Filling pump from insulin vial - negative pressure trick

This topic came up on an OmniPod thread, but it can benefit any pump user, so I am putting it in its own thread…

Any liquid contains gases dissolved in it. So for example, if you put 1ml (100 units) of insulin in your pump, a very small amount of the volume you are putting in is not insulin, it is just dissolved gas.

William Henry was a 19th century English chemist. Henry’s law states:

The amount of dissolved gas in a liquid is proportional to its partial pressure above the liquid.


We want a bolus to deliver a precise amount of insulin. We want 1 unit of insulin to be 1 unit of insulin, with less dissolved air. Make sense so far?

So to reduce the amount of dissolved gas, I put the vial under negative pressure. This reduces the amount of dissolved gas in the insulin.

To put a vial under negative pressure, when I first open a vial, I removed about 200 units of air right at the beginning. And after doing that, whenever I withdraw insulin, I put in only 2/3 of the air in, compared to the amount I withdraw.

For example, after doing that, if I take out 60 units of insulin from the vial, I would only put in 40 units of air. This keeps the vial under a constant negative pressure, which means less gas is dissolved in the insulin.

Make sense?

This does not work with pens, since the pressure automatically gets equalized. It only works with vials.

In theory, for any pump that contains a reservoir kept at a constant pressure, and is not exposed to air, the insulin cannot have more gas dissolved in it - because it is no longer exposed to air, it is kept in a closed container. There may be some air that can eventually become dissolved back in the insulin, as there is no perfectly closed system. However the trick is easy and costs nothing, so I do this with every vial.

Tagging @jbowler and @Trying who were in on the original conversation.


Thanks, @Eric, for making is so understandable! I was always told to put in the same amount of air that I withdrew of insulin! Of course, I never was explained as to why! Since I often have issues w/ air bubbles, I’m going to try the negative pressure trick, using 2/3 of air compared to the withdrawn insulin, plus the 200U of air right off the bat when opening a vial. This is the first I’ve heard of such a trick!! Thank you!!!


I’ll try it too, with the next vial.

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The idea there is that it is easier to withdraw insulin from a vial that is under positive pressure.


Yes. It will be a bit harder to pull the insulin out. You will feel the syringe plunger try to suck back into the vial. That’s a good sign, you know you have negative pressure.

The negative pressure trick is used in general by labs, just for withdrawing stuff, because it prevents spray back!


Eric, do you find that this method creates fewer visible bubbles that you have to tap to the top of the syringe and then squirt out or is it just that you know that you have less invisible devolved gas in the insulin?
I haven’t tried your method yet, but I seem to remember that sometimes when it’s hard to pull the syringe back, I end up with “champagne” bubbles, for some reason.

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A couple of things…

When you have too much negative pressure, the vacuum can cause air can seep back in behind the plunger seal. And a wider syringe barrel - like the ones that come with the omnipods - can create more of a vacuum because you would have to pull it slower to allow the pressure to equalize, compared to a narrower barrel.

Additionally, the plunger seals on the omnipod syringes are kind of cheap. So more air can sneak past them.

I don’t use the normal syringe, the one that comes with the omnipod. Because it has a much wider barrel, when you pull back quickly it creates a vacuum, and air can seep back in from behind the plunger.


I find those bubbles to be horrible with the omnipod syringe. So I always use a regular 100 unit syringe. They are cheap.

The BD ones are very good. Buy a pack of these and try them out. Only about $5 for a pack of 25.

So that :arrow_up: is the first thing to try.

I’d suggest trying those syringes for a month, and just see if it seems like you are getting less bubbles. After you get comfortable with that, then I’d suggest trying the negative pressure in the vial.

I am not 100% percent sure who else it was here who stopped using the omnipod syringes, and had better success. @Aaron maybe?

Reference post:


Yup - @TTnyc I gave up on the omnipod syringe at @Eric ’ s suggestion. The omnipod syringe is cheap has a plastic stopper that just creates bubbles. Using a normal insulin syringe with the rubber stopper thing is much better and does not have the same issue with making bubbles or getting bubbles stuck at the top of the syringe.

Unrelated, one of my boys wanted to create a “robot” that used a pair of syringes full of water to control the motion. I gave him a couple of omnipod syringes and even he didn’t want to use the omnipod syringes because they were crap :slight_smile:


I am reviving this old thread because we have quite a few people new to pumping who will find this helpful. Using the allergy syringes to fill the Pod reservoirs has improved my wear time considerably.


@CatLady Thanks for reviving this! I hadn’t read previously and while I don’t currently use a pump, I may try the Omnipod in near future. I’m surprised the vial stopper and syringes will hold enough negative/positive pressure to make much difference, though I doubt it takes a lot to be helpful, but trust Eric and Chris know their stuff.