Standard temperature and pressure

Imagine trying to compare scientific experiments or industrial measurements if everyone used different rulers. That's essentially the challenge 'Standard Temperature and Pressure,' or STP, aims to solve for gases and liquids. It provides crucial reference points for consistency across various fields, from chemistry labs to aerospace engineering. However, what constitutes 'standard' can be surprisingly varied, leading to a fascinating story of precision and consensus—or lack thereof. Standard Temperature and Pressure (STP) provides essential benchmarks for consistent scientific and industrial measurements, especially for substances whose volumes change with conditions. Despite its singular name, there isn't one universal STP; various organizations have established their own definitions, creating a complex landscape of reference conditions. Accurate calculations, particularly for the molar volume of gases, critically depend on explicitly stating which specific standard temperature and pressure conditions are being used.

Source: Wikipedia

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Imagine trying to compare scientific experiments or industrial measurements if everyone used different rulers. That's essentially the challenge 'Standard Temperature and Pressure,' or STP, aims to solve for gases and liquids. It provides crucial reference points for consistency across various fields, from chemistry labs to aerospace engineering. However, what constitutes 'standard' can be surprisingly varied, leading to a fascinating story of precision and consensus—or lack thereof.

The Need for Standards

In science, industry, and commerce, precision is paramount. When we talk about gases or liquids, their volumes change dramatically with temperature and pressure. Without a common baseline, comparing data—say, the output of a chemical reaction or the flow rate of natural gas—becomes nearly impossible.

This is where 'Standard Temperature and Pressure,' or STP, comes in. It's meant to provide a universal set of reference conditions, allowing researchers and engineers worldwide to speak the same language when reporting measurements. But here's the twist: there isn't just one STP.

A Shifting Definition

Historically, the International Union of Pure and Applied Chemistry, or IUPAC, defined STP as a temperature of 0 degrees Celsius (273.15 Kelvin) and an absolute pressure of exactly one atmosphere. This was a widely accepted benchmark for many years.

However, in 1982, IUPAC updated its definition. While the temperature remained 0 degrees Celsius, the standard pressure shifted to exactly one bar, which is 100,000 pascals. This subtle change — a bar is slightly less than a standard atmosphere — meant that calculations relying on 'STP' now needed to specify which version was being used.

Beyond STP, IUPAC also introduced 'Standard Ambient Temperature and Pressure,' or SATP, set at a warmer 25 degrees Celsius (298.15 Kelvin) and one atmosphere. The National Institute of Standards and Technology, NIST, similarly uses 20 degrees Celsius and one atmosphere for some of its standards, often called 'Normal Temperature and Pressure' or NTP.

Many other organizations have their own 'standard' conditions. From the International Organization for Standardization (ISO) to the American Petroleum Institute (API), different industries and regions have tailored benchmarks to suit their specific needs, often driven by typical ambient conditions or historical practices.

The International Standard Atmosphere

In fields like aeronautics and fluid dynamics, a different kind of standard is crucial: the International Standard Atmosphere, or ISA. This isn't just about a single point, but a model of how pressure, temperature, density, and even the speed of sound change with altitude.

At standard mean sea level, ISA specifies a temperature of 15 degrees Celsius, a pressure of 101,325 pascals (which is exactly one atmosphere), and a density of 1.2250 kilograms per cubic meter. It also defines a temperature decrease rate of 6.5 degrees Celsius for every kilometer of altitude gained.

Beyond the Lab: Standard Laboratory Conditions

You might also hear about 'standard laboratory conditions.' This term is deliberately chosen to be distinct from STP, even though it sounds similar. Why? Because the actual ambient temperature and pressure in a lab can vary significantly based on geography, climate, and whether air conditioning or heating is used.

For instance, a lab in Sydney, Australia, might define its standard conditions as 25 degrees Celsius and 100 kilopascals, reflecting its local climate. This highlights that while official standards exist, practical working conditions often necessitate their own internal benchmarks.

