There are three ways of performing tests on smoke detectors: electronic test, function test, sensitivity test. Duct detectors are smoke detectors inside a box attached to a duct. A duct detector samples the air inside the duct to see if there's smoke.
Magnets and keys test the electronics. The Electronic Tests show what the smoke or duct detector will do if the smoke detector senses smoke. Will the electronics shut down air handler fans, close dampers, close fire doors, etc. Magnets activating magnetic switches, or keys activating key-switches, fall into this category.
Magnets and keys activate and test the electronics; magnets and keys do not test the smoke sensing chambers in either area smoke detectors or duct smoke detectors.
In addition to testing the electronics, a Function Test also tests the smoke chamber. A function test uses artificial smoke from a can. The can atomizes oil to create a cloud of particles similar to real smoke. Inside the smoke chamber, these particles act like real smoke to initiate the alarm.
A function test is used to find out if an area smoke detector or duct smoke detector will sense smoke. A function test, however, does not find out how sensitive the smoke detector or duct detector is to smoke.
The Sensitivity Test shows exactly how much smoke it takes to activate a smoke detector into alarm. To do that, a cloud chamber is placed around a smoke detector. Smoke is slowly added to the cloud chamber, and when the smoke detector goes into alarm, the amount of smoke that was in the cloud chamber shows the sensitivity of the detector.
A case of smoke test cans (for function testing) can cost $120.00. By comparison that's not really expensive.
A sensitivity tester, complete with a cloud chamber and all the bells and whistles required to properly test smoke testers, costs more than $5,000.00 . . . and then you still buy specialized cans of artificial smoke. The Sensitivity Test takes five to ten times as long to test a single spot-type smoke detector than the Function Test takes.
But then, no one makes a cloud-chamber sensitivity tester that can be used in the field for duct smoke detectors.
True Testing of Duct Smoke Detectors
To find out what happens when a magnet or key is used on a smoke detector or a duct smoke detector, use a magnet or a key. Fires do not carry magnets or keys around with them, so to find out if a duct smoke detector senses smoke, test using canned smoke.