Tandem made a revolutionary change to the insulin decay model when they introduced Control-IQ. It more closely resembles how insulin is actually used by the body.
Previously, every Pump to my knowledge used a linear decay model:
You used up insulin at the same consistent rate. We basically set a number for DIA willy nilly based on when we FELT the insulin was no longer effective. Usually somewhere between 2.5 and 4 hours, after the major insulin peak wore off. But it wasn’t true… There was always a long-acting tail that wasn’t accounted for and our insulin usage is ANYTHING but consistent.
With control-ig, Tandem changed the decay model to be an exponential decay with a delayed start. I don’t have a specific picture from Tandem, but the graph is going to look a lot more like a rollercoaster. Starting out at the baseline, peaking activity 30 minutes after bolus, then slowly tapering off to zero at the five hour mark. It’s actually approaching zero for a very long time, hence your pump will display 0 IOB long before the 5 hour mark, and really aligns well with anyone previously using a DIA between 2.5-3.5 hours. It’s also why they locked the DIA in at 5 hours, because they don’t expect us to understand the change they made here.
EDIT: Came back to say that the very first section of this graph isn’t a true representation, just the closest I could find. The start of the graph actually should be up high, even with the peak, but it doesn’t really start to significantly decay until after 30 minutes or so.
You can see how the new model better aligns with real insulin usage:
There are other complications with Control-IQ, because it is often adjusting your insulin. If it delivers a correction bolus or increases your basal, it will add the extra insulin to your IOB. If it suspends delivery or reduces basal, it will subtract that insulin from the IOB. The ticking clock telling you when your bolus expires is no longer relevant. Your IOB is now a living value telling you and the pump exactly how much potential the insulin you have onboard has to lower your blood sugar.
However, the moment you toggle Control-IQ off, it reverts back to the old linear decay model with the ticking clock you define.