So were you pulling that much insulin out of the tubing or the actual cartridge?
Right from the cartridge.
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Thats a bummer, Iāve been spoiled with Medtronics pump letting me run past 0, for about 10 units.
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Yeah, I had that happen to me overnightāpump zeroed out several hours before I got up, having forgotten I needed to do a set/reservoir change the previous evening. But when I woke up, my BG was right where it normally would be, even though the pump said I was on empty. I thought Iād experienced a miracle cure! Turned out, no, it hits zero while thereās still insulin in the pipeline and it keeps going for long enough that I was still getting my basal. Ah well.
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Yes, my 5xx models all had 8 units remaining when the counter reads zero. Now that Iāve got a 670, I donāt yet know how much insulin will be left when it reads zero.
For those of you not familiar with Medtronic pumps, you can use all the insulin in the reservoir until the plunger hits bottom, so when the pump first says there are zero units left, you can still pump the 8 units mentioned above. Once the counter reads zero, it remains at zero as you use up (optionally) the additional 8 units.
If you overfill the Medtronic reservoir and insert into the pump, it primes the tubing so you can start with 300 units and not have to prime more than 1 unit during the priming process on the pump.
It has done that twice, the other few times Iād run it to zero, then be annoyed at the alarm every 30 seconds.
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Tandem does not recommend prefilling but I have takled to both a Tandem Rep and a tech support person and they acknowledge people do it they just endorse the practice. Due to my hectic work schedule and frequent travel, I have had to change out cartridges in all kinds of places: trains, busses, office break room, and yes, a bathroom in Paris. This is why if I know I am going to run out later in the day (or next day) and I know I will not be in an ideal environment for filling a cartridge, I prefill one and bring it with me. I did that with my animas pump and a few times now with the Tandem. I am careful with how I pack it.
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As a general rule I only fill a day in advance. I have heard that that after a few days the plastic in the cartridge can change the insulin. The other thing I have been told is that if I do prefill a cartridge is to be sure that I have brought it to room temperature before loading it on. The Tandem rep seems fixated in the insulin temperature.
So cool, thank you for sharing this video.
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They may not condone the practice of prefilling cartridges to avoid liability. I have never prefilled simply because I am close to all my supplies pretty much all the time.
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I just tried to run a bit of an experiment to try to quantify the difference between how much insulin is in the cartridge and what my t:slim X2 reports.
Yesterday at 8:30 am, I filled a cartridge with 160 units ā¦ as close as I could āeyeballā. That is probably +/- 2 units.
Then I filled the tubing with 11.6 and the cannula with 0.3, so I think that I had 148.1 u (+/- 2) available. 21 hours later, I have injected 12.7 u bolus and 10.3 u basal. Accordingly, I should have 125.1 units left in the pump (+/- 2 u of fill uncertainty).
My pump just in the last few minutes went from reading 110 u left to reading 105 units. At this point, the pump is somehow āroundingā to 5 units ā¦ but there are several types of rounding. What rounding does Tandem use? I do not know!!!
Here, is the type of rounding that I would use and why. Caution: this is my speculation and NOT fact. I DO NOT know what rounding algorithm Tandem uses. However, without getting overly mathematical, there are three basic types of rounding: true rounding, floor rounding, and ceiling rounding. If using true rounding, a reading of between 102.5 and 107.49 rounds to 105. Floor rounding would read 105 to about 109.9 as 105 and ceiling rounding would read 100.1 to 105 as 105.
Floor rounding is the most conservative because you always have more than the displayed amount ā¦ which is what I would do IF I designed insulin pumps. But, I do NOT and Tandem did not ask for my opinion.
However, IF Tandem uses floor rounding, and I just saw the transition from 110 to 105 units remaining, that means that my pump thinks that it has about 109.something that is still pumpable. And by my calculation, I have 125.1 +/- 2 units in the pump.
To me that says that approximately 15 +/- 2 units is either unpumpable or held in reserve. That is surprisingly close to the amount that another member extracted from an āemptyā cartridge.
Maybe others can repeat this experiment or maybe someone else knows for sure what type of rounding Tandem uses when they display remaining insulin to the ānearestā 5 units. But my little experiment leads me to believe that all but about 15 units of the original available amount is āusableā. Of course, the 12-ish units to fill tubing and cannula is also not usable, so if I fill 160 units I can only actually use 130-ish of that between tube/cannula fill and āunpumpableā reserve.
Thanks,
John
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Doesnāt the Omnipod also prevent fully emptying of the reservoir, like Tandem users complain about with their pumps? Glad the Medtronic goes to the last drop, although I donāt really know yet how my 670 will react once the counter reads zero, but the previous pumps would all totally empty the reservoir. I donāt want to get caught away from the house with any empty reservoir to test if the 670 will go below zero units displayed.
Does the 670 use the same syringe type of reservoir that all of their other pumps have used? If, so I see no reason the 670 pump would act differently when the reservoir is extremely low.
Yes it does. If the software wonāt let it go below zero, that would be a reason for it not to act as the earlier pumps.
Hopefully, they didnāt mess with that part of the pump.
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Thanks for the detailed response. I was told by the diabetic educator that the pump will (depending on how much is in there at the time) measure in units of five. I had told her that I had bolused a couple of times and the pump kept showing a steady 85 units without subtracting the units of insulin. About an hour later, it finally gave me a reading of 80 units. In other words, it did not subtract the amount until I had used five units. When the pump has less insulin in it then it will show the remaining amount in single units (except of course the lost reservoir of 20 units).
Iād be very frustrated with a pump that couldnāt properly report the quantity currently in a reservoir.
Itās quirky at first. You get used to it over time. Iāve never had an instance with the pump where I didnāt trust the information it was displaying.
If it was ever wrong, Iād not like it. 
Just like I wish that there were fewer button presses needed on the 670. Itās always a matter of choosing the least objectionable product, as nothing is perfect. Much of the reason I donāt have a Tandem pump is due to their inability to more quickly process my pump request, given that I told them my 530 was broken, which makes it all that more disturbing that they failed to get the paperwork done quickly.