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The sensing chamber of the XP95 ionisation smoke monitor operates
in exactly the same way as the Series 60 ionisation smoke detector.
The air in the dual chamber is irradiated to produce ions that travel
to the positive and negative electrodes and hence create a current
flow in the chambers. Smoke entering the outer chamber causes a
drop in the current flow and an increase in the voltage measured
at the junction between the outer and the inner chambers.
Unlike the Series 60, XP95 does not operate on
a threshold basis. The analogue voltage signal produced in he sensing
chamber is converted to a digital signal by the electronic circuitry
and transmitted to the control equipment on interrogation. The microprocessor
in the control equipment then compares the signal with stored data
and initiates a pre-alarm or fire alarm as smoke density increases.
When the equipment determines that a fire condition exists, it instructs
the monitor to switch on its indicator LED and the pre-planned alarm
routine is initiated.
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Isolators are designed to protect the XP95 loop in the event of
a short circuit fault. They divide a loop of fire monitors and ancillary
devices into groups of 20 as a rule, so that, in the event of a
short circuit, no more than 20 devices will be inoperable. The XP95
isolator has particularly low non-isolating resistance.
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When activated, the XP95 manual call point not only interrupts the
polling cycle to indicate to the control panel that it has been
operated, but also reports its address. Thus an alarm and its location
can be reported in less than 0.2 seconds.
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The XP95 optical smoke monitor uses the same optical arrangement
as the Series 60 optical smoke detector, with an internal pulsing
LED and a photo-diode at an obtuse angle. In clear air conditions
the photo diode in the XP95 monitor receives no light from the LED
and produces a corresponding analogue signal. This signal increases
when smoke enters the chamber and light is scattered onto the photo-diode.
The signal is processed by the electronic circuitry and transmitted
to the control equipment in exactly the same way as in the case
of the ionisation smoke monitor.The optical smoke monitor is externally
identical to the ionisation monitor but is distinguished by having
a clear indicator LED which emits red light when the monitor is
in alarm.
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The XP95 temperature monitor is distinguishable from XP95 smoke
monitors by its low air flow resistance case which allows good contact
between the sensing thermistor and the surrounding air.The device
monitors temperature by using a single thermistor network which
provides a voltage output proportional to the external air temperature.
The voltage signal is processed and transmitted to the control equipment
in the same way as in the case of the ionisation smoke monitor.
A temperature monitor for use in ambient safe temperatures
of up to 50 degrees celsius and which reaches the alarm level at
90 degrees celsius is also available.
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