Drilling a hole in the wall to hang a picture and hitting a pipe is an expensive hassle. It's no wonder that the search for apps capable of showing what's behind the plaster is so popular. The problem is that the app store is full of apps promising to see pipes, and most don't deliver—not because the program is bad, but because the phone doesn't have the necessary sensor.
This guide separates what's real from what's hype. You'll understand which smartphone sensor is used, what it actually detects, why it will never see a PVC pipe, and what the correct procedure is when the hole needs to be truly secure.
The only sensor that exists for this: the magnetometer.
The cell phone has a magnetometer, the sensor that makes the compass work. It measures the magnetic field around the device. When an object made of ferromagnetic material—iron, steel, nickel—is brought close, it distorts this field, and the application transforms this distortion into an audible or visual warning.
It's the same principle as apps for metal detector. And here's the most important point: the usable range is between 2 and 10 centimeters, depending on the size of the part and the quality of the sensor. There's no app that can extend this, because the limitation is the hardware.
Apps that advertise "radio waves," "ultrasound," or "wall radar" on your phone are describing technology the device doesn't possess. Similarly, those that show an image of piping superimposed on the wall using augmented reality are simply drawing on the screen—the line is created from your finger marks, not from reading the inside of the wall.
What can we really find?
With a good magnetometer app and a steady hand, you can locate:
- Nails and screws: In drywall, the screws that hold the sheet in place mark the exact line of the stud. It's the most reliable way to find where the structure is.
- Metal profiles and uprights: Plasterboard partitions and ceilings use galvanized steel profiles, which provide a clear signal.
- Steel conduit and shallow metal boxes: In older or commercial installations, metal conduit is common.
- Shallow galvanized iron piping: In older buildings, where iron pipes still exist, it's possible to detect a signal if they are just a few centimeters from the surface.
- Slab and beam reinforcement: The reinforcing steel in the structure can be detected when the concrete cover is thin.
What your cell phone can't do
You don't see any PVC, PEX, or PPR pipes. This is the most serious limitation for anyone who wants to avoid damaging the plumbing, because the vast majority of Brazilian residential plumbing is made of plastic. Plastic does not respond magnetically: to a magnetometer, a PVC pipe full of water and a piece of empty wall are the same thing.
It doesn't measure depth. The app says "there's metal around here," not "there's metal 3 cm away.". It does not distinguish between materials.The nail, profile, iron pipe, and hinge on the other side of the wall all give the same type of signal. Does not detect live wiring., Because the field of a low-current wire is too weak for the sensor. And It doesn't work on thick concrete walls., where everything is too deep.
Add to that the possibility of false positives: magnets from phone cases, speakers, electrical panels, metal furniture on the other side, and even the phone's own speaker can all interfere with the reading.
Applications that exist and function within this limit.
- Metal detector (Smart Tools co.): Free on Android, it displays the reading in microtesla with a pointer and alarm.
- Metal detector (ExaTools): Free on Android, with sensitivity adjustment and history.
- Physics Toolbox Magnetometer: free for Android and iphone, displays the raw graph of the field, without embellishment.
- Smart Metal Detector: Free on iPhone, simple reading on microtesla.
- FLIR ONE It is Seek Thermal: They are free, but only work with the brand's thermal camera attached to the cell phone. With the accessory, it's possible to see the layout of hot water pipes by the temperature difference in the wall — including plastic pipes.
How to drill safely for real
The app is just one more check, never the only one. The procedure that truly reduces risk combines a few simple things:
- Look for the building's plumbing and electrical plans. If it's a newly built property, they'll be available.
- Avoid high-risk areas: never drill vertically above or below an electrical outlet, switch, faucet, or water shut-off valve, nor horizontally next to them. That's where the plumbing runs up and down.
- Turn off the circuit breaker for that room and shut off the water supply before drilling.
- Drill shallow and slowly, gradually increasing the depth, using the correct drill bit for the material.
- Photograph the exposed walls during any renovation. That photo is worth more than any app once everything is plastered.
The right equipment when the hole is critical.
If the job is extensive, or if the wall has a history of shoddy wiring, a handheld wall detector is the way to go. Models from Bosch, Stanley, and Vonder locate ferrous and non-ferrous metal, wood, and live wiring, and indicate approximate depth—three things a cell phone can't do. They are affordable devices and solve most household problems.
For plastic piping, hidden leaks, or buried networks, the service requires a professional. The techniques used include electronic geophone, tracer gas, inspection camera, and thermography. Leak detection companies do this routinely, and it's much cheaper than breaking through the wall in the wrong place.
Frequently Asked Questions (FAQ)
- Is there an app that detects PVC pipes in walls?
- No, using only the phone's sensors. Plastic is not ferromagnetic and does not generate a signal in the magnetometer. With an attached thermal camera, it's possible to see the trace of hot water, and a wall detector with dielectric mode helps in some cases.
- Does the app work on any wall?
- It works best on drywall and thin plaster. On thick concrete, the sensor's range of a few centimeters is insufficient.
- Can the results be trusted for professional use?
- No. It serves as an initial indication. On-site, the appropriate equipment is a wall detector, and for concealed plumbing, a specialized service is needed.
- Do I need technical knowledge to use it?
- No. Just calibrate the compass, remove the case, and slowly move the device. The difficult part isn't using it, it's interpreting it: the app warns you that there's metal, but it doesn't say what it is.
- Do these apps cost anything?
- The apps mentioned here are free, with ads. Be wary of paid apps that promise to see inside walls: the sensor is the same as the free ones.
Conclusion
No app can turn your cell phone into an X-ray of walls. What does exist is a magnetometer capable of pinpointing ferrous metal a few centimeters away — useful for finding nails, screws, studs, and steel conduit, but useless for PVC pipe, which is precisely the most common type found in Brazilian homes.
Use the app as a first check, respect the risk zones around electrical outlets and faucets, turn off the circuit breaker and water shut-off valve, and drill slowly. When the job is larger, invest in a wall detector or call someone with a geophone and thermal camera. It's cheaper than repairing a burst pipe.
