Added dew point, updated example

Author: kattni

adafruit_bme280.py

@@ -104,6 +104,16 @@ def _read_temperature(self):
         self._t_fine = int(var1 + var2)
         #print("t_fine: ", self.t_fine)
 
+    @property
+    def dew_point(self):
+        """The dew point in Celsius using the Magnus forumula. Constants from Sontag, 1990."""
+        b = 17.62
+        c = 243.12
+        gamma = (b * self.temperature /(c + self.temperature)) + math.log(self.humidity / 100.0)
+        dewpoint = (c * gamma) / (b - gamma)
+        return dewpoint
+
+
     @property
     def temperature(self):
         """The compensated temperature in degrees celsius."""

examples/bme280_simpletest.py

@@ -9,9 +9,9 @@
 bme280 = adafruit_bme280.Adafruit_BME280_I2C(i2c)
 
 # OR create library object using our Bus SPI port
-#spi = busio.SPI(board.SCK, board.MOSI, board.MISO)
-#bme_cs = digitalio.DigitalInOut(board.D10)
-#bme280 = adafruit_bme280.Adafruit_BME280_SPI(spi, bme_cs)
+# spi = busio.SPI(board.SCK, board.MOSI, board.MISO)
+# bme_cs = digitalio.DigitalInOut(board.D5)
+# bme280 = adafruit_bme280.Adafruit_BME280_SPI(spi, bme_cs)
 
 # change this to match the location's pressure (hPa) at sea level
 bme280.sea_level_pressure = 1013.25
@@ -21,4 +21,5 @@
     print("Humidity: %0.1f %%" % bme280.humidity)
     print("Pressure: %0.1f hPa" % bme280.pressure)
     print("Altitude = %0.2f meters" % bme280.altitude)
+    print("Dew point = %0.1f C" % bme280.dew_point)
     time.sleep(2)