TLDR: If you need to find studs behind plaster walls, buy this and follow the printed directions. It works!

Details: The plaster in your walls is spread over wood slats called "lath." Behind the lath are the studs. You can't use a regular stud finder because it can't tell the difference between the lath and the studs. However, the Zircon MetalliScanner m40 uses powerful magnets to precisely located the *nails* that fix the lath to the studs. When you find a nail, you know you've found a stud! There is a wide setting for quickly scanning a large area and a narrow setting for homing in on a precise location. The Zircon MetalliScanner m40 is so sensitive it even picked up a reading from the lead in the pencil marks I on made on the wall to indicate where to drill. NOTE: read the directions carefully, because this device will also detect wiring, plumbing, gas lines, and other metal behind the wall that is best left undisturbed.

I have two other deep scan stud finders and they could not detect studs consistently in our plaster walls. This works perfectly for our application. Finds the studs every time.

I kept getting metal indications at the same spot on every board I was checking but could nor find the source until I realized it was picking up the hinge underneath my table. Love it. Great value.

The squeaks on my second story floor have been growing steadily worse for several years now, and I was determined to get rid of them. About a year ago I picked up a Squeek No More kit, but did not have much success because I could not find my floor joists. I had tried stud finders, the drill with a coat hanger, and hammer and listen methods to try and find the joists but I was not having any luck. I did not want to peel the carpet up, so I gave up at the time. This year, as the squeaks grew louder still and seemed to multiply I gained new motivation to try again. After some more research I came across the idea of using a metal detector to find the nails in the joists, and finally found the Zircon m40-FFP Handheld Electronic Metal Detector. It is a bit pricey, but I figured if it allowed me to fix my floors, it would be well worth it.

I am so glad I gave it a try! It took a few attempts to learn how to use the tool, but I quickly was able to lay out where I thought the joists were based on the readings. I measured and sure enough they were 16" apart. I tried the Squeek No More kit again, and this time it actually worked!!! I am overjoyed! It's making me love my old house again.

The Zircon's dual broad and fine sensing modes where very helpful in narrowing down the center of the joists, and it's indicator lights are simple to understand. It's lightweight yet sturdy and comfortable to hold. Be sure to keep it in the same orientation you start sensing with to avoid false readings. You can move side to side or forward/backwards, just do not rotate or tilt while in use.

I have been using this for several months and I am able to use it so that it is 100% correct all the time. How you use it and what you are trying to detect makes a difference. I use it to quickly scan the surface of wood for ferrous metals before I put it into a thickness planer. If a thin nail has been driven into a board and I am trying to detect it from the other side (detecting the pointed end of the nail), the Wizard needs to be within 1/2" of the end of the nail. If I am trying to detect a large mass of metal (like an I-beam :) it will detect it from 5+ inches away.

It gives many, many false positives (that is, indicates there is metal there when there isn't). The Wizard is constantly adjusting itself to accomodate the substrate the metal might be embedded in, which is always changing since you are moving the Wizard around. However how the device shows false positives is different than how it shows genuine positives (that is, metal that really is there). When I see false positives I move it back and forth over an area that doesn't have metal to let the device "set itself to 0" (i.e., the lights go out). Then slowly going back over the area in question will reveal the truth.

I am very happy with this device. I'm only giving it 4 stars because of the useless, too generalized, information given from the manufacturer about the parameters for which it will detect metal.