GB2054153A

GB2054153A - Gas Monitors

Info

Publication number: GB2054153A
Application number: GB7924894A
Authority: GB
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1979-07-17
Filing date: 1979-07-17
Publication date: 1981-02-11
Also published as: DE3025742A1; AU5944880A; ZA803390B

Abstract

A gas monitor which activates an alarm when a particular gas concentration is detected has means to inhibit alarm actuation when the detected concentration does not result solely from a concentration change. As described a low oxygen content (17%) alarm is prevented from actuation during sudden drops in ambient pressure. The inhibition operates with a sudden pressure drop curve B but not with a steady oxygen content drop curve A. Inhibition is produced when the rate of change of oxygen concentration exceeds a predetermined value e.g. the time for the oxygen content to decrease from 19% to 18% is less than a predetermined value. <IMAGE>

Description

SPECIFICATION Gas Monitors This invention relates to gas monitors.

In particular, although not exclusively, the present invention relates to monitors for sensing the oxygen level in air.

It is known for an oxygen monitor to comprise a sampling cell which derives an electrical output signal taken to be indicative of the oxygen level in the sampled air, the output signal activating an alarm should it reach a preselected level.

Unfortunately, the derived output signal of such a cell is affected by a sudden change in the pressure of the sampled air. Consequently, for a period of time after a change in the pressure of the sampled air the output signal tends not to be indicative of the true oxygen level of the sampled air.

Such a problem is high lighted in the case of oxygen detectors used in underground mines where a mine operator carrying the gas monitor can pass through ventilation doors from a zone of relatively high pressure e.g. the intake mine ventilation system into a zone of relatively low pressure e.g. the return mine ventilation system.

The resulting sudden decrease in the pressure of the sampled mine air tends to cause the output signal derived from the sampling cell to indicate an erroneously low oxygen level in the sample intake and therefore the alarm may be activated when at a safe oxygen level.

An object of the present invention is to provide an improved gas monitor which tends to overcome or reduce the above mentioned problem.

According to the present invention a gas monitor comprises a sampling cell arranged to derive an output signal indicative of a desired condition of the sampled gas, alarm means adapted to be activated when the desired output signal reaches a preselected value, and means sensing changes in the derived output signal to determine changes in the output signal arising substantially from changes in the desired condition from those changes in the derived output signal not arising solely from changes in the desired condition, the means permitting activation of the alarm means only when the derived output signal reaches the preselected value due to a change in the desired condition.

Preferably, the sampling cell derives an output signal indicative of the oxyen level of sample air.

Advantageously, the means discriminates changes in the derived output signal arising substantially from changes in the oxygen level of the sampled air from changes in the derived output signal arising from a change in the pressure of the sampled air, the means permitting activation of the alarm only when the derived output signal reaches the preselected value due to changes in the oxygen level of the sampled air.

Preferably, the means senses the rate of change of the derived output signal.

Conveniently, the means senses the rate of change of the derived output signal to discriminate between changes in oxygen level from changes in pressure.

Advantageously, the means inhibits the alarm until the sampling cell detects a correct oxygen level.

Alternatively, the means inhibits the alarm until a preselected interval after the change in the derived output signal owing to a pressure change is sensed.

Preferably, the gas monitor comprises monitor means adapted to receive the derived output signal.

Conveniently, the monitor means is adapted to display the oxygen level of the sampled air as indicated by the derived output signal.

Preferably, the monitor means is adapted to display the oxygen level only when activated in a display mode.

Conveniently, the monitor means operates independently of the means for sensing the causes of changes in the derived output signal.

By way of example only, two embodiments of the present invention will be described with reference to the accompanying drawings in which Figure 1 is a front view of a part of an oxygen level monitor constructed in accordance with a first embodiment of the present invention; Figure 2 is an incomplete sectional view of the monitor of Figure 1, looking in the same direction as Figure 1; Figure 3 is an incomplete sectional view of the monitor of Figure 1, the viewing direction being normal to that of Figure 1; Figure 4 shows graphs illustrating two typical derived output signals against time for changes in oxygen level and in pressure of the sample gas; Figure 5 is an electrical circuit diagram of the monitor of Figure 1; and Figure 6 is a block electrical circuit diagram of an alternative arrangement for a second embodiment of oxygen level monitor.

