max vacuum on a zero tank

[DS Maple]

Maybe this has been discussed before, but I couldn't find it in the threads if this is the case. Anyway, is there a maximum vacuum pressure that can be run with Zero tanks? I heard a figure of 18 inches of mercury, but was thinking this sounded low.

[MERIDIAN MAPLES]

I've had up to 20 in. on my vacuum gauge on my tanks. I've got (2) 400 gal tanks that I've put up to 20 in. My neighbor tried pumping out his zero tank while he had vacuum on it. That didn't work out for him too well though. He sucked the tank right in!!! Pretty scarey!!