Measuring PressureSPH4C – Unit #4 Hydraulic and Pneumatic Systems

Learning Goals and Success CriteriaToday I will be able to…

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

Static 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 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

Barometers

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.

Barometers

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 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.

Siphons

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.

Manometers

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 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 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?

Gauge pressure example

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

(b) What is the absolute pressure applied?

More Practice

Read: Pg. 239-246Practice Problems: Pressure practice sheet