In my previous post, I was writting about how to graph temperature and humidity from AM2302 sensor on RasberryPi. In addition, we will add dewpoint monitoring. We need two variables to calculate dewpoint, temperature and humidity. I took some already made scripts and combined them together to fit my needs. I wanted to calculate dewpoint completely with bash and bc, but since I’m to lazy, I just used the python script from this blog.

Get to the root shell (we don’t want to type sudo everytime):

sudo -s

Create python script (for dewpoint calculations):

pico /opt/dewpoint.py

Paste the code, save and exit (CTRL + C), Y, ENTER

import sys
import numpy as np

# approximation valid for
# 0 degC < T < 60 degC
# 1% < RH < 100%
# 0 degC < Td < 50 degC

# constants
a = 17.271
b = 237.7 # degC

# sys.argv[0] is program name
T=float(sys.argv[1])
RH=float(sys.argv[2])


def dewpoint_approximation(T,RH):

    Td = (b * gamma(T,RH)) / (a - gamma(T,RH))

    return Td


def gamma(T,RH):

    g = (a * T / (b + T)) + np.log(RH/100.0)

    return g


Td = dewpoint_approximation(T,RH)
print Td

Make the script executable:

chmod +x /opt/dewpoint.py

Create plugin file:

pico /etc/munin/plugins/dewpoint

Paste the code, save and exit (CTRL + C), Y, ENTER

#!/bin/sh

case $1 in
config)
cat <<'EOM'
graph_title Dewpoint
graph_vlabel Celsius
graph_category AM2302
dewpoint.label Temperature
dewpoint.draw AREASTACK
dewpoint.colour 403075
EOM
exit 0;;
esac

humidity=$(/opt/lol_dht22/loldht 7 | grep -i "humidity" | cut -d ' ' -f3)
temperature=$(/opt/lol_dht22/loldht 7 | grep -i "temperature" | cut -d ' ' -f7)

printf "dewpoint.value "
python /opt/dewpoint.py $temperature $humidity

chmod +x /etc/munin/plugins/dewpoint

Open munin-node file:

pico /etc/munin/plugin-conf.d/munin-node

Add the line at the end of the file, save and exit:

[dewpoint*]

Restart services:

munin-node-configure
/etc/init.d/munin-node restart