[mcjhn (OP)]

Can you use water instead of mercury in a barometer? If not, why not?

[Geezer]

You are in luck. You can! However, because water is a lot less dense than mercury, the water column has to be about 30 feet tall  []

[mcjhn (OP)]

brilliant thanks a lot!

I was puzzling over that for a while

[syhprum]

Another problem is that water puts out much more vapour than Mercury and contaminates the vacuum at the top making the device temperature sensitive.