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The first time I saw a portable ultrasonic flow meter (UFM) in action was about three decades ago. I wasn’t impressed then — the instrument was bulky and not very accurate. Well, things have improved immensely. Today’s devices identify the correct spacing for transducers, evaluate signal strength between transducers, and even assess the quality of the reading. They can serve as a powerful tool. For instance, flow measurements — in combination with differential pressure readings — are very useful for determining fouling, e.g., in heat exchangers, or establishing priorities in networks, like cooling water systems. In one balance, the error was only 1 gal/min!
Let’s start with the basics of a UFM. It consists of a transmitter, and two or four transducers; the transducers send and receive sound waves through a known pipe diameter and thickness. The velocity of the signal is shortest in the direction of flow; most UFMs will report a negative flow if the transducers are reversed. By default, the meters are set to water. If all you want is flow rate, then you don’t need a precise number for the fluid temperature; for mass flow measurement, use an infrared gun, measure the pipe surface temperature, and add 2°F for fluids hotter than ambient or subtract 2°F for colder fluids.
To attach the meter, you’ll need chain, rail or simply duct tape (my personal choice) and suitable paste or gel. Other necessary items include: 1) a wet wipe to clean the pipe (however, that may not suffice if the pipe is covered in bird droppings); 2) a Vernier caliper to measure the precise distance between the transducers to the nearest 0.001 in.; 3) a smartphone with a camera; 4) a flashlight; 5) a pocket knife; and 6) a wire brush. Also, find a messenger bag or knapsack to carry these items. Lastly, get a lightweight but sturdy lawn chair to hold the meter and the bag of gear, so you’re not trying to balance everything on a hot pipe, etc. Obviously, you’ll need a gas meter and, perhaps, a hot work permit to use the meter.
UFMs can measure flowing streams of slurries and even viscous liquids within reason. Like most liquid flow meters, 5–10% vapor is an approximate limit; UFMs are even more sensitive. Fully-developed turbulent flow (i.e., a Reynolds number, Re, >106) provides the best readings; I measured a 13% difference in readings, 180° apart, at Re = 7.2 ×105 for a vertical pipe. Laminar measurements require a longer length of straight pipe.
Choosing the correct transducer size can make your job a lot easier. The small ones work best for pipe diameters from ½ in. to 2 in.; medium transducers suit pipes up to 12-in. diameter, although I suppose you could use them for larger pipes, too. A chain with a turnbuckle is the easiest to use for small diameters but very difficult for pipe beyond 3 in. diameter; rails are useless in situations without large clearance around the pipe.
I prefer duct tape with medium transducers because they produce a strong signal for most of the pipe diameters of interest. The trick with tape is gluing all sides of the transducer. Add the paste or gel next. Then, strap the assembly to the pipe, securing the transducer tangentially to the wall. Use plenty of paste! Install the base transducer, then use the caliper to set the second one, repeating the gluing process.
Here’re some other tips when taking measurements. First, ensure the pipe is round and its surface smooth and free of rust and dents. Second, avoid hangs, as I call them, where you could drop chains or other equipment and never see them again — grating comes to mind; I always put a board above grating to address this risk. Third, don’t even think about making measurements on ladders; hauling 30 pounds of gear up to the top while you’re balancing on your toes to clip the transducers into place is a recipe for an accident! Last, use an ultrasonic thickness gauge, if needed, to verify the actual pipe wall thickness.
Finally, always remember a horizontal set-up is superior to a riser (flow upward) and vastly superior to a downer (flow downward); always use the biggest transducers you can; and, lastly, contact is everything.
