1. Field of the Invention
The present invention relates to a catalytic combustion hydrogen gas sensor which is used for hydrogen gas detection.
2. Description of the Related Art
Hitherto, as a hydrogen gas sensor for detecting a hydrogen concentration in a gas phase, semiconductor gas sensors and catalytic combustion gas sensors are widely used. Particularly, the catalytic combustion gas sensor is a sensor for detecting a hydrogen gas by converting reaction heat produced in burning the hydrogen gas at a sensor surface to electric signals. And the catalytic combustion gas sensor has a feature that its structure is simple and its output signal has a linear characteristic.
A catalytic combustion gas sensor disclosed in Japanese Unexamined Patent Publication No. 10-90210 is shown in FIGS. 14 and 15. A detection element 21 of this gas sensor composes a combustion body 22 for burning a hydrogen gas and a heating resistor 23 for heating the combustion body 22 by the Joule heat produced in accordance with energization of the heating resistor 22.
The combustion body 22 is formed of a directive material such as alumina into bead which contains a combustion catalyst such as palladium or platinum. And, the heating resistor 23 is realized by a metal coil, for example, platinum, having a high temperature coefficient of resistivity.
This detection element 21 is incorporated in a measuring circuit shown in FIG. 15 together with a compensation element 24. The compensation element 24 is formed into almost the same configuration as the detection element 21 except that the combustion body 22 does not contain a catalyst such as palladium or platinum.
The measurement circuit includes a bridge circuit composed of the detection element 21, the compensation element 24, and fixed resistors 17, 18, in order to obtain a voltage Vc across the output terminals “c” and “d”, determine resistance variations in the heating resistor 23 from the voltage Vc, and detect the hydrogen gas concentration based on these resistance variations.
A temperature characteristic and a humidity characteristic of the compensation element 24 are almost the same as the detection element 21. But since the compensation element 23 does not have the activity of a combustion catalyst, it does not react with the hydrogen gas. The bridge circuit shown in FIG. 15, comprises a series combination of the detection element 1 and the compensation element 2 across terminals “a” and “b”, and a series combination of the fixed resistances 17, 18, across terminals “a” and “b”. And, a variable resistance 19 for adjusting equilibrium is connected between the terminals “a” and “b”. And an intermediate tap of the variable resistance 19 is connected to a node between the fixed resistances 17, 18. A direct-current power sources E1 is connected in series a switch SW and a variable resistance 20 across the terminals “a” and “b”. Therefore, a voltage being applied across the terminals “a” and “b” is regulated by adjustment of the resistance of the variable resistance 20.
In this measuring circuit, a current passing through the heating resistor 23 varies by adjustment of the variable resistance 20 to regulate an amount of heat produced. When using thus configured hydrogen gas sensor, the resistance values of the variable resistance 20 is first adjusted in an atmosphere containing no hydrogen gas to heat the combustion body 22 to a predetermined temperature (for example, 300 to 500° C.), and variable resistance 19 is adjusted to maintain the equilibrium state of the bridge circuit. Thereafter, when the hydrogen gas arrives at the combustion body 22, the hydrogen gas is burnt and the electric resistance of the heating resistor 23 increases. On the other hand, since the compensation element 24 does not have the activity of a hydrogen combustion catalyst, the hydrogen gas is not burnt in the compensation element 24 and electric resistance of the compensation element 24 does not vary. Therefore, a difference of electric resistance is produced between the detection element 21 and the compensation element 24, and a bridge voltage is generated between the output terminals “c” and “d”. Since this bridge voltage is proportion to the gas concentration of the hydrogen gas, the gas concentration of the hydrogen gas is detected by this bridge voltage.
In recent years, such a catalytic combustion hydrogen gas sensor is expected to be used for monitoring the leakage of a hydrogen gas of fuel or monitoring and controlling a proper feed rate of hydrogen to a fuel cell, particularly in fuel cell vehicles.
By the way, generally, in fuel cell vehicles, electric power is supplied to the hydrogen gas sensor from a battery while the fuel cell vehicles are driving. But electric power cannot be supplied to the hydrogen gas sensor continually while the fuel cell vehicles are not driving in order to prevent a battery from going dead, and in the meantime, it becomes impossible to detect a hydrogen gas by a hydrogen gas sensor. Accordingly, in order to detect a hydrogen gas by a hydrogen gas sensor immediately after the start of driving of fuel cell vehicles to monitor the leakage of a hydrogen gas or monitor and control a proper feed rate of hydrogen to a fuel cell, it is required that hydrogen gas is detected precisely upon energization of hydrogen gas sensor.
However, only after a delay from a time of application of voltage to the heating resistor 23 to a time at which the combustion body 22 is heated to a predetermined temperature. Accordingly, it usually took a waiting time of more than a dozen seconds to several tens seconds before the precise hydrogen concentration can be measured after energization of the hydrogen gas sensor.