feat(bme280): Add dew point calculation using Magnus formula by nedseb · Pull Request #322 · steamicc/micropython-steami-lib

nedseb · 2026-03-30T09:47:19Z

Closes #316

Summary

dew_point() method: computes dew point temperature in degrees Celsius from current temperature and humidity using the Magnus formula (Alduchov & Eskridge, 1996 constants: a=17.625, b=243.04)
math.log imported lazily inside the method to avoid RAM usage when not needed
README: new Dew Point section in API reference, comparison table updated
2 new mock tests: plausible value check (~14.11 C from mock data), consistency verification against direct formula

Depends on #321 (altitude branch) — both branch from the same base.

Test plan

54 mock tests pass (make test-bme280)
Hardware validation

Copilot

Pull request overview

Adds derived environmental calculations to the BME280 driver and updates docs/examples/tests to expose them.

Changes:

Add BME280.dew_point() using the Magnus formula (Alduchov & Eskridge constants).
Add altitude support via BME280.altitude() and a configurable sea_level_pressure reference.
Update README, example script, and scenario tests to cover the new APIs.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
`lib/bme280/bme280/device.py`	Introduces `sea_level_pressure`, `altitude()`, and `dew_point()` methods.
`tests/scenarios/bme280.yaml`	Adds mock scenario coverage for altitude and dew point.
`lib/bme280/examples/weather_station.py`	Switches altitude calculation to `sensor.altitude()` and documents `sea_level_pressure`.
`lib/bme280/README.md`	Documents Altitude/Dew Point APIs and updates the feature comparison table.

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Copilot · 2026-03-30T09:50:24Z

+        t = self.temperature()
+        rh = self.humidity()
+        gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)
+        return 243.04 * gamma / (17.625 - gamma)


dew_point() calls temperature() and humidity() separately. In sleep/forced mode this will trigger two measurements via _ensure_data(), and it can also mix readings from different conversions. Consider using a single read() (or read_one_shot() if you want a fresh forced conversion) to get temperature/humidity from the same sample and avoid the extra I2C traffic and power usage.

Suggested change

t = self.temperature()

rh = self.humidity()

gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)

return 243.04 * gamma / (17.625 - gamma)

t, _, rh = self.read()

gamma = log(rh / 100.0) + 17.625 * t / (243.04 + t)

return 243.04 * gamma / (17.625 - gamma)

Fixed: dew_point() now uses a single self.read() call to get temperature and humidity from the same measurement, avoiding double trigger and inconsistent readings.

Copilot · 2026-03-30T09:50:24Z

+        from math import log
+
+        t = self.temperature()
+        rh = self.humidity()


dew_point() uses log(rh / 100.0) without guarding against rh being 0.0 (possible with the current humidity compensation which clamps to 0). This can raise a math domain error at runtime. Clamp RH to a small positive minimum (and optionally cap to 100%) before taking the log, or define a clear behavior/error for RH<=0.

Suggested change

rh = self.humidity()

rh = self.humidity()

# Clamp RH to a small positive minimum and a maximum of 100% to

# avoid math domain errors from log(rh / 100.0) when rh <= 0.

rh = max(0.01, min(100.0, rh))

Fixed: added rh = max(rh, 0.01) before log() to prevent math domain error when humidity compensation clamps to 0.

Copilot · 2026-03-30T09:50:24Z

+
+        Uses the ICAO barometric formula with ``sea_level_pressure`` as
+        reference (default 1013.25 hPa, adjustable).
+        """


altitude() divides by self.sea_level_pressure without validating it. Since this is a public attribute, it’s easy for users/tests to set it to 0 or a negative value and trigger a ZeroDivisionError or complex results. Consider validating sea_level_pressure > 0 (e.g., raise a clear exception) before computing altitude.

Suggested change

"""

"""

# Validate sea_level_pressure before using it as a divisor to avoid

# ZeroDivisionError or invalid results if it has been set to a

# non-positive value by user code.

if not hasattr(self, "sea_level_pressure") or self.sea_level_pressure <= 0:

raise ValueError(

"sea_level_pressure must be a positive value for altitude calculation"

)

Not addressed: same rationale as PR #321 — no validation on internal attributes per project convention. See discussion on PR #321.

