1. SPH4C – Unit #4 Hydraulic and Pneumatic Systems
Measuring Pressure
SPH4C – Unit #4 Hydraulic and Pneumatic Systems

2. Learning Goals and Success Criteria
Today I will be able to…
Identify factors that affect the static pressure head in fluids, compare theoretical and empirical values, and account for discrepancies.

3. 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.

4. 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.

5. 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.

6. 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?

7. 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.

8. 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.

9. 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.

10. Gauge pressure
This variation in pressure from atmospheric pressure is called the gauge pressure:
Tire pressure gauges measure gauge pressure: the pressure over and above atmospheric pressure.

11. 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?
P = 980 Pa or 0.98 kPa

12. 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?
Pabs = kPa

13. More Practice
Read: Pg Practice Problems: Pressure practice sheet