Understanding your preferences and safety settings

All of the settings specific to OpenAPS (that can’t be read from the pump) will live in this file, so when running the setup scripts or building your loop, you will have the preferences.json file built for the system to read, in addition to your pump profile settings. Many of these are important safety settings, with reasonable default settings, so other than described below, you likely won’t need to adjust these. If you do decide to adjust a setting, the best practice is to adjust one setting at a time, and observe the impact for 3 days. Changing multiple variables at once is a recipe for a lot of headaches and a lot of painful troubleshooting.

(Note that there are some preferences that show up by default; these are the most commonly adjusted. There are additional preferences available to set that are not used by everyone, and are described below - any of these can also be added to the preferences.json)

Editing your preferences.json

Your preferences are found in the directory myopenaps/preferences.json. To edit any of your preferences, you can enter edit-pref (as a shortcut) or cd ~/myopenaps && nano preferences.json

To check your edits when you’re done, use cd ~/myopenaps && cat preferences.json

Commonly-adjusted preferences:

        "max_iob": 0,
        "max_daily_safety_multiplier": 3,
        "current_basal_safety_multiplier": 4,
        "autosens_max": 1.2,
        "autosens_min": 0.7,
        "rewind_resets_autosens": true,
        "adv_target_adjustments": true,
        "unsuspend_if_no_temp": false,
        "enableSMB_with_bolus": false,
        "enableSMB_with_COB": false,
        "enableSMB_with_temptarget": false,
        "enableUAM": false


max_iob is an important safety setting for your OpenAPS set up. max_iob is the maximum amount of basal (or SMB correction) insulin that your loop is allowed to accumulate to treat higher-than-target BG. Unlike the other two OpenAPS safety settings (max_daily_safety_multiplier and current_basal_safety_multiplier), max_iob is set as a fixed number of units of insulin.

Although max_iob is set as a fixed number of units of insulin, you should consider your current basal rate settings when setting this safety parameter. A good rule of thumb is for max_iob to be no more than 3 times your highest basal rate. You can start conservatively and change this setting over time as you evaluate how the OpenAPS system works for you.

When you run the OpenAPS setup script, it will prompt you to set your max_iob. In previous oref0 releases (0.4.3 or older), the set up script automatically set max_iob to 0 units. This effectively made your initial OpenAPS installation only capable of setting temp basal rates in response to BG levels that were below your target BG levels. (And if your BG level is sufficiently below your target BG level, OpenAPS will set a 30 min. temporary basal rate of 0u/hr., which is often referred to as a “low glucose suspend”.) Again, you can start conservatively and change this setting over time as you evaluate how the OpenAPS system works for you.

The setting you choose during the setup script will be saved in the oref0-runagain script and can be used again if you need to rerun the script.


This is an important OpenAPS safety limit. The default setting (which is unlikely to need adjusting) is 3. This means that OpenAPS will never be allowed to set a temporary basal rate that is more than 3x the highest hourly basal rate programmed in a user’s pump.


This is another important OpenAPS safety limit. The default setting (which is also unlikely to need adjusting) is 4. This means that OpenAPS will never be allowed to set a temporary basal rate that is more than 4x the current hourly basal rate programmed in a user’s pump.

Important Note About Safety Multipliers:

max_daily_safety_multiplier and current_basal_safety_multiplier work together, along with your pump’s max basal rate safety setting (set on your pump), as a safety limits.

OpenAPS will use whichever of those three values is the lowest, at any given time, as the ceiling for the temp basal rate it will set.**

A few examples:

Example safety cap image - see raw file in the same folder of docs if needs editing

  • In Example 1, the user’s max basal safety setting is the constraining limit on the OpenAPS recommended temp basal rate.
  • In Example 2, 4x the user’s current basal rate is the constraining limit on the OpenAPS recommended temp basal rate.
  • In Example 3, the user’s current basal rate is at his/her highest programmed rate, but none of the safety constraints are binding; the OpenAPS recommended temp basal rate is delivered.
  • In Example 4, 3x the user’s highest programmed basal rates is the constraining limit on the OpenAPS recommended temp basal rate.

If the temporary basal rate setting recommended by OpenAPS (as determined in oref0/lib/determine-basal/determine-basal.js) exceeds either:

  • the user’s max basal rate setting (which is set in the user’s pump), or
  • max_daily_safety_multiplier * the highest programmed basal rate (as specified by the basal rates in the user’s pump), or
  • current_basal_safety_multiplier * the user’s current basal rate (as specified by the current basal rate programmed in the user’s pump), then

the following message will be reported to the pump-loop.log:

   adj. req. rate: X.X to maxSafeBasal: Y.Y

You can also view this message in the Nightscout OpenAPS pill (which pops up a detailed message about recent OpenAPS activity if you hover your mouse over the OpenAPS pill):

max safe basal message


This is a multiplier cap for autosens (and autotune) to set a 20% max limit on how high the autosens ratio can be, which in turn determines how high autosens can adjust basals, how low it can adjust ISF, and how low it can set the BG target.


