If a scan of your monitor came back with a wrong digit, nothing is broken — you have met the hardest problem in reading device displays. This post explains it honestly, because you should know what the tool can and cannot promise.
Why are blood pressure monitor displays hard to read by software?
Monitor displays use seven-segment digits: seven bars that switch on and off to form a number. There is no font, no letter shapes, no spacing rules — just bright bars on a grey field. Software trained on printed and handwritten text has very little to hold on to. A 3 with a faint bottom bar looks like a 7; an 8 with a weak middle looks like a 0.
Purpose-built readers for these displays reach roughly 96% accuracy in real-world photos. General-purpose vision models are noticeably worse and less consistent. That is the state of the art, not a shortcoming of one app.
Does that mean the numbers in my diary might be wrong?
Not if you check them. Monitor2Log never saves a scanned number by itself: every value is shown to you for confirmation, and nothing is stored until you confirm it. Fields the scanner was unsure about are marked Check this one so you look there first.
This is deliberate. A promise of "99% accurate" is unverifiable across every monitor, room and camera. A promise of "you check every number before it is saved" is one we can keep every time.
Which conditions make a scan fail?
In rough order of how often they cause trouble:
- Glare across the display, usually from a window or a ceiling light behind you.
- Angle — photographing from the side makes segments overlap.
- Low battery contrast, when the display is dim and grey-on-grey.
- Motion blur from photographing before the reading has settled.
- Reflections of the phone itself in a glossy screen.
Handwriting, counter-intuitively, is often easier to read than a display, because letters and digits written by hand still have shapes a model recognises.
How do I photograph a monitor so it reads correctly?
- Hold the phone straight on, directly above or in front of the display.
- Fill the frame with the display, but keep the whole number visible with a little margin.
- Turn off the flash — it creates the glare you are trying to avoid.
- If there is glare, tilt the monitor a few degrees, not the phone.
- Wait for the reading to stop changing before you shoot.
Should I scan the monitor or my notebook?
If you already keep a paper log, scan the notebook page. One photo yields a week or a month of readings, handwriting reads well, and you review the whole batch at once — including any row the scanner flagged as uncertain. Scanning the monitor is best for a single reading you want to log right away.
Either way, the last step is the same: compare each number with what you can see, and confirm it yourself.