The Molar Volume Conundrum

One of the most critical applications of STP is in calculating the molar volume of a gas—that's the volume occupied by one mole of any gas. Stating a molar volume without also stating the specific temperature and pressure conditions is like giving a speed limit without specifying miles or kilometers per hour; it's practically meaningless and can lead to serious errors.

For many gases around STP, we can use the elegant simplicity of the ideal gas law to calculate molar volume with remarkable accuracy. This fundamental equation links pressure, volume, temperature, and the amount of gas.

Here, P is pressure, V is volume, n is the number of moles, R is the ideal gas constant, and T is temperature in Kelvin. Rearranging this, the molar volume (V/n) becomes RT/P. This equation truly shows why temperature and pressure are so vital.

PV = nRT

Let's see how different STP definitions lead to different molar volumes. If we take the ideal gas constant R as 8.3145 L·kPa/(mol·K):

Notice how even small changes in the defined standard conditions result in measurable differences in molar volume. This underscores why clarity is so important.

At 0 °C (273.15 K) and 101.325 kPa (1 atm), the molar volume is 22.414 dm³/mol. At 0 °C (273.15 K) and 100.000 kPa (1 bar), the molar volume is 22.711 dm³/mol. At 25 °C (298.15 K) and 101.325 kPa (1 atm), the molar volume is 24.466 dm³/mol.

Adding to the potential confusion, technical literature sometimes fails to specify whether it's using the ideal gas constant (R) or the specific gas constant (Rs). The specific gas constant is derived by dividing the ideal gas constant by the molecular mass of the particular gas, making it unique for each gas.

The Takeaway: Always Specify

Given the multitude of 'standard' conditions, the most crucial lesson is simple: always, always specify the reference temperature and pressure when presenting data related to gas or liquid volumes. Omitting these details can lead to misunderstandings, errors, and wasted effort.

From chemical reactions to aerospace design, the concept of Standard Temperature and Pressure is fundamental for consistent, comparable scientific and industrial work. It's a testament to how even seemingly simple ideas can reveal layers of complexity and the human quest for precision in a variable world.

Article

Standard temperature and pressure

Standard temperature and pressure (STP), or standard conditions for temperature and pressure, are various standard sets of conditions for experimental measurements used to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted. Other organizations have established a variety of other definitions.

In industry and commerce, the standard conditions for temperature and pressure are often necessary for expressing the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases vary significantly with temperature and pressure): standard cubic meters per second (Sm3/s), and normal cubic meters per second (Nm3/s).

Many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often substituting the term with older "normal conditions", or "NC". In special cases this can lead to confusion and errors. Good practice always incorporates the reference conditions of temperature and pressure. If not stated, some room environment conditions are supposed, close to 1 atm pressure, 25 °C (77.00 °F), and 0% humidity.

Definitions

Main definitions

In chemistry, IUPAC changed its definition of standard temperature and pressure in (/wiki/1982):

• Until 1982, STP was defined as a temperature of 273.15 K (0.00 °C; 32.00 °F) and an absolute pressure of exactly 1 atm (101.325 kPa). • Since 1982, STP has been defined as a temperature of 273.15 K (0.00 °C; 32.00 °F) and an absolute pressure of exactly 1 bar (100,000.000 Pa).

IUPAC also defines SATP (Standard Ambient Temperature and Pressure) as a temperature of 298.15 K (25.00 °C; 77.00 °F) and an absolute pressure of exactly 1 atm (101.325 kPa).

NIST uses a temperature of 20 °C (293.15 K; 68.00 °F) and an absolute pressure of 1 atm (101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP). However, a common temperature and pressure in use by NIST for thermodynamic experiments is 298.15 K (25.00 °C; 77.00 °F) and 1 bar (100.000 kPa). NIST also uses 15 °C (288.15 K; 59.00 °F) for the temperature compensation of refined petroleum products, despite noting that these two values are not exactly consistent with each other.

The ISO 13443 standard reference conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) and 1 atm (101.325 kPa); by contrast, the American Petroleum Institute adopts 60 °F (15.56 °C; 288.71 K).

