In a mechanical room with no bars
What works
Readings you enter are held on the phone. Lose signal halfway through a test and the numbers keep going in. When you are back in range they sync once, automatically, with no duplicates — a half-entered test is never thrown away.
The app opens with no signal. Once you have used it on a phone, it opens on that phone again with the radio off, and any screen you have already visited since our last update comes back — your due list, a site, a test you opened this morning.
What doesn't work
A screen you have never opened on that phone is not there. It was never downloaded, so you get this page instead of it. We will not show you an empty test form with nothing behind it — a form you could start filling in against the wrong device is worse than a closed door.
The rest of this website does not work offline, on purpose. Prices and the pages about what we support change, and serving you a cached one from three weeks ago would be its own kind of wrong. Offline is about the app.
Why we are telling you this
Because it is the thing you would otherwise discover in a basement. The market leader's own customers raise the same limitation in public reviews — testers who say they still carry paper and a pen because the database is unreachable without cell service.
We are describing a limitation we share, not scoring a point. Ours is narrower — the app opens and readings survive — but a screen you have never opened is still a screen you have never opened.
What this means on a job
Open your due list once before you go down, and the sites on it come with you. Everything after that is local: pick the device, enter the readings, finish the test. The report renders and sends when you have signal again, and the deadline was already calculated from the notice date, not from when the phone reconnected.