[Physics Class Notes] on Torr Pdf for Exam

A torr is the non-SI unit of pressure on the absolute scale. The unit torr was named after the Italian Physicist and Mathematician Evangelista Torricelli, who discovered the principle of the barometer in 1644. 

Progressively, 760 mmHg got recognition as the “standard” atmospheric pressure. The unit of barometric pressure, i.e., one millimeter of mercury, also written as 1 mm Hg was named in honor of Torricelli, which is equal to 1 torr.

In this article, we will learn about the unit ‘torr’ in detail with a basic understanding of this unit with conversions.

[Image to be added soon]

SI Units of Pressure

The air around us has weight, and it exerts a force against everything it touches. The pressure we feel is called atmospheric pressure, or air pressure. We define pressure as the force exerted on a surface by the air above it as gravity pulls it to Earth.

The pressure is measured in various units,  now, we will discuss various units of pressure and their conversion factors below.

The SI unit of pressure is the pascal, symbolized as Pa. It is defined as one newton per meter square. 

Other units of pressure terms of SI units are as follows:

• The bar (symbolized as a bar) is defined as 105 Pa exactly.

• The atmosphere (symbol: atm)is defined as 101,325 Pa or 1.01325 mPa.

• The torr (symbol: Torr) is defined as 1/760 of the atmosphere or atm.

Since we are focusing on the unit Torr, so let’s understand what torr is.

Definition of Torr?

A torr is a non-SI unit for pressure e that is equal to 1/760 of the atmospheric pressure.

Torr is equal to 1 mmHg in a barometer and 133.322368 in pascals. 

So, torr is equal to two units, i.e., torr in mmHg and torr in Pascal.

This unit is based on the absolute scale, which is 1/760 of the standard pressure unit, i.e.,  1 atm or one atmospheric pressure. One atmospheric pressure or atm is equal to 1.01325 kPa. 

Point to Note:

1. The unit bar is used in meteorology to report atmospheric pressures.

2. The torr is a more agreeable unit for low pressures that are used in high-vacuum physics and engineering.

History of Torr

Historically, 1 torr was supposed to be the same as one “millimeter of mercury (mmHg), but subsequent redefinitions of the two units, i.e., torr and mmHg made them slightly different by less than 0.000015%. 

The torr is not part of the International System of Units or SI; however, it is often united with the metric prefix ‘milli’ to name a unit, i.e., one millitorr or mTorr or 0.001 Torr.

Conversion Factors in Torr

The mmHg by definition is as follows:

133.322387415 Pa or 13.5951 g/cm3 × 9.80665 m/s2 × 1 mm, which is estimated with known accuracies of the density of mercury and standard gravity.

Torr Definition

The definition of torr is discussed in the above text. 1 Torr is equal to 101325/760 Pa. 

After solving, we get the decimal form of this fraction as 133.322368421052631578947, which is an infinitely long, periodically repeating decimal (repetend length: 18).

The relationship between the torr and the millimeter of mercury on the scale is mentioned below:

Important  on Torr

• The difference between one millimeter of mercury (one mmHg) and one torr, and also between one atmosphere or 101.325 kPa) and 760 mmHg or 101.3250144354 kPa is less than one part in seven million or less than 0.000015%; this minute difference is negligible for most applications outside metrology.

• In medicine, mmHg measured with a sphygmomanometer is the “gold standard” unit for blood pressure measurement.

Manometric Units of Pressure

• Manometric units are considered millimeters of mercury or centimeters of water that rely on the density of a fluid and falsely adopt acceleration due to gravity (g). The use of these units is discouraged nowadays.

• However, manometric units are routinely used in the field of medicine and physiology, and they continue to have their diverse usage in areas like weather reporting and scuba diving.

Point to Remember

At present, it is many times told that Manometric results in medicine are sometimes given in torr; however, This is entirely incorrect information because the Torr and the mmHg are not the same things.

Fun Fact

Torricelli attracted notable attention in his time when he showcased the first mercury barometer to the general public. He was credited for giving the first modern explanation of atmospheric pressure.