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)
0.1111
1110
2110
319
418
517
617
814
913

1-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
0.1947
1944
2942
3939
4936
5932
6929
8919
9912

2-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
0.134182
134170
234165
334155
434140
534125
634115
83475
93446

3-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
0.182440
182420
282410
382390
482375
582340
682290
882195
982130

4-inch Orifice Plate

System Static Pressure (in.wg.)Min Leakage (cfm)Max Leakage with Cobra Tester (cfm)
0.1175680
1175650
2175645
3175625
4175600
5175540
6175465
8175350
9175250

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.