Use the graph or tables below to determine the range of air leakage you can test with the Cobra using our standard orifice plates.

Note that the data is based on laboratory tests with short lengths of hard pipe attached to the duct leakage tester. Actual field performance will likely result in reduced performance due to: (1) amount of flexible duct used; (2) number of bends in flexible duct; (3) size of duct system; (4) line voltage. To account for flexible duct and 90 degree bends, use our program that takes these in-line losses into account.

1/2-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
1110
218
318
417
517
616
813

1-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
1938
2936
3933
4930
5926
6921
899

2-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
134155
234150
334145
434135
534125
634100
83435

3-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
182415
282410
382390
482375
582340
682290
882115

4-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
1175655
2175645
3175630
4175600
5175545
6175465
8175200

Min Leakage is the recommended minimum flow rate based on a 0.4 in.wg. pressure drop across the orifice plate. For example, approximately 82 cfm through the 3-inch orifice plate results in a 0.4 in.wg. so 82 cfm is the recommended minimum leakage. Though it possible to use the orifice plates at pressure drops lower than 0.4 in.w.g, note that accuracy of the reading is decreased.