Figures 1, 2 and 3 show the oxygen level monitor comprises a casing 1 having a front panel 2 including an indicator window 3, three light emitting diodes 4, 5 and 6 which when lit indicate a fault condition, a low battery condition and an inhibit condition, respectively. The front panel also includes two test buttons 8 and 10 which when manually depressed switch the monitor to a read oxygen mode and a cancel fault mode, respectively.

The indicator window 3 is a multi-coloured arrangement comprising a red danger zone indicating a low oxygen level in the sample air (below 17%) a mid range yellow zone indicating an oxygen level condition at a preselected value, typically substantially 17% to 19% by volume of the sample air, and a green safe zone indicating a high oxygen level (19% to 23%) in the sample air.

The indicated oxygen level may or may not be indicated numerically. It is to be understood that all numerical values used in this specification such as oxygen level or time periods are for example only and may be varied in different applications and installations.

Within the casing 1 the oxygen level monitor comprises a sampling cell 15 having a fine bore inlet piece 16 inducing air to be drawn towards the sampling element of the cell which is urged into contact with the inlet piece by a resilient pad 17. A clip 1 8 retains the cell in position. The sampling cell derives an electrical output signal indicative of the oxygen level in the sample air.

The casing 1 5 also encloses a rechargeable battery unit 20 which is rechargeable via a jack plug socket 21, a circuit board arrangement 22, 23 and an alarm buzzer 24 adapted to be activated should the indicated oxygen level reach a preselected value, for example should the indicated oxygen level fall to reach 17% or below.

Figure 4 illustrates typical graphs A and B showing the change in the derived output signal from the sampling cell arising from a change in the oxygen level and from a change in the pressure of the sample air, respectively. Graph A indicates that when the oxygen level falls from one level to another level the derived output signal falls until it reaches a value indicative of the new oxygen level, the rate of change in the derived output signal being relatively gradual as indicated in Figure 4. Point 0 on Figure 4 indicates the instant at which the oxygen level is changed and the value of the derived output signal indicative of the initial oxygen level.

Graph B indicates the change in the derived output signal following a sudden fall in the pressure of the of the sampled air. From Figure 4 it can be seen that following the instant of pressure change the derived output signal falls relatively rapidly until a trough is reached and after which the derived output signal gradually returns towards its initial value.

Figure 4 also indicates the preselected alarm level i.e. the output level at which the alarm buzzer is activated to indicate an oxygen insuficiency in the sample air As seen in Figure ss the ds, ived output signal ean reach the preselat-;ised alarm level due to a change in the pressure of the sample air as well as due to a fall in the ocygen level of the sampled air.

Figure 4 also shows two trip levels at 1 9% and 1 X350 being a relatively high trip and a relatively low trip respectively. As will be explained below in connection with the electrical circuitry, the rate at which the signals corresponding eo oxygen level or pressure change occurs between the high and low trip can be used to discriminate between decompression changes and genuine oxygen changes.

Reference is directed to Figure 5, wherein the previously mentioned battery unit 20 can be seen to comprise sbt rechargeable cells. Diodes Dl, D2 and D3 protect the cells from incorrect polarity connection during recharging. An off/on switch for the power supply is indicated by SW1 and a current limiting resistance by R1.The positive supply rail is at +V volts and the negative at --V volts. Diodes D4 and D5 shunt the battery unit 20 in order to prevent circuit damage if the cells become connected in reverse polarity.

A constant voltage supply generator is indicated by 100 and comprises a Zener diode ZDl, an amplifier 40, NPN transistor TR 1 and PNP transistorTR2. The Zener diode ZD1 provides a reference voltage with respect to the positive supply rail and the amplifier 40 compares the reference with the output from the generator.

The transistors control the voltage of the generator output to three volts below the positive supply rail in response to the amplifier output. The constant voltage output is conveniently considered to be 0 volts. Resistors R2, R3 and R4 are associated with the amplifier 40 and the Zener diode ZD1 (which is in parallel with capacitor Cl) draws its current through a resistor R5. A capacitor C2 is connected in parallel with the serially connected resistors R3 and R4 between the +V volt and O volt rails.

The sampling cell 1 5 is connected to the regulated output of the regulator 100 and acts as a current source. The cell 1 5 has a resistor R6 associated therewith and thus provides a varying current to the inverting input of a current to voltage connecting operational amplifier 41. The non-inverting input to the amplifier 41 is the constant voltage output from the regulator 100.

