The biggest advantage that OpenAPS has to offer in my opinion is its incredible ability to reduce high blood glucose values (BG’s), while simultaneously virtually wiping lows from the table. Even though it is possible to reduce highs and prevent lows without automation, it takes a lot of work and careful planning; not to mention the huge amount of time you need to invest to monitor your BG, calculate possible adjustments, and then carry out these adjustments. It’s simply unrealistic to expect any human to react to changes in their BG every five minutes like OpenAPS does.

In addition to always being on top of my BG like I could never be, OpenAPS is much better at calculating and admitting precise corrections. While I could theoretically achieve the same level of precision, this would require me to carry a calculator; as well as tediously navigating my way through the temporary basal menu every time my sensor gives me a new value.

Another thing OpenAPS does very well is predicting the effect of past treatments before adjusting its insulin dose recommendations. Where I would simply look at my current BG and go from there, OpenAPS considers how much insulin is still active in my system as well as the trend of my BG (i.e. is it going up or down? and how much?) before doing anything. Sometimes, this behavior makes it look like nothing is happening because it is reducing my basal rate even though my BG is too high. However — looking at the prediction graph — you can see that it expects my values to drop by themselves based on the insulin that is still active in my body.

Even though OpenAPS is designed with security at its core, you can tell it to get on with it and hurry up when you’re done with that high value you’ve been trying to correct for the past two hours. I can tell it to deviate from my regular target (instead aiming for a lower BG), and administer small boluses (called SMB, or supermicrobolus), to get me plummeting down to reasonable levels. The first few times you do this, it’s very scary to see how fast it can make your values drop. However, the system will still operate with the same security considerations it always does. I have never dropped below the configured target while doing this. The ability to do this is locked away by default, and should only be enabled after fully understanding the danger of letting your rig give you so much insulin without manual review. Where the default basal adjustment mode can give me up to 2.4 units per hour, SMB can easily go up to 6 units per hour in my case.

A great advantage of using OpenAPS is its ability to correct very messy situations. I can go to bed with a too high BG after eating two large servings of pasta, and sleep easy — knowing that I will wake up in-range the next morning. This was virtually impossible before using OpenAPS because I would either have to sacrifice my sleep (hence, eliminating the ‘sleep easy’ part of my statement) to check and correct my value during the night, or ignore the values and instead wake up feeling bad from the high BG.

Many people are hesitant about using OpenAPS at night — because you are literally trusting your unconscious body to a machine — and I was too when I first started using it. However, OpenAPS is filled to the brim with safety checks, precautions, and limits, to reduce the risk of giving too much insulin. The OpenAPS Reference Design explains all the safety considerations that are part of OpenAPS. By default, the system runs oref0 (OpenAPS Reference Design Zero). Oref0 can only issue temporary basals below a defined limit. This limit is defined as the lowest value of:

The pump’s maximum temp basal rate

3x the maximum daily scheduled basal rate

4x the current scheduled basal rate

Furthermore, it will turn off basal delivery after the user has given themselves a bolus and wait for enough data before issuing a new temporary basal command, to avoid interfering with the user input.

Some of the safety settings available in OpenAPS

If only adjusting temporary basal rates is not enough, it is possible to enable oref1. Oref1 can deliver boluses, which makes it a very risky but extremely powerful addition to OpenAPS. Fortunately, oref1 contains several additional safety checks to ensure you never get too much insulin. It always checks the pump status before and after calculating the required dose to ensure no insulin was given during this time, and sets a temporary basal of 0 for the duration required to offset the just delivered insulin. So, even if the rig suddenly loses connection to the pump, the user is never in danger of dropping too low.

OpenAPS is completely open-source, and can be found on GitHub.