# A manometer/atm problem...how do you get the atm?

"an open-ended manometer is carried up in a hot-air balloon from sea-level to an altitude of 5 km. calculate the air pressure, in atm, at this new altitude if the mercury level has risen 35 mm in the open-end side of the tube. show all you caculations. (discount any changes in temperature.)"

1 atm = 760 mm Hg = 101.3 kPa = 14.69 psi

(Change in pressure) is proportional to (change in measured volume)

If the actual change in the manometer is 35 mm (which is a little hard to believe, when you've traveled 5 km upward!)

(760 - 35)/(760) = 0.9539 atm = 96.63 kPa = 14.01 psi

If the actual change is 350 mm = 35 cm (which is a more realistic value)

(760 - 350)/(760) = 0.5395 atm = 54.64 kPa = 7.92 psi
Something is wrong in your question.
At a height of 5km, the mercury level should have changed by 350mm (not 35mm), or do you mean 35cm (not mm)?

Start with the fact that at sea level the atm = 1 and the mercury stands at 760mm.
At the specified height (5km), the mercury will have risen by a certain percentage. Calculate that percentage.
The atm will then have fallen by the same percentage.
You think you can do that by yourself.?
(you should end up with something like 0.53atm)
I can't see how your manometer is working.
The open end should be open to the atmosphere at ground level and the other end connected to the pressure you're measuring in order to get the pressure difference. ??

If. at ground level, you connect the 'open' end to an empty cylinder with normal atmospheric pressure in it, then take it up in the balloon with the other end open to the outside atmosphere, it might work to give you the true pressure difference.
(The cylinder should be insulated against the colder atmosphere found at higher altitudes to prevent pressure-drop due to cooling).

In fact, on your way up, you should be able to read off the pressure change for each 1,000 feet or metres of height above ground.

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