Past uses

Before 1918, many professionals and scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being 15 °C (288.15 K; 59.00 °F) and 101.325 kPa (1.00 atm; 760 Torr). During those same years, the most commonly used standard reference conditions for people using the imperial or U.S. customary systems was 60 °F (15.56 °C; 288.71 K) and 14.696 psi (1.00 atm) because it was almost universally used by the oil and gas industries worldwide. The above definitions are no longer the most commonly used in either system of units.

Current use

Many different definitions of standard reference conditions are currently being used by organizations all over the world. The table below lists a few of them, but there are more. Some of these organizations used other standards in the past. For example, IUPAC's new value is the mean atmospheric pressure at an altitude of about 112 metres, which is closer to the worldwide median altitude of human habitation (194 m), and follows the metric system (Pascal is a metric unit, while atmosphere is not).

Natural gas companies in Europe, Australia, and South America have adopted 15 °C (288.15 K; 59.00 °F) and 1 atm (101.325 kPa) as their standard gas volume reference conditions, used as the base values for defining the standard cubic meter. Also, the International Organization for Standardization (ISO), the United States Environmental Protection Agency (EPA) and National Institute of Standards and Technology (NIST) each have more than one definition of standard reference conditions in their various standards and regulations.

Comparison table

<table><thead><tr><th>Temperature</th><th>Pressure</th><th>Humidity</th><th>Publishing or establishing entity</th></tr></thead><tbody><tr><td>°C</td><td>°F</td><td>kPa</td><td>mmHg</td><td>psi</td><td>inHg</td><td>%</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td> </td></tr><tr><td>0</td><td>32</td><td>100.000</td><td>750.06</td><td>14.5038</td><td>29.530</td><td></td><td>IUPAC (STP) since 1982</td></tr><tr><td>0</td><td>32</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td></td><td>NIST, ISO 10780, formerly IUPAC (STP) until 1982</td></tr><tr><td>15</td><td>59</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td>0</td><td>ICAO's ISA, ISO 13443, EEA, EGIA (SI Definition), Air density 1.225 kg/m³</td></tr><tr><td>20</td><td>68</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td></td><td>EPA, NIST.</td></tr><tr><td>22</td><td>71.6</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td>20–80</td><td>American Association of Physicists in Medicine</td></tr><tr><td>25</td><td>77</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td></td><td>IUPAC (SATP), EPA</td></tr><tr><td>20</td><td>68</td><td>100.000</td><td>750.06</td><td>14.5038</td><td>29.530</td><td>0</td><td>CAGI</td></tr><tr><td>15</td><td>59</td><td>100.000</td><td>750.06</td><td>14.5038</td><td>29.530</td><td></td><td>SPE</td></tr><tr><td>20</td><td>68</td><td>101.3</td><td>760</td><td>14.69</td><td>29.9</td><td>50</td><td>ISO 5011</td></tr><tr><td>20</td><td>68</td><td>101.33</td><td>760.0</td><td>14.696</td><td>29.92</td><td>0</td><td>GOST 2939-63</td></tr><tr><td>15.56</td><td>60</td><td>101.33</td><td>760.0</td><td>14.696</td><td>29.92</td><td></td><td>SPE, U.S. OSHA, SCAQMD</td></tr><tr><td>15.56</td><td>60</td><td>101.6</td><td>762</td><td>14.73</td><td>30.0</td><td></td><td>EGIA (Imperial System Definition)</td></tr><tr><td>15.56</td><td>60</td><td>101.35</td><td>760.21</td><td>14.7</td><td>29.93</td><td></td><td>U.S. DOT (SCF)</td></tr><tr><td>15</td><td>59</td><td>99.99</td><td>750.0</td><td>14.503</td><td>29.53</td><td>78</td><td>U.S. Army Standard Metro</td></tr><tr><td>15</td><td>59</td><td>101.33</td><td>760.0</td><td>14.696</td><td>29.92</td><td>60</td><td>ISO 2314, ISO 3977-2, ASHRAE Fundamentals Handbook</td></tr><tr><td>21.11</td><td>70</td><td>101.3</td><td>760</td><td>14.70</td><td>29.92</td><td>0</td><td>AMCA, air density = 0.075 lbm/ft3.</td></tr><tr><td>15</td><td>59</td><td>101.3</td><td>760</td><td>14.70</td><td>29.92</td><td></td><td>FAA</td></tr><tr><td>20</td><td>68</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td></td><td>EN 14511-1:2013</td></tr><tr><td>15</td><td>59</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td>0</td><td>ISO 2533:1975 ISO 13443:2005, ISO 7504:2015</td></tr><tr><td>0</td><td>32</td><td>101.325</td><td>760.00</td><td>14.6959</td><td>29.921</td><td>0</td><td>DIN 1343:1990</td></tr></tbody></table>

