Nuts and Volts - August 2015

58 August 2015

Gas sensor collection and measurement occurs once every 150 seconds for the duration of the collection session. This is dedicated by the number of total samples requested. Each 150 second collection cycle is marked by a red LED active for the purge cycle, followed by a green LED for the measurement cycle.

Prototype Operation

The air quality system application is implemented using Raspberry Pi Python 2. 7 code with ISaAC, Tinker, and SMTP, MYSQL, and File I/O Python libraries. Once the application is launched under IDLE, a Monitoring and Control GUI appears similar to what is shown in Figure 9. The application was kept simple and built to help DIYers understand and evaluate these new gas sensors for indoor environments.

Measurements made during the application process are automatically time tagged, converted to voltage, and if optionally logged, PPM. The data can be maintained using an MYSQL database to support analysis and trending.

Let’s clarify what is meant by a sample in our system.

A sample is a collection of all selected gas measurements (both voltage and calculated PPM). It is comma delimited and terminated with a carriage return and line feed. Each sample starts with a full date/time string tag that captures the start time of the first gas value in the sample.

The sample is ASCII formatted as follows (voltage and PPM are in decimal): YYYY:MM:DD:MM:SS, CO voltage, CO PPM, LPG voltage, LPG PPM, VOC voltage, VOC PPM, CR, LF. If a gas is not selected, then a blank will exist between the appropriate commas.

A good way to understand the application’s functionality is to walk through the application’s applied use of the Python Tinker GUI widget set and its associated features. Notice, first of all, that the GUI display is broken into four main sections: Gas Selection, Log Control, Acquisition Control, and Advanced Control.

Let’s start with Gas Selection. Here, check boxes help you select which of the three sensors are candidates for measurement.

The next section (Log Control) provides a file name where the session measurements will be stored (if the Enable Log checkbox is selected). If it is not selected, the session will simply dump all the measurements into the Python shell for direct viewing.

Acquisition Control sets the number of samples for the collection session. The user sets this by typing a sample number in the Entry box. Upon hitting the “Run” button, an estimated completion time is automatically displayed and logging commences if enabled. The application will automatically set a file log name in the File Name entry box. The file name uses the measurement start time as an example: Gas_Log_YYYY_MM_DD_MM_SS.txt.

With each sample collected by the system, the sample’s number box will decrement, and the collection session will end at the zero sample.

Once the session ends, a user can view the file log by hitting the “View Log” button. The file log content appears as a scrollable text box widget. Under Advanced Control, the log results can be stored in an MYSQL database (gas_database.db) for long term archival by hitting the “Update DB” button.

The user can also elect to send a copy of the log file using the “Send Email” button to the recipient’s email address listed in the email entry box address. The “from address” in this email will be fixed to your DIY air monitor system.

When in doubt about what is going on, examine the outputs that are printed to the Python Shell; it captures all the action going on “under the hood.” That is about it for operation — simple enough!