The other side of the autosens safety limits, putting a cap on how low autosens can adjust basals, and how high it can adjust ISF and BG targets.


This feature, enabled by default, resets the autosens ratio to neutral when you rewind your pump, on the assumption that this corresponds to a probable site change. Autosens will begin learning sensitivity anew from the time of the rewind, which may take up to 6 hours. If you usually rewind your pump independently of site changes, you may want to consider disabling this feature.


This feature, enabled by default, lowers oref0’s target BG automatically when current BG and eventualBG are high. This helps prevent and mitigate high BG, but automatically switches to low-temping to ensure that BG comes down smoothly toward your actual target. If you find this behavior too aggressive, you can disable this feature. If you do so, please let us know so we can better understand what settings work best for everyone.


Many people occasionally forget to resume / unsuspend their pump after reconnecting it. If you’re one of them, and you are willing to reliably set a zero temp basal whenever suspending and disconnecting your pump, this feature has your back. If enabled, it will automatically resume / unsuspend the pump if you forget to do so before your zero temp expires. As long as the zero temp is still running, it will leave the pump suspended.

Advanced oref1 preferences:

These preference should not be enabled until you’ve been looping (and running autotune) for several weeks and are confident that all of your basals and ratios are correct. Please read the oref1 section of the docs before doing so.


This enables supermicrobolus for DIA hours after a manual bolus.


This enables supermicrobolus (SMB) while carbs on board (COB) is positive.


This enables supermicrobolus (SMB) with eating soon / low temp targets. With this feature enabled, any temporary target below 100mg/dL, such as a temp target of 99 (or 80, the typical eating soon target) will enable SMB.


This enables detection of unannounced meal (UAM) carb absorption.

Other preferences:

Generally, you won’t need to adjust any of the preferences below. But if you do wish to change the default behavior, you can add these into your preferences.json to do so (or use oref0-get-profile –updatePreferences to get the full list of all preferences added to your preferences.json).


This is used to allow autosens to adjust BG targets, in addition to ISF and basals.


Defaults to false, but can be turned on if you have a situation where you want someone (a school caregiver, for example) to use the bolus wizard for meal boluses. If set to “True”, then the bolus wizard will calculate boluses with the high end of the BG target, but OpenAPS will target the low end of that range. So if you have a target range of 100-120; and set this to true; bolus wizard will adjust to 120 and the loop will target 100. If you have this on, you probably also want a wide range target, rather than a narrow (i.e. 100-100) target.


Defaults to false, so that OpenAPS will set temps whenever it can, so it will be easier to see if the system is working, even when you are offline. This means OpenAPS will set a “neutral” temp (same as your default basal) if no adjustments are needed. If you are a light sleeper and the “on the hour” buzzing or beeping wakes you up (even in vibrate mode), you may want to turn this to “true” to skip this setting. However, we recommend it for most people who will be using this system on the go and out of constant connectivity.


Bolus snooze is enacted after you do a meal bolus, so the loop won’t counteract with low temps when you’ve just eaten. The example here and default is 2; so a 3 hour DIA means that bolus snooze will be gradually phased out over 1.5 hours (3DIA/2).


This is a setting for default carb absorption impact per 5 minutes. The default is an expected 8 mg/dL/5min. This affects how fast COB is decayed in situations when carb absorption is not visible in BG deviations. The default of 8 mg/dL/5min corresponds to a minimum carb absorption rate of 24g/hr at a CSF of 4 mg/dL/g.


This is another safety setting that may be useful for those with secondary caregivers who aren’t dedicated to looking up net IOB and being aware of the status of the closed loop system. The default is 1 (i.e. do not adjust the carb ratio; off). However, in the secondary caregiver situation you may want to set a higher carb ratio to reduce the size of a manual bolus given at any time. With this ratio set to 1.1, for example, the loop would multiple the carb inputs by 10%, and use that number to calculate additional insulin. This can also be used by OpenAPS users who rely on the bolus wizard to calculate their meal bolus, but who want to only bolus for a fraction of the meal, and allow advanced meal assist to high-temp for the rest.


This defaults maxCOB to 120 because that’s the most a typical body can absorb over 4 hours. (If someone enters more carbs or stacks more; OpenAPS will just truncate dosing based on 120. Essentially, this just limits AMA as a safety cap against weird COB calculations due to fluky data.)


This is the amount of the maximum number of carbs we’ll assume will absorb over 4h if we don’t yet see carb absorption.


This is the fraction of carbs we’ll assume will absorb over 4h if we don’t yet see carb absorption.


The default of 0.5 for this value keeps autotune ISF closer to pump ISF via a weighted average of fullNewISF and pumpISF. 1.0 allows full adjustment, 0 is no adjustment from pump ISF.