The output from the amplifier 41 is fed back to the inverting input via a resistor R7 thereof and both inputs have a protective resistor associated therewith denoted by R8 and R9.

The output from the amplifier 41 is connected to the inverting input of a further operational amplifier 42 via a resistor R 10. The non-inverting input is connected to the 0 volt rail from the voltage regulator via a resistor R1 1. The amplifier 42 is similar to the amplifier 41 and has a feedback resistor R12. The output from the amplifier 42 is to a display driver 43 (including a resistor ladder network which is not shown) which is provided with a ten element light emitting diode (LED) display 44. The varying voltage signal from the amplifier 42 reaches the display driver 43 on pin 5 via a variable resistor VRl used to calibrate the upper value of signal (oxygen content) displayed by the display 44. The variable resistor VR 1 is connected to the 0 volt rail. Pin 5 of the driver 43 is connected to the 0 volt rail via capacitor C3.

A further variable resistor VR2 is connected to the inverting input of the amplifier 42 via a resistor Rl 6. A resistor R15 is connected between the variable resistor VR2 and the positive supply rail. The resistor VR2 is used to see the lower value of display by the unit 44 as will be explained in operation below.

The display driver 43 is not normally active to drive the display 44 and only becomes active when actuated via pin 3 when the voltage on the base of a transistor TR3 connected thereto falls.

One of the ways the voltage on the base of the transistor TR3 can fall is when a test switch SW2 for the display is operated. Closure of the test switch SW2 causes the positive and negative supply rails to be connected together via resistors R17 and R14 and the base voltage is sampled from between these resistors. The base of TR3 is also connected to and can be actuated by a change in the output of an inverter G2. The connection is via a resistor R18 and a protective diode Dl 2. Pin 5 of the driver 43 is connected to a comparator G1 via a resistor R19. A reference for the comparator G 1 is sampled via a resistor R20 from a variable resistor VR3 connected to the positive supply rail via a resistor R2 1.The value of reference voltage is set so as to be that corresponding to an oxygen level (for example 17%) at which alarm should be sounded by the monitor. Thus the signal at pin 5 of driver 43 is normally above the reference from VR3 but when it falls below this, the output of comparator G1 switches from low to high. Such a switching causes the invertor G2 to have an output which switches from high to low and thus affects the base of transistor TR3 so that the display 44 is energised. The output from G2 is also fed to alarm via an OR gate G3 as will be described subsequently.

The input to pin 5 of driver 43 from variable resistor VR 1 is tapped for feeding to the inverting and non-inverting inputs of comparators G4 and G5, respectively via resistors R22 and R23 respectively. Reference voltages for the comparators G4, G5 are provided via resistors R24, R25 respectively from parallel variable resistors VR4 and VR5, respectively, both of which are in series with a resistor R26 between the positive and 0 volt supply rails.The reference voltage for the comparator G4 corresponds to a relatively higher oxygen level of for example 19% and the reference value for the comparator G5 to a relatively lower oxygen level for example 18%, the normal percentage being 2150. The output from the gate G4 is connected to a monostable integrated circuit IC1 via an invertori5, 66.

Operation of the invertor G6 causes the monostable circuit lC1 to derive a positive pulse of duration determined by the value of a variable resistor VR6 as will be described below. The pulse if fed to an AND gate 67 as is the output of the G5, the latter being effected via an invertor OS.

The AND gate G7 is connected to pin 11 of a latch IC2 which can be reset from the output of gate G6 via a diode D6. The latch IC2 latches to a high output on pin 1 3 and this is gated with the alarm output into the OR gate G3. Pin 13 also is connected to the base of an NPN transistor TR7 as will be mentioned below. The output from G3 is to a further OR gate G9 where it is gated with a square wave alternating signal.

The square wave alternating signal is provided by an astable multivibrator iC3, the period being determined by the values of a resistor R27 and a capacitor C5. The astable IC3 can be inhibited on pin 9 via diodes D7 and D8 as will be described below. The output from G9 is via an invertor 610 to a further OR gate G 11. The output of the gate G 1 is fed via resistor R28 to a transistor TR4 which controls an alarm AL1 shunted by a diode D9. The other input to the OR gate G1 1 is from a latch IC4. The latch is set by an OR gate G 12 which has an input from a low battery detection circuit and from a fault detection circuit.

