Cross sensitivity
The cross-sensitivity represents the sensitivity of a measuring device to other sizes than the measured quantity represents; the latter is the size to be measured.
A variable that is not a measured variable but influences the information about the measured value supplied by the measuring device is called an influencing variable ; it is also known as a disturbance variable. It causes the measured value to change simply because the influencing variable changes. One of the goals of any measuring device development should be to keep cross-sensitivities low. Cross-sensitivity to disturbance variables is one of the most important properties of a measuring device.
Incomplete selectivity also contributes to cross-sensitivity , as it is e.g. B. occurs in gas sensors . These often respond to gases other than the one to be detected.
A very common influencing factor is temperature .
- An electrical resistance of pure metals increases with temperature. In the resistance thermometer , this phenomenon is used as a characteristic measuring principle for measuring the temperature. When measuring the electrical voltage using a moving-coil measuring mechanism , the same phenomenon represents a cross-sensitivity via the resistance of the moving coil.
- The spring constant that determines the elastic behavior of a steel spring has a negative temperature coefficient. As the temperature rises, the hardness of the spring decreases, and the display of a spring balance or moving-coil measuring mechanism increases despite the unchanged measured variable.
It is often prescribed at which ambient temperature a measuring device is to be operated in order to avoid a temperature influence on the adjusted state.
Other important influencing variables are humidity , air pressure , electric field or magnetic field . During gas analysis, gas components other than those that are to be measured can lead to cross-sensitivities in the measuring device: For example, certain mercury measuring devices react to sulfur dioxide . Measuring devices for the detection of benzo [ a ] pyrene often show cross-sensitivities with regard to ozone . In chemiluminescence methods for the determination of nitrogen oxides , ammonia and carbon dioxide act as disturbance variables. The concentrations of other gases also play a role in gas analysis . For example, the acetylacetone method for the quantitative determination of formaldehyde only exhibits cross-sensitivities to acetaldehyde when its concentration exceeds 50 times the concentration of formaldehyde; Similarly, a sulfur dioxide concentration of more than 10 mg / m 3 leads to significant lower results.
Manufacturers of measuring devices are often required to describe all known sources of interference. Testing laboratories have to assess the relevant disturbance variables and report on them. Cross-sensitivities of measuring devices for monitoring air pollution are usually not allowed to exceed a certain percentage of the limit value .
See also
Individual evidence
- ↑ DIN EN 15859: 2010-08 air quality ; Certification of automatic devices for monitoring dust collectors on stationary sources; Minimum requirements and test procedures; German version EN 15859: 2010 . Beuth Verlag, Berlin, p. 6.
- ↑ Dieter Sautter, Hans Weinerth (Ed.): Lexicon Electronics and Microelectronics . 2nd, updated and expanded edition, VDI-Verlag Düsseldorf, 1993, ISBN 3-18-401178-X , p. 359.
- ↑ Jens Korell, Hanns-Rudolf Paur, Helmut Seifert: Measurement of mercury in coal smoke gases. In: Hazardous substances - cleanliness. Air . 68, No. 4, 2008, ISSN 0949-8036 , pp. 141-148.
- ↑ Harald Creutznacher, Karl Huber: Ozone cross-sensitivity in the immission measurement of particulate matter-accumulated benzo [a] pyrene. In: Hazardous substances - cleanliness. Air. 62, No. 7/8, 2002, ISSN 0949-8036 , pp. 319-324.
- ↑ DIN EN 14792: 2006-04 Emissions from stationary sources; Determination of the mass concentration of nitrogen oxides (NOx); Reference method: chemiluminescence; German version EN 14792: 2005 . Beuth Verlag, Berlin, p. 18.
- ↑ VDI 3862 sheet 6: 2004-02 measurement of gaseous emissions; Measurement of formaldehyde by the acetylacetone method (Gaseous emission measurement; Measurement of formaldehyde by the acetylacetone method) . Beuth Verlag, Berlin, p. 13.
- ↑ DIN EN 15859: 2010-08 air quality ; Certification of automatic devices for monitoring dust collectors on stationary sources; Minimum requirements and test procedures; German version EN 15859: 2010 . Beuth Verlag, Berlin, p. 18.
- ↑ VDI 4202 sheet 1: 2010-09 Minimum requirements for automatic immission measuring devices for suitability testing; Point-related measurement methods for gaseous and particulate air pollutants (Performance criteria for performance tests of automated ambient air measuring systems; Point-related measurement methods for gaseous and particulate air pollutants) . Beuth Verlag, Berlin, p. 23.