International Standard Atmosphere

Standard temperature and pressure

In aeronautics and fluid dynamics the "International Standard Atmosphere" (ISA) is a specification of pressure, temperature, density, and speed of sound at each altitude. At standard mean sea level it specifies a temperature of 15 °C (59 °F), pressure of 101,325 pascals (14.6959 psi) (1 atm), and a density of 1.2250 kilograms per cubic meter (0.07647 lb/cu ft). It also specifies a temperature lapse rate of −6.5 °C (−11.7 °F) per km (approximately −2 °C (−3.6 °F) per 1,000 ft).

The International Standard Atmosphere is representative of atmospheric conditions at mid latitudes. In the US this information is specified the U.S. Standard Atmosphere which is identical to the "International Standard Atmosphere" at all altitudes up to 65,000 feet above sea level.

Standard laboratory conditions

Standard temperature and pressure

Because many definitions of standard temperature and pressure differ in temperature significantly from standard laboratory temperatures (e.g. 0 °C vs. ~28 °C), reference is often made to "standard laboratory conditions" (a term deliberately chosen to be different from the term "standard conditions for temperature and pressure", despite its semantic near identity when interpreted literally). However, what is a "standard" laboratory temperature and pressure is inevitably geography-bound, given that different parts of the world differ in climate, altitude and the degree of use of heat/cooling in the workplace. For example, schools in New South Wales, Australia use 25 °C at 100 kPa for standard laboratory conditions. ASTM International has published Standard ASTM E41- Terminology Relating to Conditioning and hundreds of special conditions for particular materials and test methods. Other standards organizations also have specialized standard test conditions.

Molar volume of a gas

Standard temperature and pressure

It is as important to indicate the applicable reference conditions of temperature and pressure when stating the molar volume of a gas as it is when expressing a gas volume or volumetric flow rate. Stating the molar volume of a gas without indicating the reference conditions of temperature and pressure has very little meaning and can cause confusion.

The molar volume of gases around STP and at atmospheric pressure can be calculated with an accuracy that is usually sufficient by using the ideal gas law. The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:

• Vm = 8.3145 × 273.15 / 101.325 = 22.414 dm3/mol at 0 °C and 101.325 kPa • Vm = 8.3145 × 273.15 / 100.000 = 22.711 dm3/mol at 0 °C and 100 kPa • Vm = 8.3145 × 288.15 / 101.325 = 23.645 dm3/mol at 15 °C and 101.325 kPa • Vm = 8.3145 × 298.15 / 101.325 = 24.466 dm3/mol at 25 °C and 101.325 kPa • Vm = 8.3145 × 298.15 / 100.000 = 24.790 dm3/mol at 25 °C and 100 kPa • Vm = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi (or about 0.8366 ft3/gram mole) • Vm = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lbmol at 60 °F and 14.73 psi

Technical literature can be confusing because many authors fail to explain whether they are using the ideal gas constant R, or the specific gas constant Rs. The relationship between the two constants is Rs = R / m, where m is the molecular mass of the gas.

The US Standard Atmosphere (USSA) uses 8.31432 m3·Pa/(mol·K) as the value of R. However, the USSA in 1976 does recognize that this value is not consistent with the values of the Avogadro constant and the Boltzmann constant.