please i need the information badly thanks
Please explain what you mean with distance range in the context of a gas sensor. The gas has to get to the sensor, it cannot detect gas even 10cm away. It will react as soon as it hits the sensor (and reached some density), not earlier.
this is what i mean, my professor wants to know the diameter range of a gas sensor or how long its distance/range to detect a gas, thanks for your reply
so in that case, the distance effectively reading a gas is... o cm. has to be actually ON the sensor
ok i got the point now, it can only be detected once the gas hits the sensor, thank you for all your reply its a big help 
More to the point it's a factor of the concentration of the original source of the gas release and the direction of air currents towards the gas sensor, as the gas concentration will dilute as distance between gas source and detector increase. So several variable factors are involved, sensor sensitivity, gas concentration, direction of wind currents, etc. In most gas sensor applications the reading triggers an alarm based on some minimum safety specification to set off an alarm rather then pretending it's some kind of absolute accurate gas concentration measurement. If the area to monitor is larger then more sensors should be deployed to cover the area needing the protection alarm or measurement.