Misuse of the word “since” has become quite common.
Bad Example: “Since we have Friday off, please update your status report on Thursday.”
According to Webster’s Ninth New Collegiate Dictionary, the preferred definition of “since” is:
“from a definite past time until now” or “at a time in the past after.”
Good Example: “Since the 1950’s, rock music has morphed into many sub-genres.”
If in doubt, substitute “ever since” or “since the time” in your mind, and if it still makes sense, use “since.”
The definition of “because” is: “for the reason that.”
Good Example: “Because the equipment was corroded, it needed to be replaced.”
If in doubt, substitute “due to the fact that” in your mind, and if it still makes sense, use “because.”
Some sentences could go either way, depending on what you mean.
Example: “Since they installed the new equipment, they haven’t had any problems.”
Do you mean “ever since” they installed it, or “due to the fact that” they installed it?
If you mean both, use “since.”