Units of Pressure

(interestingly, while the ‘Bowie’ isn’t one of them, Mercury is related)

Some Historical PerspectiveRemember my demo in class with the upside-down flask of water? If you make that big enough, you can use it to measure pressure:

This is the world's simplest barometer.

When air pressure goes up, morewater gets pushed in.

When air pressure goes down, waterflows out and the height gets shorter.

vacuum

air pressure

ProblemIf you use water, it needs to be >10 m tall.

Solution: use a liquid that is much more dense...

....mercury!

Now it only needs to be less thanone meter.

Measure the height of the column, andyou have your pressure in units of mmHg

Unit Number OnemmHg: Literally, the height of a column of mercury that the current pressure could support.

Also called 'torr': named after Torricelli, who invented the mercury barometer.

These are the same unit, just with different names.

5232 mmHg = 5232 torr

Some More HistoryThe barometer is good for measuring atmospheric pressure, but what if you want to measure inside a container?

Use a manometer:

One side is attached to your container

The other side is open to the air

If the pressures are the same, the sides are even.

Some More HistoryThe barometer is good for measuring atmospheric pressure, but what if you want to measure inside a container?

Use a manometer:

But if your container has higher pressure,the mercury gets pushed down.

The difference in height tells you how much higher the container is than the atmosphere.

Unit Number TwoAtmospheres (atm): because sometimes it's nice to just have atmospheric pressure have a value of one.

(since atmospheric pressure varies from day to day and with elevation, your atmospheric pressure is not guaranteed to be 1 atm, but it will be close)

1 atm = 760 mmHg

Unit Number ThreePascals (Pa): METRIC/ SI!!

Absolutely necessary for physics to make units match up for equations ( 1 Pa = 1 kg/m*s2)

1 atm = 101,300 Pa

Unit Number Fourbar: Actually just a 100,000 Pa. Used as a replacement for atm because 1 bar is approximately atmospheric pressure. But generally a little bit lower.

1 bar = 750 mmHg = 0.986 atm

You will also see millibar used (mbar). Which is actually just 100 Pascals.

Unit Number FivePounds per square inch (psi): Don't use this except in your tires.

1 atm = 14.7 psi

Some Really Obscure OnesMeters of Sea Water: used by divers.

Barye: used by people who still measure volume in hogsheads.

Technical atmosphere (at): used by people who hate all that is good and right in the world.

Feet of water (ftH2O): now they're just mocking me.

Which Ones Will We Use?mmHg: still widely used for historical reasons

atm: convenient for relating to something all around you.

Pa: metric/ SI

Summary• Lots of units of pressure.

• Many of them are crazy.

• You should recognize bar, torr, and psi, but we will not use them.