GB2313691A

GB2313691A - Heater with CO gas sensor

Info

Publication number: GB2313691A
Application number: GB9611414A
Authority: GB
Inventors: John Francis Lindsay
Original assignee: Zellweger Analytics Ltd
Current assignee: Honeywell Analytics Ltd
Priority date: 1996-05-31
Filing date: 1996-05-31
Publication date: 1997-12-03
Anticipated expiration: 2016-05-31
Also published as: GB9611414D0; GB2313691B

Abstract

A heater 1 such as a paraffin heater, has a carbon monoxide sensor module 16 located in the air stream to a burner 11 . As a result, the sensor is responsive to the ambient carbon monoxide concentration, rather than the elevated concentration found in the air stream containing the combustion products as is the case in conventional designs. The heater is arranged to be inoperative with the sensor module removed. The sensor module includes a compensation component which is used to tailor carbon monoxide concentration threshold values to the sensitivity of the sensor . The thresholds are used to identify various alarm conditions.

Description

Heater with Gas Sensor Description The present invention relates to a heater liable to produce carbon monoxide during combustion of its fuel and having a sensor for detecting carbon monoxide.

Heaters using paraffin or other hydrocarbons as a fuel are well-known. It is also known that carbon monoxide is produced by paraffin heaters. Carbon monoxide is poisonous and can be fatal if inhaled in sufficient quantities. For this reason, carbon monoxide sensors have been incorporated into heaters to sound an alarm and extinguish the heater's flame, if the concentration of carbon monoxide approaches harmful levels.

In the prior art, the carbon monoxide sensors are located in or near the path of the combustion products. However, this arrangement suffers from a disadvantage in that is gives an inaccurate measure of the carbon monoxide concentration in a room containing a heater. It is this concentration which is experienced by people in the room.

According to the present invention, there is provided a heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, an air path to the burner along which air flows to the burner to support combustion thereat and a carbon monoxide sensor, wherein the carbon monoxide sensor is located for sensing the concentration of carbon monoxide in the air in said air path. The carbon monoxide sensor may be an electrochemical, semiconductor, optical or colorimetric type.

Since the sensor is mounted upstream of the burner, it gives a more accurate reading of the carbon monoxide concentration in the room. Furthermore, it is less prone to overheating.

According to a second aspect of the present invention, there is provided a carbon monoxide sensor module comprising one part of a releasable electrical connector, a battery and a carbon monoxide sensor. Preferably, the carbon monoxide sensor comprises an electrochemical sensor.

Advantageously, a carbon monoxide sensor module includes a compensation component, wherein the compensation component has a value having a predetermined relationship to the sensitivity of the sensor. The compensation component may comprise a resistor.

According to the present invention, there is further provided a heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, a part of an electrical connector for receiving a compensation component-containing module according to the present invention, and a plurality of comparators for comparing the output of the sensor with respective reference values, wherein the reference values are generated in dependence on the value of the reference component. Thus, the control electronics of the heater do not need to be re-calibrated when the sensor is replaced. Instead, the "calibration" step is performed during manufacture of the module.

Preferably, a heater, according to the present invention, includes means for extinguishing the flame of the burner, wherein the means for extinguishing the flame of the burner is responsive to the output of one of the comparators to extinguish the flame when the sensed carbon monoxide concentration exceeds a predetermined level, e.g. 850 - 1150 ppm.

Preferably, a heater, according to the present invention, includes means for extinguishing the flame of the burner and a delay, wherein the delay is arranged to produce an output at a predetermined period after a change of state of the output of one of the comparators and means for extinguishing the flame of the burner is responsive to the output of the delay to extinguish the flame said predetermined period after the sensed carbon monoxide concentration exceeds a predetermined level, e.g. 80 - 110 ppm.

