Humidity and temperature monitoring with a Raspberry Pi

Continuing my quest to protect my Caterham 7 using my Raspberry Pi, I bought a bargain box of 37 assorted sensors for £21. One of the devices supplied was a DHT11, capable of monitoring temperature and humidity. A quick internet search led me to discover the very useful pigpio libraries and daemon. Wiring up the DHT11 to the Pi’s GPIO pins was simple – 3 wires, 1 to the 3V3 supply, 1 to a ground pin and the data pin of the device to GPIO pin 17. The author of the pigpio library also provides some example code for the sensor.

This was enough to demonstrate that I could get it to work, but the accuracy of the DHT11 was woeful. It’s advertised as having an accuracy of +/- 2 Celsius, but my experiments suggested that the one I had acquired for the princely sum of 57p had an accuracy of around +/- 5 Celsius. As I also wanted the sensor to work in an unheated garage, another limitation was that the DHT11 is unable to read values below freezing.

Encouraged, I decided to invest a further £6 in a plug compatible DHT22 sensor. This has an operating range of -40 Celsius to 125 Celsius, with an accuracy of +/- 0.5 degrees. Initial tests suggested that it was far more accurate than the DHT11. While the sensor produces a slightly different data stream to the DHT11, it was close enough for the example code to have been written in a way that it would still work.

As part of my experiments, I made a small modification to the example code (testDHT.c) to calculate the dew point from the temperature and relative humidity readings. I’ve also changed the way it outputs data, to write it to a file rather than the screen.

void cbf(DHTXXD_data_t r)
{
   FILE* fp;
   char buff[100];
   time_t now;
   float rdwpt,rtemp,rhum;
   extern float rdewpt_();
 
   if (r.status == 0) {
       now = time (0);
       strftime (buff, 100, "%Y%m%d-%H%M", localtime (&now));
       rtemp=r.temperature;
       rhum=r.humidity;
       rdwpt=rdewpt_(&rtemp,&rhum);
       fp=fopen("readings.txt", "a");
       fprintf(fp,"%s ",buff);
       fprintf(fp,"%.1f ", rtemp);
       fprintf(fp,"%.1f ", rhum);
       fprintf(fp,"%.1f", rdwpt);
       fprintf(fp,"\n");
       fclose(fp);
   }
}

While it would have been simple enough to write a dew point calculation in C, I decided to write it in FORTRAN instead. Hey, the more people who get to love FORTRAN the better. Here’s the function I wrote, which uses the Magnus formula. It’s declared in the C fragment above as an external function that returns a float. Parameters are passed to FORTRAN by address, rather than by value.

C
C     DEWPOINT CALCULATION FROM TEMPERATURE IN CELSIUS - RTEMP
C                               RELATIVE HUMIDITY - RHUM
C     USING THE MAGNUS FORMULA WITH CONSTANTS SUITABLE FOR A
C     TEMPERATURE RANGE OF -45 TO 60 CELSIUS
C
C     AUTHOR: TJH 28-01-2017
C
      REAL FUNCTION RDEWPT(RTEMP,RHUM)
      REAL RH
      RH = (LOG10(RHUM)-2)/0.4343+(17.62*RTEMP)/(243.12+RTEMP)
      RDEWPT = 243.12*RH/(17.62-RH)
      RETURN
      END

Compiling and linking the C and FORTRAN code using gfortran:

gfortran -Wall -pthread -o DHTXXD *.c *.f -lpigpiod_if2

produces an executable that can create a data file. This example was created using the command:

./DHTXXD -g17 -i600

which reads the data from GPIO pin 17 every 10 minutes.

20170128-2009 6.9 72.0 2.2
20170128-2019 6.9 71.7 2.1
20170128-2029 6.8 71.3 2.0
20170128-2039 6.8 71.4 2.0
20170128-2049 6.8 71.7 2.0
20170128-2059 6.8 71.4 2.0
20170128-2109 6.7 71.1 1.8

This provides data in a suitable format to use some relative simple gnuplot commands to create a charts. The first example plots the relative humidity readings over time, with the larger spikes in the relative humidity data correlating to the garage door being opened while it was raining.

set xdata time
set xlabel "Time"
set ylabel "Relative Humidity %"
set timefmt '%Y%m%d-%H%M'
set format x '%H%M'
plot 'readings.txt' using 1:3 title 'Relative Humidity' with linespoints

HumidityThe second example graphs the air temperature reading directly from the DHT22 with the dew point temperature calculated from the Magnus formula.

set xdata time
set xlabel "Time"
set ylabel "Temperature in degrees Celsius"
set timefmt '%Y%m%d-%H%M'
set format x '%H%M'
plot 'readings.txt' using 1:2 title 'Air Temperature' with linespoints, \
     'readings.txt' using 1:4 title 'Dew Point' with linespoints

Air temperature and dew pointI’m going to be keeping an eye on the data, to understand if it might be beneficial to seal the garage door more effectively than at present and invest in a dehumidifier.

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