T:slim x2 Reservoir Amount

I’ve been using an X2 for years. When filling the reservoir with more than 180 units, it’s always read “180+” until the last two changes. It’s now reading “120+”, and after some time it shows 150 units until the capacity drops below 150 units.
I called Tandem and was told that’s how it’s supposed to work. I asked about the change from 180+ to 120+ and wasn’t given an answer other than “it’s with 30 units”. “Within thirty units of what?” I asked. No response.
Anyone else come across this?

IMO the Tandem reservoir system is wasteful and inaccurate. It is related to the baggie reservoir and how the pump dispenses insulin. I just filled a reservoir with approximately 140 units and used 11 to fill the tubing. My pump now says I have 60+ units. I know that it will get more exact once I have used 10 units but my guess is that it will never get close to 120 units. There is a lot of insulin that “disappears” and is unusable with the Tandem system. I am lucky that I have no OOP cost for insulin with Medicare, but I do miss my Medtronic and Animas pumps that more accurately measured the insulin in my reservoirs and allowed me to use most of in the insulin loaded into the pump. There is a lot of like about Tandem but I do wish that we had more choice in pumps these days.


I called Tandem back and had a long discussion with a second support person. She told me they’ve had a number of complaints like mine and they have an update coming some time before summer that, among other things, will address the issue I raised. Do I believe her? We’ll see.
Laddie I haven’t noticed any “disappearing” insulin but will keep watch to see if I notice anything.

The “disappearing insulin” happens a few ways.

One, when my pump says the cartridge is close to zero units, I can always pull at least 20 units out of the cartridge with a syringe. So that is 20 units that can’t be used by the pump. Some people put that insulin in their next cartridge. I don’t because I already use my cartridges for 5-6 days due to being a low insulin user and I think it is old enough by then.

The second disappearing act is that I put in 140 units according to the syringe this morning and used 11 units to fill the tubing. My pump still says 60+ units although I probably haven’t yet used 10 units. I’ll try to remember to report back, but I bet the cartridge will eventually register at about 80 units. So 140 units inserted minus 11 to fill the tubing and then 10 units used. So in my book I should have at least 110 units of insulin in the cartridge. Or to be generous to Tandem with rounding errors, I would say at least 100 units. But it is unlikely that I will see more than 80. So that is another 20+ units of insulin that disappeared.

As I said above, I have no out-of-pocket cost for insulin with Medicare paying 80% of the cost and my Medigap plan paying the remaining 20%. But someone is paying that cost. Plus not everyone has good insurance coverage for their insulin.

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That sounds exactly right. Every cartridge has 15-20 units of unusable insulin remaining, per Tandem and the instruction manual.

My understanding is that this is right on par with Omnipod and Medtronic pumps, too. I haven’t personally used either of the other systems, but I’ve heard repeatedly through the years that some residual and unusable insulin is normal. I haven’t been able to find a spec for this in the Medtronic manual, but a quick Google search says 28 units wasted for Medtronic. I’m not sure how credible the paper was, though, because it in general is reporting much higher wastage than I would expect.

I don’t know if it applies to you or not, but I also know a lot of people lose insulin to the syringe. There’s a lot of wasted space in them! Those Excel branded ones with the less-waste tongue, don’t make a lick of difference. The just washed more than the BD ones did to start. The directions say to remove the air bubble from the cartridge with the insulin-loaded syringe and flick it up to the top. I find this air bubble perfectly fills the wasted space in the syringe. By the time I’ve injected all the insulin into the cartridge, and I’ve reached the air bubble, the syringe is fully seated to it’s hilt. Yet, I read all the time about people just removing the air bubble with an empty syringe and ejecting it into the air. Thus all that insulin, I would guess 20 units or so, is stuck in the syringe.

I certainly agree that there is 20-ish units of insulin left in the plastic bag when the Tandem pump says “zero”. I also like Robyn’s point about “waste” when filling and purging a syringe multiple times a day.

I only know of one other bagged delivery system: the bag used in boxed wine. In my experience, a typical box of wine holds the equivalent of 4 bottles of wine: 4 x 750 ml = 3 liters. By the time a box of wine “runs dry”, you can pull the bag out of the box and fully squeeze and drain it. I find that there is typically two glasses of wine that is still in the bag. That is about 300 ml … about 2 glasses of wine or about 10% of the original volume.

Some may claim that equating my box of wine to our life-extending insulin is silly … but if there is 10% loss in a box of wine, it’s not surprising that there is at least that much “reserve” in our t:slim cartridge. After all, we all EXPECT that we will get insulin, and not air, all the way down to cartridge zero.

Have a good evening.


