I have been actively using Loop since April 2016 and active in the OpenAPS community for over a year.
As more and more people jump on the open source automated pancreas’ bandwagon, I have have watched many people struggle with getting their settings right. Along the way I have observed some folks doing what I am calling “max basal clipping”. This is the practice of preventing lows by clipping off the Max Basal rate.
I am a big fan of using a Max Basal rate that you are comfortable with for safety, but I don’t find comfort in people clipping max basal and calling that a solution to the yoyo effect that happens when your settings are incorrect.
The yo-yo effect can be seen when Looping and BG spikes, then drops, then spikes and drops over and over again.
When a bolus is taken BG will rise 99 times out of 100. It is all about minimizing that rise (preBolus and eatingSoon are two ways to help). If a bolus does not contain enough insulin (ie IC ratio not aggressive enough) then BG will rise quickly and Loop will high temp shortly thereafter. But many times the ISF ratio is too aggressive and thus the high temp delivers a little more insulin than you might want and do it a litter later than you might want (spread out of 30m starting 15–20 mins after eating means it is following the food instead of leading it). As soon as the rise starts to turn downward then Loop zero temps because it’s got too much following insulin. Then BG drops fairly rapidly and eventually lower than one might want (since ISF was too aggressive). Then the yo-yo starts happening because Loop has been zero temping for 60–90 minutes and one may carb correct. Which results in a high temp that uses too aggressive of an ISF and then the ups and down continues.
One solution to hypothetically consider (I am not a doctor) might be to soften ISF and make IC more aggressive. This will result in the same or a tad more insulin up front to help with the BG spike after a meal sooner but not give too much that causes a low. The result is the high temp should go away completely.
Others have been using a clipping solution where instead of making ISF softer they just cap the Max Basal as a number lower than they have used previously. The reason I disagree with this solution is in the end your settings should drive your BG as close to optimal as possible. Loop should only be high or low temping on occasion when things are just a bit off (carb counting errors, a little extra exercise, carb absorption times being slightly different than expected). Loop should be cruise control, not constantly overriding your driving by slamming on the gas and then slamming on the breaks with high and zero temping fore each and every meal.
Update Jan 10: Max Basal Clipping does not impact Bolus recommendations either. So if you’ve clipped away some of the basal Loop thinks it should deliver that missing insulin will be recommended when you press the bolus button (on it’s own or part of another carb entry). If Loop wants to give you more insulin than you think you need — adjust your settings to match what you need, don’t clip your temp basal max.