Det-Tronics R7404 - System Sensitivity Considerations

7.1 9 95-8242
SYSTEM SENSITIVITY CONSIDERATIONS
Figure8showstheapproximaterelationbetweencounts
per second (cps) and distance.  From thiscurveit can
be seen, for example, that a 4 ft2 (0.37 m2) gasoline fire
at 60 feet (18 meters) will normally cause the detector to
generate 20 cps. The same fire at 40 feet (12 meters) will
generate about 50 cps. If a 2 ft2 (0.18 m2) fire at 20 feet
(6 meters) will generate about 100 cps, the same fire at
70 feet (21 meters) will generate about 8 cps. Because
of the complexity of the combustion process, the UV
tube count rate generated by different size fires viewed
from the same distance is difficult to predict with a high
degree of precision. In general, however, if a fire doubles
in size, the tube count rate is increased by approximately
60 percent.
NOTE
The count rate of any given detector will depend
upon the sensitivity of the sensor tube, the type
and amount of fuel, the distance between the
detector and the re, as well as various other
factors. Figure 8 illustrates the relative response
of a “typical” C7050 to various size gasoline res
based on minimum sensitivity standards for DE1888
UV sensor modules. Sensor modules with higher
sensitivity are available. Consult the Field Support
Group at Det-Tronics for information or assistance
concerning a specic detector tube type or a
specic combustible material.
Selection of controller sensitivity and time delay to be
used in a given application is dependent on the level of
hazard present and the action to be taken in the event
ofre.  The adjustable sensitivityandtime delayof the
R7404 allows it to meet the requirements of virtually any
application.
Thesystemcanbeadjustedtovarioussensitivitylevels
by programming the controller to respond at a pre-
determined detector tube count rate. This count rate is
dependent upon the intensity of the ultraviolet radiation
reaching the detector, which is a function of fuel, flame
size, distance from the detector, and the amount of UV
absorbing vapors that may be present.
Programming the controller to respond to a low count
rate results in high system sensitivity. Conversely,
programming the controller to require a high count rate
results in low system sensitivity. The presence of UV
absorbing vapors must be examined closely. Some
chemical and petrochemical vapors have very strong UV
absorption characteristics. See Table 2.
Referring to Figure 8 and considering the conditions
described above, the criteria for selecting a correct system
sensitivityadjustmentcanbeestablished.Forexample,
assume that the hazard to be protected is at a distance
of 23 feet (7 meters) from the detector. Assume that the
hazard is gasoline and that it is desired to produce an
alarm signal when a fire with a surface area of 1 square
foot (0.09 m2) develops. Reading on the horizontal
“Distance” axis of Figure 8, locate the vertical line at
approximately23feet(7meters).Followthislineuntilit
intersects the “1 square foot” curve. Note that this occurs
at the horizontal line of about 50 counts per second on
the vertical “Detector Output” axis. This means that the
controller should be adjusted to 48 cps sensitivity in
order to detect this size fire from 23 feet (7 meters). If the
detectors were located 30 feet from the hazard, it can be
seen that it would be necessary to use a more sensitive
(lower cps) setting.
The following 38 substances exhibit signicant UV
absorption characteristics. These are also generally
hazardous vapors. While generally of little consequence
in small amounts, these gases can restrict UV detection
if they are in the atmosphere in heavy concentrations. It
should also be determined whether or not large amounts of
these gases may be released as a result of a re-causing
occurrence.
Acetaldehyde Methyl Methacrylate
Acetone Alpha-Methylstyrene
Acrylonitrile Naphthalene
Ethyl Acrylate Nitroethane
Methyl Acrylate Nitrobenzene
Ethanol Nitromethane
Ammonia 1-Nitropropane
Aniline 2-Nitropropane
Benzene 2-Pentanone
1,3 Butadiene Phenol
2—Butanone Phenyl Clycide Ether
Butylamine Pyridine
Chlorobenzene Hydrogen Sulde
1-Chloro-1-Nitropropane Styrene
Chloroprene Tetrachloroethylene
Cumene Toluene
Cyclopentadiene Trichloroethylene
O-Dichlorobenzene Vinyl Toluene
P-Dichlorobenzene Xylene
If UV-absorbing gases may be a factor in a given application,
precautionary measures should be taken. Detectors can
be placed closer to the potential hazard area, and/or
the sensitivity of the detection system can be increased.
Contact the factory for further details.
Substances such as methane, propane, butane, hexane,
camphor and octane are not UV absorbing.
Table 2—UV Absorbing Gases and Vapors