gases_sampler - bruno-beloff/scs_dev GitHub Wiki

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DESCRIPTION

The gases_sampler utility reads a set of values from a South Coast Science digital front-end (DFE) board hosting an Alphasense analogue front-end (AFE) board. The also utility reads an NDIR CO2 sensor, if the appropriate configuration document is present.

The gases_sampler utility reports raw electrode voltages, temperature-compensated voltages, and gas concentrations (in parts per billion) derived according to the relevant Alphasense application notes. If the AFE board includes a Pt1000 temperature sensor, then the gases_sampler utility may also report the Pt1000 voltage and temperature.

The DFE board supports AFEs with up to four electrochemical sensors, or three electrochemical sensors plus one photo-ionisation detector (PID). Before using the gases_sampler utility, the configuration of the AFE must be specified using the scs_mfr/afe_calib utility.

Temperature must be known in order to perform a simple data interpretation. The gases_sampler utility applies an order of precedence for temperature sources, depending on availability:

Sensirion SHT in A4 package
Free-to-air Sensirion SHT
AFE Pt1000 sensor

The presence of the DFE subsystem, and the availability of the Pt1000 sensor on the AFE is specified using the scs_mfr/interface_conf utility. The configuration of Sensirion SHT sensor(s) is specified using the scs_mfr/sht_conf utility.

Gas concentrations for each sensor are adjusted according to a baseline value, which is specified using the scs_mfr/afe_baseline utility. This provides a simple way of managing zero-offset drift for each sensor expressed in parts per billion.

The gases_sampler writes its output to stdout. As for all sensing utilities, the output format is a JSON document with fields for:

the unique tag of the device (if the system ID is set)
the recording datetime in ISO 8601 format
a value field containing the sensed values

Command-line options allow for single-shot reading, multiple readings with specified time intervals, or readings controlled by an independent scheduling process via a Unix semaphore.

SYNOPSIS

gases_sampler.py [{ -s SEMAPHORE | -i INTERVAL [-n SAMPLES] }] [{ -v | -d }]

Options
--version	show program's version number and exit
-h, --help	show this help message and exit
-s SEMAPHORE, --semaphore=SEMAPHORE	sampling controlled by SEMAPHORE
-i INTERVAL, --interval=INTERVAL	sampling interval in seconds
-n SAMPLES, --samples=SAMPLES	number of samples (1 if interval not specified)
-v, --verbose	report narrative to stderr
-d, --debug	report detailed narrative to stderr

EXAMPLES

./gases_sampler.py -i10

FILES

~/SCS/conf/afe_baseline.json
~/SCS/conf/afe_calib.json
~/SCS/conf/interface_conf.json
~/SCS/conf/pt1000_calib
~/SCS/conf/scd30_conf.json
~/SCS/conf/sht_conf.json
~/SCS/conf/schedule.json
~/SCS/conf/system_id.json

DOCUMENT EXAMPLE - v0

{"rec": "2021-10-11T11:00:32Z", "tag": "scs-bgx-431", "val": {"CO2": {"cnc": 417.2}, "NO2": {"weV": 0.28388, "aeV": 0.27657, "weC": 1e-05, "cnc": 15.0}, "Ox": {"weV": 0.40332, "aeV": 0.40332, "weC": 0.00015, "cnc": 43.4}, "NO": {"weV": 0.29882, "aeV": 0.28919, "weC": 0.01382, "cnc": -192.2}, "CO": {"weV": 0.30882, "aeV": 0.27632, "weC": 0.01929, "cnc": 4.1}, "sht": {"hmd": 55.4, "tmp": 16.2}}, "exg": {"vB20": {"NO2": {"cnc": 21.6}}}}

DOCUMENT EXAMPLE - v2

{"rec": "2021-10-11T15:48:19Z", "tag": "scs-be2-3", "ver": 2.0, "src": "AFE", "val": {"NO2": {"weV": 0.29019, "aeV": 0.29494, "weC": 0.00181, "cnc": 22.8, "vCal": 15.858}, "Ox": {"weV": 0.40057, "aeV": 0.39907, "weC": 0.00083, "cnc": 51.4, "vCal": 7.123, "xCal": -0.389858}, "CO": {"weV": 0.59401, "aeV": 0.27619, "weC": 0.29396, "cnc": 1159.1, "vCal": 1236.165}, "sht": {"hmd": 49.3, "tmp": 22.9}}, "exg": {"src": "vB20", "val": {"NO2": {"cnc": 22.5}}}}

RESOURCES

Data formats/Topic: gases
ISO 8601

scs_core/gas/a4_temp_comp.py

Alphasense Application Note AAN 803-02