Hi,
There is one simple rule when adding/altering fields in a loop
packet/archive record; always take note of the unit system used by the
packet/record. Loop packets are emitted by the driver using a unit system
of the drivers choice, it does not matter what unit system is used and it
need bear no resemblance to the unit system used in the database or in any
reports (the StdConvert service takes care of converting packets/records to
the unit system used by the database and there are mechanisms within the
StdReport service for converting to whatever display units the user wishes
to use). If a WeeWX service wants to add a field to the loop packet that
service should check the unit system used in the packet and then convert
the data to be added to the appropriate units before adding the data to the
packet. In some cases it does not matter (eg humidity, radiation, direction
all use the same units across all unit systems) but obviously it matters
for temperatures, speeds, pressures etc. You can certainly have your
external source provide temperatures in C if your driver is emitting
temperatures in C but what happens if you get a new station/driver that
emits F? Such a chnage would result in your C data being treated as if it
was in F.
So back to your system. The service that you write will need to do a number
of things. First it needs to read your data and know what units are being
used (units could be hard coded in your service or read from the external
source - its your source so you control it). Then you need to check the
unit system used in the record you are augmenting and convert your data to
the units used in the record. Finally you add the converted data to the
record. Using the example you linked your code might look something like
(untested):
import syslog
import weewx
import weewx.units
from weewx.wxengine import StdService
class PondService(StdService):
def __init__(self, engine, config_dict):
super(PondService, self).__init__(engine, config_dict)
d = config_dict.get('PondService', {})
self.filename = d.get('filename', '/var/tmp/pond.txt')
syslog.syslog(syslog.LOG_INFO, "pond: using %s" % self.filename)
self.bind(weewx.NEW_ARCHIVE_RECORD, self.read_file)
def read_file(self, event):
try:
with open(self.filename) as f:
value = f.read()
value = float(value)
syslog.syslog(syslog.LOG_DEBUG, "pond: found value of %s" %
value)
except Exception as e:
syslog.syslog(syslog.LOG_ERR, "pond: cannot read value: %s" % e)
else
value_vt = weewx.units.ValueTuple(value, 'degree_C',
'group_temperature')
conv_value_vt = weewx.units.convertStd(value_vt, event.record[
'usUnits'])
event.record['extraTemp1'] = conv_value_vt.value
I have highlighted the changes in yellow. Firstly we import weewx.units as
we are going ot use some of the unit conversion functions that reside in
the WeeWX units module. The changed use of float() lets us make use of the
existing try..except to catch the error that would occur if the data read
cannot be converted to a number. The code indented under 'else' is executed
if the try..except does not encounter an error, in other words the data is
read and successfully converted to a number. Looking at the 'else' code in
detail, first we express the imported data as a ValueTuple, this is a three
element tuple that defines a value, its units and the unit group it belongs
to. This is necessary to use the WeeWX unit conversion functions. In this
case we know our imported data is in C and is a temperature. We then call
the convertStd function to convert that ValueTuple into the units used by
that particualr unit group in that particular packet. There may be no
conversion needed, but we know now that irrespective of the unit system
used in event.record, conv_value_vt contains our imported data in the same
temperature units used in event.record. So we can just simply add the value
to the record.
Hopefully that gives you some hints on a way ahead.
Gary
On Wednesday, 24 June 2020 23:03:19 UTC+10, Ashley Hinton wrote:
>
> Hey everyone.
>
> I'm having a bit of a brain freeze over how to tell Weewx what units
> (metric or US) to expect when importing a temperature figure from a text
> file.
> My set up uses Weewx version 3.9.2 running under Python2 (setup.py install
> method) on a Raspberry Pi 4, OS is Raspian Buster (Debian 10)
>
> I have two data sources at the moment:
> The main weather station is an Aercus Instruments Weathersleuth which is
> basically a fine offset observer. That uses the Intercepter plugin
> /home/weewx/bin/user/interceptor.py
> I also have two extra sensors feeding from another pi on the network using
> the One Wire (OWFS) plugin /home/weewx/bin/user/owfs.py - these are mapped
> as extraTemp1 & 2 and report as Greenhouse and Garage temperatures.
>
> As far as I'm aware all of those report the temperature as metric when
> polled by weewx and I don't recall having to modify anything.
>
> What I want to do is map a third, extraTemp3, to data from a Temper USB
> temperature sensor I've got in the cupboard where the Raspberry Pi sits
> (the one running Weewx)
>
> All well and good, i've installed a python application which when called
> will report the temperature from the Temper.
>
> > temper-poll
>
> Found 1 devices
>
> Device #0: 39.7°C 103.4°F
>
> I'm sure I can write a bash script to run temper-poll, strip out unwanted
> data, and overwrite the result to a file in /tmp/ (such as cupboard.txt) -
> once happy I'll set up a cron for the script and then a service in
> /home/weewx/bin/user as per the example:
> https://github.com/weewx/weewx/wiki/add-sensor - modifying as needed.
>
> As per notes within weewx.conf [StdConvert] has always remained as US
> units.
>
> What I can't figure out is how Weeex knows whether the incoming data, from
> any of the sensors across the system, are metric or US? is it specified
> within the service(s) themselves?
> I notice further down in the example of adding an extra sensor::
>
> That is, whatever unit system you use in pond.txt, must match the unit
> system used by the incoming record. The unit system used by the incoming
> record will be given by event.record['usUnits']
>
> Does that mean I need to add the above to my new service?
>
> In short:
> Both Interceptor and Owfs are presenting temperature data in degrees C,
> but the Weewx [stdConvert] is untouched from default (US)
> My reports are in metric, but of course that's just the reports. I could
> change it at any time without affecting the database or the incoming sensor
> data to Weewx.
> Do I need to do anything to my new service in /weewx/bin/user/ to specify
> that the data is metric or US depending on what value gets presented from
> /tmp/cupboard.txt ?
>
> I'm reluctant to just try it as I don't want incorrect data, even though
> this is just so I an easily see the temperature of the cupboard where my
> main "server" raspberry pi, router, network switch and a few other bits of
> kit reside. Ignore the alarmingly high temp as the stick is currently
> plugged directly into the pi and its a hot day here - a USB extension cable
> will give a more accurate reading of the ambient temp.
>
> Thanks for any help and guidance and of course apologies in advance if
> it's in the instructions (I did try a search - honest!)
>
> Regards
>
> Ashley
>
>
>
--
You received this message because you are subscribed to the Google Groups
"weewx-user" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to [email protected].
To view this discussion on the web visit
https://groups.google.com/d/msgid/weewx-user/964c061c-4d03-43da-9f47-284d302cffaco%40googlegroups.com.
