# The OmniPod's "reserve tank"

I have been tracking this down for a while. Finally got a somewhat official answer, which matches the tests and numbers I came up with on it.

If you are a OmniPod user, the conclusion may possibly be of use to you at some point.

The OmniPod PDM shows units remaining. But is it never exactly correct. I always noticed a bit more insulin in the pod than what is actually displayed on the PDM. But the “how much” was somewhat of a mystery.

So I started loading my pods with very specific amounts with a syringe (not using their big goofy fill syringe that doesn’t give you a clear amount). And I started doing the math to see how close the remaining amount is.

Their displayed units remaining is based on plunger position to get to the 50 unit mark, and then they use the delivery amount to keep subtracting down to zero.

But…they have a safety margin they use of 5 units. It’s kind of like the reserve tank in your car. Your fuel light goes on, but you still have some gas in there. How much?

So 5 units is the number they use. They tell you the max fill amount is 200 units, but you can actually put in 205+ (most I have done is 215) units.

When the PDM shows “X” units remaining, it’s actually more like “X + ~4.5” units.

So this might help you in a certain scenario. Suppose your display says 8 units remaining, and you are not sure you should change your pod before leaving, or wait until you get back home. You actually have closer to 13 units. So this gives you a somewhat better idea of how much insulin you really have left.

Two important notes:

1. It is never exact, because it waits until it gets to the plunger position of 50 units to start the countdown, and the plunger position of 50 units has some error either plus or minus a bit.

2. Once you get below 5 units, you don’t have any idea, because the PDM says “low” instead of an actual unit amount. So you have to make a note of it before it gets below 5 units.

But for amounts remaining between 5 and 50, you can add roughly 4.5 units, which is kind of like the pod’s safety reserve tank.

Anyway, hope this is useful. Lemme know if there are questions. My explanation was somewhat scatter-brained.

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Medtronic pumps have a safety margin too. I don’t know how much, but it still gives insulin for some time if there are “–,-” units remaining.

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Ow, this is VERY useful, @Eric !

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Great info. Thank you for posting.

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A little bit more info to share.

The omnipod does seem to have more units than what is actually displayed. Kind of their safety, so you always have a bit more than you think. There is always more than what they say is left in the reservoir.

But the other thing I have come up with is the amount of insulin it takes to prime, to fill up all the dead-space in the tubing that runs from the reservoir to the cannula, and the insulin that is perhaps wasted in the priming process.

I was surprised to find how much it is.

Dead-space is the amount of insulin that can not be used. If you depress a syringe completely, there is some that does not enter your body, it can’t leave the syringe. This is known as dead-space:

So combined in the priming process - any insulin that is wasted plus the dead-space of the pod, there is about 7.5 units.

If you load your syringe with 200 units, you only get 192.5 units delivered.

This is not to be confused with the display being off by about 5 units. These are two completely different things.

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@Eric, a question came to mind:

is there a way to KNOW that you are actually OUT, and that your Pod is not dispensing anymore?

What EXACTLY happens when your pod still has time to go before expiration but you run out of insulin?

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Yes, I’ve run them dry to test it several times. The PDM will alert you that nothing is left in it!

It will keep going until empty, then the pod’s clicking stops, and it will alert you. Then it automatically deactivates the pod so you can’t try to keep blousing.

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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

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.

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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!

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.

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)
-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.

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I look at it as a medal!

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