0 is supposed to be 1ATM IIRC, and the MMHG is how far below one ATM the pressure is. I'm pretty sure is based on how much murcury expands under pressures that low, it might be something else, but its based around mercury obviously. 1ATM=760MMG, its around 30 inches if I converted it correctly.
EDIT: my ISP is getting really slow right now, it took like 15 minutes for the post to get posted after I clicked the post link.
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