The datasheet of MQ5 sensor says, it requires a pre-heat of 24 hours. Does this mean that it needs to be powered on for 24 hours consecutively before usage? Some other websites (e.g. http://www.circuitstoday.com/interfacing-mq5-lpg-sensor-to-arduino) say that 15 minutes pre-heat would suffice. Could someone clarify this issue? Thanks.
Does this mean that it needs to be powered on for 24 hours consecutively before usage?
No, it just means that typical performance data in the datasheet has been measured after 24h. So, if you want to be within specification you must observe that 24h preheating time.
Does it mean you can't use the sensor before those 24h? Absolutely not. Given the small size of the sensor, thermal equilibrium will be almost surely reached within 30 min. And it would probably take just a few minutes to be within +/- X % of the specification (you can only know by actually measuring how much time does the output need to stabilise, which you should do).
At the end, the time to wait for thermal stabilisation of the sensor (preheating time) will depend on your application.
Only if you want a very accurate measure of gas concentration, you will need to wait those 24h (and also have a good calibration of your sensor, as well as means to compensate other environmental factors like temperature, humidity, etc.). But if you wanted to do this kind of measurement you probably won't be using a MQ-5 sensor anyway.
Take a look at this excellent technical note from Figaro Sensors to better understand the preheating time. It ISN'T a burn-in time per se, but a stabilisation time whose duration depends on several factors.
Please be clear when you mean the one time only pre-heat and when you mean turning the sensor on to take a measurement.
It is a electrochemical sensor and it needs time to burn-in. During those 24 hours, the impedance of the sensor changes a lot. After those 24 hours it can be used in a normal way and it will return consistant values.
The pre-heat should be done just once. I mean: just once in the lifespan of the sensor. After buying it, you should run the pre-heat. When the manufacturer writes in the datasheet that the pre-heat time is 24 hours, then you should do 24 hours. I suggest to connect the sensor in a normal way, with 'A' en 'B' pins connected and with the load resistor, and keep it on for 24 hours. Maybe it is already stable after a few hours, but keep it on for 24 hours to be sure.
To take a measurement, the sensor needs time to warm up. About 5 minutes might be okay. I think that 2 minutes is definitely too short.
When the sensor is always at room temperature, and the time to warm up is always the same, then the measurement is relative accurate and can be compared with other measurements. That means that when the sensor is mostly turned off to save power, only a few minutes to warm up can be used. However, I would start with at least 5 minutes.
[EDIT] The datasheets and other information from the manufacturers use the term "pre-heat" in different ways. The Technical Note in the link by @EnricBlanco uses the term "preheating" only for the "one-time-only" heating after storage to make the sensor more stable, as far as I can tell. Although it is not "one-time-only", it should be applied after storage or changing conditions as well. @EnricBlanco, try a very cheap MQ gas sensor (which is probably not preheated in the factory) and see how the impedance changes a lot during the first hours of preheating, and becomes stable after a few hours.