Copilot · 2026-03-30T09:50:25Z

+  - 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


This dew point scenario is labeled as “equals temperature at 100% humidity”, but the script doesn’t set RH to 100% (it reads the mock ~50%RH) and it computes expected from the same Magnus formula, so it’s really a formula-consistency test. Rename/update the test (and comments) to match what it asserts, or actually force RH=100% and assert dew_point() == temperature() within tolerance.

Suggested change

- 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

- name: "dew_point() matches Magnus formula for mock readings"

action: script

script: |

# Validate dew_point() implementation against the Magnus formula

# using the current mock temperature and humidity readings

from math import log

t = dev.temperature()

rh = dev.humidity()

# Compute expected dew point from Magnus formula using these values

Fixed: test renamed to "dew_point() matches Magnus formula for mock readings" and uses dev.read() for consistency with the implementation.

Copilot · 2026-03-30T09:50:25Z

+    def altitude(self):
+        """Return estimated altitude in meters from current pressure.
+
+        Uses the ICAO barometric formula with ``sea_level_pressure`` as
+        reference (default 1013.25 hPa, adjustable).
+        """
+        p = self.pressure_hpa()
+        return 44330.0 * (1.0 - (p / self.sea_level_pressure) ** 0.1903)


PR title/summary focus on adding dew_point(), but this diff also introduces the sea_level_pressure attribute and altitude() API (plus README + example + scenario updates for altitude). If this is a stacked PR that depends on #321, consider rebasing onto that branch/retargeting so this PR only contains the dew point-related changes, or update the PR description/title to reflect the additional API surface.

The branch is based on feat/bme280-altitude and will be rebased on main.

…(0).

# [0.9.0](v0.8.0...v0.9.0) (2026-03-30) ### Features * **bme280:** Add dew point calculation using Magnus formula ([#322](#322)) ([8e730ad](8e730ad))

semantic-release-updater · 2026-03-30T10:10:40Z

🎉 This PR is included in version 0.9.0 🎉

The release is available on:

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Copilot AI review requested due to automatic review settings March 30, 2026 09:47

github-actions Bot assigned nedseb Mar 30, 2026

Copilot started reviewing on behalf of nedseb March 30, 2026 09:47 View session

Copilot AI reviewed Mar 30, 2026

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nedseb force-pushed the feat/bme280-dew-point branch from 626f6b9 to 95bfb54 Compare March 30, 2026 10:03

nedseb added 2 commits March 30, 2026 12:06

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

2786b29

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

4e59955

…(0).

nedseb force-pushed the feat/bme280-dew-point branch from 95bfb54 to 4e59955 Compare March 30, 2026 10:07

nedseb merged commit 8e730ad into main Mar 30, 2026
9 checks passed

nedseb deleted the feat/bme280-dew-point branch March 30, 2026 10:10

semantic-release-updater Bot pushed a commit that referenced this pull request Mar 30, 2026

chore(release): v0.9.0. [skip ci]

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# [0.9.0](v0.8.0...v0.9.0) (2026-03-30) ### Features * **bme280:** Add dew point calculation using Magnus formula ([#322](#322)) ([8e730ad](8e730ad))

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

feat(bme280): Add dew point calculation using Magnus formula#322
nedseb merged 2 commits intomainfrom
feat/bme280-dew-point

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-        rh = self.humidity()
+        rh = self.humidity()
+        # Clamp RH to a small positive minimum and a maximum of 100% to
+        # avoid math domain errors from log(rh / 100.0) when rh <= 0.
+        rh = max(0.01, min(100.0, rh))

-        """
+        """
+        # Validate sea_level_pressure before using it as a divisor to avoid
+        # ZeroDivisionError or invalid results if it has been set to a
+        # non-positive value by user code.
+        if not hasattr(self, "sea_level_pressure") or self.sea_level_pressure <= 0:
+            raise ValueError(
+                "sea_level_pressure must be a positive value for altitude calculation"
+            )

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nedseb commented Mar 30, 2026

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