Category Archives: Theory

Anemometer coding woes

Turns out all my anemometer code so far is wrong, equations is not my strong suit and I seemed to be always trying to overwork the equation.

The above is mostly the code I am using now. I have tweaked it since to allow float seconds so I can use the same procedure to work out the gusting speed.

The above code is borrowed from Sparkfuns Weather Station Tutorial.

Moon Phases, Sunrise and Sunset

So I want the web front end to give some other data, that might be of use to people.

Since I am also into astronomy, I would like to know what phase the moon is in (not that we’ve seen all that much of it recently). Turns out some clever bloke has written a php class to tell you what phase the moon is in, source is available via github.

PHP 5.1+ contains functions for calculating sunrise and sunset. As far as I can see there’s 3 functions, date_sunrisedate_sunset and date_sun_info. The latter looks like the best bet as you don’t have to supply as many variables.

Now I just need to get the GPS coords for my flat.

Wind Sensor Testing

UPDATE2: yeah ignore the drivel in this post and check this post for correct equation for the anemometer.

UPDATE: So it turns out that I haven’t a clue about how you measure wind speed with an anemometer. I found the datasheet for the sensors, which gives 1.492mph per pulse on the anemometer over 1 second, so I have had to rework the equation and procedure below. So the screen shot below gives wrong values, but is an example of the scripts output.

Just started testing with the Anemometer N96FY. I have the anemometer connected to GPIO pin 18 and 3.3 volt supply. This wind sensor is like a switch and it’s default state is closed, meaning electricity with flow through the anemometer except for two opposing points during a full revolution where the voltage will briefly fall to zero.

I work out the equation to be

wind speed = half the circumference / time taken

Wind speed will be in metre per second. Half the circumference is 0.21994 metres.



Database Testing

I am going to use a local and remote mysql database to host the weather information. So I had to think about which variables to store and at 1 minute resolution, how much space would I need to accomplish this, as I want to store the information year on year. So I will store the following information:-

  • Date and Time of the reading
  • Humidity Outside
  • Temperature on the Humidity Sensor
  • Temperature on the Barometric Sensor
  • Humidity Inside
  • Temperature Inside
  • Wind Speed Average over 1 Minute
  • Wind Gusting Speed
  • Wind Lowest Speed
  • Light Full
  • Light Visible
  • Light Infrared
  • Barometric (Pressure)

This works out to 1,440 readings a day (60 minutes x 24 hours). Which is 525,600 readings a year, so making the database as small as possible is critical. For this I will be using the DECIMAL data type, so you can specify exactly how much information to store. So the rough table looks something like this:-

So using a local database on a desktop PC via phpMyAdmin, I used a store procedure to generate the dummy information. First create and store the procedure:-

Note: The dummy database name is “pi”. Then you can just call:-

Which will take a few minutes (~10 mins) to finish populating the database. You’ll notice on the stored procedure that I counted for 366 days, rather than 365, just to simulate what a leap year would be.

Final size on disk of the database is ~61MB. Plus a bit more for the unique index on the time_stamp column.

Issues encountered: 1. Day light saving time. I was originally trying to store the timestamp, but on the 30th March 2014 at 2am I would get 60 rows with exactly the same date and time and since the time_stamp field needs to be unique, this was a massive issue, so to cure the problem, I changed the store value to DATETIME and inserted the utc_timestamp() value, so I will probably need to convert the utc timestamp to localised time allowing for day light saving time if I need to refer to a date and time specifically.

2. Log files. During my testing and repeated procedure running, I managed to generate a >600MB log file. So disabling the log files on the testing and production machine will need to be done. Find the following in your my.ini or my.cnf file:-

find the line “log-bin=mysql-bin” and place a # and space in front of it, this will disable the log file generation.

The next step will be see how fast the hour average calls are for the last 24 to 48 hours for the chart mapping.