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.
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.