Battery powered gas level indicator

A gas level indicator incorporates two Taguchi-type sensing elements, one of which is to receive air to be sampled and the other of which is insulated therefrom. A comparator circuit receives the outputs of the two elements, and an indicator actuated by the output of the circuit announces the result of the comparison. Preferably, both sensing elements are mounted in a single housing, and an elongated pipe and a fan or aspirator provided to form a probe for drawing air to be sampled from a remote position to the one element.

This invention relates to a gas level indicator. There are indicators known 
which are fixed in position to indicate a dangerous increase in gas level 
in the ambient atmosphere. There are also portable quantitative indicators 
used, for example, to test for leaks on site. The present invention 
relates to the latter type of indicator or detector. 
Such indicators as are known are both expensive and tend to be short-lived. 
In particular, existing indicators employ an element which is subject to 
poisoning and falsification by other gaseous media, for example 
lead-carrying petrol fumes. 
It is an object of the invention to provide a portable indicator, 
preferably battery-powered, which is cheap, has a relatively long life, 
and is not subject to poisoning of the detector element. 
Accordingly, the invention provides a gas level indicator incorporating two 
Taguchi-type gas sensing elements, one of which is to receive air to be 
sampled and the other of which is insulated therefrom, a comparator 
circuit receiving signals derived from the two elements, and an indicator 
actuated by the output of the circuit. 
Such an indicator does not suffer the disadvantages of the known portable 
types, can be battery powered, and provides an accurate reading which 
eliminates the variables such as ambient pressure, temperature and 
atmospheric pollution which can otherwise lead to false readings.

Referring now to the drawings, in the embodiment of FIG. 1 a 12 volt 
battery supply feeds the circuit. In order to provide a stable voltage for 
the detector circuits, a regulator 10 is employed. This directly feeds the 
heaters of two Taguchi-type gas sensing elements 12 and 13. The sensors 
themselves are in series respectively with resistors 12A, 13A. Two 
amplifiers 14, 15 receive a fixed proportion of this constant voltage 
supply, as determined by the voltage divider 16, 17. 
Despite the changing resistance of each element 12, 13 the voltage across 
it is maintained constant by varying the total voltage output by the 
amplifiers across it and the series resistance 12A, 13A. These outputs are 
fed by resistors to a third amplifier 18. The resultant voltage from the 
two sensing elements is differentially amplified by this amplifier 18. The 
gain is conveniently set at unity by adjustment of the values of the 
resistors 20, 21, 22, 23. The output of the amplifier drives an analogue 
meter 25 to indicate gas concentration. 
One of the Taguchi-type gas sensing elements 12, 13 is used to monitor the 
ambient conditions, while the other element is positioned in air at or 
from a particular location to be tested, for example, a joint of a gas 
main. The said other element may be housed in a single casing with the 
rest of the parts of the indicator and supplied with a draught of air 
through a probe tube. FIG. 2 shows such an arrangement with two 
Taguchi-type sensing elements 12, 13 in a single casing or housing. 
Element 13 is suppied with air to be sampled by means of an elongated pipe 
41 and a fan or aspirator element 40. Alternatively, the element may be 
itself housed at the end of a long probe so that it can be introduced into 
the location to be tested. However, this is a less advantageous 
arrangement since it carries the risk of a spark from the electrical 
connections which must be made along the probe to the element. 
In either case, the effects of temperature, of humidity and of other gas 
contaminents are equalised for both probes, and a comparison can be made 
between the ambient situation and that at a particular narrow location 
required to be tested. 
The circuit includes a further section designed to prevent a non-zero meter 
reading until the unit is ready for action, i.e., until the sensor heaters 
have warmed up. A capacitor 31 is charged via a resistor 32 and discharged 
via a diode 33 when the unit is switched off. The level of voltage across 
the capacitor is monitored relative to a voltage divider 34, 35 by a 
further amplifier 36. The output of this amplifier is high when the 
voltage across the capacitor is low. This high voltage is applied via a 
diode 37 to the amplifier 18, and thus keeps its output voltage low. 
When the voltage across the capacitor 31 exceeds the voltage tapped, the 
output of the amplifier 36 changes from high to low. This reverse biases 
the diode 37, allowing the amplifier 18 to operate normally, and at the 
same time provides an indication that the meter reading can be trusted by 
illuminating a light emitting diode 38. 
The analogue meter may if required be replaced by a simple on/off 
indicator.