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Article #5:  Checking Flow TD Accuracy

July 17, 2006
We have found that one of the most commonly used applications for Flow Finder equipment has been to check the calibration of resistive flow transducers. In most of the Flow Finder product demonstrations that we've conducted over the years, one or more of the flow transducers installed on a route was found to be inaccurate or defective. While the importance of flow transducer accuracy has surprisingly been downplayed by manufacturers, we would like to give our reasons why accurate flow transducers are essential in analyzing the system and dispatching technicians. The most important reasons are:
  • The quality of a cable pressurization system is being based more and more on air flow per sheath mile of cable or per tube (3/8" tubing from a manifold or distribution panel to a cable). A poor System Quality Index (SQI) for a route can result in labor hours being expended to improve the system. An obviously inefficient use of labor hours would be to work on a good route that merely appears "bad" because of an inaccurate reading by a faulty flow transducer.

    A flow transducer reading can be verified by installing a Flow Finder on the air pipe at the transducer location. The Flow Finder will provide an accurate flow rate that can be compared with the transducer reading.
  • When a flow transducer indicates a flow increase at an air pipe manifold, a technician is typically dispatched to the field to find the cable leak. But the indicated flow increase may also be the result of a bad flow transducer, trouble on the pair, an incorrect transducer flow range input into the monitoring pair, or the installation of a transducer with an incorrect flow range.

    To begin looking for a leak without first verifying the accuracy of the flow transducer is both inefficient and frustrating. For comparison, think about the standard procedure for pressure transducer dispatches. When a pressure transducer reading drops, do you trust the reading or do you take a manual pressure reading at the transducer location?

    The same thing should apply to flow transducer readings. While you are at the flow transducer location, use a Flow Finder and Flow gauge to reconfirm that you are not chasing ghosts.
  • When the sum of the air pipe manifold flow transducer readings does not add up to the total flow at the pipe alarm panel, something is wrong. There are several possible reasons for this: 1) there's a leak somewhere in the air pipe, 2) a ghost manifold or cheater hose is installed somewhere on the pipe, 3) one of the flow transducers is bad (inoperative or inaccurate), or 4) the air pipes are transposed.

    The important thing to remember is that it is NOT OK to have flows that don't add up. For example, when the sum of the manifold flows on a route is 20 Standard Cubic Feet per Hour (SCFH) and the pipe alarm panel is flowing 50 SCFH, you've got a problem. You can't expect the index on this route to be accurate or your dispatching to be effective without first determining where the 30 SCFH is going.
So, whenever you encounter a situation where air flows don't add up, find out why. A few Flow Finders installed on a route will tell you more about your system in one afternoon than you ever thought possible.