Express pressure of 225 kPa (abs) as a gauge pressure. The local atmospheric pressure is 101 kPa. 2. Express a pressure of -6.2 psig as an absolute pressure. Use the standard atmospheric pressure. 3. Convert the following: a. 2 bar to mmHg b. 30 kPa to psi c. 550 mmHg to kPa
The Correct Answer and Explanation is:
Answers:
- Gauge Pressure:
Gauge Pressure=Absolute Pressure−Atmospheric Pressure\text{Gauge Pressure} = \text{Absolute Pressure} – \text{Atmospheric Pressure}Gauge Pressure=Absolute Pressure−Atmospheric PressureGauge Pressure=225 kPa−101 kPa=124 kPa (gauge)\text{Gauge Pressure} = 225\ \text{kPa} – 101\ \text{kPa} = \boxed{124\ \text{kPa (gauge)}}Gauge Pressure=225 kPa−101 kPa=124 kPa (gauge)
- Absolute Pressure from psig:
Standard atmospheric pressure = 14.7 psiAbsolute Pressure=Gauge Pressure+Atmospheric Pressure\text{Absolute Pressure} = \text{Gauge Pressure} + \text{Atmospheric Pressure}Absolute Pressure=Gauge Pressure+Atmospheric PressureAbsolute Pressure=−6.2 psig+14.7 psi=8.5 psia\text{Absolute Pressure} = -6.2\ \text{psig} + 14.7\ \text{psi} = \boxed{8.5\ \text{psia}}Absolute Pressure=−6.2 psig+14.7 psi=8.5 psia
- Conversions:
a. 2 bar to mmHg
1 bar = 750.06 mmHg2 bar=2×750.06=1500.12 mmHg2\ \text{bar} = 2 \times 750.06 = \boxed{1500.12\ \text{mmHg}}2 bar=2×750.06=1500.12 mmHg
b. 30 kPa to psi
1 kPa = 0.145038 psi30 kPa=30×0.145038=4.35114 psi30\ \text{kPa} = 30 \times 0.145038 = \boxed{4.35114\ \text{psi}}30 kPa=30×0.145038=4.35114 psi
c. 550 mmHg to kPa
1 mmHg = 0.133322 kPa550 mmHg=550×0.133322=73.3271 kPa550\ \text{mmHg} = 550 \times 0.133322 = \boxed{73.3271\ \text{kPa}}550 mmHg=550×0.133322=73.3271 kPa
Explanation
Pressure can be expressed in different forms depending on reference points. Absolute pressure is measured relative to a perfect vacuum, while gauge pressure is measured relative to atmospheric pressure. If the pressure is above atmospheric, gauge pressure is positive. If it is below, gauge pressure becomes negative and is called a vacuum pressure.
To convert absolute pressure to gauge pressure, subtract the atmospheric pressure from the absolute pressure. This removes the influence of the surrounding air. In the first case, an absolute pressure of 225 kPa is measured, and the local atmospheric pressure is 101 kPa. Subtracting gives a gauge pressure of 124 kPa.
In the second case, pressure is already given as a negative gauge pressure, indicating a vacuum. To find the absolute pressure, atmospheric pressure is added to this negative gauge pressure. Standard atmospheric pressure is 14.7 psi, and the gauge pressure is -6.2 psi. Their sum gives an absolute pressure of 8.5 psia.
Pressure units can be converted using fixed conversion factors. Bar is a metric unit of pressure, often used in weather and scuba diving contexts. 1 bar equals 750.06 mmHg, the unit commonly used in measuring blood pressure. For the conversion of 2 bar, multiplying by this factor yields 1500.12 mmHg.
For kPa to psi, the factor is 0.145038. This is used in industrial applications where pressure is needed in psi (pounds per square inch). Lastly, converting 550 mmHg to kPa involves multiplying by 0.133322, useful in scientific fields such as chemistry and physiology where pressure needs expression in SI units.
