Gas monitors

A gas sensor incorporating a sensor element of the heated semi-conductor type has a heater for the element connectible to a variable power source, the output of which is responsive to the concentration of gas under test.

This invention relates to gas monitors, and is particularly, although not 
exclusively, concerned with monitors capable of detecting the existence in 
an air ambient of toxic gases such as carbon monoxide. 
The present invention is concerned with monitors in which the gas sensor 
comprises an electrically heated semi-conductor element whose resistance 
varies as a function of adsorbed gas. Typically such elements contain a 
suitable doped metal oxide and examples of this type of sensor are 
disclosed in UK Patent Specification No. 1374575 and in "Solid State 
Detectors for Carbon Monoxide"--Ann. occup. Hyg. Vol. 18 pp 63-68.

Sensors of the type with which the present invention is concerned generally 
are incorporated into one arm of a bridge or other resistance-sensitive 
network so that changes in resistance brought about by variation in the 
concentration of a constituent in the ambient surrounding the sensor can 
be detected and registered. The relative sensitivity of the sensor to 
different constituents can be modified by selecting the operating 
temperature of the semiconductor element and a temperature for detecting 
specific gases is achieved by adjusting the level of the sensor heating 
current. 
It has been found, however, that the sensor temperature necessary to 
maintain the sensor free of absorbed gas provides relatively low 
sensitivity for most gases. Such lack of sensitivity is particularly 
apparent in the case where the concentration of carbon monoxide in the air 
ambient of an industrial environment is to be monitored, and it is one 
object of the present invention to improve the sensitivity of a gas 
monitor. 
According to its broadest aspect, the present invention provides a gas 
monitoring device incorporating a sensor of the Taguchi heated 
semi-conductor type having a heater for the element sensor connectible to 
a stepped variable output power source, the output of which is responsive 
to the concentration of the gas under test. 
It has been found for most gases that by lowering the temperature of the 
sensor an increase in sensitivity can be obtained notwithstanding the 
progressive sensor poisoning which occurs by the more rapid absorption of 
gas at the lower temperature. The period over which the sensor can be 
operated at the lower temperature depends both upon the temperature drop 
which is produced as well as upon the concentration of the selected gas 
which is being monitored. 
In order to avoid unnecessary operation of the sensor at the reduced 
temperature, the stepped power source conveniently is adapted to reduce 
the normal quiescent power input to the heater only when the concentration 
of the selected gas rises above a selected level. 
After operation at the reduced temperature, the increased quantity of gas 
absorbed in the sensor may be expelled by increasing the sensor 
temperature above the normal quiescent operating level at which 
equilibrium is maintained between absorbed and expelled gas. 
In a preferred embodiment of the invention the stepped variable output 
power source for the sensor heater also is arranged to respond to a 
further control signal representative of a concentration of the selected 
gas above a second higher threshold at which the sensor absorbs gas at a 
rate producing more rapid poisoning. In this case the stepped power source 
automatically increases the power applied to the heater above the 
quiescent level, to raise the sensor to a temperature at which the rate of 
gas absorption is reduced. While this higher temperature does in general 
produce a reduction in sensitivity, this reduction is compensated for by 
the higher concentration of the gas being monitored. 
An embodiment of the invention will now be particularly described by way of 
example with reference to the accompanying drawing which is a schematic 
circuit diagram of a carbon monoxide monitor incorporating multi-voltage 
heating of the sensor element. 
Referring to the drawing, the monitor includes a sensor 2 of the Taguchi 
semi-conductor type contained in a housing 4, which has a built-in 
resistance heater 6. The heater 6 which is effective to maintain the 
sensor 2 at the selected temperature is energised from a stepped, variable 
output current source 8. Source 8 is capable of providing a stepped power 
output at 10 in response to selective activation of control inputs 12, 14, 
and 16. 
A primary or secondary battery 17 energises the current source 8 together 
with the remainder of the circuit shown in the FIGURE. 
Sensor 2 is incorporated into one arm of a bridge circuit including 
potential divider 18 in known fashion. Bridge circuits incorporating 
sensors are known, a typical example being U.S. Pat. No. 3,932,807. The 
voltage output from the tap of divider 18 will reflect the resistance of 
the sensor 2 which is in turn representative of the concentration of 
carbon monoxide at the active surface of the sensor. 
Current input to the bridge circuit is conventional and is by way of a 
stabilised potential difference derived from a light emitting diode (LED) 
