centimeters of mercury (cmHg) to torr (torr) conversion

What is the centimeter of mercury?

The centimeter of mercury (cmHg) is a unit of pressure equal to the pressure exerted by a one-centimeter-high column of mercury under standard conditions. It is a scaled-up form of the millimeter of mercury (torr) used in some scientific and industrial settings.

Definition

The conventional centimeter of mercury is based on a 1 cm column of mercury with a density of 13,595.1 kg/m³ under standard gravity (9.80665 m/s²):

$$1\ \text{cmHg} = 1333.22\ \text{Pa}$$

Exactly, 1 cmHg = 1333.22387415 Pa, which is ten times the millimeter of mercury (1 mmHg ≈ 133.322 Pa). This equals 10 torr under the conventional definition.

Origin and History

The unit descends directly from Torricelli's mercury barometer of 1643, where atmospheric pressure was read as the height of a mercury column. Mercury's high density makes the column compact and easy to read. The conventional value fixes mercury's density (its value at 0 °C) and standard gravity so the unit is independent of temperature and location.

Law and Notable Facts

The centimeter of mercury is not an SI unit; the SI unit of pressure is the pascal. It relates to the standard atmosphere as 1 atm = 76 cmHg exactly (by the historical 760 mmHg definition). The millimeter of mercury (mmHg), essentially identical to the torr, remains standard in medicine for blood pressure.

Real-World Examples and Conversions

• 1 cmHg = 10 mmHg (10 torr) ≈ 1.33322 kPa.
• One standard atmosphere equals exactly 76 cmHg.
• A blood pressure of 120/80 mmHg corresponds to 12/8 cmHg.
• A vacuum of 5 cmHg absolute is about 6.7 kPa, roughly 93% below atmospheric pressure.

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.