Where is it regulated that the PSI in the air lines has to be visible?

by Martin Lackie

In other words, is it a requirement that a tag or warning sign is required that shows the line pressure?

I have looked in OSHA and found only references to just line pressure.

I know some agency somewhere has regulated it to have warning signs posted on how much is carried in the line.

I did a walk through inspection and found several lines not marked. I told the shop person they needed to mark the PSI and they asked me where it is written. I figured that's a good question.

Any help would be appreciated.
Thanks
Martin