Ultrasonic Flow Meter Installation Tips
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Consider the medium you are trying to measure with your ultrasonic flow meter. Sewage would obviously require a Doppler ultrasonic flow meter while distilled water requires a time of flight ultrasonic flow meter, however, what about applications that involve a combination of liquids or liquids with small particles? As a general rule of thumb, if the liquid flow has particles 100 PPM or greater then they generally are more suited to a Doppler ultrasonic flow meter. Anything under 100PPM, such as oil, sea water, or even gray water are more suited to a time of flight ultrasonic flow meter.
A time of flight ultrasonic flow meter will require a completely full pipe to operate. While a typical Doppler ultrasonic flow meter is more suited to a completely full pipe, it is possible under certain circumstances to use these units in partially full installations.
Transducer Installation
In an ideal situation you will require a straight length of horizontal pipe that is at least fifteen pipe diameters long. This should be free of valves, inflow/outflow diffusers and as far away from pumps as possible. When using a time of flight ultrasonic flow meter on an installation that has a T-section or pump, you may need a clear 50 pipe diameters to obtain an accurate flow reading.
Any ultrasonic flow meter will have difficultly giving you a reliable reading on a vertical pipe with a downward flow. The pipe surface must also be clean and free from rust. Also the ultrasonic flow meter transducers must be clean and have a substantial amount of couplant material to ensure a good acoustic connection.
While dealing with a horizontal pipeline, ensure that the Doppler transducers are installed on the top of the pipe. For a time of flight ultrasonic flow meter, install the transducers on the side of the pipe, this will allow more accurate flow readings as generally air bubbles are located on the top of a pipe while sediment falls to the bottom.
A typical time of flight ultrasonic flow meter will have two sets of transducers, one will be specifically for the upstream direction of the flow, while the other for the downstream. If you are unsure if the flow direction, you will normally see a negative flow rate if you have installed these incorrectly. If the transducers are installed correctly and you are still having a problem in obtaining a correct figure from your ultrasonic flow meter, try different sound path settings, this will require you to move your transducers up and down the pipe as the ultrasonic wave is reflected a different number of times on the pipe walls. Generally, the more transit paths, the more accurate your reading. However, the more transit paths can also lead to a weaker signal which can cause setup difficulties, this is why it is recommended that you always start with the simple “V” method.
Take care in setting the exact pipe measurements, transducer spacing and liquid type. If you have incorrectly programmed any of these variables in your ultrasonic flow meter you will have an incorrect reading, or no flow reading at all.
Once you have taken the time and accomplished your first ultrasonic flow meter installation, future measurements will be much easier.
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About the Author
Jim Furness is CEO of Omniinstruments Ltd, specialists in data logger and other instrumentation solutions such as ultrasonic flow meter.
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