I lost four DS18B20s before I worked out what was happening, and the failure pattern was what made it hard.

A sensor would work perfectly. Two sessions, five, sometimes a dozen. Then it would return garbage forever — not noise, not drift, just nonsense from a part that had been fine an hour earlier. I replaced the sensor and the new one worked. So I blamed the sensors, ordered a different batch, and lost those too.

Then I blamed the water. The thermowell was dry inside and the cable gland was sealed, but I checked anyway. Nothing. I blamed the wiring, reflowed every joint, shortened the lead. Nothing.

What finally gave it away was cooking at a friend’s house on a gas hob for a week with no failures at all. The variable was not the sensor, the water or the wiring. It was the cooktop.

An induction hob is a large coil throwing a serious alternating magnetic field around, and I had placed a bare semiconductor inside a metal tube directly above it. The stainless thermowell I had fitted to protect the sensor was acting as an antenna, collecting that field and delivering it to the one component I was trying to shield.

Two changes, and no failures since. First: bed the sensor in thermal paste before it goes into the well. Second: bond the steel tube itself to GND with a wire.

The ground bond is almost certainly doing the real work — it turns the sleeve from an antenna into a shield wrapped around the sensor. The paste went in for thermal coupling, and it does help there; the probe responds noticeably faster with the air gap gone. But it also holds the sensor still inside a grounded enclosure, which cannot hurt.

Several months of induction use since, no dead sensors. That is months, not years, so I will say “this fixed it for me” rather than “this is solved”. If you build one and still lose a sensor, please tell me.

The general lesson is worth more than the specific fix: a metal enclosure is only a shield if it is connected to something. Floating, it is an antenna, and it will deliver whatever it collects straight into the most sensitive thing you put inside it.