This one’s really simple. The current system is a bad one because if the email password changes or internet connectivity gets interrupted or the system sending the email dies (eg: power supply failure), the system fails because on any lack of signal it’s assumed that everything’s good. That’s a terrible design. Here’s a better one:

- External system polls the monitor and records levels.

- Local system reaches out to external system and fetches status.

- Someone has the responsibility to do rounds and record the status indicated by the local system.

External system down or indicates unacceptable levels, monitor unreachable, etc.? Local system shows a failure. Investigate. Local system down? Investigate.

You can even add redundancy by having a manager double check that the responsible employee is checking off the status plus the manager can do a daily sweep as a fallback. Throw something like PagerDuty into the mix and have multi-channel notifications on any failure that escalate over time.

How about an audible alarm and a flashing light at the dangerous location?

This seems like a solved problem, e.g. for smoke detectors.

Phones are good. So are loud, annoying alarms as well as labeled, illuminated alert lights in the potentially-hazardous area.

The alarms and lights are easy to test, too. Make pressing the self-test button part of the nightly or weekly rounds.

The notification does not need to be out-of-band.

It can be very much in-band, like a fire alarm primarily also is.

Do not design anything. Buy an already designed and approved horn with a flashing light.