I bought my Dakota tester years ago. To me, if it's not certifiable (none of the cheap ones are legal for testing when results really matter) it's not much better than visual and weight inspections. You can pay a couple hundred and still cost yourself a couple grand later. They also gave me a crash course on proper testing and have a file of what I bought, and have been great to talk to over the years when i had questions.
Personally, not having any background in how they work or thier design, I would never modify the transducer. They tell me not to. I figure they should know...lol. Good companies offer a variety of them for ferrous and non-ferrous metals and various radii. Mine has paid for itself over time but it's also saved me from making an expensive mistake or two - so I'm way ahead.

Well, art is art, isn't it? Still, on the other hand, water is water! And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know.