Arduino (Counter as a Dead man Switch)

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I add a small code that say "if flowRate" is higher that 20 open a LED and it's working.

How do you open an LED? Are you able to close it again?

I need to add a "dead man switch" this DMS will monitor the purging that append every 6 minutes.

Switches don't monitor anything. They get pressed. They get released. What, EXACTLY, is the relationship between the switch and the purging function?

But I also want to monitor the increasing PSI and if the PSI in not increasing for X PSI per second

You mean "by X PSI per second"?