Use SI constants in CO unit converter (#153187)

homeassistant/util/unit_conversion.py

@@ -91,6 +91,17 @@ _GALLON_TO_CUBIC_METER = 231 * pow(_IN_TO_M, 3)  # US gallon is 231 cubic inches
 _FLUID_OUNCE_TO_CUBIC_METER = _GALLON_TO_CUBIC_METER / 128  # 128 fl. oz. in a US gallon
 _CUBIC_FOOT_TO_CUBIC_METER = pow(_FOOT_TO_M, 3)
 
+# Gas concentration conversion constants
+_IDEAL_GAS_CONSTANT = 8.31446261815324  # m3⋅Pa⋅K⁻¹⋅mol⁻¹
+# Ambient constants based on European Commission recommendations (20 °C and 1013mb)
+_AMBIENT_TEMPERATURE = 293.15  # K (20 °C)
+_AMBIENT_PRESSURE = 101325  # Pa (1 atm)
+_AMBIENT_IDEAL_GAS_MOLAR_VOLUME = (  # m3⋅mol⁻¹
+    _IDEAL_GAS_CONSTANT * _AMBIENT_TEMPERATURE / _AMBIENT_PRESSURE
+)
+# Molar masses in g⋅mol⁻¹
+_CARBON_MONOXIDE_MOLAR_MASS = 28.01
+
 
 class BaseUnitConverter:
     """Define the format of a conversion utility."""
@@ -169,14 +180,17 @@ class BaseUnitConverter:
 
 
 class CarbonMonoxideConcentrationConverter(BaseUnitConverter):
-    """Convert carbon monoxide ratio to mass per volume."""
+    """Convert carbon monoxide ratio to mass per volume.
+
+    Using ambient temperature of 20°C and pressure of 1 ATM.
+    """
 
     UNIT_CLASS = "carbon_monoxide"
     _UNIT_CONVERSION: dict[str | None, float] = {
-        CONCENTRATION_PARTS_PER_MILLION: 1,
-        # concentration (mg/m3) = 0.0409 x concentration (ppm) x molar mass
-        # Carbon monoxide molar mass: 28.01 g/mol
-        CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER: 0.0409 * 28.01,
+        CONCENTRATION_PARTS_PER_MILLION: 1e6,
+        CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER: (
+            _CARBON_MONOXIDE_MOLAR_MASS / _AMBIENT_IDEAL_GAS_MOLAR_VOLUME * 1e3
+        ),
     }
     VALID_UNITS = {
         CONCENTRATION_PARTS_PER_MILLION,

tests/components/sensor/test_recorder.py

@@ -3626,7 +3626,7 @@ async def test_compile_hourly_statistics_changing_units_3(
             13.050847,
             -10,
             30,
-            1.145609,
+            1.16441,
         ),
         (
             "carbon_monoxide",
@@ -3636,7 +3636,7 @@ async def test_compile_hourly_statistics_changing_units_3(
             13.050847,
             -10,
             30,
-            1 / 1.145609,
+            1 / 1.16441,
         ),
         (None, "%", None, "unitless", 13.050847, -10, 30, 0.01),
         (None, "W", "kW", "power", 13.050847, -10, 30, 0.001),

tests/util/test_unit_conversion.py

@@ -119,7 +119,7 @@ _GET_UNIT_RATIO: dict[type[BaseUnitConverter], tuple[str | None, str | None, flo
     CarbonMonoxideConcentrationConverter: (
         CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER,
         CONCENTRATION_PARTS_PER_MILLION,
-        1.145609,
+        1.16441,
     ),
     ConductivityConverter: (
         UnitOfConductivity.MICROSIEMENS_PER_CM,
@@ -291,13 +291,13 @@ _CONVERTED_VALUE: dict[
         (
             1,
             CONCENTRATION_PARTS_PER_MILLION,
-            1.145609,
+            1.16441,
             CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER,
         ),
         (
             120,
             CONCENTRATION_MILLIGRAMS_PER_CUBIC_METER,
-            104.74778,
+            103.05655,
             CONCENTRATION_PARTS_PER_MILLION,
         ),
     ],