DETECTEUR DE GAZ INFLAMMABLE

DETECTOR APPARATUS OF INFLAMMABLE GAS

Abrégé anglais

ABSTRACT
In order to detect the presence of inflammable gas, a detector
element is utilized consisting of a coil made with a thin wire of a platinum
alloy, enclosed in a Davy cage, the detector is inserted in a resistor bridge
circuit and the resistance of the detector changes according to the concen-
tration of the inflammable gas so that above a predetermined concentration the
bridge becomes unbalanced to such a degree as to give rise to an electric vol-
tage signal having an amplitude proportional to the density (per cent) of gas
present. This signal is utilized to electronically control an alarm and/or
telesignaling and/or intervention device. A potentiometer resistor R3 inserted
in the bridge circuit, permits the intervention threshold of detector element
to be changed.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for detecting inflammable gas comprising in
combination: a power supply section, a resistor bridge circuit,
which includes a single gas detector element formed by a coil of
platinum alloy wire the resistance of which varies according to
the concentration of the inflammable gas, comparator means con-
nected across the bridge circuit and arranged to provide a trig-
ger signal when the bridge circuit becomes unbalanced due to the
concentration of the inflammable gas in the vicinity of the gas
detector element exceeding a predetermined threshold, a tone
oscillator connected to the comparator means for actuation by
the trigger signal and an electroacoustic transducer connected
to the tone oscillator for actuation thereby.
2. Apparatus according to claim 1 wherein said coil of
platinum alloy wire is enclosed in a Davy cage.
3. Apparatus according to claim 1 wherein said bridge cir-
cuit includes a variable resistor for adjusting the balance
condition of the bridge circuit, thereby to change the predeter-
mined threshold of the detector element.
4. Apparatus according to claim 1 including means to convey
ambient air onto the detector element.
5. Apparatus according to claim 1 wherein the power supply
section comprises a primary supply section and an ancillary sup-
ply section including a battery buffer connected to an automatic

recharge circuit and a signaling device of mains drop and battery
break down and a timer circuit to permit the intermittent opera-
tion of the apparatus.

Description

1~3750~i
The invention relates to an apparatus for detecting inflammable
gas.
It is known in both industrial and domestic use of combustible
gases that the high degree of safety needed in the operating of plants is a
technical problem of primary importance since it is necessary to minimize the
possibility of a spill and consequent fires or injury to people and damage to
property due to the toxicity of such inflammable gases.
Accordingly, prevention devices which are able to detect the pre-
sence of dangerous concentrations of gas and, thus, control the operating of
warning means or more complex emergency apparatus, are of particular importance
especially when equipment using inflammable gas is operated in closed rooms.
It is clear that a high degree of sensitivity and operation reliability are the
basic, constant characteristics that the detector device must present.
The detector apparatus of the present invention meets these
requirements to a high standard; in addition, its electronic operation provides
a quick and reliable response and great flexibility in use.
The sensing element used is essentially made up of a platinum alloy
wire wound to form a closed coil which is able to change its electrical resis-
tance as a function of the density (%) of inflammable gas surrounding the coil.
This sensing element, once inserted in a branch of the bridge circuit made up
of resistors having constant resistance is able to derive control signals
from the resistance variations which are due to the presence of traces of gas,
for the control of electronic circuits which are provided to obtain an alarm
or intervention.
The invention will now be described in greater detail with refer-
ence to the accompanying drawing in which:
-- 1 --

~3'7~
Figure 1 is a block diagram of an inflammable gas detector apparatus
according to the invention; and
Figure 2 is a circuit diagram of the detector apparatus of Figure 1.
Reduced to its basic structure and with reference to ~igure 1
of the accompanying drawings, the detector includes a power supply section
including a mains feeder A provided with a rectification bridge transformer
(shown in Figure 2) suitably stabilized by means of known electronic circuits.
The detector also includes a bridge circuit having resistors PM, a branch of
which is made up of a detector element R for picking off the gas; the detector
element is shaped as a coil consisting of a thin wire of platinum alloy enclosedin a cage of metallic net (~avy type) in order to prevent flame propagation in
case of gas ignition around the coil.
Sucking and conveying means As for the ambient air to be controlled,
including a sucking pump and canalization ducts for increasing the sensitivity
of said detector element R are also provided.
A tone oscillator G and an electroacoustic transducer cascade
connected are also provided.
More particularly, and according to a preferred embodiment of the
invention as shown in ~igure 2, the mains feeder A is provided in order to
obtain greater stability, with a transistor Tl, a Zener diode Zl and an inte-
grated circuit ICl.
The bridge circuit PM is made up of two resistors R2 and R4 having
constant resistance, a resistor R3 having a variable resistance and the above
mentioned detector R. As can be seen R2 and R3 are connected in series to form
one arm of the bridge and R4 and R are connected in series to form the other
arm of the bridge. The junctions of R2 and R3 and of R4 and R are connected
to the input terminals 2-3 of a differential amplifier Ad whose output 6 is
-- 2 --

~237505
brought to the terminal ~ of an integrated circuit IC2 of the oscillator G
which operates as a generator of low frequency oscillations. The output 3 of
integrated circuit IC2 is connected, through a capacitor C8 and a resistor R7
in series, to the above-mentioned electroacoustic device TA.
In order to avoid a possible fault in the apparatus operation owing
to a power drop, an ancillary supply section Al (Figure 1) is provided includ-
ing: a battery, buffer connected to the relevant automatic recharge circuit,
a warning device for warning of mains power drop and battery break down and a
timer circuit Tp providing for the activation of the ancillary supply section
after a predetermined interruption the length of which may be adjusted to
suit.
The operation of the device is as follows. In case of an increase
of inflammable gas concentration around the sensor R, which brings about a
resistance variation of sensor R, an electric voltage unbalance takes place
between the terminàls 2 and 3 of the amplifier Ad; and this signal, suitably
amplified, triggers the working of the oscillator G which, in turn, activates
the transducer TA.
Such operating condition will be maintained throughout the time
the sensor detects a gas percentage greater than the predetermined one. The
predetermined percentage is set up as desired by adjusting variable resistors
R3 to vary the intervention threshold i.e. the balance condition of the bridge.
Typical values of the essential components which are used in an
embodiment of the invention are as follows:
Cl = 2000 ~P R5 = 10 Kohm
C = C3 = C6 = C7 = 10 ~F R6 = 100 Kohm
C4 = 0.1 ~F R7 = 22 Ohm 1/2 W
C8 = 100 ~F Tl = BC 140

~7~37505
Rl = 470 Oh~ Zl = Zener 12 V/0.4 W
R2 = 220 Ohm Ad = llA741
R3 = 100 ~ 470 Ohm ICl = LM 340T5.0
R4 = 6. 8 Ohm IC2 = NE555