#!/usr/bin/env python

import bme680

import time

print("""indoor-air-quality.py - Estimates indoor air quality.

Runs the sensor for a burn-in period, then uses a

combination of relative humidity and gas resistance

to estimate indoor air quality as a percentage.

Press Ctrl+C to exit!

""")

try:

sensor = bme680.BME680(bme680.I2C_ADDR_PRIMARY)

except (RuntimeError, IOError):

sensor = bme680.BME680(bme680.I2C_ADDR_SECONDARY)

# These oversampling settings can be tweaked to

# change the balance between accuracy and noise in

# the data.

sensor.set_humidity_oversample(bme680.OS_2X)

sensor.set_pressure_oversample(bme680.OS_4X)

sensor.set_temperature_oversample(bme680.OS_8X)

sensor.set_filter(bme680.FILTER_SIZE_3)

sensor.set_gas_status(bme680.ENABLE_GAS_MEAS)

sensor.set_gas_heater_temperature(320)

sensor.set_gas_heater_duration(150)

sensor.select_gas_heater_profile(0)

# start_time and curr_time ensure that the

# burn_in_time (in seconds) is kept track of.

start_time = time.time()

curr_time = time.time()

burn_in_time = 300

burn_in_data = []

try:

# Collect gas resistance burn-in values, then use the average

# of the last 50 values to set the upper limit for calculating

# gas_baseline.

print('Collecting gas resistance burn-in data for 5 mins\n')

while curr_time - start_time < burn_in_time:

curr_time = time.time()

if sensor.get_sensor_data() and sensor.data.heat_stable:

gas = sensor.data.gas_resistance

burn_in_data.append(gas)

print('Gas: {0} Ohms'.format(gas))

time.sleep(1)

gas_baseline = sum(burn_in_data[-50:]) / 50.0

# Set the humidity baseline to 40%, an optimal indoor humidity.

hum_baseline = 40.0

# This sets the balance between humidity and gas reading in the

# calculation of air_quality_score (25:75, humidity:gas)

hum_weighting = 0.25

print('Gas baseline: {0} Ohms, humidity baseline: {1:.2f} %RH\n'.format(

gas_baseline,

hum_baseline))

while True:

if sensor.get_sensor_data() and sensor.data.heat_stable:

gas = sensor.data.gas_resistance

gas_offset = gas_baseline - gas

hum = sensor.data.humidity

hum_offset = hum - hum_baseline

# Calculate hum_score as the distance from the hum_baseline.

if hum_offset > 0:

hum_score = (100 - hum_baseline - hum_offset)

hum_score /= (100 - hum_baseline)

hum_score *= (hum_weighting * 100)

else:

hum_score = (hum_baseline + hum_offset)

hum_score /= hum_baseline

hum_score *= (hum_weighting * 100)

# Calculate gas_score as the distance from the gas_baseline.

if gas_offset > 0:

gas_score = (gas / gas_baseline)

gas_score *= (100 - (hum_weighting * 100))

else:

gas_score = 100 - (hum_weighting * 100)

# Calculate air_quality_score.

air_quality_score = hum_score + gas_score

print('Gas: {0:.2f} Ohms,humidity: {1:.2f} %RH,air quality: {2:.2f}'.format(

gas,

hum,

air_quality_score))

time.sleep(1)

except KeyboardInterrupt:

pass