I wonder if your values are strings rather than floating point numbers?

If so, then you would need:
*data['windSpeed_true'] = 
str(float(data['windSpeed_rel_mwv'])-float(data['SOG_kn'])* ) 

I had to run my version of "airmar.py" independent of WeeWx in order to 
more easily debug.
When I was satisfied with my tests, then I went back to WeeWx.

I had my driver working for awhile but then had some trouble related, I 
think, to the RaspberryPi not having a good clock.
After I get a real clock and get things going again, I will document my 
driver development for Airmar 200Wx modified from Rob's.

-JiM.

On Saturday, August 8, 2020 at 3:14:11 PM UTC-4 [email protected] wrote:

> Edit: I ment subtract not divide...sorry about that
>
> [email protected] schrieb am Samstag, 8. August 2020 um 21:07:44 UTC+2:
>
>> First of all thank you Jim and Rob for your patience ;-)
>>
>> Second I'd like to ask, if someone (more python versed) can tell me how 
>> to divide two values in the airmar driver. In my specific case it would be 
>> the *windSpeed_rel_mwv* - *SOG_kn.* SOG_kn is an value I extracted from 
>> the message 
>> *$GPVTD. *
>> If I use *data['windSpeed_true'] = 
>> data['windSpeed_rel_mwv']-data['SOG_kn']* I get an error. So can anyone 
>> tell me, where my mistake is?
>> What I have learned, it is not the exact or 100% right way to calculate 
>> "true wind" but I think I would be close enough ;-)
>>
>> Thank you in advance and best regards,
>>
>> Dennis
>>
>> [email protected] schrieb am Samstag, 1. August 2020 um 22:29:51 UTC+2:
>>
>>> Hi Rob,
>>>
>>> if I didn't get the technical manual for the wx unit wrong not all 
>>> messages are enabled by default. So some messages have to be activated 
>>> manually. Did anyone managed to do so? When I start minicom the sensor ist 
>>> talking but never listening.
>>> @James: how are you coming on with this topic?
>>>
>>> Thank you and best regards,
>>>
>>> Dennis
>>>
>>> [email protected] schrieb am Donnerstag, 9. Juli 2020 um 22:04:31 
>>> UTC+2:
>>>
>>>> Hi Dennis,
>>>>
>>>> One option to see the raw data in the terminal would be to put kplex 
>>>> between the usb and the weewx. Configure kplex to receive the messages 
>>>> from 
>>>> usb, and publish on a tcp server socket. Configure weewx to connect to 
>>>> that 
>>>> socket for messages.
>>>>
>>>> Kplex will log to a file if you configure it so, or you can simply 
>>>> telnet to the kplex tcp port and see the raw messages. You're looking for 
>>>> $WIVWT messages.
>>>>
>>>> http://www.stripydog.com/kplex/configuration.html
>>>>
>>>> I hope this helps,
>>>> Rob.
>>>>
>>>>
>>>>
>>>> On Thu, 9 Jul 2020 at 20:02, Dennis Füß <[email protected]> wrote:
>>>>
>>>>> Hi Jim and Rob,
>>>>>
>>>>> first of all: thank you for your fast reply.
>>>>> For the moment I have used windSpeed_rel_mwv but it turned out, that 
>>>>> the data I'm getting is the windspeed without the subtracted vessel 
>>>>> speed. 
>>>>> The main problem is, that I'm the "engineer" for the system but have 
>>>>> no possibility to test it in person on the lake, where the vessel is 
>>>>> located :-/
>>>>>
>>>>> Maybe I explain my setup:
>>>>> Airmar 150WX goes via NMEA 0183 cable to an RS422 to USB converter to 
>>>>> a raspberry pi 3 running weewx. Weewx stores the data in an influxdb and 
>>>>> grafana is showing them.
>>>>>
>>>>> @Jim: thank you. But where have you specified the mentioned data? I 
>>>>> mean in which configuration file?
>>>>>
>>>>> @Rob: you asked if I would receive the values for windSpeed_true_vess 
>>>>> etc. -> I tried to call it in grafana but it didn't show me data, so I 
>>>>> guess I don't receive them. Or is there a way to see all received 
>>>>> data/variables in terminal (probably a stupid question, but for the 
>>>>> moment 
>>>>> I don't know). When I open grafana it shows me the available data from 
>>>>> its 
>>>>> database, so I don't see any reason why available data shouldn't be 
>>>>> stored 
>>>>> in there, right?
>>>>>
>>>>> So I hope, that I didn't forget anything and stay thrilled for Jim's 
>>>>> findings ;-)
>>>>> Thank you in advance and best regards,
>>>>>
>>>>> Dennis
>>>>> Am Mittwoch, 8. Juli 2020 10:24:16 UTC+2 schrieb Rob Fuller:
>>>>>>
>>>>>> Hi Dennis,
>>>>>>
>>>>>> The device we use is at a fixed position so it's not something I've 
>>>>>> looked at before.
>>>>>>
>>>>>> Looking quickly at the code I see it does try to 
