GAS MONITOR CIRCUITS

Abstract: Circuits are described in which the sensing and the refer­ ence electrodes (12, 16) of a gas sensor housed within a gas monitor are connected together through the source and drain of a field effect transistor (FET) (20). The gate of the FET is con­ nected to a power source to apply a first voltage to the gate when the monitor is operational so that the resistance between the source and the drain of the FET is high and the two elec­ trodes are effectively isolated from each other; when the moni­ tor is not operational, however, a second voltage, which is preferably zero volts, is applied to the gate that greatly reduces the resistance between the source and the drain of the FET so that a short circuit is established between the electrodes. The circuit has output lines (32, 34) leading to an output device, e.g. a display or an alarm. A resistor (22) of large resistance is connected between the two output lines (32, 34) and, when a gas is sensed a large voltage change occurs across the resistor which can be used directly, i.e. without an intervening amplifi­ er, for driving the output device.

Description: The present invention relates to gas monitors and particularly to improvements in the electronic circuits thereof. Gas monitors are instruments that detect a gas and provide a visual or audible output according to the amount of gas sensed. Each monitor includes one or more replaceable sensors providing an electronic signal proportional to the amount of gas detected which signal is analyzed by circuitry within the monitor to give the desired output. The output may be a display, in analog or digital form, of the amount of a specific gas detected and/or it may be a printer or plotter and/or an alarm to give an audible and/or visible warning if the concentration of a gas falls to an undesirable level or if the concentration of a gas rises above a certain set threshold level. However, gas monitors need not necessarily give a direct indication of the amount of gas detected but may use the signal from one or more sensors to compute another parameter which may be displayed or printed etc.. The replaceable sensors used in the sensors of the present invention are of the type having a sensing (or working) electrode for sensing the gas, a counter electrode and a reference electrode which are connected via terminals to the circuitry within the monitor. The potential difference between the sensing electrode and the reference electrode must be controlled and in some sensors this is done by connecting these two electrodes to the two inputs of an operational amplifier either directly or through a resistor. Examples of such an arrangement can be found in British Patent Specifications 1,101,101 and 1,385,201 and in U.S. Patent Specification 3,776,832. The present invention is concerned with the electrical signal that gives a measure of the amount of gas present at the sensing electrode of a gas sensor. According to British patent Specification 1,101,101, this electrical signal is derived by measuring the voltage drop across a resistor connected in series with the sensing electrode. The value of the resistance must not be too high or else the response time of the sensor is unacceptably high and it must not be too low or else the change in voltage on sensing a gas is small and hence measurements are difficult to obtain and inaccurate. Furthermore, as the value of the resistance is reduced, the sensing system becomes more prone to noise. To overcome these problems an intermediate value resistor is chosen that results in an acceptable response time and noise immunity, but the voltage change is still too low, typically 10 microvolts per part per million of gas, to provide a useful output, for example for driving a display, for triggering an alarm or that can be plotted on a graphical plotter. The voltage drop across the resistor is therefore amplified by an amplifier which has a gain of several hundred fold to a required level. The present invention dispenses with the need to use an operational amplifier to boost the output voltage. According to the present invention, there is provided a gas monitor as defined in the accompanying claims. In order to provide noise immunity, the monitor may include a further resistor having a lower resistivity than the resistor connected between the output lines, which further resistor is connected in series between the sensing electrode and one of the inputs of the operational amplifier. Thus, one resistor can be chosen to optimise the trade-off between noise immunity and a fast response time while the output of the gas sensor is measured as the voltage drop across the other resistor. By making the resistor between the output lines highly resistive, a large voltage drop is generated across the second resistor which is sufficiently large to provide a direct output, i.e. it is not necessary to provide an operational amplifier to boost the output voltage. This is possible because the impedance between the inputs of the operational amplifier is high as is the impedance of the measuring means. The present invention will now be described, by way of example only, with reference to Figs 1 and 2 of the accompanying drawings all of which show gas monitor circuits in accordance with the present invention. In Figure 1, a gas sensor 10 is shown having a sensing electrode 12, a counter electrode 14 and a reference electrode 16 connected to terminals 12a, 14a and 16a respectively of a gas monitor. The reference electrode 16 is connected to the negative input of an operational amplifier 18, and also to the source of a field effect transistor (FET) 20. The sensing electrode 12 is connected through a resistor 22 to a line 32 and the drain of the FET 20 is either connected directly to line 32 (Figure 1) or between the sensing electrode 12 and the resistor 22 (Figure 2). A voltage V p is applied to the gate of the FET 20 via line 23 when the gas monitor is operational but no voltage is applied when the monitor is not operational. When no voltage is applied to the FET, the resistance between the drain and the source of the FET is small (a few hundred ohms) and so the sensing and reference electrodes are effectively shorted together. When in operation, the monitor applies a voltage V p to the gate of the FET which causes the resistance between the source and the drain of the FET to increase to a high value at which the leakage current between the drain and source is at most 100 microamps but generally it should be of the order of tens of nanoamps. This removes the short circuit between the sensing and the reference electrodes. The FET 20 may be an n-channel FET, in which case voltage V p must be negative with respect to the drain or the source or a p-channel device, in which case the voltage V p must be positive. The use of an FET to short circuit the sensing and reference electrodes together when the monitor is not switched on and to break the short circuit automatically when the monitor is operational greatly simplifies the circuitry and makes it much cheaper. In the Figure 1 circuit, the reference electrode 16 is connected to the negative input of an amplifier 18, while the output of the amplifier is connected directly to the counter electrode 14. A first resistor 22 is connected between the sensing electrode 12 and a first output line 32 and a second resistor 34 is connected between the first output line and a second output line 36. The positive input to the amplifier is connected to the first line 32. The voltage drop across the second resistor 34 is measured through output lines 32 and 36 by a measuring device 38 of high impedance which may be a display, a printer, a plotter or an alarm trigger. The Figure 2 circuit contains the same components fulfilling the same function as those of the Figure 1 circuit and therefore the same reference numerals have been used in both Figures. When gas is present at the sensing electrode, a potential difference is established between the sensing and reference electrodes and hence between the inputs of operational amplifier 18 causing a current to flow at the amplifier output and hence also between the sensing and counter electrodes, the magnitude of which is proportional to the amount of gas present at the sensing electrode. Because the impedance between the terminals of an operational amplifier is large and because the measuring device 38 is also of high impedance, the current that is caused to flow through the resistor 34 is approximately the same as that flowing through the sensor cell. By using a resistor 34 of high resistance, a large voltage drop between lines 32 and 36 can be achieved which is sufficient to drive a printer or display 38 directly without using an operational amplifier. The resistor 34 may have a resistance of 0.5 to 500 kohms, preferably 1 to 100 kohms. For example, when resistor 34 has a resistance of 10 kohms, the voltage drop across resistor 34 may be approximately 1mV per part per million of gas detected. Resistor 22 has a resistance of between 0 and 200 ohms and provides noise immunity to the circuit by altering the relative potentials between the sensing- and the counter- electrodes in response to noise. The low resistivity of the first resistor 22 also provides a fast response time.