The low battery detection circuit comprises a comparator G13 which samples the 0 volt rail via a resistor R29 and compares this with a value of battery voltage sampled via a resistor R30 from a variable resistor VR3 1 connected between the positive and negative supply rails. Hysteresis for the comparator is provided by a resistor R32. The value of voltage sampled from between the rails is chosen so that at minimum acceptable battery voltage, the comparator G 13 will switch lower.

The output of the comparator G 13 is connected to the OR gate G12 via an invertor G14. When the invertor G14 switches high in response to the change in output from G13, the astable multivibrator lC3 is inhibited via diode D7 and pin 9. Also the biasing voltage on the base of a transistor TR5 connected in series with a light emitting diode LED1 and a resistor R33 is changed (via a resistor R34) so that the transistor TR5 conducts and the light emitting diode LED1 conducts.

The fault detection circuit comprises a comparator G1 5 which compares the output of the operational amplifier 42 with a reference voltage dropped across a diode D10 in series with a resistor R35 between the 0 volt and negative rails. The output from the comparator G15 is connected via an invertor G16 to the OR gate 612. A transistor TR6, resistors R36, R37 and light emitting diode LED2 are provided in the fault circuit in a similar way to the transistor TR5 resistors R33, R34 and light emitting diode LED1 in the low battery circuit. The comparator G15 effectively determines whether there is an open or short circuit across the cell 15 or its associated circuitry 41, 42.

A fault or low battery condition, which causes the audible warning to be continually sounded as will be explained below, can be acknowledged to remove the noise. Acknowledgement is effected by closing a switch SW3 in series with a resistor R38 between the positive and negative supply rails. Closure of the switch resets the latch IC4 via pin 4. Diodes Dl2, D13, pass the resetting pulse to lC2 and le4, respectively.

The transistor TR7 controls a light emitting diode LED3 in a similar manner to TR5 and LED1 orTR6 and LED2. The diode LED3 is connected in series with a resistor R6 1 and is energised whenever pin 13 of IC2 goes high via resistor R60 to the base of TR7.

A resetting signal for the latches IC2 and IC4 is provided by a resetting circuit comprising capacitor C7 and resistor R39 which has a diode D11 in parallel therewith. When the monitor is switched on, the resetting circuit provides a one second pulse which resets latch IC2 on pin 10 and latch lC4 on pin 4 via diode D12 and D13 respectively. This ensures that CMOS latches which could start in either state are correctly set when power is switched on.

Operation of the circuitry of the monitor is now described. The light emitting diodes in the unit 44 have to operate to display a maximum value of 23% oxygen (which is also displayed when a greater percentage is in the atmosphere) and a minimum of 14% oxygen, the alarm being sounded at 17%.

The display driver operates on a signal range of 75 to 525 millivolts (owing to the resistor ladder network) and this range has to be displayed as 14% to 23% oxygen. The lower value of signal is determined by putting the cell 15 into say a 12.5% oxygen atmosphere and adjusting the variable resistor VR2 so that no light emitting diode is illuminated. The first diode then comes on when 14% oxygen is reached (i.e. 75 millivolts).

The upper value of the display is simply set by putting the cell 1 5 into a 23% oxygen and varying VR1 until the appropriate diode is energised i.e.

23% LED.

The operation of the display can be tested by actuation of the switch SW2, but the display is not normally on and only otherwise actuated when the comparator G1 switches. In safe oxygen levels, therefore, the monitor is quite and the display off.

Now suppose the oxygen level falls to below 17%, the preselected alarm level. The comparator G1 then switches from a low to high output because the signal wiped from VR2 falls below the reference voltage from VR3. Consequently, the output from the invertor G2 switches from high to low, thereby switching the output from OR gate G3 from high to low [unless there is another high gated from latch IC2 to inhibit as will be explained below], thus permitting the astable multivibrator lC3 to pass its square wave signal through the OR gate G9.The square wave drive signal then passes through the invertor G10 and the OR gate G1 1 [unless there is a high gated from latch IC4 to inhibit in which case the astable IC3 would also be inhibited via D7 or D8 as will be described below] to provide an intermittent drive for the audible alarm AL1 via TR4.

