | | | | ultrasonic flow meter. Ultrasonic jelly should also be |
| object moving towards, or away from the | | | | smeared under the sensor to ensure a solid contact |
| measurement point. Normally liquids with a minimum | | | | on the pipe. |
| of 100ppm concentration at 100microns particle size | | | | The biggest advantage to using an ultrasonic flow |
| or bigger would be suitable for a Doppler ultrasonic | | | | meter is maintenance. Older insertion meters usually |
| flow meter. One stumbling block of most Doppler | | | | require the plant to be shut down while installing. |
| ultrasonic flow meter systems is that they are less | | | | However an ultrasonic flow meter can be installed |
| accurate in low flow conditions. | | | | without stopping the process. Also as an ultrasonic |
| A transit time ultrasonic flow meter is ideal for clean | | | | flow meter has no moving parts it cannot wear out. |
| fluids such as water and oil. These meters are more | | | | Both Doppler and transit time portable ultrasonic flow |
| advanced than Doppler ultrasonic flow meter types | | | | meter kits are available from Omni Instruments for |
| due to the advanced calculations used to determine | | | | hire or purchase. |
| the flow rate. Two transducers are used, the first | | | | By Richard Burgess |
| sends an acoustic signal which is bounced off of the | | | | OmniInstruments. |