From 2786b29f39475be2d118459c526c982b5696550b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20NEDJAR?= <sebastien@nedjar.com>
Date: Mon, 30 Mar 2026 11:46:35 +0200
Subject: [PATCH 1/2] feat(bme280): Add dew point calculation using Magnus
 formula.

---
 lib/bme280/README.md        | 12 +++++++++++-
 lib/bme280/bme280/device.py | 13 +++++++++++++
 tests/scenarios/bme280.yaml | 27 +++++++++++++++++++++++++++
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/lib/bme280/README.md b/lib/bme280/README.md
index 5b91f94f..9432bb9e 100644
--- a/lib/bme280/README.md
+++ b/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 | ❌ | ❌ | ❌ |
diff --git a/lib/bme280/bme280/device.py b/lib/bme280/bme280/device.py
index a0108e07..35f85ac1 100644
--- a/lib/bme280/bme280/device.py
+++ b/lib/bme280/bme280/device.py
@@ -409,3 +409,16 @@ 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.
+        """
+        from math import log
+
+        t = self.temperature()
+        rh = self.humidity()
+        gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)
+        return 243.04 * gamma / (17.625 - gamma)
diff --git a/tests/scenarios/bme280.yaml b/tests/scenarios/bme280.yaml
index fca42d32..0aa9ced7 100644
--- a/tests/scenarios/bme280.yaml
+++ b/tests/scenarios/bme280.yaml
@@ -630,3 +630,30 @@ 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() equals temperature at 100% humidity"
+    action: script
+    script: |
+      # Override humidity compensation to return ~100%RH
+      # At 100% RH, dew point should equal temperature
+      from math import log
+      t = dev.temperature()
+      rh = dev.humidity()
+      # Compute expected dew point from actual mock values
+      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]

From 4e599550e6dc94e0db93c7da128f6aa870e54d5f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20NEDJAR?= <sebastien@nedjar.com>
Date: Mon, 30 Mar 2026 12:00:12 +0200
Subject: [PATCH 2/2] fix(bme280): Use single read() in dew_point and clamp RH
 to avoid log(0).

---
 lib/bme280/bme280/device.py | 7 ++++---
 tests/scenarios/bme280.yaml | 9 +++------
 2 files changed, 7 insertions(+), 9 deletions(-)

diff --git a/lib/bme280/bme280/device.py b/lib/bme280/bme280/device.py
index 35f85ac1..c2ef8bab 100644
--- a/lib/bme280/bme280/device.py
+++ b/lib/bme280/bme280/device.py
@@ -414,11 +414,12 @@ 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.
+        current temperature and relative humidity readings.  Both values
+        come from a single ``read()`` call to ensure consistency.
         """
         from math import log
 
-        t = self.temperature()
-        rh = self.humidity()
+        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)
diff --git a/tests/scenarios/bme280.yaml b/tests/scenarios/bme280.yaml
index 0aa9ced7..c849596a 100644
--- a/tests/scenarios/bme280.yaml
+++ b/tests/scenarios/bme280.yaml
@@ -642,15 +642,12 @@ tests:
     expect_true: true
     mode: [mock]
 
-  - name: "dew_point() equals temperature at 100% humidity"
+  - name: "dew_point() matches Magnus formula for mock readings"
     action: script
     script: |
-      # Override humidity compensation to return ~100%RH
-      # At 100% RH, dew point should equal temperature
       from math import log
-      t = dev.temperature()
-      rh = dev.humidity()
-      # Compute expected dew point from actual mock values
+      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()