(Bob, from a previous thread)
I phoned Firex tech support and asked about using smoke cans for testing their alarms.
The black cannisters of smoke cans contain a silicon based propellant that blocks up the sensors if it is not given in the correct dosage using a hood. Firex sell this sort of spray:-)
The solo spray cans are fine.
The nice guy on the phone also pointed out that many people squirt the smoke into the grill on the front of the detector. This grill is only there to allow the siren to work properly and reach the required dB rating. All Firex smoke alarms have their smoke intakes through the outer edge of the detector.
One other point that he mentioned was that people often use the test button to test the alarm before using the spray. The test button on some models also acts as a hush button and so no amount of canned spray will make the alarm sound if the spray is used within 10 minutes of a push button test.
Adam