Define the home of pressureDefine and also convert amongst the units of pressure measurementsDescribe the operation of usual tools because that measuring gas pressureCalculate press from manometer data

The earth’s setting exerts a pressure, together does any kind of other gas. Although we do not normally notification atmospheric pressure, we space sensitive to pressure changes—for example, as soon as your ear “pop” during take-off and landing while flying, or once you dive underwater. Gas pressure is resulted in by the pressure exerted through gas molecule colliding v the surface of objects (Figure 1). Return the force of every collision is really small, any kind of surface that appreciable area experience a large number the collisions in a brief time, i m sorry can an outcome in a high pressure. In fact, typical air press is solid enough to crush a steel container once not well balanced by equal pressure from inside the container.

You are watching: Assume the atmospheric pressure is 1 atm


Figure 2. Although (a) one elephant’s load is large, developing a very big force top top the ground, (b) the figure skater exerts a much greater pressure ~ above the ice because of the small surface area of she skates. (credit a: modification of work by Guido da Rozze; credit b: change of work-related by Ryosuke Yagi)

The SI unit of press is the pascal (Pa), with 1 Pa = 1 N/m2, whereby N is the newton, a unit that force characterized as 1 kg m/s2. One pascal is a tiny pressure; in numerous cases, it is an ext convenient come use devices of kilopascal (1 kPa = 1000 Pa) or bar (1 bar = 100,000 Pa). In the united States, press is often measured in pounds of pressure on one area of one square inch—pounds per square inch (psi)—for example, in auto tires. Press can also be measured utilizing the unit atmosphere (atm), which originally represented the mean sea level air press at the approximate latitude that Paris (45°). Table 1 provides some info on these and also a few other common units for push measurements

Table 1. Push UnitsUnit Name and AbbreviationDefinition or relation to various other Unit
pascal (Pa)1 Pa = 1 N/m2

recommended IUPAC unit

kilopascal (kPa)1 kPa = 1000 Pa
pounds every square inch (psi)air press at sea level is ~14.7 psi
atmosphere (atm)1 atm = 101,325 Pa

air push at sea level is ~1 atm

bar (bar, or b)1 bar = 100,000 Pa (exactly)

commonly used in meteorology

millibar (mbar, or mb)1000 mbar = 1 bar
inches that mercury (in. Hg)1 in. Hg = 3386 Pa

used through aviation industry, also some weather reports

torr ext1 torr=frac ext1 ext760 extatm

named after ~ Evangelista Torricelli, inventor that the barometer

millimeters of mercury (mm Hg)1 mm Hg ~1 torr

Example 1

Conversion of push UnitsThe unified States nationwide Weather organization reports press in both inches of Hg and millibars. Convert a pressure of 29.2 in. Hg into:

(a) torr

(b) atm

(c) kPa

(d) mbar


This is a unit switch problem. The relationships in between the miscellaneous pressure systems are given in Table 1.

(a) 29.2cancel extin Hg imes frac ext760 torr29.92cancel extin Hg= ext742 torr

(b) 742cancel exttorr imes frac ext1 atm760cancel exttorr= ext0.976 atm

(c) 742cancel exttorr imes frac ext101.325 kPa760cancel exttorr= ext98.9 kPa

(d) 98.9cancel extkPa imes frac1000cancel extPa1cancel extkPaphantom ule0.4em0ex imes phantom ule0.4em0exfrac1cancel extbar100,000cancel extPaphantom ule0.4em0ex imes phantom ule0.4em0exfrac ext1000 mbar1cancel extbar= ext989 mbar

Check your Learning

A common barometric pressure in Kansas City is 740 torr. What is this press in atmospheres, in millimeters of mercury, in kilopascals, and also in bar?

We deserve to measure atmospheric pressure, the force exerted by the environment on the earth’s surface, with a barometer (Figure 3). A barometer is a glass tube the is closed in ~ one end, filled through a nonvolatile fluid such together mercury, and then inverted and also immersed in a container of that liquid. The setting exerts press on the liquid external the tube, the shaft of fluid exerts push inside the tube, and also the push at the liquid surface ar is the same inside and outside the tube. The elevation of the liquid in the pipe is thus proportional to the press exerted by the atmosphere.


Figure 4. A manometer deserve to be offered to measure the push of a gas. The (difference in) height between the fluid levels (h) is a measure up of the pressure. Mercury is normally used since of its huge density.

Example 3

Calculation of pressure Using a Closed-End Manometer

The pressure of a sample of gas is measured v a closed-end manometer, as displayed to the right. The fluid in the manometer is mercury. Recognize the press of the gas in:

(a) torr

(b) Pa

(c) bar


Example 4

Calculation of push Using an Open-End Manometer

The push of a sample of gas is measured at sea level with an open-end Hg (mercury) manometer, as presented to the right. Determine the pressure of the gas in:

