I inserted a new Dexcom G7 sensor before the old one expired. I started the new sensor about an hour after I had inserted it. I was surprised to see values from old and new sensor during the overlapping period on the Dexcom app.
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I have done that trick but not seen the overlap.
How did you get the phone app to “see” the new G7?
Maybe my app needs an update… 
Thanks for posting this.
@TEH4PWR, the overlapping data showed up AFTER I paired the new sensor. I assume that the app downloaded the values from the new sensor after pairing, not before. The sensor start time displayed by the app was the insertion time, not the time when I paired the sensor. I think going forward I will overlap at least 30 minutes so that I always get a reading.
@Helmut
The new sensor starts it´s warm up-period the second you insert it. I´m told it´s some kind of magnet that register that the sensor has been inserted. It has nothing to to do with your pump or your phone. The devices you use to see your readings just catch up.
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One of the (maybe the last) insertion steps says something like “push the top of the sensor for 10 seconds.” I’ve always assumed that’s what turns the built in transmitter on. If it were constantly looking to see if it’s been inserted into a body it would use up a lot of its battery life just sitting on the shelf before it’s inserted.
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@Greg_T, I don’t think that I ever pressed down 10 seconds. First couple of sensors I did not press down at all. Big mistake. For G6 I read that pressing down was bad. I definitely had better results with the G6 when not pressing down. G7 is different. Pressing down is a must. Live and learn.
Yes, pressing down when installing the G7 is a must. Pressing down after that is asking for a compression low, just like the G6.
Oh, ha, I thought that this step was to help the adhesion. This makes much more sense though. 
I normally try to do a 12 hour overlap. My experiences with the G6 convinced me that the G6 took about 24 hours to settle but there was no overlap. The G7 is certainly a lot better and it’s certainly not as scary to do a change with no overlap (as the G6 forced).
The G7 should retain 24 hours of data (at least I think that is the spec) so if something can get at that data it can show up to 24 hours of comparison data after a new sensor is started. It just depends on what the app, or the receiver, chooses to show.
It is nice to see that Dexcom are now showing the comparison data; that speaks much more for their confidence in their sensors than any amount of marketing (well, to me.)
What is annoying me at present though is that I can’t “start” a new sensor (the G7 starts itself) without stopping the current one using the receiver. Stopping a sensor does stop it delivering readings; the sensor is still up and running, it will do bluetooth, but it won’t, apparently, report any readings. I hope they will fix this.
Nope 
After you have done an insert pick up a steel paper clip, look for the silver thing in the inserter on the outer perimeter of the white nylon and hold the paper clip to it. It’s a neodymium (or similar) magnet. It holds the electrical supply within the sensor off until the sensor is a sufficient distance away.
A word of warning: those magnets have caused serious injury to small children because if you swallow enough of them you can clog up your gut (they stick together).
That magnet was good observation. Yes, multiple magnets can be hard on your digestive system. But I remember feeding plastic coated magnets to dairy cows when I was a kid. It was supposed to keep barbed wire and other metal that unselective eaters picked up in their stomach to corrode away rather than float around and make holes elsewhere in the digestive tract.
You can use a G7 sensor with up to three devices simultaneously. If you stop it on one device (your pump) does that stop it on other devices (a receiver or phone)? On the G6 I don’t think stopping on one device effected other devices. But on the G6 entering the calibration code on a new device after the startup period had ended could kill the sensor (it would claim it was being restarted), so this type of experiment would mean running for ten days with just the pump connected to the CGM. Since I like seeing my BG on my watch I didn’t try running both a new and an old sensor in parallel. And you could only think about doing this when starting up a new transmitter.
I haven’t tried it with the G7, but the built in transmitter makes it a lot simpler to do. And the code you enter is a pairing code (device identifier) rather than a calibration code, so the G7 may well allow this without requiring you to wait for the next sensor before pairing both devices to the same sensor again.
The problem arises if we take apart the inserter (I do this for disposal). The single magnet in the inserter is reasonably well attached and the lid can be screwed back on, but if the inserter is taken apart (to put the needle in a sharps container) the magnet is loose and too many of them are a problem.
If you stop it on one device (your pump) does that stop it on other devices (a receiver or phone)?
Yes; after the transmitter is stopped from the receiver the other devices (xDrip+) will report it as stopped. I made this mistake using the receiver; I tried to check a new sensor and ended up killing the old one because the receiver UI does not allow a “new” connection without stopping the old one.
As I understand what @Navid (xDrip+) has said with xDrip+ there is no point stopping the G7; just change the pairing code to pick up the new sensor. I believe it should be possible to swap back and forth between the two but I haven’t tried that; it costs too much if it doesn’t work!
xDrip+ doesn’t handle the multiple readings; it only shows one reading at a time. This is a pity; I can’t see any reason why xDrip+ should not keep reading two or more sensors at the same time apart from the fact that it would probably be a major change to the code.
On the G6 I don’t think stopping on one device effected other devices.
