For the moment, I have 2 Dexcom G4 receivers (because the button was getting pesky on the original), so they sent me a new receiver and I have a month to send the original one back. So of course, I have them both calibrated and they're both showing slightly different readings about 5-10 ish points apart. Why in the world should they be different at all though?? Shouldn't the transmitter be sending the exact identical readings to both of them?? Now I'm tempted to keep both receivers as long as I can to see how big the differences could become. And I did calibrate them both at the exact same time with the exact same blood glucose results from my meter. Thoughts??
I've been running two Dex G4 receivers since I received my second one last November. I don't think it's unusual at all that there's a slight variance in the numbers displayed on the two receivers. The signal that the transmitter sends is indeed the only signal that each of the receivers is tuned to. Radio frequency signals can and do degrade over the gap between each transmitter and each receiver. The signal that receiver #1 receives can be slightly different than the one that receiver #2 receives due to its relative distance from the transmitter as well as its exact location.
Once each receiver gets the latest update from the transmitter, it then processes that update using the calibration that you input to it on a regular basis. It would be pretty hard, if not impossible, to feed each receiver the same calibration number at the "exact" same time. Humans cannot sense precise time the way computers do.
I envision that each receiver micro-processor executes its own algorithm that is customized and updated as the system "learns" how to interpret the series of calibrations it receives.
In fact, I'm surprised at how close my receivers run to each other and how often they match almost exactly. Right now, my primary receiver is reading 123 with a sideways arrow and my secondary receiver reads 124 with a sideways trend arrow. And I just came back from a 45 minute walk with only the primary receiver with me.
I like having a second receiver to bridge the gap once per week during the two-hour blackout period. Sometimes it's a nuisance, for example, during the first 24-48 hours when the sensor and system can be erratic and sounding false high and low alarms. I still like the redundancy, however, especially when I'm sleeping. One receiver can be blocked from getting a good signal but it's not likely that the second receiver suffers the same problem.
Wow--great explanation--thank you, Terry!
Good explanation. I would take issue with one point. I don't think that "degradation" of the signal depending on distance will affect the reading. The transmitter and receiver are digital devices so the (analog) sensor current reading (in nanoamps or whatever) will be digitized and sent to the receiver. If the signal is good enough to be receivable the data will not be degraded.
On a slightly different point. I have noticed that when I start a new sensor (or re-start an old one) I will give it the 2 fingerstick calibration readings and the first reading from the CGM will ALWAYS be lower than both of the fingersticks (e.g. on last start, fingersticks 7.3 and 7.6, cgm reading 7.0)
Anyone else have this???
Joel
Yup, I think the algorithm is designed to have the CGM report on the low side so that you don't miss on that side and have a catastrophic event and try to blame Dexcom.
I don't find in general that the CGM shows a particular bias either low or high. It's usually amazingly accurate (within 10% of a fingerstick) but the variation can be either way.
I'd just offer the slight correction that signal variations present no potential for error in the received data. The connection is digital, no doubt with various bit error detection and correction algorithms applied, so at the receiver it either receives the data correctly, or it rejects it and in effect receives nothing for that update.
The rest of your post nails the problem -- two receivers can not be reliably calibrated at exactly the same time. This introduces two possible variances in their calibration, both of which affect the algorithm used to calculate BG levels: The actual sample(s) recieved by the two recievers may not be the same during the calibration (i.e. you lock in a BG value on receiver #1 and right after that both receivers get a new update before you lock in that same BG value on the receiver #2). Second, the timing is critically important as well. Calibrations are time-stamped. Even with the same last update value in memory, one of the receivers will be calibrated a bit later than the other. Its time-stamp will be different. In calculations, the calibrated BG value entered will be just a bit longer from the last sample value recieved. This will affect the result of calculations.
So, it's is basically impossible to get two receivers to sync on a single transmitter.
Great explanation.
Is is also possible that if one of the Dexcom receivers is an old one it may be running an earlier version of the software?
When I was considering changing from a MM pump/sensors to the Animas Vibe + Dexcom, one of the reps for the company that represents Dexcom in the UK suggested I might be better off with a separate receiver because the Dexcom receiver was using later version of the software than the version embedded in the Vibe.
Joel
My understanding of the transmitter battery life was that the clock started ticking the second you removed the transmitter from the magnet irregardless of whether you are even wearing it or not. If storing your transmitter won’t save battery life then I doubt that using two receivers will drain it faster. I don’t KNOW this, just speculating. Transmitter life does seem to vary a bit so hard to say why. Mine lasted a little over a year but when it died it only lasted 2 1/2 days after the first low battery warning.
My guess is that the transmitter is continually sending a signal and the receiver is just processing that signal into useful information. It would appear that the receiver just checks every 5 minutes because if it misses a reading then you get a gap. And the timing seems to correlate with when a sensor is started or calibrated.
So G4 transmitter just transmits, one-way link.
Bit more technically; it goes to sleep/idle mode for 5 minutes(to save power), then it wakes up, transmits transmitter id + raw glucose value and goes back to sleep.
So having many receivers wont affect the battery life.