Gas Analyzer

Topics: Pressure, Air pollution, Gas Pages: 17 (5081 words) Published: August 25, 2013
Gas analysis techniques

Terms for gas analysis techniques
 Concentration
The term concentration describes the amount of a substance, expressed as mass, volume, or number of particles in a unit volume of a solid, liquid, or gaseous substance e.g. alcohol in beer or oxygen in air. Different units are in use to describe concentration in gases: Mass concentration

Concentration expressed in terms of mass of substance per unit volume [g substance/m3 gas volume] ¢ Volume concentration
Concentration expressed in terms of gaseous volume of substance per unit volume [cm3 substance/m3 gas volume] Part concentration
Concentration expressed as number of particles of substance per a certain number of particles

In flue gas analysis both the terms mass concentration and part concentration are common and used in parallel. The mass unit is gram (and mg, g, see table 15) and the most popular expression for part concentration is ppm (parts per million). "ppm" means "x number of parts in a million parts". ppm is usually used for low concentrations; larger concentrations are expressed in "percent" (%), see table 16. Consequently the concentration of a gaseous pollutant is expressed  either using (or mg or g etc.) with reference to a definite gas volume, usually cubic metres (m3), e.g. 200 mg/m3 or using ppm without any reference, e.g. 140 ppm Please note

Because of the variation of a gas volume with temperature and pressure changes it is necessary to use one of the following alternatives for describing a concentration value:  additional specification of gas temperature and pressure values existing during measurement or conversion of the measured concentration value into the corresponding value at standard zero conditions, see the following chapter. After conversion, the volume is expressed as standard volume (standard cubic meter, Nm3 or m3N). Standard zero conditions of a gas

The volume of a gas depends on its actual temperature and pressure. To achieve comparable results a standard zero volume has been defined: A gas has its standard zero volume at a pressure of 1013 mbar (hPa) and a temperature of 273 K (corresponding to 0°C). Conversion of concentration values

Conversion of a measured value to standard conditions
The conversion of an actual measuring value (status 1) to standard conditions (status 2) is performed using the formula

with the following expressions:
Status 1
Conditions during measurement

 | T1 : Gas temperature during measurement
(273 + actual temperature in °C)
p1 : Gas pressure during measurement in hPac1 :
c1 : measured concentration value|

Status 2
Standard conditions
 | T2 : Standard zero temperature (=273 K)
p2 : Standard zero pressure (=1013 hPa)
c2 : Concentration converted to standard
conditions|
Example
The value "200mg/m3" at conditions of 35 °C and 920 hPa
results, after conversion into standard conditions, in the value "248,4 mg/Nm3".

Conversion of ppm to mass concentration [mg/m3]
ppm ( parts per million) is a very common concentration unit as expression for a relation of particles in a gas volume. Simultaneously the unit mass concentration is used. A concentration value in [ppm] can be converted to the corresponding value expressed as mass concentration [mg/Nm3] using the standard density of the gas as a factor. For that the dilution of the gas by air (excess air, specially added air or false air from leaks) must be considered by using the oxygen concentration as reference. All measured values must be in reference to a certain oxygen content ("reference O2"). Only concentration values with identical oxygen reference values are comparable to each other! Therefore, in official regulations, limit concentration values of pollutants are always specified together with a certain oxygen reference value. The actual oxygen concentration value is also required for the conversion calculation as measure for the actually existing gas dilution level.  

Conversion formulas for...
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