standard atmospheres (atm) to torr (torr) conversion

What is the standard atmosphere?

The standard atmosphere (atm) is a unit of pressure defined as a fixed reference value close to the average atmospheric pressure at sea level. It is widely used in chemistry, physics, engineering, and diving to express pressures relative to typical sea-level conditions.

Definition

The standard atmosphere is defined exactly as 101,325 pascals:

$$1\ \text{atm} = 101325\ \text{Pa}$$

This is equivalent to 1013.25 hectopascals (millibars), 760 millimeters of mercury (torr), and about 14.6959 pounds per square inch. The value was fixed by the 10th General Conference on Weights and Measures (CGPM) in 1954.

Origin and History

Early pressure measurement grew from Evangelista Torricelli's 1643 barometer experiments, which showed the atmosphere supports a mercury column about 760 mm high. The "atmosphere" became a convenient reference for a whole unit of ambient pressure. In 1954 the CGPM adopted the exact value 101,325 Pa, based on a 760 mm mercury column at 0 °C under standard gravity, to remove the temperature and location dependence of earlier definitions.

Law and Notable Facts

The standard atmosphere is accepted for use with the SI but is not an SI unit; the SI unit of pressure is the pascal. IUPAC now recommends the bar (100,000 Pa) rather than the atmosphere as the standard pressure for reporting thermodynamic data, though "atm" remains common. Note that the technical atmosphere (at) is a different unit equal to 98,066.5 Pa.

Real-World Examples and Conversions

• Average sea-level air pressure is very close to 1 atm (101.325 kPa).
• Ocean pressure increases by roughly 1 atm for every 10 meters of seawater depth, so a diver at 30 m experiences about 4 atm total.
• A typical car tire inflated to 32 psi holds about 2.2 atm of gauge pressure.
• The pressure inside a champagne bottle is roughly 6 atm (about 608 kPa).

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.