Preferably, a heater, according to the present invention, includes an alarm means, wherein the alarm means is responsive to the output of one of the comparators to indicate an alarm condition when the sensed carbon monoxide concentration exceeds a predetermined level. More preferably, a delay is arranged such that the alarm condition is indicated by the alarm means a predetermined period after the alarm condition is signalled by the output of said comparator. The alarm means may produce audible or visible or audible and visible alarm signals. Means for transmitting the alarm condition to a central station may also be provided.

According to the present invention, there is still further provided a heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, a part of a releasable electrical connector for receiving electrical conneaor part of a module according to the present invention and interlock means for inhibiting operation of the burner if the parts of the releasable electrical connector are separated.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a heater according to the present invention; Figure 2 is a front view of the heater of Figure 1 with the front wall cut away; and Figure 3 is a schematic diagram of the circuitry of the heater of Figure 1.

Referring to Figures 1 and 2, a paraffin heater 1 comprises a case 2, having a bowed front face 2a, first and second rectangular end faces 2b, 2c, a top face 2d, a flat rear face 2e and a bottom face 2f. A rectangular aperture 3 extends across the front face 2a and is covered by a grill 4. Three parallel slots 5 are provided at each end of the top face 2d. The bottom margin of the front face 2a is cut away over substantially the whole width of the front face to provide an opening 6.

The interior of the heater 1 is divided into an upper chamber 7 and a lower chamber 8 by first and second plates 9,10 extending horizontally from midway along the end walls 2b, 2c. A burner assembly 11 extends vertically between the free ends of the first and second plates 9, 10, from a wick adjusting mechanism 12, which is in turn mounted on a fuel tank 13. A circuit board 14, containing control circuitry, a carbon monoxide sensor module 15 and a sounder 16, is located at the bottom of the lower chamber 8 beside the fuel tank 13. A temperature sensor 17 is mounted to the first plate 9 in the upper chamber 7 and is elearically coupled to the circuit board 14.

When the heater 1 is in operation, air for supporting combustion is drawn in through the opening 6 as indicated by the arrows marked A in Figure 1 and then up through and beside the burner assembly 11, as shown by the arrows marked B in Figure 2. The hot combustion produas escape from the heater 1 through the slots 5 along the paths indicated by the arrows marked C in Figure 1.

The control circuitry of the heater 1 will now be described with reference additionally to Figure 3. In the interests of clarity, only those circuit elements necessary for the understanding of the present invention are shown in Figure 3. However, it is to be understood that the control circuitry includes power supply distribution lines, bias generating circuits and buffers.

Referring to Figure 3, the sensor module 15 is provided with contact pins and is releasably plugged into a corresponding socket mounted to the circuit board 14. The sensor module 15 comprises an electrochemical carbon monoxide sensor 20, a nine volt battery 21 and a compensation resistor 22. The purpose of the compensation resistor 22 will become apparent from the following description.

With the sensor module 15 in place, the output of the sensor 20 is coupled to the input of a sensor cell amplifier 23. The sensor cell amplifier 23 is powered by the battery 21. The compensation resistor 22 is connected between the sensor 20 and an input to a threshold generating circuit 24. The threshold generating circuit 24 generates three threshold voltages which are applied to the inverting inputs of first, second and third comparators 25, 26, 27 respectively. The non-inverting inputs of the first, second and third comparators 25, 26, 27 receive the output of the sensor cell amplifier 23. The threshold voltage applied to the first comparator 25 is arranged to correspond to a carbon monoxide concentration of approximately 45 ppm. The threshold voltage applied to the second comparator 26 is arranged to correspond to a concentration of approximately 95ppm and the threshold voltage applied to the third comparator 27 corresponds to a concentration of approximately 1000 ppm.

When the sensor 20 is exposed to gas, it generates a current. The compensating resistor 22 is chosen so that the resulting signal applied to the threshold generating circuit 24 compensates for any variation in the sensor sensitivity so that the thresholds voltages applied to the first, second and third comparators 25, 26, 27 are appropriate for the particular sensor 20.

The output of the first comparator 25 is coupled to control an oscillator 28 which in turn drives a first LED 29.

