World Patents Pending ©AirSense Technology Ltd. 1999
ISSUE 2.4
Page 4
T E C H N I C A L
.
M A N U A L
Many principles have been used to detect fire and are still in use, although some are
only used in special applications. The following comments describe some of the more
commonly used principles:
All fires give off heat, and sensing the temperature of the air at a given point in a given
zone can indicate the presence of a fire occurring in it. The method is simply to sense
if the temperature rises above a pre-determined trigger level and give a warning. If the
trigger temperature is low, relatively early warning can be given. Unfortunately an
unwanted alarm could be generated by heating systems being switched on or a heat
wave occurring, taking it above the trigger temperature. If the trigger temperature is
high, the fire may be well alight before warning is given. Such a system is unlikely to
be ideal in many applications.
In order to improve on this situation, the ‘Rate of Rise’ temperature detector is more
often used. These sensors do not measure the actual temperature, but the rate at
which it increases. In this detector, the background temperature is largely ignored, but
if the rate at which the temperature rises is greater than a given amount, it is assumed
that this is due to a fire. Unfortunately, for general use, the unwanted alarm conditions
still occur even if not so readily as with the simple heat detector; also the fire must be
well alight to generate the rates of rise of temperature required to trigger them.
Heat detectors must either be strategically placed with respect to potential fire sources
or have a very high placement density in order to be effective. The heat of the fire must
also be transmitted to them rapidly.
Carbon products are given off in the form of smoke and gasses when a fire occurs. If
a small sample of air is ionised by means of a radioactive substance, then the ions will
allow a small electrical current to flow between two electrodes placed in the sample.
If the combustion products mix with the air in the sample then they will inhibit the
movement of the ions that then as a result de-ionises, and the electrical current will
decrease. This is the operating principle of the Ionisation detector. A low concentra-
tion in the air of the products of combustion is required to effect a detectable change
in the current, and this can make the detector quite sensitive. Unfortunately the
current is also effected by draughts in the air and to a lesser degree by humidity,
atmospheric pressure and temperature. Although these unwanted effects can be
Background
Ionisation Detectors
Heat Detectors