However, as has been mentioned above, the value of signal could fall if the cell was to undergo a decompression. In such a situation, the signal would fall more rapidly than in the case of decreasing oxygen and also, if the oxygen level in the decompressed zone was satisfactory, the signal would rise again (as shown in curve B of Figure 4).

Thus, if false alarms owing to decompression rather than lack of.exygen are to be avoided, the alarm must be inhibited by some means which can determine the rate at which the oxygen signal falls.

This is where the period of the monostable circuit IC1 becomes relevent.

- Suppose, therefore, that the oxygen signal value tapped from the wiper of VR1 falls to below a value of signal corresponding to 19%. In this situation, the comparator G4 will detect that the oxygen signal has fallen below its reference signal wiped from VR4. Consequently the output of G4 will switch from low to high causing the output of invertor G5 to switch from high to low to trigger the monostable IC1 via pin 6. The monostable ICi then derives a positive pulse on pin 10 which is gated with a signal from G8 in the AND gate G7 and a latching pulse is passed by AND gate G7 to latch IC2 only if there is a high from G8.A high on the invertor G8 can only occur if the output of comparator G5 switches from high to low. This will occur when the voltage wiped from VR1 falls below the voltage wiped from VR5 i.e. the oxygen level falls below 18%. Thus, it will be appreciated that the latch only switches if the time taken for the oxygen signal to fall from 19% to 18% oxygen is less than the period of the monostable. The period of the monostable is chosen so that the latch is actuated only if the fall of the oxygen signal is caused by decompression, since as has been explained above, such a fall is quicker than when oxygen content decreases.

When the latch is actuated, the pin 13 switches from low to high, thus holding the output of OR gate G3 high even if the comparator G2 should cause the other input to go low.

Consequently, the alarm is inhibited because the high output of OR gate G3 holds the output of OR gate G9 high and prevents the square wave from the astable IC3 from passing.

The latch IC2 is not reset (apart from if the monitor is switched on) until a high appears on pin 10. This high is derived from the invertor G6 when the output of the comparator G4 switches from high to low when the oxygen level again rises above 19% and the signal wiped from VR1 exceeds the reference from VR4. Thus the alarm is no longer inhibited after the signal following curve B returns to normal after a decompression.

From the above description it will be appreciated that the monitor circuitry is able to discriminate between decompression and a genuine falling oxygen signal. The LED3 is energised while pin 13 is high so t.hat an operator realises the instrument is inhibited.

The operator also is informed if the battery level of the monitor becomes too low or if a fault condition (open or short circuit) arises in the cell 1 5 (or associated circuitry 41, 42). Operation in case of fault is now described.

In a fault condition, the comparator G15 switches from high to low output, because the output from amplifier 42 rises above the reference value of voltage dropped across the diode D 10, and thereby causes the invertor G16 to switch from low to high and the diode LED2 to be energised via transistor TR6. The high signal from G16 switches the OR gate G12 to a high output and thereby actuates the latch IC4 via pin 3. The high from G16 also inhibits the astable IC3 via diode D8 and pin 9.

When latched, the latch IC4 has a high output on pin 1 which passes through OR gate 611 1 to actuate the alarm AL1 in a continuous sound via the transistorTR4. In order that an operator can acknowledge the faulty monitor to cut off the alarm, the switch SW3 is provided. Actuation of the switch SW3 resets the latch via pin 4 so that the alarm is cut off. However, the light emitting diode LED2 remains actuated until the fault condition is repaired even though SW3 has been closed.

The low battery level circuit operates in a similar way to the alarm circuit, visual warning being given by LED 1 and the comparator G13 (corresponding to G15) switching from low to high when the supply voltage from the battery falls to a predetermined low level, this being determined when the voltage tapped from resistor R31 is less than the regulated three volts from the regulator 100.

As has been mentioned above, a one second pulse is provided by C7, R39 and D11 when the monitor is actuated, in order that the latches are correctly set.

Reference is now directed to Figure 6 wherein alternative method of operating latch IC2 to inhibit the alarm in the case of decompression is shown. In Figure 6, like reference characters are used for like parts in Figure 5.

Figure 6 it can be seen that the connections to the inverting and non-inverting terminais of both comparator G4 and G5 have been reversed, thus reversing the logic outputs of invertor G6 and G8 in operation. The output from invertor G6 is fed to the monostable circuit IC1 which has an output to a NAND gate G18 via a NAND gate G19 connected to operate as an invertor. The period of the monostable IC1 is determined by the value of a resistor R40 and a capacitor C10. A diode Di 3 is connected in parallel with the resistor R40.

