Thanks for your answers.
Wawa, I don't have a capcitor , i doubt a smoothing capacitor will help when the ADC output is so high... but i'll give it a shot . it will take time for me to get a capacitor.
Wawa and outsider,
Regarding the sensors: i have 2 sensors: one was bought in Amazon. i got the other one from the laboratory in the college i study at. so these sensors are not from the same batch.
My project is a autonomous robot (4WD) which detects fire and extinguishes it. It's also supposed to take the temperature into considerations and alert in case of high temperatures.
I've considered purchasing a digital (DHT11 or DHT22) sensor, but the fact this sensor comes with a header file with ready functions will not be appreciated by the proffessors. It's much easier to deal with analog sensor (as i have plenty of work with other sensors ahead of me).
Any other ideas? 