Here is the further developed version of my simple perl-script. It now parses the output of gatttool and displays it in readable form.
You should rename the file from 121gw.txt to 121gw.pl.
Then you can run it via:
gatttool -b 88:6B:0F:73:XX:XX --char-write-req -a 0x0009 -n 0300 --listen | ./121gw.pl
It reads the output of gatttool and parses the packet the multimeter sends.
It detects a lot of error but not all of them, so it's something to play with but until the bluetooth transmission is fixed in the firmware I would not rely on the data.
If the meter is in VA mode most of the data is wrong. This can't be fixed in the script and is also the case in the Android app.
The output looks like this:
1516048737.255 2017-08 00042 Voltage DC (V) 4.4652 V _TempC 24.0
1516048737.795 2017-08 00042 Voltage DC (V) 4.4648 V _TempC 24.0
1516048738.065 2017-08 00042 Voltage DC (V) 4.4648 V _TempC 24.0
1516048739.010 2017-08 00042 Voltage DC (V) 4.4643 V _TempC 24.0
1516048739.280 2017-08 00042 Voltage DC (V) 4.4643 V _TempC 24.0
1516048739.550 2017-08 00042 Voltage DC (V) 4.4640 V _TempC 24.0
1516048739.820 2017-08 00042 Voltage DC (V) 4.4640 V _TempC 24.0
1516048740.157 2017-08 00042 Voltage DC (V) 4.4639 V _TempC 24.0
At the beginning is a unix timestamp when the record was received.
The year and month is always the same since the meter doesn't seem to send the current one.
The third column is the "serial number" of the meter. You can set this value on the meter by cycling through the setup until you reach the display with 00000. If you long press SETUP you can change the digits to some other number. This number is send by the meter in every bluetooth frame.
Here is some PoC i created (unfortunately no sources yet, as its too ugly and mostly just PoC, it is linux specific, C++), idea that might be handy for EE:
Measuring resistance in auto range mode, it is a bit slow to get to correct range.
https://www.youtube.com/watch?v=7mnrvE271fA&feature=youtu.beI used cheapo chinese microscope ($70) and, sure, 121GW
Not sure if worth to continue this app.