GB1586967A - Gas detection apparatus - Google Patents

Description

(54) IMPROVEMENTS IN GAS DETECTION APPARATUS (71) We, HORSTMANN GEAR GROUP LIMITED formerly known as THE HORSTMANN GEAR COMPANY LIMITED, a British Company of Newbridge Works, Bath, BA1 3EF, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to gas detection equipment.

There are now available certain semiconductor sensor elements which are responsive to the presence of a particular gas or gases in the ambient environment, for example by changes of resistance thereof. The invention aims to provide a gas detector unit incorporating such a sensor elements and which has good reliability and performance. In particular, it is a disadvantage that during the warm up period of the sensor element, such a unit may give false alarms.

Accordingly, the invention provides a gas detector unit comprising a semiconductor sensor element sensitive to the presence of a gas to produce an output signal, alarm signal output means for operating an alarm and including a comparator capable of two output conditions in dependence upon the presence of a concentration of a gas sufficient to cause a predetermined change in said output signal, one of said output conditions being effective to cause operation of an alarm, a delay element producing a delay element output for a predetermined time period after switching on the unit, and a gate receiving said delay element output which blocks the output of the alarm signal output means until the expiry of said predetermined time period.

The accompanying drawing shows a practical embodiment of a gas detector unit according to the invention, in block diagram form.

Power from a d.c. supply passes through a diode bridge 1 and filter 2. The diode bridge 1 prevents the possibility of damage or malfunction due to the connection of a supply of incorrect polarity. The filter 2 prevents noise on the d.c. supply causing malfunction. The inclusion of the diode bridge and filter also allows operation from a low voltage a.c. supply such as might be obtained from the mains via a small transformer.

A stabilized voltage supply for the gas detector element heater is provided by a voltage stabilizer 3 to maintain a stable operating point in spite of supply voltage variations. This stabilized supply is used as a reference to obtain a second stabilized voltage from a further voltage stabilizer 4 for the sensor 5 of the gas detector element to thus further maintain a stable operating point. The output voltage 6 of the sensor 5 is compared in a comparator 8 with a preset reference voltage 7 (derived from the second stabilizer 4). The detection of a predetermined concentration of gas will cause the output voltage 6 to exceed the preset voltage 7 and the alarm will sound.

To prevent false alarms when the gas detector is first energised, before the heater element has reached its correct operating temperature, a delay circuit 9 is included.

This circuit 9 prevents the alarm from sounding by controlling a diode gate 10. An inhibit signal 11 is fed to the gate until a fixed time has elapsed after switch on.

When this time has elapsed an indicator 12 is illuminated also, to show that the gas detector is operating.

In addition to these features, the circuit can be equipped with an output relay which will allow the remote switching of signal alarms, gas valves, etc. in the event of a fault condition being detected.

The unit described includes both the advantageous delay feature, and also be nefits from the fact that the comparator 8 provides a simple GO/NO GO indication depending upon the signal 6 from the sensor reaching a certain level. This is much more reliable than the direct use of the analogue value of the signal 6 to operate an alarm.

WHAT WE CLAIM IS: 1. A gas detector unit comprising a semiconductor sensor element sensitive to the presence of a gas to produce an output signal, alarm signal output means for operating an alarm and including a comparator capable of two output conditions in dependence upon the presence of a concentration of a gas sufficient to cause a predetermined change in said output signal, one of said output conditions being effective to cause operation of an alarm, a delay element producing a delay element output for a predetermined time period after switching on the unit, and a gate receiving said delay element output which blocks the output of the alarm signal output means until the expiry of said predetermined time period.

2. A gas detector unit as defined in claim 1, wherein said sensor element suffers change of resistance in the presence of a gas.

3. A gas detector unit as defined in claim 1, wherein said comparator compares an output signal from the sensor element with a stabilised voltage.

4. A gas detector unit as defined in claim 3, which includes a source of stabilized voltage which is fed to the sensor element and to the comparator.

5. A gas detector unit as defined in claim 1 wherein said delay element provides a signal to an indicator which, after expiry of said predetermined time period, shows said unit to be operational.

6. A gas detector unit substantially as herein described and as illustrated in the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. nefits from the fact that the comparator 8 provides a simple GO/NO GO indication depending upon the signal 6 from the sensor reaching a certain level. This is much more reliable than the direct use of the analogue value of the signal 6 to operate an alarm. WHAT WE CLAIM IS:

1. A gas detector unit comprising a semiconductor sensor element sensitive to the presence of a gas to produce an output signal, alarm signal output means for operating an alarm and including a comparator capable of two output conditions in dependence upon the presence of a concentration of a gas sufficient to cause a predetermined change in said output signal, one of said output conditions being effective to cause operation of an alarm, a delay element producing a delay element output for a predetermined time period after switching on the unit, and a gate receiving said delay element output which blocks the output of the alarm signal output means until the expiry of said predetermined time period.

2. A gas detector unit as defined in claim 1, wherein said sensor element suffers change of resistance in the presence of a gas.

3. A gas detector unit as defined in claim 1, wherein said comparator compares an output signal from the sensor element with a stabilised voltage.

4. A gas detector unit as defined in claim 3, which includes a source of stabilized voltage which is fed to the sensor element and to the comparator.

5. A gas detector unit as defined in claim 1 wherein said delay element provides a signal to an indicator which, after expiry of said predetermined time period, shows said unit to be operational.

6. A gas detector unit substantially as herein described and as illustrated in the accompanying drawings.