The Impulse is an older PI from Fisher. The newer ones by Garrett, Whites and Tesoro are much better in my experience and don't eat batteries as fast. Though the Impulse was better in it's day on battery usage compared to the rest of the PI's made back then.
The Impulse also loves to find iron and will find it deep. In fresh water (you didn't say where you were wanting to use it, as that is important) bobby pins can last decades so you will dig deep holes for them. Time you could better spend searching for the good stuff.