20 which displays zener diode characteristics. Diode 20 forms part of a 
series chain of LED's 20,22,24 connected between the positive and negative 
rails of the battery supply 17. Diode 22 provides an indication of the 
charge level of battery 17 where diode 24 provides a potential drop 
enabling diodes 20 and 22 to provide the operating potential differences 
required. 
The output from potential divider 18 is applied simultaneously to known 
differential comparator amplifier 26 and to differential amplifiers 28 and 
30. Differential amplifiers 28 and 30 are arranged to provide voltage 
outputs when the input from divider 18 respectively exceeds relatively 
lower and higher levels. Amplifier 28, for example, is arranged to provide 
an output when the input signal exceeds a value representative of a first 
lower threshold level of carbon monoxide of about 25 p.p.m. in the ambient 
being monitored. With the first lower threshold exceeded, the output from 
the amplifier 28 constitutes a control signal via line 12 effective to 
switch the stepped current source 8 from a quiescent output voltage of 5.0 
volts to a lower output voltage of 4.25 volts. With the sensor of the 
Taguchi type (Type 812 manufactured by Figaro Japan, for example), a 
heater voltage of 5.0 volts produces a sensor temperature at which 
adsorbed and expelled carbon monoxide are maintained substantially in 
equilibrium but with a relatively low sensitivity to carbon monoxide. At 
the temperature produced by the reduced voltage of 4.25 volts, the 
sensitivity to carbon monoxide is increased by a factor of about 5 
notwithstanding that progressive poisoning of the sensor occurs due to a 
higher rate of gas adsorption accompanied by a lower rate of emission. 
Unless the carbon monoxide concentration exceeds about 800 p.p.m. the 
lower temperature can be maintained for a sufficient period to provide 
meaningful measurement without significant loss of sensitivity. 
Variations in the concentration of carbon monoxide, reflected in the output 
voltage from potential divider 18, are applied to comparator amplifier 
module 26. Amplifier module 26 has a plurality of individual amplifier 
circuits effective sequentially to activate a line matrix 32 of light 
emitting diodes which provide an indication of concentration. Selected 
outputs of amplifier 26 and representative of specific concentrations, are 
effective to activate one or more alarm devices such as an audio 
transponder 34. The audio transponder may be replaced by a light emitting 
device which operates continuously or intermittently. 
An output from potential divider 18 representative of a high concentration 
of carbon monoxide, for example 800 p.p.m. or more, sufficient to produce 
rapid poisoning of the sensor 2, is effective to activate differential 
amplifier 30 which, in turn, activates the stepped current source 8 via 
line 14, to produce a higher output voltage of 6.5 volts to the heater. As 
hereinbefore described, the increased sensor temperature at this input to 
the heater, while providing a lower sensitivity to carbon monoxide, 
significantly reduces the rate at which the sensor becomes poisoned, and 
maintains stability during the period in which measurement is being made. 
To maintain the appropriate high level display, notwithstanding the 
reduction in the sensitivity of the sensor at the high temperature 
operation produced by a heater voltage of 6.5 volts, a signal is applied 
directly from the higher level control signal to the display circuit to 
maintain it in alarm condition. 
A timer 36, activated when the circuit is first switched on, is effective 
to control the current source 8 to provide a heater voltage which 
maintains the sensor 2 at a temperature at which it can expel gas adsorbed 
during the preceding non-operating period. 
A bridge linearisation circuit 38 is connected to a comparator amplifier 26 
and to differential amplifiers 28 and 30 in known fashion to ensure a 
constant sensitivity of the monitor as a whole, notwithstanding changes of 
sensitivity in the sensor 2 brought about by changes in operating 
temperature. Further compensation for change of sensitivity with 
temperature may be provided by a thermistor responsive to the temperature 
of the sensor housing and effective to apply a compensating signal to the 
circuit 38, in known fashion. 
To ensure that the heater voltage returns to 5.0 volt operation when the 
gas concentration falls to the level at which the first change of heater 
voltage occurs, it is necessary to arrange that reverse switching occurs 
at a level which is increased in direct relationship to the increase in 
sensitivity produced by the initial reduction in heater voltage. This may 
be achieved by feeding a signal from the 4.25 heater control signal to the 
first switching reference level of the comparator amplifier. 
It will be appreciated that while the invention has been described with 
reference to the detection of carbon monoxide in air, it is equally 
applicable to the detection of other gases. It will also be appreciated 
that the temperature at which the sensor is operated may be varied 
according to the gas being detected and according to the specific 
sensitivity which is required in different ambients.