millimeters of mercury (mmHg) to torr (torr) 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 torr?

Torr is a unit of pressure measurement commonly used in vacuum applications. Let's delve into its definition, origin, and relevance.

Definition of Torr

The torr is a unit of pressure defined as 1/760 of standard atmospheric pressure. In other words, 760 torr is approximately equal to one atmosphere (atm).

$$1 \text{ torr} \approx \frac{1}{760} \text{ atm}$$

It is also nearly equal to one millimeter of mercury (mmHg). More precisely:

$$1 \text{ torr} \approx 1 \text{ mmHg}$$

Origin and History

The torr is named after Italian physicist and mathematician Evangelista Torricelli (1608–1647), who invented the barometer in 1643. Torricelli's experiment demonstrated that air pressure could support a column of mercury, paving the way for pressure measurement.

Relation to Pascal (Pa)

The pascal (Pa) is the SI unit of pressure. The relationship between torr and pascal is as follows:

$$1 \text{ torr} \approx 133.322 \text{ Pa}$$

Therefore, to convert from torr to pascals, you can use the formula:

$$\text{Pressure in Pa} = \text{Pressure in torr} \times 133.322$$

Real-World Examples and Applications

Torr is commonly used in fields that involve vacuum systems, such as:

• Vacuum pumps: Vacuum pump performance is often rated in torr or millitorr (mTorr). For example, a roughing pump might achieve a vacuum of 10$^{-3}$ torr.
• Scientific instruments: Mass spectrometers, electron microscopes, and other analytical instruments require high vacuum conditions, often specified in torr or microtorr (µTorr).
• Semiconductor manufacturing: Vacuum processes, such as chemical vapor deposition (CVD) and sputtering, use vacuum levels measured in torr to control deposition rates and film quality.
• Space research: Simulating space environments requires extremely low pressures, which are measured in torr or even smaller units like picotorr (pTorr).
• Vacuum Furnaces: Sintering, brazing, and heat treating of materials at reduced pressures, which improves the properties of the final product.

Interesting Facts

• While torr and mmHg are often used interchangeably, they are technically slightly different due to variations in the definition of standard gravity.
• The unit "micron" (µ) is sometimes used as a unit of pressure, where 1 micron = 1 mTorr.
• The lowest pressure ever achieved in a laboratory setting is on the order of $10^{-17}$ torr.