bme280_xxx.py: Add methods for altitude and dew_point

The Documentation is extended accordingly

README.md

@@ -5,7 +5,7 @@ for use with MicroPython on Pycom of ESP8266 boards. It is also compatible with
 the BMP280 which provides the same interface but temperature + pressure only.
 
 Two different variants of the library are supplied. bme20_int.py uses integer
-arithmetic. bme280_float.py uses float arithmetic for the compensation  of the
+arithmetic, bme280_float.py uses float arithmetic for the compensation  of the
 raw values. The results are (almost) the identical, but the format of the
 returned values differs.
 
@@ -24,21 +24,37 @@ bme= BME280(i2c=i2c, mode=BME280_OSAMPLE_8, address=BME280_I2CADDR)
 mode is the setting for oversampling of the humidity value, address the i2c
 address used.
 
-## Property
+## Properties
 
-### values = BME280.value
+### values = BME280.values
 
 The `values` property is a convenience function that provides a tuple of
 human-readable string values to quickly check that the sensor is working.
 In practice, the method to use is `read_compensated_data()` which returns
 a `(temperature, pressure, humidity)`-tuple
 
+### altitude = bmp.altitude
+Altitude in m. The altitude is calculated based on the value given to
+the property sealevel (see below). Obviously, this value does not have to be the
+sealevel pressure, but any pressure you may select, for instance to measure
+altitude difference in general.
+
+### bmp.sealevel = sealevel  
+### sealevel = bmp.sealevel
+Setting and getting the pressure for altitude calculation.  
+The default is 101325 Pa, but you can use your local
+QNH in Pa, or set a local pressure to determine altitude difference.  
+
+### dew_point = si7021.dew_point
+Returns the dew_point temperature (°C) calculated from the actual temperature and humidity.  
+
 ## Methods
 
 ### values = read_compensated_data(result = None)
 
-Values is an array of either integers of floats, holding the values of temperature,
-pressure and humidity. The format differs for integers and floats:
+Values is an array of either integers (bme280_int.py) of floats (bme280_float.py),
+holding the values of temperature, pressure and humidity.
+The format differs for integers and floats:
 
 #### Integer formats:
 * `temperature`:  the temperature in hundredths of a degree Celsius. For example,
@@ -76,10 +92,10 @@ Copy `bme280.py` onto the board. Then:
 # On pycom devuces that is P9 = SDA, P10 = scl
 #
 import machine
-import bme280
+import bme280_float
 
 i2c = machine.I2C()
-bme = bme280.BME280(i2c=i2c)
+bme = bme280_float.BME280(i2c=i2c)
 
 print(bme.values)
 ```

bme280_float.py

@@ -69,6 +69,7 @@ class BME280:
         if i2c is None:
             raise ValueError('An I2C object is required.')
         self.i2c = i2c
+        self.__sealevel = 101325
 
         # load calibration data
         dig_88_a1 = self.i2c.readfrom_mem(self.address, 0x88, 26)
@@ -186,6 +187,39 @@ class BME280:
 
         return array("f", (temp, pressure, humidity))
 
+    @property
+    def sealevel(self):
+        return self.__sealevel
+
+    @sealevel.setter
+    def sealevel(self, value):
+        if 30000 < value < 120000: # just ensure some reasonable value
+            self.__sealevel = value
+
+    @property
+    def altitude(self):
+        '''
+        Altitude in m.
+        '''
+        from math import pow
+        try:
+            p = 44330 * (1.0 - pow(self.read_compensated_data()[1]
+                                   / self.__sealevel, 0.1903))
+        except:
+            p = 0.0
+        return p
+
+    @property
+    def dew_point(self):
+        """
+        Compute the dew point temperature for the current Temperature
+        and Humidity measured pair
+        """
+        from math import log
+        t, p, h = self.read_compensated_data()
+        h = (log(h, 10) - 2) / 0.4343 + (17.62 * t) / (243.12 + t)
+        return 243.12 * h / (17.62 - h)
+
     @property
     def values(self):
         """ human readable values """

bme280_int.py

@@ -69,6 +69,7 @@ class BME280:
         if i2c is None:
             raise ValueError('An I2C object is required.')
         self.i2c = i2c
+        self.__sealevel = 101325
 
         # load calibration data
         dig_88_a1 = self.i2c.readfrom_mem(self.address, 0x88, 26)
@@ -189,6 +190,41 @@ class BME280:
 
         return array("i", (temp, pressure, humidity))
 
+    @property
+    def sealevel(self):
+        return self.__sealevel
+
+    @sealevel.setter
+    def sealevel(self, value):
+        if 300 < value < 1200: # just ensure some reasonable value
+            self.__sealevel = value
+
+    @property
+    def altitude(self):
+        '''
+        Altitude in m.
+        '''
+        from math import pow
+        try:
+            p = 44330 * (1.0 - pow((self.read_compensated_data()[1] / 256)
+                                   / self.__sealevel, 0.1903))
+        except:
+            p = 0.0
+        return p
+
+    @property
+    def dew_point(self):
+        """
+        Compute the dew point temperature for the current Temperature
+        and Humidity measured pair
+        """
+        from math import log
+        t, p, h = self.read_compensated_data()
+        t /= 100
+        h /= 1024
+        h = (log(h, 10) - 2) / 0.4343 + (17.62 * t) / (243.12 + t)
+        return (243.12 * h / (17.62 - h)) * 100
+
     @property
     def values(self):
         """ human readable values """