(a) mm Hg

(b) atm

(c) kPa


Measuring Blood Pressure

Blood press is measured utilizing a machine called a sphygmomanometer (Greek sphygmos = “pulse”). It consists of an blow up cuff to restrict blood flow, a manometer to measure up the pressure, and also a technique of determining once blood circulation begins and also when it becomes impeded (Figure 5). Since its invention in 1881, it has been critical medical device. There room many varieties of sphygmomanometers: manual ones that require a stethoscope and also are supplied by clinical professionals; mercury ones, used when the many accuracy is required; less accurate mechanical ones; and also digital ones that deserve to be provided with little training yet that have limitations. When using a sphygmomanometer, the cuff is placed around the top arm and also inflated till blood circulation is completely blocked, then gradually released. As the love beats, blood forced through the arteries causes a increase in pressure. This rise in pressure at which blood flow begins is the systolic pressure—the top pressure in the cardiac cycle. As soon as the cuff’s pressure equates to the arterial systolic pressure, blood flows past the cuff, creating audible sounds that have the right to be heard utilizing a stethoscope. This is complied with by a diminish in press as the heart’s ventricles prepare for another beat. As cuff pressure continues to decrease, at some point sound is no much longer heard; this is the diastolic pressure—the lowest pressure (resting phase) in the cardiac cycle. Blood pressure units indigenous a sphygmomanometer are in regards to millimeters the mercury (mm Hg).


Figure 6. Meteorologists use weather maps come describe and also predict weather. Areas of high (H) and also low (L) push have big effects top top weather conditions. The gray present represent places of consistent pressure known as isobars. (credit: alteration of occupational by national Oceanic and Atmospheric Administration)

In terms of weather, low-pressure systems happen when the earth’s surface ar atmospheric press is lower than the surrounding environment: Moist wait rises and also condenses, creating clouds. Movement of moisture and air within miscellaneous weather fronts instigates most weather events.

The environment is the gaseous layer the surrounds a planet. Earth’s atmosphere, which is around 100–125 kilometres thick, is composed of approximately 78.1% nitrogen and 21.0% oxygen, and also can be subdivided further into the regions presented in number 7: the exosphere (furthest native earth, > 700 km above sea level), the thermosphere (80–700 km), the mesosphere (50–80 km), the stratosphere (second lowest level of ours atmosphere, 12–50 km above sea level), and the troposphere (up come 12 km above sea level, around 80% the the earth’s setting by mass and also the great where many weather occasions originate). As you go higher in the troposphere, wait density and also temperature both decrease.

The press of a sample that gas is measured with an open-end manometer, partially displayed to the right. The liquid in the manometer is mercury. Presume atmospheric pressure is 29.92 in. Hg, determine the pressure of the gas in:torrPabar
The push of a sample the gas is measured in ~ sea level with an open-end mercury manometer. Assuming atmospheric push is 760 mm Hg, determine the pressure of the gas in:mm HgatmkPa
How would certainly the use of a volatile liquid affect the measurement of a gas making use of open-ended manometers vs. Closed-end manometers?

Selected Answers

1. The advanced of a knife that has actually been sharpened has actually a smaller sized surface area than a dull knife. Because pressure is pressure per unit area, a sharp knife will exert a higher pressure through the very same amount of force and also cut through material more effectively.

3. Lying under distributes her weight end a larger surface area, exerting less pressure on the ice compared to standing up. If friend exert less pressure, you are much less likely to break through thin ice.

5. Transform 615 mm Hg to environments using 760 mm Hg = 1 atm. Use 1 atm = 101.325 kPa in the second part. ext615 mm Hg imes frac1 extatm760 extmmHg=0.809 extatm ext0.809 atm imes frac101.325 extkPa1 extatm=82.0 extkPa

7. 32.0cancel extlbcancel extin^ ext-2 imes frac1cancel extatm14.7cancel extlbcancel extin^ ext-2phantom ule0.4em0ex imes phantom ule0.4em0exfrac ext101.325 kPa1cancel extatm=2.2 imes 10^2 extkPa

9. Identify: 14.7 lb in–2 = 1 atm88.8cancel extatm imes frac ext14.7 lb extin^ ext-21cancel extatm=13.1 imes 10^3 extlb extin^ ext-2

11. (a) 29.97cancel extin. Hg imes frac ext101.325 kPa29.92cancel extin. Hg= ext101.5 kPa;

(b) 28.0cancel extin. Hg imes frac ext760 torr29.92cancel extin. Hg= ext711 torr; 762 – 711 = 51 torr drop

13. (a) 26.4cancel extcm imes frac10cancel extmm1cancel extcmphantom ule0.4em0ex imes phantom ule0.4em0exfrac ext1 torr1cancel extmm= ext264 torr;

(b) ext264 torr imes frac101, ext325 Pa ext760 torr=35, ext200 Pa;

(c) 264cancel exttorr imes frac ext1.01325 bar760cancel exttorr= ext0.352 bar

15. The pressure of the gas equals the hydrostatic pressure as result of the pressure of the environment at sea level minus a obelisk of mercury of elevation 13.7 cm. The press on the left is as result of the gas and also the push on the ideal is because of the atmospheric press minus 13.7 cm Hg).

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(a) In mm Hg, this is: 760 mm Hg – 137 mm Hg = 623 mm Hg;

(b) ext623 mm Hg imes frac ext1 atm ext760 mm Hg=0.820 atm;

(c) ext0.820 atm imes frac ext101.325 kPa ext1 atm= ext83.1 kPa

17. With a closed-end manometer, no readjust would it is in observed, because the vaporized liquid would contribute equal, opposing pressure in both eight of the manometer tube. However, v an open-ended manometer, a higher pressure reading of the gas would be derived than expected, because Pgas = Patm + Pvol liquid.