The output of the second comparator 26 is applied to a first timer circuit 30.

The output of the first timer circuit 30 is applied to the input of a second timer 31 and also to the sounder 16. The first and second timers 30, 31 are configured to time periods of the order of 4 to 6 minutes. The output of the second timer 31 is applied to an input of a NOR gate 32.

The output of the third comparator 27 is also applied to an input of the NOR gate 32.

The positive terminal of the battery 21 is coupled to the inverting input of the fourth comparator 33. The non-inverting input of the fourth comparator 33 is held at a reference voltage, having a value corresponding to the output voltage of the battery 21 towards the end of its life. The output of the fourth comparator 33 is applied to an input of the NOR gate 32.

The positive terminal of the battery 21 is also coupled to the inverting input of a fifth comparator 34. The non-inverting input of the fifth comparator 34 is held at a reference voltage slightly higher than that applied to the noninverting input of the fourth comparator 33. The output of the fifth comparator 34 drives a second LED 35.

The temperature sensor 17, which may be a thermistor circuit, is coupled to an input of a sixth comparator 36. The sixth comparator 36 compares the output from the temperature sensor 17 with a reference voltage corresponding to a dangerously high temperature. The output from the sixth comparator 36 is applied to an input of the NOR gate 32.

The output from a fuel level sensor (not shown), such as a float sensor in the tank 13, is applied to drive the third LED 37 and to trigger a third timer 38 when the fuel in the tank 13 falls below a predetermined level. The third timer 38 has a period of approximately 2 seconds and its output is connected to the sounder 16.

The sounder 16 includes circuitry 40 which responds to an input signal by driving a speaker 41 to omit 2 second bursts of sound in a cycle having a period of 7 seconds.

The output of the NOR gate 32 is applied to a control element in the wick adjusting mechanism.

Power for the control circuitry, except for the sensor cell amplifier 23, and the wick adjusting mechanism 12 is provided by a battery 42.

The operation of the heater of Figures 1 to 3 will now be described.

The operation of the heater under normal conditions is conventional.

Therefore, the present description will concentrate on the special features of the present embodiment.

Under normal operation, the sensor 20 responds to the concentration of carbon monoxide in the air drawn into the heater 1 and produces an output current proportional to that concentration. The output from the sensor 20 is amplified by the sensor cell amplifier 23 and is applied to the first, second and third comparators 25, 26, 27. The compensation resistor 22 is selected during manufacture of the module 15 such that the threshold voltages, applied to the first, second and third comparators 25, 26, 27, correspond to the voltages output by the sensor cell amplifier 23 at carbon monoxide concentrations of 45, 95 and 1000 ppm respeaively. In this way, sensitivity variations between sensors 20 can be accommodated.

If the carbon monoxide concentration, sensed by the sensor 20, rises above 45 ppm, the output of the first comparator 25 changes from low to high, switching on the oscillator 28 which then causes the first LED 29 to flash.

If the carbon monoxide concentration increases further and reaches a level of 95ppm, the output of the second comparator 26 goes high triggering the first timer 30. After four to six minutes, the output of the first timer 30 goes high causing the sounder 16 to sound an alarm. The change of level of the output of the first timer 30 triggers the second timer 31. After four to six minutes, the output of the second timer 31 goes from low to high causing the output of the NOR gate 32 to go from low to high also. The change of state of the NOR gate 32 causes the wick adjusting mechanism to retract the wick in the burner assembly 11, thereby extinguishing the flame.

If the carbon monoxide concentration rises rapidly to 1000 ppm, the wick will be retracted immediately as a result of the change in level of the output of the third comparator 27 and the resultant change of state of the output of the NOR gate 32.

Should the module 15 be removed, the outputs of the fourth and fifth comparators 33, 34 will both go high. The output of the fifth comparator 33 will then cause the output of the NOR gate 32 to go high causing the wick to be retracted. The change in the output of the fifth comparator 34 will turn on the second LED 35. If the module 15 is not present when an attempt is made to start the heater, the wick mechanism 12 is disabled, preventing the wick being raised. Since, the wick cannot be raised, the burner 11 cannot be lit.

