03. MEDIDAS PRESIÓN. Parte 1/4

Summary

The video introduces pressure measurement in industrial processes, covering pressure units, pressure references, differential pressure, hydrostatic pressure, atmospheric pressure, pressure losses, and fluid behavior in pipe restrictions.

1. What pressure is and why it is measured

• Pressure is force applied over an area. In the International System of Units, it is measured in pascals: 1 Pa = 1 N/m².
• Industrial pressure instruments may display different units, so technicians need to recognize common units and their approximate conversions.
• Pressure measurements can also help determine other process variables indirectly, including tank level, flow rate, filter clogging, and—in more complex systems—density.
• Differential pressure across a filter, for example, can indicate how clogged it is. Differential pressure can also be used to infer flow or liquid level.

2. Pressure types and instrument connections

The pressure value depends on the reference used:

• Absolute pressure is measured from a perfect vacuum.
• Gauge (relative) pressure is measured from atmospheric pressure, which serves as zero for most ordinary pressure gauges.
• Vacuum pressure describes pressure below atmospheric pressure; it is often shown as a negative gauge pressure.
• Differential pressure is the difference between pressures at two points.

A differential-pressure transmitter can be connected in different ways:

• For absolute pressure, one port is sealed under vacuum and the other is connected to the process.
• For gauge pressure, one port is open to the atmosphere and the other is connected to the process.
• For a vacuum reading displayed as a positive value, the ports are swapped so the process vacuum reaches the low-pressure port.
• For differential pressure, both ports connect to separate process points, such as opposite sides of an orifice plate.

The presenter notes that differential-pressure transmitters used this way may have limits on the absolute pressures they can measure.

3. Hydrostatic pressure and level measurement

• Pressure in a liquid depends on the height of liquid above the measurement point and on the liquid’s density.
• The relationship is expressed as hydrostatic pressure = density × gravity × height: (p = \rho g h).
• At the same depth, a denser liquid exerts more pressure. The video compares water and kerosene to illustrate this.
• A differential-pressure transmitter can measure the pressure at the bottom of a tank relative to a reference pressure. If the liquid’s density is known or compensated for, the pressure difference can be used to determine the liquid level.

4. Atmospheric pressure, boiling, and pump cavitation

• Atmospheric pressure is the pressure exerted by the column of air above us. It varies with altitude and weather.
• At higher altitudes, lower atmospheric pressure means water boils at a lower temperature.
• If pressure falls sufficiently at a pump’s suction while the temperature remains high enough, some liquid can vaporize. When the vapor enters the pump and pressure rises, it condenses again, creating damaging pressure shocks.
• This phenomenon is called cavitation. Low-pressure switches on a centrifugal pump’s suction can help protect it by detecting an excessive pressure drop.
• The video gives approximate atmospheric-pressure references, including 1 atmosphere ≈ 101.3 kPa, along with other rounded values.

5. Pressure drop in pipes

Moving fluid loses energy through friction, which appears as a pressure drop. The presenter identifies these influences:

• Longer pipes produce greater pressure loss.
• Larger pipe sections generally reduce friction and pressure loss for the same flow.
• Higher flow rates increase friction and pressure loss.
• More viscous fluids produce greater friction and pressure loss. Heating fuel can reduce its viscosity and make it easier to pump.

6. Restrictions, Bernoulli’s principle, and Venturi applications

• At a narrowing in a pipe, fluid speed increases and pressure decreases, as described by Bernoulli’s principle.
• This pressure reduction can be used to draw in or mix another fluid. The video gives examples including foam-agent injection in fire-protection systems and the air–fuel mixing function of a car carburetor.
• Some pressure recovers after the narrowing, but not all of it, because friction and other losses remain.

7. Common pressure units

The video highlights units commonly encountered in industrial settings: bar, kg/cm², psi, centimetres of mercury, and kilopascals. The presenter offers rounded conversions for practical orientation:

• 1 bar ≈ 1 kg/cm²
• 1 bar ≈ 14.5 psi
• 1 bar ≈ 76 cm of mercury
• 1 bar ≈ 100 kPa

These are approximate values intended for quick comparison.

Speakers and sources featured

• Presenter/instructor: One speaker explains the material; the subtitles do not name the presenter. The video metadata attributes the channel and video to Electrocontrol / Jose Ignacio Lorenzo.
• Named concepts referenced: Bernoulli’s principle and the Venturi effect. No additional speakers are heard in the subtitles.