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Definitions

Calibration gas

Gas used for adjusting the sensitivity (deflection) of the detected gas. It is a gas mixture of known composition (measured component and suitable residual gas).

Sensitivity

Ratio between a change in output variable observed on the measuring instrument and the change in input variable required for this.

Linearity error of devices with linear characteristics

Deviation of measured characteristic from a linear reference characteristic.

The linearity is an important variable particularly for instruments which use a measuring effect with nonlinear characteristic and where the measured characteristic is linearized electronically.

Cross-sensitivity

Measure for the selectivity of a gas analyzer with regard to interfering components.

It is the ratio between the displayed value of the interfering component and the displayed value of the measured component; both have the same concentration.

In the case of analyzers where the total concentration of different materials is measured (e.g. total hydrocarbon concentration) and where the individual components are weighted differently in the measuring result, these factors are specified in equivalents of a master component (e.g. CH₄ equivalents for the total hydrocarbon measurement) and not as cross-sensitivity.

Time response

The dynamic response of an analyzer is characterized by its response time and dead time. The response time is the time which passes until the output variable remains constantly within defined limits following an abrupt change in the input variable. The response time is usually understood as the time required to reach 90 (T₉₀) or 95% of the expected display.

Units of measurement

Vol%

Volume proportion in % of measured component, based on the sample gas.

ppm (vpm)

Parts per million, i.e. one proportion of the measured component per 10⁶ proportions of the sample gas (corresponds to 10^-4 %).

In gas analysis technology, ppm is usually understood as volume concentrations. The dimension unit vpm is frequently used for unequivocal identification:

1 vpm = 1 cm³ / m³

Example: 1 000 vpm = 0.1 vol.% = 1 dm³ / m³

mg/m³

Mass of measured component in mg referred to 1 m³ of sample gas at 1 013 hPa and 20 °C.

Example: 1 vpm = 1 cm³ / m³ corresponds to:

(molecular weight of component / molecular volume of component) · (mg / m³)

Weight concentration

Specification of measured values in weight concentrations is not common with gas analysis. Weight concentrations can only be determined in exceptional cases. The dimension unit mg/m³ does not mean weight concentration.