As the battery 21 approaches the end of its useful life, its output voltage will decline. When it declines to a first level, the output of the fifth comparator 34 will change state causing the second LED 35 to light up. A user will see that the second LED 35 is on and understand that the module 15 should be replaced. If the module 15 is not replaced at this point, the output voltage of the battery 21 will continue to decline, eventually causing the output of the fourth comparator 33 to go high. The NOR gate 32 responds to the change in state of the output of the fourth comparator 33 by sending its own output high, thereby causing the wick to be withdrawn.

If the temperature in the heater 1 rises to a dangerous level, the sixth comparator 36 will respond to the output of the temperature sensor 17 crossing the threshold value by sending its output high, causing the output of the NOR gate 32 to go high as well. As before, the output of the NOR gate 32 going high causes the wick to be withdrawn.

If the level of fuel within the fuel tank 13 falls below a threshold, a float switch applies a voltage to the third LED 37 turning it on, and at the same time triggers the third timer 38. The output of the third timer 38 goes high for approximately 2 seconds, sufficient to cause the sounder 16 to issue one 2 second burst of sound.

The present invention has been described with reference to an embodiment employing simple components. However, the functions of the circuitry, shown in Figure 3, may be largely replaced by a suitably programmed microprocessor or microcontroller.

Claims

1. A heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, an air path to the burner along which air flows to the burner to support combustion thereat and a carbon monoxide sensor, wherein the carbon monoxide sensor is located for sensing the concentration of carbon monoxide in the air in said air path.

2. A carbon monoxide sensor module comprising one part of a releasable electrical connector, a battery and a carbon monoxide sensor.

3. A carbon monoxide sensor module according to claim 2, wherein the carbon monoxide sensor comprises an electrochemical sensor.

4. A module according to claim 2 or 3, including a compensation component, wherein the compensation component has a value having a predetermined relationship to the sensitivity of the sensor.

5. A module according to claim 4, wherein the compensation component comprises a resistor.

6. A heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, a part of a releasable electrical connector for receiving electrical connector part of a module according to any one of claims 2 to 5 and interlock means for inhibiting operation of the burner if the parts of the releasable electrical connector are separated.

7. A heater for burning a fuel which is liable to produce carbon monoxide as a combustion product, the heater comprising a burner, a part of an electrical connector for receiving a module according to claim 4 or 5, and a plurality of comparators for comparing the output of the sensor with respective reference values, wherein the reference values are generated in dependence on the value of the reference component.

8. A heater according to claim 7, including means for extinguishing the flame of the burner, wherein the means for extinguishing the flame of the burner is responsive to the output of one of the comparators to extinguish the flame when the sensed carbon monoxide concentration exceeds a predetermined level.

9. A heater according to claim 7 or 8, including means for extinguishing the flame of the burner and a delay, wherein the delay is arranged to produce an output at a predetermined period after a change of state of the output of one of the comparators and means for extinguishing the flame of the burner is responsive to the output of the delay to extinguish the flame said predetermined period after the sensed carbon monoxide concentration exceeds a predetermined level.

10. A heater according to claim 7, 8 or 9, including an alarm means, wherein the alarm means is responsive to the output of one of the comparators to indicate an alarm condition when the sensed carbon monoxide concentration exceeds a predetermined level.

11. A heater according to claim 10, including a delay arranged such that the alarm condition is indicated by the alarm means a predetermined period after the alarm condition is signalled by the output of said comparator.

12. A heater according to claim 10 or 11, wherein the alarm means produces an audible alarm signal.

13. A heater according to claim 10, wherein the alarm means produces a visible alarm signal.

14. A heater substantially as hereinbefore described with reference to the accompanying drawings.

15. A carbon monoxide sensor module substantially as hereinbefore described with reference to the accompanying drawings.