thats the good thing about spring scales. their "k" constant never changes and unless you bind them or pull them too much, you will repeatedly get the same measurements of the same fish. You just have to really be careful not to over extend it or bind them too much or you change your "K" constant and therefore get repeatedly similar, but inaccurate, measurements. So basically, you can weight the same fish ten times, and get the same number all ten times, but the weight is wrong because your K for the spring has changed. Rust and other "wear and tear" attributes affect the constant as well. But like mentioned, spring scales dont break, and thats the one thing that cant be beat, ie no batteries required