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The new mobi pump looks like it has a clear syringe looking reservoir. I’m thinking they got tired of everyone complaining about the reservoir that no one really knows how to gauge.

Now if they would only just release it already.

Usually if I let the cartridge go all the way empty… or should I say I am lucky enough to have the opportunity because it is a big gamble as to how long my site will actually last… I cant pull much if any insulin out of it.

The one thing I DO have issues with is the fact that the cartridges usually can’t fit the full 300u they are supposed to. I will occasionally get one that all the insulin shoots right in, but usually it seems the internal bag can get stuck and not open all the way and it will push a good 20 - 30 units back. I usually don’t even fill the syringe all the way to 300, just a little under. And yes I get all the air out of the cartridge first :grin:

I long ago got used to the vague cartridge level readings. I also clean my pump using alcohol and will clean the sensor thingy as well which does seem to help some. Usually it starts on 240+ and then once I have used some it will give an exact number.

I kept waiting for their new pump to come out and it seemed like it was going to be years so after my pump warranty ran out and the battery wasn’t lasting well I just got another X2. That pump sounds cool, and I would LOVE to have a USB-C port like it is supposed to have as the Micro USB on the t:slim is pretty crappy in my opinion. Even with good lighting and looking right at the plug it can take a couple tries before I get it lined up properly to go in as it get’s snagged on the edges of the port. It would be nice to be able to pop the cable in without worrying about direction and also in dim light.

The one issue I would have with that pump though is the fact it doesn’t have a 300u reservoir. 200 is just not enough for me to go 3 days on.

200 u might be enough if you were giving Toujeo once daily to supplement your basal but I don’t know how that would impact the automatic functioning of control-IQ!

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It’s not just your opinion. The micro-usb is the primary reason I refuse to get another X2, even though I’m out of warranty, too. It might be a small detail to most, but to me, it really makes the pump feel out-dated.

I highly recommend getting one of those magnetic charging cables. I ripped the port cover off and just leave the micro-usb adapter in the pump at all times. Then when I need to charge, the magnet just makes the cable snap into place with ZERO effort. Mine came with lots of cables and adapters, so I’ve got cables all over the house and use them in most of the small electronics, like headphones/ear buds. It’s not quite powerful enough for my phone, though.

This is the one I got, but I got a better price as an Amazon lightning deal:



I too dislike micro_usb because I normally notice that my battery is low either when I am driving or just after I’ve gone to bed. In either case, funmbling to find ip/down is a pain … and your “self-aligning solution would be a big win.

Follow up question: will you magnetic adapter support USB data transfer as well or only charging?



Mine does not do data transfer, but a quick search says there’s are some that do. The data capable ones seem to have even worse reviews than the regular ones, though. I’m suspecting that the data transfer isn’t very reliable. And Tandem seems to be especially finicky about the data cable, so I have no clue if you could get the pump to recognize a magnetic one.

I also haven’t had a reason to plug the pump into a computer since 2019! So that wasn’t on my radar.

FYI, I can understand the mixed reviews. It’s a really nice convenience, but they’re all en mass made in China quality. I’ve done really well with mine, though. Only one small problem after about 8 months of use. Mine came with a LOT of tips, like 5 or 6 of each lightning, micro-usb, and usb-c. The only issue I’ve had is that one of the micro-usb tips came apart. The magnet stayed stuck to the cable and I had a half-naked bit left in the pump. I couldn’t fix it, so I’m down one tip, but no other problems. (Well, other than wrapping myself up like a mummy in the 10-foot cord by the bed.)

Wow a 200 unit cartridge actually can account for 66 units per day. My sites won’t absorb more than 50 units. They just crap out. I’m currently using about 45 units a day. And I’m a big person. 6’3 and 200 lbs.
if 200 isn’t enough you must be taking more than66 units per day.
I have started taking 10 units of lantus to make my pump work more predictably. So I’m only pumping 35 units a day now.

I know everyone has different insulin needs, but I’m surprised your site will allow that much insulin to be delivered in one place and not crap out.

What kind of sets do you use and where do you put them in?

I’ve struggled with this volume issue for many years. I have never found a solution beyond taking lantus.

Is that okay in the shower? I always wondered if the port cover is waterproof. Waterproof phones have exposed USB ports, so I guess it’s possible to protect electrically and physically. I’ve got sfew mag cables around. I’ll give it a try. Thanks!

I’ve never actually worn my pump in the shower, but I’ve dropped it in the bathtub often with no issues.

The port has nothing to do with waterproofing. Yes, water can touch the charging port with no issue, so long as it’s within conditions the electronic device is rated for. The port is made from non-corrosive metal, which isn’t affected by the water.
Waterproofing is usually all about the rubber gaskets inside the seams and openings, that keeps water away from more gentle components.

