The slot round the sensor

What you are looking at

Turn the board over and there is an arch of nothing round the middle: a slot one millimetre wide, running up one side of the sensor, across the top and down the other side. What it leaves behind is a tongue of board 4.9 mm wide, with the sensor on the end of it, joined to the rest only at the header end.

That arch is in the board outline file — the same file the fabricator uses to cut the outside edge. It is not a crack and it is not a scratch. It was drawn on purpose.

The problem it solves

A temperature sensor measures the temperature of itself. That is not a limitation of this part, it is what "temperature sensor" means: the element reports how warm the element is, and you are trusting that the element is as warm as the thing you wanted to know about.

A circuit board is a sheet of fibreglass with copper on it. Copper carries heat thousands of times better than air does, and even the fibreglass carries it about ten times better. So a sensor soldered to a board that is plugged into a project is thermally connected to the regulator turning spare volts into heat, the USB socket, and the microcontroller — all of them warmer than the room, and all of them reaching the sensor faster through the board than the room reaches it through the air.

Without the slot, all 23.2 mm of board width is a path. With it, the only path is 4.9 mm of tongue, and the heat has to travel further to use it. A fifth of the cross-section and a longer route means most of it is lost sideways into the air before it arrives.

What it does not solve

Everything off the board. The slot stops the board conducting heat to the sensor; it does nothing about the sensor sitting in the warm air above your project, which is usually the larger effect.

So the slot buys you the right to care about placement:

• Get the board out of the breadboard, on four long jumper wires.
• Keep it away from regulators, LEDs, displays and anything else warm, and below them rather than above if you have the choice: warm air rises into whatever sits over it.
• Give it a few minutes after moving it. Temperature takes a couple of seconds to catch up and the board it is bolted to takes longer.
• Leave the slot unobstructed. It is a thermal break, and a blob of hot glue or a cable tie across it puts the path back.

Why the tongue cannot be cut free

The obvious next thought is to finish the job and snap the tongue off. It does not work: the sensor's five connections — SDA, SCL, VCC, ground and ADDR — all run up that tongue as copper traces. It is the only reason it is still attached at all, and the isolation stops exactly where the wiring has to begin.