millimeters of mercury (mmHg) to pascals (Pa) conversion

What is millimeters of mercury?

Millimeters of mercury (mmHg) is a unit of pressure, often used in medicine (especially blood pressure) and meteorology. It represents the pressure exerted by a column of mercury one millimeter high at a standard temperature. Let's delve into its definition, history, and applications.

Definition and Formation

Millimeters of mercury (mmHg) is a manometric unit of pressure. Specifically, it's the pressure exerted at the base of a column of mercury exactly 1 millimeter high when the density of mercury is 13,595.1 kg/m³ and the local acceleration of gravity is exactly 9.80665 m/s². It's not an SI unit, but it is accepted for use with the SI.

While not an official SI unit (Pascal is the SI unit for pressure), mmHg remains widely used due to its historical significance and practical applications, especially in fields like medicine.

History and Torricelli's Experiment

The unit originates from Evangelista Torricelli's experiments in the 17th century. Torricelli, an Italian physicist and mathematician, invented the mercury barometer in 1643. He filled a glass tube with mercury and inverted it into a dish of mercury. The mercury column would fall, leaving a vacuum at the top, and the height of the column was proportional to the atmospheric pressure. This led to the standardized measurement of pressure using the height of a mercury column. Read more about it in Britannica.

Relation to Other Units

• Pascal (Pa): The SI unit of pressure. 1 mmHg is approximately equal to 133.322 Pascals.

  $$1 \, mmHg \approx 133.322 \, Pa$$

• Atmosphere (atm): A standard unit of pressure. 1 atm is equal to 760 mmHg.

  $$1 \, atm = 760 \, mmHg$$

• Torr: Named after Torricelli, 1 Torr is very close to 1 mmHg. For most practical purposes, they are considered equivalent.

  $$1 \, Torr \approx 1 \, mmHg$$

Real-World Examples and Applications

• Blood Pressure: In medicine, blood pressure is commonly measured in mmHg. For example, a blood pressure reading of 120/80 mmHg indicates a systolic pressure of 120 mmHg and a diastolic pressure of 80 mmHg. The first number represents the pressure in the arteries when the heart beats (systolic pressure) and the second number represents the pressure in the arteries between beats (diastolic pressure).

• Atmospheric Pressure: Meteorologists often use mmHg to report atmospheric pressure. Standard atmospheric pressure at sea level is 760 mmHg. Changes in atmospheric pressure are often precursors to changes in weather.

• Vacuum Gauges: Many vacuum gauges, particularly older or specialized instruments, display pressure in mmHg. Low pressures in vacuum systems, such as those used in scientific experiments or manufacturing processes, are often expressed in mmHg or fractions thereof (e.g., milliTorr, which is approximately 1/1000 of a mmHg).

• Aircraft Altimeters: Aircraft altimeters use atmospheric pressure to determine altitude. While the actual scale on the altimeter might be in feet or meters, the underlying pressure measurement is often related to mmHg.

Important Considerations

While mmHg is widely used, it's essential to be aware of its limitations:

• Temperature Dependence: The density of mercury varies with temperature, so precise measurements require temperature corrections.
• Local Gravity: Although standardized, the local acceleration due to gravity can vary slightly depending on location, potentially affecting accuracy.

What is pascals?

Pascal (Pa) is the SI unit of pressure, defined as the force of one newton acting on an area of one square meter. This section will delve into the definition, formation, historical context, and practical applications of Pascal.

Pascal Definition

The pascal (Pa) is the SI derived unit of pressure used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. It is defined as one newton per square meter.

$$1 \ Pa = 1 \frac{N}{m^2}$$

It can also be described using SI base units:

$$1 \ Pa = 1 \frac{kg}{m \cdot s^2}$$

Formation of Pascal

Pascal as a unit is derived from the fundamental units of mass (kilogram), length (meter), and time (second). Pressure, in general, is defined as force per unit area.

• Force: Measured in Newtons (N), which itself is defined as $kg \cdot m/s^2$ (from Newton's second law, $F=ma$).
• Area: Measured in square meters ($m^2$).

Thus, Pascal combines these: $N/m^2$ which translates to $(kg \cdot m/s^2) / m^2 = kg/(m \cdot s^2)$.

Blaise Pascal and Pascal's Law

The unit is named after Blaise Pascal (1623-1662), a French mathematician, physicist, inventor, writer, and Catholic theologian. He made significant contributions to the fields of mathematics, physics, and early computing.

Pascal's Law (or Pascal's Principle) states that a pressure change occurring anywhere in a confined incompressible fluid is transmitted throughout the fluid such that the same change occurs everywhere.

Mathematically, this is often represented as:

$\Delta P = \rho g \Delta h$

Where:

• $\Delta P$ is the hydrostatic pressure difference
• $\rho$ is the fluid density
• $g$ is the acceleration due to gravity
• $\Delta h$ is the height difference of the fluid

For further reading about Pascal's Law, you can refer to Pascal's Law and Hydraulics.

Real-World Examples

Here are some examples of pressure measured in Pascals or related units (like kilopascals, kPa):

• Atmospheric Pressure: Standard atmospheric pressure at sea level is approximately 101,325 Pa, or 101.325 kPa.
• Tire Pressure: Car tire pressure is often measured in PSI (pounds per square inch), but can be converted to Pascals. For example, 35 PSI is roughly 241 kPa.
• Hydraulic Systems: The pressure in hydraulic systems, like those used in car brakes or heavy machinery, can be several megapascals (MPa).
• Water Pressure: The water pressure at the bottom of a 1-meter deep pool is approximately 9.8 kPa (ignoring atmospheric pressure). The Hydrostatic pressure can be determined with formula $\Delta P = \rho g \Delta h$. Given that the density of water is approximately 1000 $kg/m^3$ and the acceleration due to gravity is 9.8 $m/s^2$
• Weather Forecasts: Atmospheric pressure changes are often reported in hectopascals (hPa), where 1 hPa = 100 Pa.