The pump is only rated for 3 feet deep for 30 minutes. It’s not WATERPROOF, but it’s plenty safe from accidental plunges and splashes… No matter what kind of charger I’m using.

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Awesome! Thanks. I dug my mag cables out and I’m using them now. I don’t recall exactly I gave up on them, but I may find out. I will probably us them on my pump regardless. Cheers!

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I’m about 6ft and 250lbs. My total daily dose is usually in the 60-90 range. I do frequently have trouble with sites lasting less than 3 days, sometimes they only make it 1 day if they happen to be in just the wrong spot (have literally had some that failed within a day and I moved it just a little bit away from the first spot and they lasted 3 days perfectly). I have to take my pump off quite a bit so the only practical place for me to put the sites is my stomach. I have been on a pump for about 23 years, so I did start to realize several years ago that the typical areas I have always used which was the middle portion of my stomach had started getting the hard buildup under the skin which was causing more failures and slow absorption. I started using the middle areas for my CGM and then started putting the pump sites in the upper and lower areas. That has worked for the most part but I still have leakage failures a lot of times.

If I recall Tandem said that the cover is mostly there for dust and doesn’t affect the water resistance. I’m sure they use similar ports as resistant phones do. I still prefer to keep a good flap on there though lol.

@Robyn_H I will have to give the magnetic thing a shot some time. I never really thought of that.

Ok so then I think perhaps adding a long acting insulin to your schedule could help you.
You could ask your doctor. Also some type 1s are trying to use metformin to reduce their insulin needs.

I have land mines all over. I gave up on my belly for years because the scar tissue was too much. I moved to my hips and top of my butt. But I still have failures.

For me it’s pure volume, more than 50 units in a day causes the site to fail prematurely. So taking lantus has really helped my sites to stay good for 3 days.

At some point I may have to give up pumping if I have so much scar tissue that it becomes a real insurmountable problem.

I think the mobi pump will open up more areas to pump when it comes out, because it won’t be necessary to look at it

I have my Tslim but no training till May 4. So am trying to absorb (no pun intended) lots of info here. I use 300 ml full in my Medtronic and go 4 days. It doesn’t sound like I will be able to do that with the tslim but will sure try. I use about 55u/day and w wastage in tubing & getting a few bubbles out that uses it all. I have always used all the space on my abdomen so it is still good, since 2000. Knock on wood! I’m not thin!
I suspect that pulling anything off the pump would invalidate its warranty. Re Robin_H. But am now ticked to hear ‘another’ negative about tslim, the usb port. Sheesh. I read only positive stuff about the tslim until I decided to switch to it and now all these issues. The bag not filling always or not reporting accurate insulin amount, and now this. I don’t know how Tandem could have increased sales so rapidly with inaccurate and crummy insulin so-called “cartridge” which to me a bag is not a cartridge.
Just trying to listen in and learn. This is another useful thread. Thanks.

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My only complaints about the T:Slim is that it is the fixed range used by the algorithm, but that can be dealt with by adjusting basal rate, carb ratio and ISF.

As to the amount of insulin being reported correctly, when I first change the cartridge it displays +60u. It’s not a problem because I know I filled with 125u - 13.3u for tubing and cannula fill. So far I have never had bubbles in the line after doing a fill.

While I have gone 4 days on a site, 3 days is recommended. My estimate is for 3 days you would fill with 200 to 225u and not 300u.

If you don’t like the pump, return it within 30 days. That wasn’t possible for me because I was sent the pump just prior to Christmas/New Years and my trainer was off.

I hope you get a stand up trainer like mine. She also gifted me with a Dexcom transmitter. I doubt that’s a usual thing, but I was worried about the transmitter in use was going to die soon, and I was having a problem with the DME people.

That’s my big beef, I have to deal with 2 DME companies, one for CGM the other for pump supplies.

One other thing, see if your trainer will give you a sample of infusion sets. I was sent 90 days worth of AutoSoft 90 with 9mm cannulas. My preferred location is on the thighs. No go, caused pain. I requested a change to AutoSoft XC with 6mm cannulas. That took care of the discomfort (a medical term for pain), and the XCs are easier to disconnect and reconnect when showering.

As to people reporting problems with Tandem, there are pros and cons to any system. Some love Medtronics, or Pods or Tandem, while others hate them. I chose Tandem because it works with Dexcom and was put off be the reports of dismal results with the Medtronic CGM + having to calibrate - what is it - 2 times per day.

Pods scared me, because I am active and thought that I would clumsily knock them off on doorways or what ever. But you ask, What about the pump? Well I came up with my own solution that has worked very well so far, for me.