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Article #2:  Air Flow Restriction

June 26, 2006
A concern that has been voiced in the past about our Flow Finders is that they restrict air flow. Some people believe that this restriction is significant enough to reduce cable pressure and actually jeopardize air pressure protection in the cable. Although we don't agree with this assessment, we recognize that it's a very real concern. Therefore, in this Flow Finder Tip we'd like to clarify our views on restriction and try to eliminate any uneasiness among those of you who might be using or are planning to use Flow Finders.

Flow Finders contain a calibrated internal orifice which creates a very slight, but measurable pressure differential as air flows through the device. Two tank valves installed on top of the Flow Finder above the orifice provide the means of measuring this differential. One valve measures uphill or incoming pressure; the other measures the downhill pressure on the back side of the orifice. This slight pressure differential is then converted by the Flow Gauge into a highly accurate flow rate. Because you can actually see the orifice inside of the Flow Finder, the question of a restriction has been raised. We acknowledge that a slight restriction does occur; otherwise, there could be no pressure differential to be measured.

You may not realize that this same principle pertains to resistive flow transducers which, of course, have been in use for many years in the telco industry. An orifice in the flow transducer—one that is not visible to the eye—causes a slight restriction in order to measure flow. The orifice that causes the pressure differential in a Flow Finder has approximately the same pressure drop as the orifice used in a standard flow transducer. In fact, all Flow Finders meet Bell Communication Research (Bellcore) Technical Reference TR-TSY-000183 for acceptable pressure drop. This acceptable pressure drop is less than or equal to the pressure drop created by a flow transducer.

One way to reduce concerns about Flow Finders restricting air flow and lowering cable pressure is to recall when you installed flow transducers. Did you see any drop in cable pressures? Probably not. And because Flow Finders and flow transducers use the same concept to measure flow, you won't see a drop in cable pressures when installing Flow Finders. There are a number of devices already installed in your system that generate a greater pressure drop than Flow Finders. For example, check valves, air pipe manifolds, and 3/8-inch tubing all impact the cable pressurization system. However, the effect is so small, that removing them accomplishes absolutely nothing.

When a drop in delivery pressure occurs in a pressurization system, we can almost guarantee that it's caused by a high flow condition (air pipe leaks or cheater hoses) and not the hardware installed in the system. Using Flow Finders on high flowing routes allows you to track down leaks and cheater hoses—helping to raise pressures, not bring them down.

Now that we've provided you with some information about the restriction issue, we hope you will try Flow Finders and see for yourself what takes place. Call us and we'll be glad to discuss arrangements for a field trial and share some eye-opening experiences we've had with technicians who have used the product. Also, don't hesitate to let us know if you have any questions about the information in this Tip of the Week, the Flow Finders in general, or any of our other products and services.