Figure 9-2.--Mercury thermostat and housing.

spaces that house combustible stores. Figure 9-2 is an

illustration of a mercury thermostat and its housing. The

thermostats require a free circulation of air for proper

operation. Barriers that obstruct the free circulation of

air should never be placed around thermostats, and

thermostats should not be installed in the path of supply

ventilation. As many thermostats as needed for the

prompt detection of high temperature can be connected

to any one line. Figure 9-3 is an illustration of three

thermostats connected in parallel on one line.

Mercury thermostats are available in three

temperature ratings: 105F, 125F, and 150F. The

Figure 9-3.--Three mercury thermostats connected in

105F thermostat is normally installed in magazines,

parallel.

while the 125F and 150F thermostats are normally

installed in storerooms and paint lockers. The

thermostats are identical except for the differences in

(fig. 9-4, view A) is installed on the overhead in the

temperature ratings. When replacing a defective

compart- ment or space to be protected. A four-pin

thermostat, you should always replace it with one

polarized plug fits into a socket base, allowing easy

having the same temperature rating.

replacement (fig. 9-4, view B). The major units of the

detector head are the inner and outer chambers and the

DETECTORS

cold cathode tube (fig. 9-4, view C). The detector

compares the air in the inner chamber with the air in

Detectors used with the alarm, safety, and warning

the outer chamber. When combustion gases and/or

systems include ionization detectors and dual-purpose

smoke are present in the air of the outer chamber, the

detectors.

cold cathode tube fires and supplies the current to

initiate an alarm.

Ionization Detectors

The air in the inner and outer chamber is made

Ionization detectors are used with the com-

conductive by a small quantity of radium (fig. 9-4, view

bustible gas and smoke detector system (circuit 4F)

C). Alpha particles given off by the radium have the

to detect the presence of combustible gases and

ability to ionize air into positive ions and negative

smoke. The combustion gas and smoke detector head

electrons. If this ionized air is introduced into an electric

Figure 9-2.--Mercury thermostat and housing.

9-6