# The OmniPod's "reserve tank"

**URL:** <https://forum.fudiabetes.org/t/the-omnipods-reserve-tank/1957>\
**Category:** Pump\
**Tags:** omnipod\
**Created:** [September 12, 2017, 10:30pm UTC](https://forum.fudiabetes.org/t/the-omnipods-reserve-tank/1957 "2017-09-12T22:30:17Z")\
**Posts on this page:** 1\
**Showing post:** 1

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**Author:** ![Eric](https://yyz2.discourse-cdn.com/flex030/user_avatar/forum.fudiabetes.org/eric/32/9209_2.png) [@Eric](https://forum.fudiabetes.org/u/Eric)\
**Post date:** [September 12, 2017, 10:30pm UTC](https://forum.fudiabetes.org/t/the-omnipods-reserve-tank/1957/1 "2017-09-12T22:30:17Z")

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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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