Air Quality - Gas contents - 2
This webpage provides the following information:
Measurement/ Detection of Gasses CO, NH3 and NO2 by sensor MiCS6814
As start a basic configuration which is rather broadband, with a combined gas sensor which in (limited) quality and (limited) sensitivity is compatible with the dust-sensor GP2Y10
Sensor type MiCS6814 is best available, already implemented on a break-out PCB
With multiple sensors it measures & reports gas level divided in 3 signals for CO, NH3 and NO2, as a kind of spectrum-survey aimed at
"detection of pollution from automobile exhausts and for agricultural/industrial odors" [quote from datasheet].
Relative to the single value measured by the companion type MiCS2714,
this latter capability of type MiCS6814 seems useful for a role as 'general alerter',
and for some 'practical learning' about the contents of the environment.
The 3 analogue signals from the MiCS6814 are read by the channels 0 till 2 of an ADC type ADS1115.
Till the availablity of a simple, local calibration & scaling, these ADC-signals initially will be scaled on the 10 bits of the ADC => 0 till 1024 units,
directly compatible with the setting for readout of ADS1115: fullscale = 1024mV => 1unit = 1mV
When calibration has been found, either the controlling ESP8266, or the next stage in Domoticz for each type of gas will translate/scale the measured amount of units into microgram/m3.
Adjacent to the gassensor an extra (simple) thermosensor LM35 measures the temperature of the air flowing over the gassensor:
the sensor LM35 is physically fitted close and hovering over the gassensor for a local measurement of temperature as close as possible.
It's analogue output is a linear voltage of 10mV/degree, read by channel 3 of the same ADS1115:
also this 3rd channel of the ADC applies the setting of fullscale = 1024mV => 1unit = 1mV => resolution = 0.1 degree
anticipating the measurement of positive temperatures.
The cluster of Gassensor+Thermosensor is free-hanging in the housing in front of the dustsensor:
no protection against temperature, moisture and dust from ambient air, as would be required for more stable measurements.
ToDo-List for the 'basic' configuration
To be investigated & to be developed:
- Calibration concept =
data logging => offsets + transfer function,
to become compatible with RIVM's AQI-classsification.
- Environmental control =
stable air flow circuit with control for heating/drying and for filtering of the air/gas for gassensor & dustsensor,
to compensate effects of temperature & humidity, respectively dust (related to gas measurements):
First feasible approach seems:
- a tube/canal in front of the entry of the GP2Y10-sensor,
- enclosing the cluster of gassensor MiCS6814 + thermosensor LM35,
- with heater at entry of tube/canal,
- with heater power PWM-controlled from the LM35-info
- NO dustfilter, being too complicated to fit & maintain
[Such filter should be positioned in the air flow 'before' the gas-sensor:
however conflicting with the present position of the gas-sensor cluster in front of the dust-sensor,
would block good operation of the dust-sensor,
which is now located 'behind' the gassensor]
Replace BME280 by BME680 =
addition of very basic VOC-measurement for awareness of outdoor VOC-level.
VOC is becoming an important aspect for indoor environment, but might also be significant outdoors.
The BME680 is equivalent to BME280 for Temperature/Humidity/Pressure.
Addition of this measurement to this setup is a first step extending the measurement catalogue to cover the VOC-aspect.
For an 'Extended Configuration' the concept must be developed from scratch:
still in infancy.
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