On the G6 the equivalent operation would be to stop the transmitter; the sensors couldn’t be stopped or started (only a “sensor session”). However I can’t remember a way of stopping the transmitter, I think that’s new on the G7.
Neither does the G7 actually shut down; it’s still there and it is still visible via bluetooth. As soon as the magnet is released the G7 is on until the battery expires. I think the G6 is the same except that it is always on. The problem is that the G7 unnecessarily reports that it is “stopped” when there is no reason for it to respond to that part of the protocol; it can just keep sending readings because it doesn’t have anything else to do. (Or does it start mining bitcoin?)
When I’m in the 12-hour grace period at the end of a session and my phone prompts me to “replace sensor”, I put in the new pairing code and get the new sensor’s readings on my phone while still getting the old sensors readings on my T-slim pump. Not sure it would work the other way around (new sensor on pump/old sensor on phone) or outside of the 12-hour period when the replace sensor prompt comes. That’s the only way I’ve done it.
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Does anyone know how to get this to work with the Dexcom receiver? IRC “replace sensor” doesn’t appear while the (old) sensor is still active but even after it has expired “replace sensor” on the receiver says that it will “stop the current sensor session”.
Clearly it does work from what you say (@Helmut’s original post just showed that the app retrieves and displays the history from the new sensor/transmitter.)
Well, one thing that works with the G6 and might work with the G7 (haven’t tried) is turning off Bluetooth on one of your devices for a full 15 minutes at the time you stop it on the other device. So, you’d turn off Bluetooth on your pump, and end the session on your receiver and start a new session on your receiver. Wait 15 or better 20 minutes and turn Bluetooth back on on your pump. The pump should pick up the old session and miss the signal that you stopped the session on the other device because the Bluetooth was off.
Can’t attest to this working with the G7 but it definitely works with the G6 if you have two sensors using different transmitters.
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I did some experiments yesterday when I reached end-of-life on my previous G7. This was scary.
I did what I have always done since I started using the G7 and whacked the new one in around 12 hours before the old one went AWOL (hey, I’ve got two arms, why not?)
I think waited for an hour or so and, after that, invoked xDrip+ and changed the transmitter ID [the pairing code] to the new number. Perfect, I was on the new sensor, except it was reading very-very-low (off by about 40mg/dL).
I stuck with that for a while, more than 30 minutes, then I swapped the “Transmitter ID” back to the previous sensor.
At this point xDrip+ stopped working. I could not reconnect to the original transmitter, it reported what it loosely described as an authentication problem (there was nothing more meaningful in the error log).
After a few hours investigation by a very experienced debugger of computer software [me] I found out that, maybe, it’s necessary to “unpair” all of the G7 sensors on Android to get xDrip+ to work correctly.
So, executive summary; swapping between sensors is no big deal if the thing you are using actually works. It just works if the app works. Alas there is still some work to do in xDrip+.
FYI, I have never had to “Gently press on top of sensor for 10 seconds” (step 6b in printed instructions) with G7 sensors and using the phone app. When I insert the new sensor, my phone gets a pairing request from the new sensor within seconds. Once I select pair, the phone app shows the sensor warmup countdown and reading appear when the countdown finishes.
I wonder whether the G7s go “live” when inserted (see magnet discussion below) rather than requiring the manual "press & hold) step with older G7 sensors. Anyone know?
The G7 design hasn’t changed since I’ve been using it; it always had the neodymium magnet in the inserter.
The G6 did the same thing with the sensor connection pads; when those were connected the first time the transmitter got switched on. I never saw a “live” G6 transmitter before the first sensor was activated.
My personal opinion is that the magnet is over-engineering; the G7 should be able to do the same thing without the expensive magnet. It just requires a certain amount of imagination (in other words the solution is obvious).
So far as I can determine the G7 does not shut down if the magnet is reconnected (I haven’t conducted detailed experiments; I’m not being paid and the whole thing strikes me as overly complex.)
The “pressing gently” stuff is to do with the sensor itself, not the transmitter.
Thx for your response. So the sensor “bluetooth “radio” gets power from the insertion needle movement effect on the magnet which opens the circuit to the battery.
Somewhere I read a Dexc item stating the “press & hold” activated the sensor but I must have misread it.
Unfortunately these products have to pass gov’t approval for use by anyone in almost sny physical condition. So things are more complicated than needed for at least half the users.
Mark44
Yes. There’s a teardown (of a sort) here:
https://www.reddit.com/r/dexcom/comments/zjssvi/inside_of_the_g7/
If I got the geometry right the relevant component is the one labelled P33 at the top left (on the first picture). My guess is that when the magnet goes away it opens (or closes) and the battery management unit in the G7 then switches everything on. I don’t think it can be turned off; I’ve reinserted G7 sensors into the inserter and they did not seem to turn off.
The Omnipod is broadly the same; when enough insulin is injected into the 'pod it turns on. I suspect that might be a simple mechanical switch (a one-way one, as in “flip a switch”).