Add sea level pressure calculation

documentation/boards/enviro-weather.md

@@ -11,6 +11,7 @@ Enviro Weather is a super slimline all in one board for keeping a (weather) eye
 |Temperature|`temperature`|celcius|°C|`22.11`|
 |Humidity|`humidity`|percent|%|`55.42`|
 |Air Pressure|`pressure`|hectopascals|hPa|`997.16`|
+|Adjusted Sea Level Air Pressure|`sea_level_pressure`|hectopascals|hPa|`1014.06`|
 |Luminance|`luminance`|lux|lx|`35`|
 |Rainfall|`rain`|millimetres|mm|`1.674`|
 |Rainfall Average|`rain_per_second`|millimetres per second|mm/s|`1.674`|

enviro/boards/weather.py

@@ -3,7 +3,7 @@
 from breakout_ltr559 import BreakoutLTR559
 from machine import Pin, PWM
 from pimoroni import Analog
-from enviro import i2c, activity_led
+from enviro import i2c, activity_led, config
 import enviro.helpers as helpers
 from phew import logging
 from enviro.constants import WAKE_REASON_RTC_ALARM, WAKE_REASON_BUTTON_PRESS
@@ -190,14 +190,27 @@ def get_sensor_readings(seconds_since_last, is_usb_power):
   ltr_data = ltr559.get_reading()
   rain, rain_per_second = rainfall(seconds_since_last)
 
+  # Adjust pressure to calculated sea level value if set to in config
+  pressure = bme280_data[1] / 100.0
+  temperature = bme280_data[0]
+
   from ucollections import OrderedDict
-  return OrderedDict({
-    "temperature": round(bme280_data[0], 2),
+  readings = OrderedDict({
+    "temperature": round(temperature, 2),
     "humidity": round(bme280_data[2], 2),
-    "pressure": round(bme280_data[1] / 100.0, 2),
+    "pressure": round(pressure, 2),
     "luminance": round(ltr_data[BreakoutLTR559.LUX], 2),
     "wind_speed": wind_speed(),
     "rain": rain,
     "rain_per_second": rain_per_second,
     "wind_direction": wind_direction()
   })
+
+  if config.sea_level_pressure:
+    logging.info(f"  - recorded temperature: {temperature}")
+    logging.info(f"  - recorded pressure: {pressure}")
+    sea_level_pressure = round(helpers.get_sea_level_pressure(pressure, temperature, config.height_above_sea_level), 2)
+    logging.info(f"  - calculated mean sea level pressure: {sea_level_pressure}")
+    readings["sea_level_pressure"] = round(sea_level_pressure, 2)
+
+  return readings

enviro/config_defaults.py

@@ -23,6 +23,19 @@ def add_missing_config_settings():
   except AttributeError:
     warn_missing_config_setting("wifi_country")
     config.wifi_country = "GB"
+
+  try:
+    config.sea_level_pressure
+  except AttributeError:
+    warn_missing_config_setting("sea_level_pressure")
+    config.sea_level_pressure = False
+
+  try:
+    config.height_above_sea_level
+  except AttributeError:
+    warn_missing_config_setting("height_above_sea_level")
+    config.height_above_sea_level = 0
+
 
 def warn_missing_config_setting(setting):
     logging.warn(f"> config setting '{setting}' missing, please add it to config.py")

enviro/config_template.py

@@ -55,3 +55,8 @@
 
 # compensate for usb power
 usb_power_temperature_offset = 4.5
+
+# sea level pressure conversion (adjusts measured pressure output for mean sea level value)
+sea_level_pressure = False
+# height in metres
+height_above_sea_level = 0

enviro/helpers.py

@@ -98,3 +98,10 @@ def get_saturation_vapor_pressure(temperature_in_k):
       temperature_in_k *
       (a1*v + a2*v**1.5 + a3*v**3 + a4*v**3.5 + a5*v**4 + a6*v**7.5)
   )
+
+# Calculates mean sea level pressure (QNH) from observed pressure
+# https://keisan.casio.com/exec/system/1224575267
+def get_sea_level_pressure(observed_pressure, temperature_in_c, altitude_in_m):
+# def sea(pressure, temperature, height):
+	qnh = observed_pressure * ((1 - ((0.0065 * altitude_in_m) / (temperature_in_c + (0.0065 * altitude_in_m) + 273.15)))** -5.257)
+	return qnh