Reply To: HTU21D answers the scan but every humidity read comes back empty

Wiring is fine, your read is too early. In no-hold mode the HTU21D NACKs its read address until the measurement is done, which is exactly what the analyser shows. A 12-bit humidity reading takes up to about 16 ms, and with the default resolution and some margin you want more than 15. Temperature is the slow one at 50 ms, and you probably copied the delay from a humidity-only example and got lucky the other way round.

I poll instead of guessing:

Wire.beginTransmission(0x40);
Wire.write(0xF5);
Wire.endTransmission();
uint8_t n = 0;
for (int i = 0; i < 10 && n < 3; i++) {
  delay(10);
  n = Wire.requestFrom(0x40, 3);
}
if (n < 3) { /* skip humidity this round */ }

And when a read still fails, leave the key out of that minute’s message or send null. Don’t send 0, Hermetry stores it as a real 0 % and your alarm rule will love that. null just means no reading.