>>>>>> parse windAngle_true_vess and windSpeed_true_vess from the $WIVWT 
>>>>>> message. 
>>>>>> Does your device send that?
>>>>>>
>>>>>> Otherwise I'm waiting to see what James Manning comes up with ;-)
>>>>>>
>>>>>> Kind regards,
>>>>>> Rob.
>>>>>>
>>>>>> On Tue, 7 Jul 2020 at 20:38, Dennis Füß <[email protected]> wrote:
>>>>>>
>>>>>>> Hi Rob,
>>>>>>>
>>>>>>> maybe you can help me out since Aleksandar seems to be gone for a 
>>>>>>> while.
>>>>>>> I have an Airmar 150WX up and running but i doesn't seem to give me 
>>>>>>> any values for *windSpeed_true_vess*. Or in other words I don't get 
>>>>>>> the windspeed subtracted from the vessel's speed. 
>>>>>>>
>>>>>>> Do you have any idea, how to accomplish this?
>>>>>>>
>>>>>>> Thank you and best regards in advance,
>>>>>>>
>>>>>>> Dennis
>>>>>>>
>>>>>>> Am Mittwoch, 10. Juni 2020 09:12:48 UTC+2 schrieb Rob Fuller:
>>>>>>>>
>>>>>>>> Hi James,
>>>>>>>>
>>>>>>>> I haven't looked at this for a while, but anyhow it was working 
>>>>>>>> okay. At Marine Institute we did something like this:
>>>>>>>>
>>>>>>>> wget -O bin/weewx/drivers/airmar.py 
>>>>>>>> https://raw.githubusercontent.com/fullergalway/weewx-airmar-150wx-driver/master/airmar.py
>>>>>>>>  
>>>>>>>> && chmod 755 bin/weewx/drivers/airmar.py
>>>>>>>>
>>>>>>>> In the weewx conf we included these lines
>>>>>>>> [Station]
>>>>>>>> ...
>>>>>>>> station_type = Airmar
>>>>>>>>
>>>>>>>> [Airmar]
>>>>>>>>     model = Airmar 200WX
>>>>>>>>     port = socket://10.11.104.5:10000
>>>>>>>>     driver = weewx.drivers.airmar 
>>>>>>>>
>>>>>>>> I hope this helps!
>>>>>>>> Rob.
>>>>>>>>
>>>>>>>> On Tue, 9 Jun 2020 at 20:50, 'James Manning - NOAA Federal' via 
>>>>>>>> weewx-development <[email protected]> wrote:
>>>>>>>>
>>>>>>>>> Hi Rob-
>>>>>>>>>
>>>>>>>>> First, thanks MUCH for posting your "airmar.py" on github.
>>>>>>>>>
>>>>>>>>> I am a very novice weewx user.  I just got the "simulator" working 
>>>>>>>>> on my RaspberryPi and now I want to hook up my Arimar 200WX.
>>>>>>>>> From what I understand,  I need to take two basic steps:
>>>>>>>>>
>>>>>>>>> 1) modify the weewx.conf to point to new station_type and set 
>>>>>>>>> driver=weewx.drivers.airmar
>>>>>>>>> 2) put the "airmar.py" in my "user" directory which I think is 
>>>>>>>>> /home/pi
>>>>>>>>>
>>>>>>>>> Is that right?
>>>>>>>>> -JiM.
>>>>>>>>>
>>>>>>>>> On Wednesday, May 20, 2015 at 12:05:58 PM UTC-4, Rob Fuller wrote:
>>>>>>>>>>
>>>>>>>>>> Thanks Aleksandar,
>>>>>>>>>>
>>>>>>>>>> I needed to make a couple of small changes for it to work with 
>>>>>>>>>> our device (Airmar 200WX). Mostly:
>>>>>>>>>>
>>>>>>>>>> 1. use pyserial socket:// url, since we are exposing the 
>>>>>>>>>> weatherstation using ftdi driver and kplex
>>>>>>>>>> 2. added some detail in the log messages
>>>>>>>>>> 3. added some leniency on the checksum verification (" F" == "0F")
>>>>>>>>>>
>>>>>>>>>> For the 200WX I'm still getting a couple of warnings, but haven't 
>>>>>>>>>> looked into it yet, and generally things seem ok. The warnings are:
>>>>>>>>>>
>>>>>>>>>> weewx[11]: airmar: Wrong data format for windchill_rel ''
>>>>>>>>>> weewx[11]: airmar: Wrong data format for windchill ''
>>>>>>>>>> weewx[11]: airmar: Wrong data format for heatindex ''
>>>>>>>>>>
>>>>>>>>>> I put the source on github for the moment here if someone is 
>>>>>>>>>> running with it:
>>>>>>>>>> https://github.com/fullergalway/weewx-airmar-150wx-driver
>>>>>>>>>>
>>>>>>>>>> Thanks again,
>>>>>>>>>> Rob.
>>>>>>>>>>
>>>>>>>>>> On Tuesday, April 28, 2015 at 11:20:47 AM UTC+1, Aleksandar 
>>>>>>>>>> Tsankov wrote:
>>>>>>>>>>>
>>>>>>>>>>> Hi,
>>>>>>>>>>> I followed the mwall's advice and went back to original 
>>>>>>>>>>> wxservices.py. The logic for deciding which windSpeed and windDir 
>>>>>>>>>>> to be 
>>>>>>>>>>> used is putted in the driver.
>>>>>>>>>>> Now everything works as supposed.
>>>>>>>>>>> Since I want to have some real-time application running on 
>>>>>>>>>>> tablet, is it possible weewx to generate Weather Display's 
>>>>>>>>>>> clientraw.txt 
>>>>>>>>>>> file in order WDL (or something similar) to be used?
>>>>>>>>>>> I'm attaching the new version of driver.
>>>>>>>>>>>
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>>>>>>>>>  
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>>>>>>>>> .
>>>>>>>>>
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>>>>>
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