Claims: A gas monitor having terminals (12 a , 14 a , 16 a ) for providing electrical connection to the sensing-, counter- and the reference-electrodes (12, 14, 16) of a gas sensor (10), which monitor comprises: (a) an operational amplifier (18) having a first input connected to the reference electrode terminal and a second input connected to the sensing electrode terminal and its output connected to the counter electrode terminal; (b) a measuring device for providing a measure of the amount of gas sensed at the sensing electrode; (c) a first output line (32) connected to the sensing electrode terminal; characterised in that (d) a resistor (34) is connected between said first output line (32) and a second output line (36), said second input of said operational amplifier being connected to a point between said resistor (34) and said sensing electrode terminal; (e) the voltage drop across said resistor (34) is measured through said output lines by said measuring device. A gas monitor having terminals (12 a , 14 a , 16 a ) for providing electrical connection to the sensing-, counter- and the reference-electrodes (12, 14, 16) of a gas sensor (10), which monitor comprises: (a) a measuring device for providing a measure for the amount of gas sensed at the sensing electrode terminal; characterised in that (b) the monitor comprises an operational amplifier (18) having a first input connected to the reference electrode terminal, a second input connected to the sensing electrode terminal and its output connected to its second input; (c) a first output line (32) is connected to the counter electrode terminal; (d) a resistor (34) is connected to said first output line; (e) a second output line is connected to said resistor (34); (f) the voltage drop across said resistor (34) is measured through said output lines by said measuring device. A monitor as claimed in claim 1 or claim 2, which includes a further resistor (22) having a lower resistivity than the resistor (34) connected between the output lines (32, 36), which further resistor (22) is connected between the sensing electrode terminal (12 a ) and the second input of the operational amplifier (18). A gas monitor as claimed in any one of claims 1 to 3, which includes a gas sensor (10).

Inventors: BUCKENHAM, HOWARD, GOTLEY, PAUL, JONES, GARETH JEREMY
Applicants: NEOTRONICS LIMITED
Application year: 1986
Main classification: G01N27
IPC codes: G01N27