The output from invertor G8 is fed to the NAND gate G18 which has an output connected to a NAND gate G20 connected as an invertor and which in turn is connected to pin 2 of a latch/timer unit CS. The timer part of the latch/timer IC5 has its period controlled by the value of a resistor R42 and capacitor Cii. The monostable IC1 and latch timer IC5 are powered from the positive and negative supply rails.

The operation of the circuitry of Figure 6 is to inhibit the alarm AL1 for a fixed period if a decompression should occur, but then to end the inhibition after the fixed period, whatever the value of detected oxygen level.

In operation, if the oxygen signal falls below a value representing 1 9%, then the output of invertor G6 switches from low to high. If the output signal continues to fall and goes below 18% then the output of invertor G8 switches from high to low. Switching of invertor G6 gives rise to a positive pulse from monostable IC1 which is inverted by NAND gate G19 and the feed to NAND gate G18. If the pulse is still in existence when the invertor G8 switches low, then a high pulse appears at the output of NAND gate G18 until the monostable IC1 switches.Thus, the rate of fall of signal determines whether a pulse appears at the output of NAND gate G 8 so that decompressions are discriminated from genuine falls in oxygen levels. The positive pulse output from NAND gate G18 is inverted by the NAND gate G20 and fed to the latch timer IC5.

When the latch/timer IC5 receives such a pulse, it derives an inhibiting signal for the alarm on pin 3. The inhibiting signal lasts for a preselected duration (about 10 seconds), after which time the alarm is sounded unless the signal is above the alarm level. Consequently, this circuit has the advantage that it does not require a latch to be reset by the comparator G4 so that if a depressurization should be followed by a drop in oxygen level 'unlikely as this may be) the alarm is not inhibited.

The output from invertor G8 is connected to the latch/timer IC5 to reset it so that if the oxygen signal should rise above 18% then the inhibition of the alarm is stopped immediately, whatever portion of the time delay has expired.

From the above description it can be seen that an improved gas monitor is provided.

Claims

1. A gas monitor comprising a sampling cell arranged to derive an output signal indicative of a desired condition of the sampled gas, alarm means adapted to be activated when the desired output signal reaches a preselected value, and means sensing changes in the derived output signal to determine changes in the output signal arising substantially from changes in the desired condition from those changes in the derived output signal not arising solely from changes in the desired condition, the means permitting activation of the alarm means only when the derived output signal reaches the preselected value due to a change in the desired condition.

2. A gas monitor as claimed in claim 1, in which the sampling cell derives an output signal indicative of the oxygen level of sample air.

3. A gas monitor as claimed in claim 2, in which the means discriminates changes in the derived output signal arising substantially from changes in the oxygen level of the sampled air from changes in the derived output signal arising from a change in the pressure of the sampled air, the means permitting activation of the alarm only when the derived output signal reaches the preselected value due to chan.ges in the oxygen level of the sampled air.

4. A gas monitor as claimed in claim 3, in which the means senses the rate of change of the derived output signal.

5. A gas monitor as claimed in claim 4, in which the means senses the rate of change of the derived output signal to discriminate between changes in oxygen level from changes in pressure.

6. A gas monitor as claimed in claim 5, in which the means inhibits the alarm until the sampling cell detects a correct oxygen level.

7. A gas monitor as claimed in claim 5, in which means inhibits the alarm until a preselected interval after the change in the derived output signal owing to a pressure change is sensed.

8. A gas monitor as claimed in any one of claims 1 to 7, in which the gas monitor comprises monitor means adapted to receive the derived output signal.

9. A gas monitor as claimed in claim 8, when dependant upon claims 2 to 7, in which the monitor means is adapted to display the oxygen level of the sampled air as indicated by the derived output signal.

10. A gas monitor as claimed in claim 8 when dependant upon claims 2 to 7, or claim 9, in which the monitor means is adapted to display the oxygen level only when activated in a display mode.

ii. A gas monitor as claimed in claim 8, 9, or 10, in which the monitor means operates independently of the means for sensing the causes of changes in the derived output signal.

12. A gas monitor substantially as hereinbefore described and as shown in Figures 1 to 5 or Figure 6 of the accompanying drawings.