The OmniPod's "reserve tank"

Will the PDM alert you when you are at official zero, or at real empty? Is that a special sound alarm? How exactly does it tell you?

The pod’s lack of clicking may not help us because my son is rather oblivious to the world around him :slight_smile:

Yeah, the clicking doesn’t really help alert you, other than just the fact you will know nothing is left in there.

It alerts you when you get to real zero. I tracked this by noting when it displayed a number like 8 or so, still above the “low” warning. And then I kept track of the bolus amounts until it got below what should have been zero. And it kept clicking and even recorded those amounts as delivered. Generally about 5 units more than what you think is left.

But when it gets to real zero, there is no question about it. You will be notified! Alarm and a deactivated pod. You won’t miss it, don’t worry!

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Awesome, thank you!

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3 posts were split to a new topic: Shutting off Omnipod deactivation alarm: suggestions

I have realized over the past few days how valuable this measurement is. Thanks, @Eric!

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Medtronic appears to have a safety net of close to 10 units. I usually bolus 20 units or better. I have often done a 20 unit bolus when my 723 Revel said there were only 10 units left. Of course within a short period I will be installing a newly filled reservoir.

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With Animas and Tandem - zero means zero.

It never occurred to me this was not the same across the board.

:smiley_cat:

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I’m not sure I follow. If you withdraw 200U from the vial and expel the air from the syringe, then in all you have 200U plus X (the dead space in the hub and the needle itself). But after you fill your pod or other reservoir, your 200U will have gone into the pump, and you’ll still have X in the dead space. The only “waste” is therefore from priming (and I think I’ve read that with an Omnipod that’s less than 1U).

Do some pumps account for the priming amount in their calculation of insulin remaining, and others don’t?

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The way I understand it is:

  • If you withdraw 200U from a vial, 7.5 will go to deadspace and 192.5 is available to fill the pod.

But if you measure 200U with the syringe you can deliver the 200U, because the syringe really measures 200U + deadspace.

@Eric, is this correct?

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A few things actually. This is a bit confusing, it may take me a few posts to explain it well, but let me try and then please let me know if you have questions…


First of all, the omnipod syringe has some deadspace, about 6 units. But that doesn’t really account for anything, because in order to fill the syringe up, you would need to put in enough to get past the dead space. If you wanted to get the syringe plunger drawn back to 100 units, it actually takes 106 units to get there. But if you were to inject it all out into the pod, you’d only get 100 units out. So the syringe deadspace doesn’t even enter into the equation.

Using green food coloring for the pictures to illustrate everything here:

syringe dead space ~ 6 units, but doesn’t really matter:




Now, here is where it gets a bit confusing.

Hang with me after the pictures. There is some fuzzy math the omnipod does which I will try to explain!

And then I will give an easy summary in case the numbers are making your eyes glaze over.


The pod itself has some deadspace. And also, some insulin is wasted from priming. But how much is the total insulin not delivered? The combination of wasted insulin from priming and deadspace?

The amount of insulin not delivered (pod deadspace and wasted from priming) is 7.45 units.

I used a real syringe with exact unit markings for my testing. So I could see exactly what I was putting in there, rather than using the omnipod fill syringe.

A real syringe much more accurate for measuring!
image

I measured the amount put in the pod (real amount of units), and then recorded everything that was delivered.

7.45 units less than what I actually put in the pod was delivered before the pod was empty.

Here are some pictures with food coloring to illustrate the deadspace:

After pressing the plunger all the way down (not talking about the syringe, but in the actual pod reservoir shown here), you can see there is still some food coloring at the end.

That is insulin that will stay in the pod and would not be delivered.
image

Food coloring stains!

A look at the parts.


So here is the fuzzy math they use.

They have a “reserve tank”. That amount is about 5 units (actually it is about 4.5 units). And they also know that they are going to be delivering 7.45 units less than what you first put in.

So suppose you start with 85 units. The pod status will show 50+ units remaining
when you first start. After you deliver 30 units, the units remaining will show…43 units!

85 - 30 = 55 units…but they say 43 units remaining. Fuzzy math!

Here is how they get that. They take away the 7.45 units of deadspace/primed/wasted insulin. And they also take away the ~5 unit reserve tank.

So here is the actual formula:

85u (what you started with)
-7.45u (deadspace)
-5u (approximate reserve)
-30u (delivered so far)
= 42.55 units they say is remaining, rounded to 43 for the display.

I have tested this several times. There may be a small amount of variability in the amount of insulin they can’t deliver, but my most careful and accurate test for it came out to be 7.45 units of deadspace/primed/wasted insulin.


One more thing to confuse it a bit more!

You can actually put more than 200 units in the pod to start with. I have put in as much as 215 units (just to test it out!).

So because they know there is deadspace, and because they advertise that it will hold 200 units, they actually let you put in more than 200 units, so you can actually get 200 units out of it.

Make sense?




Here is the simplified version:

  • There is about 4.5 units more left in the pod than what they display on units remaining status.

  • Whatever amount you put in the pod, you get 7.45 units less than that delivered.

  • You can put more than 200 units in it. If you do that, put in 208 units for example, you can actually get 200 units delivered.

Questions?

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Total sense! Thank you for this!

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The cost of science. My fingers are stained green from the food coloring. :crazy_face:

image

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I look at it as a medal!

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Just a quick buzz gets the green off…
image

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Here is another way of displaying the deadspace/primed/wasted insulin.

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Sorry for opening up a super old thread, but I just wanted to share that I googled this topic (Omnipod low reservoir, or something) and this thread was the first google result! And was exactly what I needed. I was so excited to find my answer from a trusted source :blush:

I’m almost on empty and am hoping to make it until I get home in a couple hours (would rather not do a pod change at work if I can avoid it). Based on the info shared above, I should have no trouble!:+1:

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Teenagers are the best are they not? They never keep you bored, that is for sure.

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So how did it work out for you?!

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Like a charm!

I knew I had a couple units, which was enough to get me by, barely. I had never run one so low though, so I was a little concerned it would just flake out on me - I guess I wasn’t sure if I should trust it once it showed “low” rather than a number. Was glad to read that not only would it not stop early, but I actually had a few spare units in there. I think I ran it to about 1 unit before I had to shut it off ahead of a run, so I couldn’t test it out any further.

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I kept trying to google to find out if the Omnipod tells how many units are in the reservoir and found this thread. It is helpful. Thank you for the info with in it.
I wish the Omnipod would tell how many units it has instead of always saying “over 50 units” and then doing a count down below 50. But I will have to get used to it. Each pump has its things about it that a person likes and dislikes.

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