I didn't reverse-engineer the protocol for my blood pressure monitor in 24 hours

If you've got one of these machines at home it seems you really want to run it a couple of times. Often I find the first reading I get is quite alarming but then if I sit for a bit longer and run it another 1-2 times I get a very normal reading.

Also, they can read high if the batteries are low.

A big reason people's first measurement is often high is the positioning while putting the cuff on, adjusting to the pose, and then settling in place, a process which can spike pressure. Ideally you prepare yourself (cuff on, in place with your arms at rest, etc) and sit there and relax for five minutes before you take the first measurement, making it legitimately reflective of the at-rest state.

This often isn't possible, but it's a reason doctor visits usually drop you in a room by yourself with a machine that will take multiple measurements with a lengthy delay between. Though that's where the white coat thing comes into play and people are stressed about a doctor coming in, etc.

FWIW, some "smarter" machines (e.g., [0, 1]) support taking three readings with a minute pause in-between and then spit out a single reading.

[0] https://www.garmin.com/en-US/p/716808/ [1] https://www.withings.com/us/en/bpm-connect

Actually that is what my doctor told me to do, this seems standard procedure to measure twice every time with 1-2 minutes in between.

The $50 I got from the pharmacy does it too, if the reader doesn't want to spend $100+ on a device.

I wonder why that would be? Presumably if the batteries are low then the pressure the machine "thinks" it's inflated the cuffs to is higher than the actual pressure...