Hi Thanks so much for your help, I have a better understanding and have managed to get it working by modifying your example as needed. Initially WeeWX crashed with a syntax error on *else *but guessed it needed to say *else:* after noticing *try:* (without googling it)
In my weewx.conf I have an entry under the generic labels section: extraTemp3 = The Core Temperature (we nickname the cupboard where the router, etc sits as "the core") Under [engine] [[service]] I added: user.cupboardtemp.CupboardService I'm not bothering to plot any graphs as I just want to quickly see how the cupboard is doing heat-wise on the main report: http://www.achinton.co.uk It turns out that temper-poll (which polls the Temper USB device) has a -c option which just returns the value in degrees C, I didn't need to filter anything before writing to the text file which the WeeWX service then reads. At the moment I have a cron which runs every 10 minutes executing a bash script which contains: */usr/bin/python2 /usr/local/bin/temper-poll -c > /tmp/cupboardtemp.txt* I should imagine there are more elegant ways of getting the data from the Temper USB directly within a WeeWX service, the temper-poll program was written in python as well so certainly something I'll have a play around with. Plus learning some Python is something I keep promising myself. Thanks again, Ashley On Thursday, 25 June 2020 at 14:00:03 UTC+1 gjr80 wrote: > 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 weewx-user+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/weewx-user/986fc9bb-745e-445a-ad95-83ed428098acn%40googlegroups.com.
