Well, @Michel … it isn’t a simple answer to give. A lot depends on how Dexcom implements it. It is technically possible to connect the transmitter to any Apple Watch running Watch OS4. Watch OS4 adds the Core Bluetooth API that Dexcom will use. Per my sources at Apple, they are asking Dexcom to display on any watch running Watch OS4.
It is technically possible to send the data from the sensor directly to any Apple Watch. I have established a connection between my S1 watch and the G5 sensor. However, without the app updates, there is currently no way to display the data (Yeah, my team does BT for a living).
The thing gained on Series 3 watches is the ability to upload data to the cloud (Clarity) directly and to Share without the phone nearby.
For S1 and S2, the watch would need to sync the data to the phone when it came back in range in order to get it into Clarity, and you would lose real-time sharing. Keep in mind the transmitter itself can store a certain amount of data when not connected to the phone, which is how they fill gaps on short disconnects. The Watch has enough memory to store another chunk of data. I haven’t worked out the exact amount of time one could go without the phone.
All that said, Kevin Sayer (Dexcom CEO) claims the G5 can connect to 2 Bluetooth devices at the same time. However, their current implementation only allows 1 device and the receiver. Dexcom’s tech guys claim the receiver channel is not Bluetooth, but I haven’t torn apart a receiver to verify one way or the other. If there are 2 Bluetooth channels, they could easily connect to both a watch and phone/pump/“other device”.
I have to believe they have 2 available channels since Kevin Sayer says setting changes and calibrations would still require a phone. Even with the S3 watch, the phone still needs to be connected.
But then Dexcom does not have a good track record for implementing things that make sense, even when technically feasible. Think of that awful update when they bypassed the vibrate switch on the iPhone. You would be surprised at some of the “possible fixes” their lead firmware engineer considered, before ignoring it all together.
We will have to wait and see.