The duct traverse with a pitot tube or an air foil is the most accurate way for the air balance technician to determine the quantity of air in a duct system or portion there of. At one time traverse readings were taken on all fans, supply, exhaust and return. The air balancer used the readings as a base line to formulate his outlet balancing and to determine if there was significant air leakage in the system. There are basically four reasons why duct traverses are not used often in the industry.
Air balancing is about accuracy so the first reason for the limited use of a traverse is there is not ample room in most systems to get a reliable reading. The traverse should be taken six to 10 duct diameters downstream from any elbow, transition or branch. Try and remember the last system that you had this much room. You can take a traverse with less room, but the accuracy will be in question.
The second reason the air balance technician doesn’t take a traverse is duct work doesn’t leak as much as it used to. Suppliers and installers have come to realize the value of sealing the ductwork before it is put into operation. The need for the air balancing technician to verify leakage has all but gone away. There are some high and medium pressure systems that should still be tested with a traverse, but the vast majority of low pressure systems no longer require the traverse
The third reason and air balance technician will not use a traverse is time. In most cases you are going to spend any where from 30 minutes to an hour locating the traverse, drilling the appropriate holes, taking the reading and analyzing the result. On most jobs you do not have this time.
The fourth reason is the accuracy of other air balance equipment. Over the last 20 years the flow hood has become almost if not as reliable as the traverse for taking readings at outlets. Most suppliers of flow hoods suggest that their hood will accurately measure to +/- 2%.