# Raspberry Pi Driver for Adafruit HTU21D-F
# Go buy one at https://www.adafruit.com/products/1899
# written by D. Alex Gray dalexgray@mac.com
# Thanks to egutting at the adafruit.com forums
# Thanks to Joan on the raspberrypi.org forums
# This requires the pigpio library
# Get pigpio at http://abyz.co.uk/rpi/pigpio/index.html
# 
# Copyright (c) 2014 D. Alex Gray
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
import time
import pigpio
import math

pi = pigpio.pi()

# HTU21D-F Address
addr = 0x40

# i2c bus, if you have a Raspberry Pi Rev A, change this to 0
bus = 1

# HTU21D-F Commands
rdtemp = 0xE3
rdhumi = 0xE5
wtreg = 0xE6
rdreg = 0xE7
reset = 0xFE

def htu_reset():
	handle = pi.i2c_open(bus, addr) # open i2c bus
	pi.i2c_write_byte(handle, reset) # send reset command
	pi.i2c_close(handle) # close i2c bus
	time.sleep(0.2) # reset takes 15ms so let's give it some time

def read_temperature():
	handle = pi.i2c_open(bus, addr) # open i2c bus
	pi.i2c_write_byte(handle, rdtemp) # send read temp command
	time.sleep(0.055) # readings take up to 50ms, lets give it some time
	(count, byteArray) = pi.i2c_read_device(handle, 3) # vacuum up those bytes
	pi.i2c_close(handle) # close the i2c bus
	t1 = byteArray[0] # most significant byte msb
	t2 = byteArray[1] # least significant byte lsb
	temp_reading = (t1 * 256) + t2 # combine both bytes into one big integer
	temp_reading = math.fabs(temp_reading) # I'm an idiot and can't figure out any other way to make it a float 
	temperature = ((temp_reading / 65536) * 175.72 ) - 46.85 # formula from datasheet
	return temperature

def read_humidity():
	handle = pi.i2c_open(bus, addr) # open i2c bus
	pi.i2c_write_byte(handle, rdhumi) # send read humi command
	time.sleep(0.055) # readings take up to 50ms, lets give it some time
	(count, byteArray) = pi.i2c_read_device(handle, 3) # vacuum up those bytes
	pi.i2c_close(handle) # close the i2c bus
	h1 = byteArray[0] # most significant byte msb
	h2 = byteArray[1] # least significant byte lsb
	humi_reading = (h1 * 256) + h2 # combine both bytes into one big integer
	humi_reading = math.fabs(humi_reading) # I'm an idiot and can't figure out any other way to make it a float
	uncomp_humidity = ((humi_reading / 65536) * 125 ) - 6 # formula from datasheet
	# to get the compensated humidity we need to read the temperature
	temperature = read_temperature()
	humidity = ((25 - temperature) * -0.15) + uncomp_humidity
	return humidity

Create HTU21DF.py