Weewx Engine Packet Conversion Mapping Issue

[Patrick Mendiuk]

HI,

I have noticed some weird MQTT data from my Davis wind speed sensor.  The wind_speed value from the Davis packet data is not being published via MQTT.  I started adding some debug logging and in engine.py I noticed that wind_speed_raw is being mapped into windSpeed.  Here is the log output:

Apr 11 11:31:08 raspberrypi weewx[11331]: rtldavis: data: 11:31:08.118616 4005E6FFC1000032 64053 0 0 0 2 msg.ID=0

Apr 11 11:31:08 raspberrypi weewx[11331]: rtldavis: data_pkt: {'wind_speed': 2.2351944444444443, 'curr_cnt0': 64053, 'curr_cnt3': 0, 'curr_cnt2': 0, 'curr_cnt1': 0, 'wind_speed_raw': 5, 'channel': 1, 'bat_iss': 0, 'wind_speed_ec': 5.0, 'wind_dir': 318.55731225296444}

Apr 11 11:31:08 raspberrypi weewx[11331]: engine: event.packet: {'txBatteryStatus': 0, 'outTempBatteryStatus': 5, 'dateTime': 1586629868, 'windDir': 318.55731225296444, 'windSpeed': 2.2351944444444443, 'usUnits': 17}

Apr 11 11:31:08 raspberrypi weewx[11331]: engine: converted_packet: {'windDir': 318.55731225296444, 'windSpeed': 5.0, 'outTempBatteryStatus': 5, 'txBatteryStatus': 0, 'dateTime': 1586629868}

Apr 11 11:31:08 raspberrypi weewx[11331]: engine: converted_packet_unit: {'txBatteryStatus': 0, 'outTempBatteryStatus': 5, 'dateTime': 1586629868, 'windDir': 318.55731225296444, 'windSpeed': 5.0, 'usUnits': 1}

Apr 11 11:31:08 raspberrypi weewx[11331]: restx: MQTT: call record: {'txBatteryStatus': 0, 'outTempBatteryStatus': 5, 'maxSolarRad': None, 'dateTime': 1586629868, 'windDir': 318.55731225296444, 'windSpeed': 5.0, 'rainRate': 0, 'usUnits': 1}

Apr 11 11:31:08 raspberrypi weewx[11331]: restx: _datadict: {'txBatteryStatus': 0, 'outTempBatteryStatus': 5, 'maxSolarRad': None, 'dateTime': 1586629868, 'windDir': 318.55731225296444, 'windSpeed': 5.0, 'rainRate': 0, 'usUnits': 1}

Apr 11 11:31:08 raspberrypi weewx[11331]: restx: MQTT: get record: {'txBatteryStatus': 0, 'hourRain': 0.0, 'outTempBatteryStatus': 5, 'maxSolarRad': None, 'dateTime': 1586629868, 'windDir': 318.55731225296444, 'windSpeed': 5.0, 'rain24': 0.0, 'dayRain': 0.0, 'rainRate': 0, 'usUnits': 1}

The expected value is being written to the database and used in the html files.

Thanks,

Patrick

[Rich Bell]

Patrick,

I think this is due to unit conversions. The packet data is units METRICWX (usUnits: 17), which is m/s. The data in the MQTT payload is US (usUnits: 1), which is mph. 

Converting 2.2352 m/s * 2.237 = 5 mph.

rich

[Patrick Mendiuk]

Thanks Rich,

So I guess the rtldavis packet has mixed units and the wind speed in m/s is rounded to integer mph values.  The windSpeed data written to the database isn't rounded.  Do you have any idea which dictionary is being used?

Thanks,

Patrick

[gjr80]

Davis stations emit wind speed as integer mph values. So it’s not really a case of m/s being converted and rounded to the nearest mph value, rather it is the other way around. rtldavis is taking any integer mph value and converting it (precisely) to m/s and then later on when converting to mph the original integer mph value is returned.

Gary

[Patrick Mendiuk]

Thanks Gary,

The ultrasonic sensor resolution has tainted me.  I should have looked at the Davis specifications first :) 

Sounds like a good excuse to build a replacement for the ISS board.

Thanks,

Patrick