Standard volume

The standard volume (in particular standard cubic meters , standard liters , etc.) is a volume unit commonly used in pneumatics , process and gas engineering . It is used to compare gas quantities that are present at different pressures and temperatures ( operating status , operating volume ). For this purpose, the gas quantities are each converted to the same standard condition , e.g. B. with the help of the state number .

Classification and demarcation

For historical reasons, fluid quantities are mainly given in the unit of a volume to this day (at least in the past, “gauging” was much easier than determining the corresponding fluid mass). Since the volume of a certain gas mass depends on the states of pressure and temperature , they must always be specified when specifying the volume of a certain gas mass. Since gas quantities are otherwise not comparable, we have agreed on various precisely defined reference states, so-called norm or standard states.

When measuring gas quantities, “ standard conditions ”, “standard conditions” and “reference conditions” are generally synonymous terms. In contrast to the standard conditions, the pressure and temperature that are actually present in a gas or gas process are often referred to as "operating conditions", and the associated volume flow as "operating volume flow ".

When using the same reference status, (standard) volumes or (standard) volume flow rates can be compared directly with one another. Unfortunately, in the meantime there have been a large number of different reference states, which can lead to misunderstandings and misinterpretations when specifying "standard volumes" and "standard volume flows". Therefore, when specifying a standardized volume, the underlying standard or standard conditions should always be mentioned. With the consistent specification and use of gas masses (and gas mass flow rates) not instead of standardized volumes this problem exists, the naming of a reference state is not necessary here.

Physical standard condition

In addition to many other standard conditions or reference states defined later, the following standard conditions were defined in DIN 1343 as early as 1940 , which have been established worldwide under the term physical standard state :

Standard pressure p _n = 101 325 Pa = 1 013.25 h Pa = 101.325 k Pa = 1.01325 bar
Standard temperature T _n = 273.15 K = 0 ° C

Information such as standard cubic meters, standard liters, standard cubic centimeters etc. mostly refer to this "physical standard condition" according to DIN 1343, albeit v. a. Internationally, often different standard conditions apply:

International Standard Atmosphere & technical standard condition

Other common reference states or standard conditions for volume and flow measurement are:

ISO 2533 ( ISA = International Standard Atmosphere defined or adopted by the International Civil Aviation Organization (ICAO); also common in the trade in pressurized gas cylinders):

p _n = 1.01325 bar = 1013.25 hPa; T _n = 288.15 K = 15 ° C

ISO 6358 / ISO 8778 ( Standard Reference Atmosphere , mainly used in USA) according to DIN 1945-1 ( technical standard condition ):

p _n = 1.0 bar; T _n = 293.15 K = 20 ° C

Notation

According to PTB , units must not contain any additional information about the type of physical quantity . Common spellings such as B. "Nm³", "m _n ³", "m³ (n)", "nm³" etc. are therefore not permitted.

In order to identify standardized volume flow information as such, the associated symbol should be modified accordingly. DIN 1343 suggests the index 'n' for this (e.g. V _n =…).

Connection with the crowd

The standardized volume can be converted directly into a mass by multiplying it by the associated standard density:

{\ displaystyle V _ {\ text {n}} \ cdot \ rho _ {\ text {n}} = m}

Standard volumes (flows) with different reference conditions can then be converted into one another using the gas masses (flows).

For an ideal gas , the standard density is calculated according to the thermal equation of state of ideal gases as:

{\ displaystyle \ rho _ {\ text {n}} = {\ frac {p _ {\ text {n}}} {R _ {\ text {i}} \ cdot T _ {\ text {n}}}}}

Here are:

${\ displaystyle p _ {\ text {n}}}$ the absolute pressure and the absolute temperature of the defined standard or norm condition ${\ displaystyle T _ {\ text {n}}}$
${\ displaystyle R _ {\ text {i}}}$ the specific or individual gas constant of the gas.

Under the physical standard conditions z. B. air has a density of = 1.293 kg / m ³ . ${\ displaystyle \ rho _ {\ text {n}}}$

literature

Erwin Ruppelt (Hrsg.): Compressed air manual. Vulkan Verlag GmbH, Essen 2003, ISBN 3-8027-2548-4 .
Ulrich Wernekinck: Gas metering and gas billing. 3rd edition, Vulkan Verlag GmbH, Essen 2003, ISBN 3-8027-5617-7 .

Web links

Flow measurement in gases What is a standard cubic meter? (accessed on January 21, 2016)
Molar mass, standard molar volume, molar concentration (accessed on January 21, 2016)