Imperial/US or Metric
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The default value for the system of measurement is determined the first time you run the application based on your regional settings (set in the iPhone’s Settings Application under General->International->Region Format). Any changes you make to this setting within Trimix Tools will persist for future invocations of Trimix Tools. For example, if you live in Canada but dive with Imperial/US equipment, just change this setting within Trimix Tools and it will stay set until you change it again.
You can rearrange the items on the tab bar from the “More” tab. Just select the “Edit” button and drag the tools into the order that you prefer. It might be preferable to have the Settings tab handy if you frequently change systems of measurement.