Datalogging with Luca

For our datalogging assignment Luca and I wanted to test air quality. We chose the Gas Sensor MQ135, which measures levels of Ammonia, Sulfide, and Benze steam, and other harmful gases. These sensors are used in air quality control equipments for buildings/offices, etc.
In order to calibrate the sensor’s readings, we needed to measure the temperature and the humidity, so we also used a TMP36 temperature sensor and an HIH4030 humidity sensor. (Find specifications here and or at sensor report we created for the TMP36, the HIH4030 and the MQ-135).

We first tested each sensor separately (only one set on the board) getting good values; but when we tested all together we got higher values on the TMP36. This was because the MQ135 was taking current, changing the voltage. Tom Igoe told us about the “Secret Voltmeter” (Peter Knight). We used this code to measure the Arduino’s internal voltage rail. This gives us more accurate results because it takes into account that the voltage coming into the sensors is not exactly 5V.

To set the sensors using Arduino Duemilanove + Arduino SD shield:

Images from the Serial Monitor. This shows readings from all three sensors:

Arduino sample code

Our biggest impediment was understanding how to interpret the sensor’s output. We didn’t initially understand why one sensor’s output should be converted to another unit of measurement while another can be left as a value from 0-1023. Through talking to our teacher, Tom, and Zeven (ITP resident), we came to understand that some values need to be converted to human readable values.  For example, the TMP36′s readings are much easier to understand if they are converted to Farenheit or Celsius. The MQ135′s output does not need to be converted necessarily. We are more interested in relative change for the MQ135, so the values can be read with any unit of measurement.

Another issue was that we couldn’t initialize the micro SD card. So we took our values from the serial monitor (copy paste into excel).

We first measured  in my living room (where I don’t have windows), from 12:00 – 12:10 pm, taking readings every 10 seconds.
Next we measured outside in may backyard (raining) from 12:40-12:50, again every 10 seconds. It took about 20 minutes for the temperature and humidity sensors to adjust when we moved outside. They need a at least 10-20 min to get accurate readings in a new environment.

Graphs of our readings:

Here are graphs of the readings while the sensors were adjusting to the new outdoor environment. The temperature readings fall slowly, while the humidity rises, and the gas readings are a bit erratic: