Standard conditions of pressure and temperature

The standard conditions of pressure and temperature, commonly referred to as normal conditions or standard conditions, are a set of standard conditions of atmospheric pressure and temperature for laboratory experimental measurements that are established to allow comparisons between different sets of measured data.

According to the Dictionary of the Spanish language, standard is a condition "that serves as a type, model, norm, pattern or reference", while normal is a condition "that serves as a norm or rule". Both terms are therefore synonymous and refer to standardized conditions.

It is usual to speak interchangeably of "standard conditions" and "normal conditions" to refer to those conditions, but there is usually no agreement on the values established as normal or standard. This is because each body establishes its own denomination and standard conditions. Thus, the temperature of 0 °C can be as standard or normal as that of 20 °C. For example, both the RAE and the NIST talk about "normal conditions", but each one of them establishes a different value as normal for the temperature. The IUPAC, however, usually speaks of "standard conditions" to refer to standardized measurement conditions, although it is also its own conditions established as a standard.

The most widely used standard conditions are those of the International Union of Pure and Applied Chemistry (IUPAC) and those of the National Institute of Standards and Technology (NIST), although these are not universally accepted. Other organizations have established a variety of alternative definitions for their standard conditions of reference. Thus, the Dictionary of the Spanish language defines "normal conditions" as follows:

"Standard substances, established by agreement, which are taken as a reference to define the physical condition of a body and which correspond to zero degrees Celsius and a pressure atmosphere. »

In chemistry, the IUPAC in its Golden Book has established two standards, which are currently identical:

• Standard Temperature and Pressure (TPE, or STP): Temperature of 273.15 K (0 °C) and pressure of 10⁵(1 bar or 0.986 923 27 atm); normally employed in gas volume reports. Please note that calibrated flow meters with standard gas volumes per time unit often indicate volumes at 25 °C, not at 0 °C.
• Standard conditions for gases: Temperature of 273.15 K (0 °C) and pressure of 10⁵(1 bar or 0.986 923 27 atm). Previously IUPAC recommended for gases a standard pressure of 1 atm (equivalent to 1.01325 × 10⁵Pa), but currently recommends that the use of 1 atm as a pressure value should be interrupted.

In thermodynamics, NIST establishes the following conditions:

• Normal temperature and pressure (TPN, or NTP): A temperature of 20 °C and an absolute pressure of 1 atm.

The ISO 13443 standard for natural gas establishes the following conditions:

• Standardized reference conditions: 15 °C temperature and 1 atm pressure.

In industry and commerce, standardized conditions of temperature and pressure are often necessary to define standard reference conditions for expressing volumes of gases and liquids and related quantities, such as volumetric flow rate (volumes of gases vary significantly with temperature and pressure). However, many technical publications (books, magazines, advertisements for equipment and machinery) simply state "normal conditions" without specifying them, which often leads to confusion and errors. Good practice always incorporates the reference conditions of temperature and pressure.

Standard conditions

There are currently many different definitions of standard reference conditions used by organizations around the world. The following table shows some of them, but there are more. Some of these organizations used other standards in the past. For example, the IUPAC until 1982 defined standard reference conditions as 0 °C and 101 325 Pa (1 atm).

International Standard Atmosphere

In aeronautics and fluid dynamics the "International Standard Atmosphere" (ISA) is a specification of pressure, temperature, density, and speed of sound based on altitude. The International Standard Atmosphere is representative of atmospheric conditions in mid-latitudes.