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Understanding pressure measurement is crucial in fluid mechanics. There are three main types of pressure: gauge pressure, absolute pressure, and vacuum pressure. Each has its unique application and meaning.

• Gauge pressure measures the pressure difference between the system and the atmospheric pressure. It's often used in everyday applications like tire pressure measurements.
• Absolute pressure, on the other hand, reflects the total pressure exerted on a system, including atmospheric pressure. It's the sum of gauge pressure plus atmospheric pressure.
• Vacuum pressure refers to pressures below the atmospheric level and is thus considered a negative gauge pressure.

Measuring these pressures accurately is essential in many practical scenarios, such as in engineering and meteorology, where precise fluid dynamics calculations are required.

Vacuum pressure is a term used when the pressure in a space is below the atmospheric pressure. This is effectively a negative gauge pressure.
When you hear that something measures '40 kPa vacuum,' it means the pressure inside the system is 40 kPa less than the atmospheric pressure. This is why it's expressed as a negative value in pressure calculations.
Vacuum pressure plays a huge role in systems such as pumps and engines, where below-atmospheric pressures are needed for movement and operation.

Gauge pressure indicates how much pressure is exerted by a fluid compared to the local atmospheric pressure. Since it does not include atmospheric pressure, the gauge pressure can be positive or negative.

• Positive gauge pressure implies that the system's pressure is greater than the surrounding atmosphere.
• Negative gauge pressure, or vacuum pressure, indicates that the system's pressure is less than atmospheric pressure.

For instance, in the context of the original exercise, a gauge pressure of -40 kPa informs us that the pressure inside the pump is 40 kPa less than the atmospheric pressure, hence creating a vacuum situation inside.

Absolute pressure is a comprehensive measure of any pressure system. It represents the true pressure within a system, accounting for both the internal pressure and the atmospheric pressure.
Absolute pressure is particularly significant because it provides the complete picture. Unlike gauge pressure, it can never be negative, as it includes the constant atmospheric baseline.
In the previous exercise, absolute pressure is calculated by adding negative gauge pressure to the local atmospheric pressure, thus: The absolute pressure = atmospheric pressure - vacuum pressure = 100 kPa - 40 kPa = 60 kPa (abs). This value gives us meaningful insight into the actual pressure within the system.