application of BMS or other suitable sensors to access openings.
d) Utility inlets/openings which have open fluid flow (sewers,
culverts, etc.) are challenges for the security system designer. Present
technologies to apply include: BMS on access covers; electric cable fence
sensors on inlet/outlet gratings; fabrication of metal gratings which do not
materially affect water flow but which have a sense wire within the grating;
and the use of pulsed infrared beams or other highly directional bistatic
sensors above the fluid level.
4.2 Volumetric Sensors - Interior This term is used to denote a sensor
which detects a change of state (normally motion) within a protected space.
Some volumetric sensor types are called "active" in that they flood an area
with a type of energy (e.g., ultrasonic, microwave, infrared beam) and
detect a change in that energy state. Other volumetric sensor types are
"passive" in that they monitor the change in state of a particular energy
which exists in the protected space, e.g., heat (infrared), sound (audio),
or light shades (CCTV), and detect a change in that energy state.
Volumetric sensors are restricted generally to detection within a confined
space inside a structure. Some models do have range limitations or other
detection pattern limitations which make positioning a critical factor in
determining sensor performance. The security system designer may have
selected the optimal sensor for a particular application, but lack of a
proper installation specification. Its improper placement, or lack of
quality installation supervision, will result in poor sensor performance and
poor protection of the desired area. Volumetric sensors may be used by
themselves or in conjunction with point sensors or other volumetric sensors
to provide a higher level of security. While very effective in proper
applications, they are subject to defeat by an intruder who knows and
understands what phenomena they sense and what their limitations are. Too
often, misapplication of volumetric sensors or poor installation facilitates
intruder success. While the highest levels of interior security may be
satisfied by multiple combinations of different volumetric sensors, such
sensor configurations may induce nuisance alarms through mutual interference
if complementary sensors are not selected. In summary, volumetric sensors
not only facilitate the task of the security system designer but also
challenge him to assure proper application.
4.2.1 Infrared Sensors. These types of sensors may be active or passive.
The "active" models are similar in operation to photoelectric beams and are
discussed in the paragraph titled "Photoelectric Sensors." This section is
limited to "passive" infrared sensors. Passive infrared (PIR) sensors
function on the principle that all objects emit infrared energy or heat as a
function of their ambient temperature. The sensor can detect sudden
temperature change in an object or the introduction of an object with a
different temperature (e.g., human body) within its field of view. If this
temperature exceeds the specific level or threshold, it will cause an alarm.
Since infrared energy is a form of invisible light, passive infrared sensors
generally consist of mirrors or lenses and mirrors which collect the emitted
infrared energy and focus it on a sensing element which converts the sensed
energy into an electrical impulse. This sensing element may be a
thermistor, thermopile, or pyroelectric element. Since the human body
radiates in the