Added example of how to calculate Indoor Air Quality (IAQ).

examples/air-quality.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python
+
+import bme680
+import time
+
+print("""Estimate 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
+
+""")
+
+sensor = bme680.BME680()
+
+# 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) / (100 - hum_baseline) * (hum_weighting * 100)
+
+            else:
+                hum_score = (hum_baseline + hum_offset) / hum_baseline * (hum_weighting * 100)
+
+            # Calculate gas_score as the distance from the gas_baseline.
+            if gas_offset > 0:
+                gas_score = (gas / gas_baseline) * (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