A hot wire (hot bead) anenometer is often used to measure breath eg in respiration masks. They're a wire or thermistor slightly heated by the cuirrent passing through them. When air is passed over them, they're cooled and the resistance change gives a measure of air speed. Need correcting for temperature if accuracy is required.

@alowhum nano bootloaders have changed recently. They now use the same one as Uno, used to be an older version. I too struggled with this at first but now realise that, if you use a recent ARduino IDE (eg 1.8.8) and select 'nano' as the device, you can then further select 'processor' and have a choice of ATmega 168 and two ATmega368, one of which is 'old bootloader'. Earlier versions of the IDE will always be 'old bootloader'. So I don't know if these boards are to the new standard (and you're trying to use them as the old ones) or the old standard (and you haven't selected that in the IDE) but you'll find the same thing with some ordinary nanos too.

@alowhum nano bootloaders have changed recently. They now use the same one as Uno, used to be an older version. I too struggled with this at first but now realise that, if you use a recent ARduino IDE (eg 1.8.8) and select 'nano' as the device, you can then further select 'processor' and have a choice of ATmega 168 and two ATmega368, one of which is 'old bootloader'. Earlier versions of the IDE will always be 'old bootloader'. So I don't know if these boards are to the new standard (and you're trying to use them as the old ones) or the old standard (and you haven't selected that in the IDE) but you'll find the same thing with some ordinary nanos too.

A hot wire (hot bead) anenometer is often used to measure breath eg in respiration masks. They're a wire or thermistor slightly heated by the cuirrent passing through them. When air is passed over them, they're cooled and the resistance change gives a measure of air speed. Need correcting for temperature if accuracy is required.

@dbemowsk It has a humidity sensor, the bare BMP180 doesn't.
It would be more fair to compare it with the BMP280E, but these are less common / more expensive.

If you don't want to sense humidity then no, it has no advantages as far as I can see.

@oneyb Thanks - that works for me now.
It does complain (of course) that it couldn't open /dev/ttyUSB0.
Is there a better way to deal with local specifics like this ? Maybe a convention of named links to specific ports that might exist ? So I could

ln -s  ~/.platformio/local/serial/Uno  /dev/serial/by-id/usb-Arduino__www.arduino.cc__0043_95335343136351D0D1E0-if00

I installed platformio a while back for experiments with ice40. I've avoided it ever since because I found it way overcomplicated (and I generally prefer CLI environments - it just seemed that by trying to tackle everything it was reinventing too many wheels and had too many dependencies).

But inspired by your delightfully simple example, I reinstalled it and had another go. It up did some updates (including to ice40, which surprised me), but failed in building blink with :

Compiling .pioenvs/mysensors_htu21d/FrameworkArduino/HardwareSerial0.cpp.o
src/mysensor-code.cc:64:1: error: 'HTU21D' does not name a type
HTU21D myHumidity;
^
src/mysensor-code.cc: In function 'void setup()':
src/mysensor-code.cc:97:3: error: 'myHumidity' was not declared in this scope
myHumidity.begin();
^
src/mysensor-code.cc: In function 'void readHTU21DTemperature(bool)':
src/mysensor-code.cc:148:16: error: 'myHumidity' was not declared in this scope
float temp = myHumidity.readTemperature();
^
src/mysensor-code.cc: In function 'void readHTU21DHumidity(bool)':
src/mysensor-code.cc:171:16: error: 'myHumidity' was not declared in this scope
float humd = myHumidity.readHumidity();
^
Compiling .pioenvs/mysensors_htu21d/FrameworkArduino/HardwareSerial1.cpp.o

It looks like something is still out of date. Is there anything obvious I missed or do I have to start digging ?

I picked up some Motorola Focus Tags. These are part of their Hubble security camera system - I think they're used to identify things that move in the camera's view.

They're 42mm discs with a CR2032 , button, a led, a piezo disc and, I think, a light sensor and an accelerometer. Processor is an nrf51822 CEAAE0.

There are two other chips I can't identify. One has only the letters NAI (the N is stylised, could be a logo) and a QR code that's too small to read. It's quite thick - it's probably an accelerometer / gyro. 16 pins, I think

The other is marked 2411 2276.1 UFC511. It has 20 pins and seems to be linked to what I initially took to be a light sensor but could be a chip antenna. In fact, given the PCB layout in the area (restricted ground plane) and the lack of any other antenna, it's probably a PA.

I got some of these from ebay, but they're fitted with nrf51802 QFAAA1 instead of a 51822. I can't find the memory sizes etc. for the 51802 - can they be used ?