It depends upon the circumstances. Millimetres of water is a much more convenient scale for, say, air handling, where differential pressures are quite low. Millimetres of mercury is more convenient for, say, measuring high vacuum in pharmaceutical primary manufacture.
So the short answer is that it depends upon what 'we' are doing, and 'we' haven't stated what it is.
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
And guess what is on the desk beside the monitor right now?
(usual disclaimer)
Ah! That's better.
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"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Hmmmm. How strange. It's visible in the northern hemisphere (Reading, UK, to be exact). Ah! [Downloads the graphic to My Documents and links from there] Try again:
__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
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Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.