feat(bme280): Add dew point calculation using Magnus formula (#322)

* feat(bme280): Add dew point calculation using Magnus formula.

* fix(bme280): Use single read() in dew_point and clamp RH to avoid log(0).

Author: nedseb

lib/bme280/README.md

@@ -186,6 +186,16 @@ sensor.sea_level_pressure_hpa = 1020.0
 
 ---
 
+### Dew Point
+
+```python
+dp = sensor.dew_point()
+```
+
+Returns the dew point temperature in **degrees Celsius**, computed from the current temperature and humidity using the Magnus formula.
+
+---
+
 ## Data-Ready Status
 
 ```python
@@ -313,7 +323,7 @@ Performs a soft reset, re-reads calibration data, and re-applies default configu
 | Standby time | ✅ | ❌ | ✅ | ❌ | ⚠️ Fixed 500ms | ❌ |
 | Altitude | ✅ | ✅ | ✅ | ✅ | ✅ | ❌ |
 | Sea-level pressure | ✅ | ✅ | ✅ | ✅ | ✅ | ❌ |
-| Dew point | ❌ | ✅ | ❌ | ❌ | ❌ | ❌ |
+| Dew point | ✅ | ✅ | ❌ | ❌ | ❌ | ❌ |
 | Soft reset | ✅ | ❌ | ❌ | ❌ | ✅ | ❌ |
 | Full reset + recalibration | ✅ | ❌ | ❌ | ❌ | ❌ | ❌ |
 | power_off / power_on | ✅ | ❌ | ⚠️ Via mode | ❌ | ❌ | ❌ |

lib/bme280/bme280/device.py

@@ -409,3 +409,17 @@ def altitude(self, pressure_hpa=None):
         """
         p = self.pressure_hpa() if pressure_hpa is None else pressure_hpa
         return 44330.0 * (1.0 - (p / self.sea_level_pressure_hpa) ** 0.1903)
+
+    def dew_point(self):
+        """Return dew point temperature in degrees Celsius.
+
+        Uses the Magnus formula (Alduchov & Eskridge, 1996) with the
+        current temperature and relative humidity readings.  Both values
+        come from a single ``read()`` call to ensure consistency.
+        """
+        from math import log
+
+        t, _, rh = self.read()
+        rh = max(rh, 0.01)
+        gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)
+        return 243.04 * gamma / (17.625 - gamma)

tests/scenarios/bme280.yaml

@@ -630,3 +630,27 @@ tests:
       result = abs(alt - 33.7) < 1.0
     expect_true: true
     mode: [mock]
+
+  # ----- Dew point -----
+
+  - name: "dew_point() returns plausible value"
+    action: script
+    script: |
+      # Mock: ~25.08°C, ~50.57%RH → dew point ~14.11°C
+      dp = dev.dew_point()
+      result = abs(dp - 14.11) < 0.5
+    expect_true: true
+    mode: [mock]
+
+  - name: "dew_point() matches Magnus formula for mock readings"
+    action: script
+    script: |
+      from math import log
+      t, _, rh = dev.read()
+      # Compute expected dew point from Magnus formula
+      gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)
+      expected = 243.04 * gamma / (17.625 - gamma)
+      dp = dev.dew_point()
+      result = abs(dp - expected) < 0.01
+    expect_true: true
+    mode: [mock]