Adjust temperature under usb power

enviro/__init__.py

@@ -97,8 +97,11 @@ def stop_activity_led():
 from machine import RTC, ADC
 import phew
 from pcf85063a import PCF85063A
+import enviro.config_defaults as config_defaults
 import enviro.helpers as helpers
 
+config_defaults.add_missing_config_settings()
+
 # read the state of vbus to know if we were woken up by USB
 vbus_present = Pin("WL_GPIO2", Pin.IN).value()
 
@@ -351,7 +354,7 @@ def get_sensor_readings():
     logging.info(f"  - seconds since last reading: {seconds_since_last}")
 
 
-  readings = get_board().get_sensor_readings(seconds_since_last)
+  readings = get_board().get_sensor_readings(seconds_since_last, vbus_present)
   # readings["voltage"] = 0.0 # battery_voltage #Temporarily removed until issue is fixed
 
   # write out the last time log

enviro/boards/grow.py

@@ -98,7 +98,7 @@ def water(moisture_levels):
           drip_noise()
         time.sleep(0.5)
 
-def get_sensor_readings(seconds_since_last):
+def get_sensor_readings(seconds_since_last, is_usb_power):
   # bme280 returns the register contents immediately and then starts a new reading
   # we want the current reading so do a dummy read to discard register contents first
   bme280.read()

enviro/boards/indoor.py

@@ -1,7 +1,9 @@
+import enviro.helpers as helpers
 import math
 from breakout_bme68x import BreakoutBME68X
 from breakout_bh1745 import BreakoutBH1745
 
+from enviro import config
 from enviro import i2c
 
 bme688 = BreakoutBME68X(i2c, address=0x77)
@@ -40,12 +42,21 @@ def colour_temperature_from_rgbc(r, g, b, c):
       ct = 10000
   return round(ct)
 
-def get_sensor_readings(seconds_since_last):
+def get_sensor_readings(seconds_since_last, is_usb_power):
   data = bme688.read()
 
   temperature = round(data[0], 2)
-  pressure = round(data[1] / 100.0, 2)
   humidity = round(data[2], 2)
+
+  # Compensate for additional heating when on usb power - this also changes the
+  # relative humidity value.
+  if is_usb_power:
+    adjusted_temperature = temperature - config.usb_power_temperature_offset
+    absolute_humidity = helpers.relative_to_absolute_humidity(humidity, temperature)
+    humidity = helpers.absolute_to_relative_humidity(absolute_humidity, adjusted_temperature)
+    temperature = adjusted_temperature
+
+  pressure = round(data[1] / 100.0, 2)
   gas_resistance = round(data[3])
   # an approximate air quality calculation that accounts for the effect of
   # humidity on the gas sensor
@@ -64,4 +75,4 @@ def get_sensor_readings(seconds_since_last):
     "aqi": aqi,
     "luminance": lux_from_rgbc(r, g, b, c),
     "color_temperature": colour_temperature_from_rgbc(r, g, b, c)
-  })
+  })

enviro/boards/urban.py

@@ -34,7 +34,7 @@ def particulates(particulate_data, measure):
   multiplier = 10 if measure >= PM0_3_PER_LITRE else 1
   return ((particulate_data[measure * 2] << 8) | particulate_data[measure * 2 + 1]) * multiplier
 
-def get_sensor_readings(seconds_since_last):
+def get_sensor_readings(seconds_since_last, is_usb_power):
   # bme280 returns the register contents immediately and then starts a new reading
   # we want the current reading so do a dummy read to discard register contents first
   bme280.read()

enviro/boards/weather.py

@@ -180,7 +180,7 @@ def rainfall(seconds_since_last):
 
   return amount, per_second
 
-def get_sensor_readings(seconds_since_last):
+def get_sensor_readings(seconds_since_last, is_usb_power):
   # bme280 returns the register contents immediately and then starts a new reading
   # we want the current reading so do a dummy read to discard register contents first
   bme280.read()

