Auto Ignition Temperature

Auto ignition temperature is the minimum temperature at which a substance will spontaneously ignite in air without any external source of ignition, such as spark or flame.

              Ignition Temperature of common liquids and gases.

Name	Ignition Temperature(OF)
Acetaldehyde	347
Acetic acid	867
Acetone	869
Acetylene	581
Ammonia	1204
Benzene	928
Butane	788
Carbon	1292
Carbon monoxide	1128
Cyclohexane	473
Diethyl ether	320
Ethane	882
Ethyl alcohol	685
Ethyl benzene	810
Ethyl ether	356
Ethylene	842
Hydrogen	932
Gasoline	536
Iso-butane	864
Iso-pentane	788
Isopropyl alcohol	750
Methane	999
Methyl alcohol	867
Naphtha	550
n-Butane	550
n-Heptane	399
n-Hexane	437
n-octane	403
n-pentane	500
Propane	842
Styrene	914
Toluene	896
Xylene-m	982
Xylene-o	867
Xylene-p	984

IONIZATION SMOKE DETECTOR

A typical ionization chamber consists of two electrically charged plates and radioactive source (typically Americium 241) for ionizing the air between the plates .The radioactive source emits alpha particles that collide with the air molecules and dislodge their electrons as molecules loser electrons ,they become positively charged ion. As other molecules gain electrons, they become negatively charged ions. Equal numbers of positive ions and negative ions are created. The positively charged ions are attracted to the negatively charged electrical plate, while the negatively charged ions are attracted to the positively charged p[late. this creates a small ionization current that can be measured by electronic circuitry connected to the plates.(normal condition in the detector)

Particles of combustion are much larger than the ionized air molecules. As particles of combustion enter an ionization chamber, ionized molecules collide and combine with them. Some particles become positively charged & some become negatively charged. As these relatively large particles continue to combine with many other ions, they become recombination centers, and the total number of ionized particles in the chamber is reduced. This reduction in ionized particles results in decrease in the chamber current i.e. sensed by the electronic circuitry monitoring the chamber. When the current is reduced by a predetermined amount, a threshold is crossed and alarm condition is established.