Measuring Pressure: Measuring Pressure: Student Success CriteriaStudent Success CriteriaI can conduct an investigation to

identify factors that affect the static pressure head in fluids, compare theoretical and empirical values, and account for discrepancies.

Measuring PressureMeasuring PressureSPH4C

Static Pressure HeadStatic Pressure Head

For any point in a static fluid, the height of the column above that point is called the static pressure head.

A dam must be thicker or stronger at greater depths to withstand the increased pressure.

Static Pressure HeadStatic Pressure Head

The formula for the pressure exerted is:

Where D is the density of the fluid,h is the height, and g = 9.8 m/s2.

Dhg

A

ghAD

A

gVD

A

mg

A

Fp

BarometersBarometers

In mercury barometers, it is the static pressure head that indicates the external air pressure.

The higher the atmospheric pressure, the higher the static pressure head.

BarometersBarometers

In mercury barometers, it is the static pressure head that indicates the external air pressure.

The higher the atmospheric pressure, the higher the static pressure head.

Question: Why do we use mercury in barometers instead of water?

Water LevelWater Level

Liquid in connected containers exposed to the same air pressure will be at the same height.

The shape and orientation of the containers makes no difference to the height.

SiphonsSiphons

This is often phrased as “water seeks its own level,” and is why water will even flow uphill in a siphon if it can reach a lower point at the end of the siphon.

Note that the siphon needs to be full of liquid to connect the two containers.

ManometersManometers

Manometers can be used to measure variations in pressure.

When more pressure is applied to the left side of the tube, there will be a difference in the heights of the liquid.

Gauge pressureGauge pressure

This variation in pressure from atmospheric pressure is called the gauge pressure:

atmabsg ppp

pressurecatmospheripressureabsolutepressuregauge

Tire pressure gauges measure gauge pressure: the pressure over and above atmospheric pressure.

Gauge pressure exampleGauge pressure example

Pressure is applied to one end of a water manometer so that the difference between the two heights is 10 cm.

(a) What is the gauge pressure applied?

(b) What is the absolute pressure applied?

Gauge pressure exampleGauge pressure example

(a) What is the gauge pressure applied?

2

3

8.9

10.0

1000

sm

m

kg

g

mh

D

Gauge pressure exampleGauge pressure example

(a) What is the gauge pressure applied?

2

3

8.9

10.0

1000

sm

m

kg

g

mh

D

Gauge pressure exampleGauge pressure example

(a) What is the gauge pressure applied?

2

3

8.9

10.0

1000

sm

m

kg

g

mh

D

Gauge pressure exampleGauge pressure example

(a) What is the gauge pressure applied?

2

3

8.9

10.0

1000

sm

m

kg

g

mh

D

kPaor

PamDhgpsm

m

kg

980.0

980)8.9)(10.0)(1000( 23

Gauge pressure exampleGauge pressure example

(b) What is the absolute pressure applied?

kPa

kPakPa

ppp atmgabs

28.102

3.101980.0

More PracticeMore Practice

Static Pressure Head Lab Activity