fig1 illustrates an embodiment of a gas detection instrument according to the invention . the instrument includes five major parts , a power supply 10 , a sensor 12 , an actuating power control unit 14 that operates the sensor , a unit 16 for measuring sensor output , and an output unit 18 , which may include an lcd display , an audible alarm , or other audio and / or visual output . the sensor 12 is a combustible gas sensor , and it may be a catalytic bead sensor or a semiconductor ( mos ) sensor , as discussed above . the power supply 10 , which is usually a battery pack , provides electricity for operation of the entire unit . the power supplied to the sensor 12 is controlled by the actuating power control unit 14 . for either the semiconductor sensor type or the catalytic bead type , the energy to the sensor , and especially to the sensing elements , may be in the form of a constant voltage , constant current , or constant power . the voltage , current , or power is selected to be adequate to heat the sensor to a desired and predetermined operating temperature . the measuring unit 16 includes circuitry which detects changes in an electrical parameter , for example the electrical resistance , of the sensing elements in the sensor , and converts the change to an output signal . typically the measuring unit includes a wheatstone bridge circuit when the sensor is of the catalytic bead type . there is a feedback link 20 , as shown by the dashed lines in fig1 , between the measuring unit and the actuating control unit , so that the power supplied to the sensor can be adjusted according to changes in sensor output signal detected by the measuring circuit . the measured results are presented by the output unit 18 , which can be an lcd , an alarm , or another device or a combination of other devices as known to those of ordinary skill in the art . when digital devices are included , the output unit should include an a / d converter and possibly a microprocessor for processing the signal from the sensor and managing output devices . fig2 illustrates the working principle of the embodiment of fig1 , showing gas concentration ( curve a ) as a percentage of lower explosive limit (% lel ), instrument response ( curve b ) from 0 to high , and electricity supply ( curve c ) as off or on . in the absence of a combustible gas ( concentration 0 ), the actuating control unit supplies electricity intermittently to the sensor ( time period 30 ). in this case the power is in the form of a plurality of substantially identical pulses of voltage . since it typically takes ˜ 2 seconds to heat a commercial catalytic bead sensor , or ˜ 1 second to heat a semiconductor sensor to the operating temperature (˜ 250 - 500 ° c . ), the duration of power supply is typically one or more seconds and the duration of rest in one cycle depends on how much power is projected to be saved . when a significant change in gas concentration ( point 32 ) causes a corresponding change in the sensor &# 39 ; s output ( point 33 ), it is detected by the measuring unit . when the change in sensor output exceeds a predetermined value , the actuating control unit instantly changes to steady state electricity ( time period 34 ), during which the sensor output keeps changing and the instrument measures output of the sensor at a more frequent basis . accordingly , instrument output devices such as lcd display and alarm , can be updated at the same frequency . in practice , it is preferred to have a portable gas detection instrument update the measured reading once a second . when no further significant change in gas concentration occurs ( point 36 ), the sensor output reaches a stable reading ( point 38 ), and the actuating control unit changes back to the pulsed power mode ( time period 40 ). the same mechanism applies when the gas concentration starts decreasing ( point 42 ). the actuating control unit switches to steady state power ( time period 44 ) and the instrument updates its output more often in order for instrument output devices to follow the rapid changes in gas concentration . once the gas concentration returns to zero ( point 46 ), the measuring circuit detects no further significant change in sensor output and this information is fed back to the actuating control unit , after which the operating power returns to intermittent or pulsed mode ( time period 48 ). the pulsed electricity is typically represented by the square waveform shown in fig2 , but many other waveforms are possible in the spirit of the invention . the threshold value for switching between intermittent current and steady state current is selected based on the signal - to - noise ratio of the output signal , the effects of ambient conditions such as temperature and humidity on the output signal , and the alarm settings of the instrument . for example , a typical commercial catalytic bead sensor possesses a transient response in the range of ± 3 % lel when there is a sudden , large change in temperature and / or humidity . the instrument alarm may be set at a level as low as 10 % lel . the threshold value for switching power applied to the sensor is preferable 3 - 5 % lel , which is higher than sensor &# 39 ; s response to changes in ambient conditions , and significantly lower than the alarm setting to allow steady state operation when sensor output signal rises in order to take full advantage of the invention . in another embodiment of the invention shown in fig3 , pulsed power is combined with trend analysis or extrapolation techniques to provide updated instrument outputs with an interval less than or equal to one second . in this embodiment , the feedback link 20 between the actuating power control unit and the sensor measuring unit is omitted . pulses of electricity are applied to the sensor on a continuous basis , even when the presence of a combustible gas has been detected . in practice , the total cycle time for a power pulse is typically between 3 - 8 seconds ; shorter times are difficult to achieve because of the minimum time required to heat the sensing elements of the sensor for reliable gas detection . this means that the time for updating the output is about 3 - 8 seconds even when the gas concentration changes quickly with time . in order to compensate for the less frequent updating , extrapolation is used to obtain new instrument output data at intervals of about 1 second . as shown in curve b of fig3 , the instrument starts to update its output ( point 60 ) after continuous changes in the gas concentration ( point 62 ) have been detected by the measuring circuit . an accurate read - out of the change in concentration usually requires at least 2 consecutive cycles after the occurrence of the change . the microprocessor in the instrument calculates the rate of change in sensor output , and predicts sensor outputs or instrument readouts for the next few seconds based on existing data and the rate of change calculated . an extrapolation method or other algorithm can be employed . the calculation method can be written into software and stored in the microprocessor . the predicted data are sent to the output unit , and are used for updating the instrument readout until an actual sensor output signal is obtained in the next power cycle . it is understood that the more actual sensor output data are obtained , the closer the predictive data are to actual gas concentration .