In curiosity, does anybody tried time bound task execution ? what I mean is can we execute a task for certain period of a time for e.g. If i want to run certain functions for a minute then how will I execute in the code?
This is a very common. No libraries are required. See these two tutorials
Using millis() for timing. A beginners guide
Demonstration code for several things at the same time
What is frustrating for me in this thread is that you have a series of seemingly unrelated problems/questions with no apparent resolution before you ask the next question.
1)How to tell a disconnected sensor from an unloaded one when both readings are 0.
2)How to manage an SD card for data storage.
3)How to enable a period for a function to run.
I can not see achieving success with this approach. I would suggest that you break things down into small tests and work them to resolution or else decide that the functionality is not going to be achieved and let us know that you are moving on.
In my opinion, your coding skills are not strong enough to be successful in this project, and you either need to get a programmer for team mate, or back up and start learning how to code from the basics.