Before installing an ultrasonic flow meter the most important consideration is the medium to be measured. Raw sewage usually requires a Doppler ultrasonic flow meter whilst distilled or clean water requires a time of flight ultrasonic flow meter. However, what about applications that use a combination of liquids or liquids with small particles? For applications where the media has particles of 100 PPM or more a Doppler ultrasonic flow meter would be the most suitable device to use. For media with particles smaller than 100PPM, such as oil, sea water, or even gray water a time of flight ultrasonic flow meter types are most suitable.
A time of flight ultrasonic flow meter requires a totally full pipe to work. While a typical Doppler ultrasonic flow meter is more suited to a completely full pipe, it is possible under certain conditions to utilise these units on partially full pipes.
Sensor Installation
In an ideal condition a straight length of horizontal pipe that is at least fifteen pipe diameters long is required. This should be free of valves, inflow and outflow diffusers and should be as far away from pumps as possible. When using a time of flight ultrasonic flow meter on an installation that has a pump or T-section, a straight length of up to 50 pipe diameters to obtain an accurate flow reading.
Most ultrasonic flow meter models are unable to measure downward flow in a vertical pipe. The pipe surface must also be clean and free from rust or paint bubbles. Also the ultrasonic flow meter transducers must be clean and have a substantial amount of couplant material to make a good acoustic connection.
For horizontal pipes a time of flight ultrasonic flow meter should have the transducers fitted 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. Doppler transducers are generally installed on the top of the pipe.
A typical time of flight ultrasonic flow meter will have two 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 liquid type, sensor spacing and pipe measurements. If you have incorrectly programmed any of these variables in your ultrasonic flow meter you will get either 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.
Jim Furness is the CEO of Omniinstruments Ltd, specialists in <a href="http://www.omniinstruments.co.uk/">data logger</a> and other instrumentation solutions such as <a href="http://www.omniinstruments.net/">Ultrasonic Flow Meter</a>.