enviro/config_defaults.py

@@ -0,0 +1,23 @@
+import config
+from phew import logging
+
+DEFAULT_USB_POWER_TEMPERATURE_OFFSET = 4.5
+
+
+def add_missing_config_settings():
+  try:
+    # check if ca file parameter is set, if not set it to not use SSL by setting to None
+    config.mqtt_broker_ca_file
+  except AttributeError:
+    warn_missing_config_setting("mqtt_broker_ca_file")
+    config.mqtt_broker_ca_file = None
+
+  try:
+    config.usb_power_temperature_offset
+  except AttributeError:
+    warn_missing_config_setting("usb_power_temperature_offset")
+    config.usb_power_temperature_offset = DEFAULT_USB_POWER_TEMPERATURE_OFFSET
+
+
+def warn_missing_config_setting(setting):
+    logging.warn(f"> config setting '{setting}' missing, please add it to config.py")

enviro/config_template.py

@@ -50,4 +50,7 @@
 auto_water = False
 moisture_target_a = 50
 moisture_target_b = 50
-moisture_target_c = 50
+moisture_target_c = 50
+
+# compensate for usb power
+usb_power_temperature_offset = 4.5

enviro/constants.py

@@ -42,3 +42,8 @@
 UPLOAD_RATE_LIMITED = 2
 UPLOAD_LOST_SYNC = 3
 UPLOAD_SKIP_FILE = 4
+
+# humidity
+WATER_VAPOR_SPECIFIC_GAS_CONSTANT = 461.5
+CRITICAL_WATER_TEMPERATURE = 647.096
+CRITICAL_WATER_PRESSURE = 22064000

enviro/destinations/mqtt.py

@@ -13,12 +13,6 @@ def upload_reading(reading):
   password = config.mqtt_broker_password
   nickname = reading["nickname"]
 
-  # check if ca file paramter is set, if not set it to not use SSL by setting to None
-  try:
-    config.mqtt_broker_ca_file
-  except AttributeError: 
-    config.mqtt_broker_ca_file = None
-
   try:
     if config.mqtt_broker_ca_file:
     # Using SSL

enviro/helpers.py

@@ -1,5 +1,5 @@
 from enviro.constants import *
-import machine, os, time
+import machine, math, os, time
 
 # miscellany
 # ===========================================================================
@@ -57,3 +57,44 @@ def copy_file(source, target):
         if not chunk:
           break
         outfile.write(chunk)
+
+# temperature and humidity helpers
+# ===========================================================================
+
+# https://www.calctool.org/atmospheric-thermodynamics/absolute-humidity#what-is-and-how-to-calculate-absolute-humidity
+def relative_to_absolute_humidity(relative_humidity, temperature_in_c):
+  temperature_in_k = celcius_to_kelvin(temperature_in_c)
+  actual_vapor_pressure = get_actual_vapor_pressure(relative_humidity, temperature_in_k)
+
+  return actual_vapor_pressure / (WATER_VAPOR_SPECIFIC_GAS_CONSTANT * temperature_in_k)
+
+def absolute_to_relative_humidity(absolute_humidity, temperature_in_c):
+  temperature_in_k = celcius_to_kelvin(temperature_in_c)
+  saturation_vapor_pressure = get_saturation_vapor_pressure(temperature_in_k)
+
+  return (WATER_VAPOR_SPECIFIC_GAS_CONSTANT * temperature_in_k * absolute_humidity) / saturation_vapor_pressure * 100
+
+def celcius_to_kelvin(temperature_in_c):
+  return temperature_in_c + 273.15
+
+# https://www.calctool.org/atmospheric-thermodynamics/absolute-humidity#actual-vapor-pressure
+# http://cires1.colorado.edu/~voemel/vp.html
+def get_actual_vapor_pressure(relative_humidity, temperature_in_k):
+  return get_saturation_vapor_pressure(temperature_in_k) * (relative_humidity / 100)
+
+def get_saturation_vapor_pressure(temperature_in_k):
+  v = 1 - (temperature_in_k / CRITICAL_WATER_TEMPERATURE)
+
+  # empirical constants
+  a1 = -7.85951783
+  a2 = 1.84408259
+  a3 = -11.7866497
+  a4 = 22.6807411
+  a5 = -15.9618719
+  a6 = 1.80122502
+
+  return CRITICAL_WATER_PRESSURE * math.exp(
+      CRITICAL_WATER_TEMPERATURE /
+      temperature_in_k *
+      (a1*v + a2*v**1.5 + a3*v**3 + a4*v**3.5 + a5*v**4 + a6*v**7.5)
+  )