What is Torr?
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.
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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:
The unit bar is used in meteorology to report atmospheric pressures.
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:
1 Torr to mmHg = 0.999999857533699... mmHg
1 mmHg to torr = 1.000000142466321... Torr
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.
FAQs on Torr
1. What is the definition of a Torr in physics?
A Torr (symbol: Torr) is a non-SI unit of pressure. It is defined as exactly 1/760 of one standard atmosphere (atm). This unit is named after the Italian physicist Evangelista Torricelli, who invented the barometer and first explained atmospheric pressure.
2. How many Torr are in one standard atmosphere (atm)?
By definition, there are exactly 760 Torr in one standard atmosphere. Therefore, the relationship is 1 atm = 760 Torr. This is a fundamental conversion factor used in physics and chemistry.
3. How do you convert between Torr, Pascals (Pa), and mmHg?
To convert between these common pressure units, you can use the following standard relationships:
- Torr to Pascal: 1 Torr is approximately equal to 133.322 Pascals (Pa).
- Torr to mmHg: 1 Torr is almost identical to 1 millimeter of mercury (mmHg). For most practical calculations, the conversion 1 Torr ≈ 1 mmHg is used.
4. In which scientific fields is the Torr unit most commonly used?
The Torr is particularly useful for measuring very low pressures, or partial vacuums. It is most commonly found in:
- High-vacuum physics and related engineering applications.
- Scientific experiments requiring precise low-pressure environments.
- Manufacturing processes for semiconductors and optical coatings.
5. Why was the Torr unit named after Evangelista Torricelli?
The unit was named in honour of Evangelista Torricelli to commemorate his pioneering work. In 1644, he invented the mercury barometer and was the first to correctly identify that the height of the mercury column was supported by the surrounding atmospheric pressure, providing the first modern explanation for it.
6. What is the main difference between a Torr and a millimeter of mercury (mmHg)?
The key difference is in their formal definitions, although their values are nearly identical. A Torr is defined precisely as 1/760 of a standard atmosphere. An mmHg is defined based on the pressure exerted by a 1 mm high column of mercury. Due to redefinitions of these standards, there is a tiny difference (less than 0.000015%), but for most applications, including CBSE-level physics, they are considered interchangeable.
7. Why is the Torr considered a non-SI unit, and what is the official SI unit for pressure?
The Torr is a non-SI unit because the International System of Units (SI) has a specific derived unit for pressure. The official SI unit for pressure is the Pascal (Pa), defined as one Newton of force per square meter (N/m²). While Torr is historically important and convenient for vacuum physics, the Pascal is the standard for scientific work under the SI system.
8. What is the pressure at Standard Temperature and Pressure (STP) in Torr?
Standard Temperature and Pressure (STP) is defined by a standard pressure of 1 atmosphere (atm). Since 1 atm is exactly equal to 760 Torr, the pressure at STP is 760 Torr. This value is a critical reference point used in gas law calculations and other scientific contexts.
