Gasmessung: invasiv oder nicht-invasiv
Destructive Methods of Gas Measurement
Measurement of gases can be done in various technologies. Beside of other parameters there are two different possibilities in changes of gas mix during measurement. The gas leaves the measurement unit with exact the same composition as entered (non-invasive measurement) or the measured gases or gas mixes are not the same after leaving the measurement, the are (partly) destroyed (invasive measurement).
In exhaust measurement and in many industrial processes the gas composition after measurement is irrelevant for process because there is no feedback of gas into the process again.
However feedback of measured gas may be necessary or volitional. At processes with small gas quantities (e.g. at biotechnology with small reactor volumes) often it is not possible to remove enough gas for a continuous gas measurement. The atmosphere inside of reactor will decrease, the process will collapse and the charge is corrupt. In this case it is necessary to preserve the gas mix at measurement.
For cost reduction often gases will be recycled. Gases extracted from a process will be refurbished and fed back into the process. Depending on application it may be necessary to measure these gases by a non-invasive technology.
Measurement with Electrochemical Cells (EC) is an invasive process. Hydrogen e.g. used in biotechnological processes and in hardening will combusted to water. Hydrogen sulfide will change to acid sulfur and water. Electrochemical Cells are inexpensive, but they have a limited live time. EC are available for many gases, e.g. O2, H2, H2S, CO, …
NDIR (non dispersive infrared) is a non-invasive measurement method. Many molecular gases (e.g. CO2, CO, SO2, NO2, CH4,…) may be determined by this technology. Measurement systems of SAXON Junkalor mostly base on this technology.
Paramagnetic measurement is a non-invasive measure method for oxygen in industrial applications. It has a very long live time, but it needs higher initial costs as an EC.
Heat conductive sensors work non-invasive. This technology is limited to gas mixes with two known gases.
Semiconductive sensors work catalyticly and combust the gas for measurement. Oxygen sensors basing on zirconium oxide, well known as Lambda-Sensor influence the gas indirectly. Oxygen concentration in gas mix is increasing, measurement is only possible at high temperatures.
A Pellistor combust the gas to measure, it belongs to the destructive (invasive) sensors.
For many process gases are invasive and non-invasive measurements available. The measurement method depends not only on the destruction of gas or gas mix. It also depends on other parameters like precision, cross sensitivities, live time and costs.