Flow Finder Tip #9

In this Tip of the Week article, we'd like to discuss why Flow Finders measure air flow more accurately than a portable flow rater. We've been asked repeatedly over the years why readings from a portable flow rater at an air pipe manifold never add up to the flow indicated by the flow transducer. There's a logical explanation for this, and we can prove it with a simple experiment.

When a flow rater is used to take a flow measurement at an air pipe manifold, its air chucks must first be installed on the manifold's pressure testing valves. Then, a shutoff valve on the manifold is closed and the air is redirected through the flow rater (as shown in the figure below). This rerouting of air flow is the actual cause of inaccurate readings. Because the air must travel through the flow rater, the air chucks, the pressure testing valves and the tubing, it is restricted considerably. This restriction reduces or slows down the "real" flow and causes a reading that is less than accurate. So, the higher the flow being measured, the greater the inaccuracy. In fact, it's not uncommon to have a flow transducer at a manifold indicate a flow of 18 Standard Cubic Feet per Hour (SCFH), while the flow rater shows a flow of 10 or 11 SCFH.

Because the Flow Finder uses a different concept for measuring air flow, this rerouting of air doesn't occur. The Flow Finder simply creates a slight pressure differential over a calibrated orifice, which can then be measured with the Flow Gauge (as shown in the figure below).

In order to see the discrepancies that occur when using a portable flow rater versus a Flow Finder, you many like to try the following experiment. Install a 0–19 SCFH Flow Finder either before or after the flow transducer on a manifold. Hook up the Flow Gauge to the Flow Finder. Now read each individual cable with a portable flow rater. As each cable is read, you'll discover that the flow indicated by the Flow Finder will be reduced each time air is rerouted through the portable flow rater (figure below). You'll also discover that trying to measure flow in an air pipe with the use of a portable flow rater and an air pipe shutoff valve is almost useless. In fact, you can only measure about 50% of the true flow. For these reasons, we believe that when it comes to verifying flow transducer readings or leak locating in the air pipe, the Flow Gauge and Flow Finders are the best tools for the job.

After you've tried the experiment described above, give us a call. We'd like to hear the results.

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