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- Measurement principles in emission measuring instruments: NDIR & electrochemical sensors | Testo
Measurement principles in emission measuring instruments:
NDIR & electrochemical sensors
Emission measuring instruments mainly use NDIR measurement cells and electrochemical sensors to reliably determine concentrations of gases such as CO, CO2, NO, NO2 or SO2 and output them in mg/m3 or vol. %. Both measurement principles have specific strengths and are each suitable for certain components, measuring ranges and instrument concepts.
NDIR measurement – infrared technology for gas analyses
NDIR stands for non-dispersive infrared measurement and is mainly used to determine gases with pronounced IR absorption bands – such as CO, CO2 or NO. The operating principle is based on molecules absorbing infrared radiation at certain wavelengths. An NDIR analyser sends infrared light through a measuring cuvette filled with sample gas. A detector measures the radiation attenuated by the gas. The gas concentration is calculated from the ratio of emitted to detected intensity using the Lambert-Beer law.
A typical NDIR measurement cell includes:
an IR light source,
a measuring cuvette with a defined length,
optical filters for selecting the relevant wavelengths,
an IR detector,
electronics for signal evaluation and concentration calculation.
In emission measuring instruments, several NDIR channels are often combined to measure different gases in parallel. Measuring ranges can be adapted to different requirements via cuvette length, optical filters and signal processing – from low ppm ranges up to the percent range.
Electrochemical sensors – compact multi-component measurement
Electrochemical sensors are mainly used for O2, CO, NO, NO2 and SO2 and are particularly suitable for compact, mobile emission measuring instruments. They consist of a measuring cell with electrolyte and electrodes. The gas to be measured diffuses into the cell and reacts electrochemically. The resulting current is proportional to the gas concentration.
Advantages of electrochemical sensors are their compact design, the ability to combine multiple sensors in one instrument and comparatively short response times. They make it possible to realise multi-gas emission measuring instruments in a portable design that can be used flexibly in everyday service work.
At the same time, electrochemical sensors have a limited service life, which depends on operating temperature, gas exposure and frequency of use. Regular checks and, if necessary, replacement of sensors are therefore part of a professional maintenance concept.
Strengths, limitations and typical applications
NDIR measurement cells are characterised by high long-term stability and selectivity for certain gases. They are predestined for stationary emission measurement systems and applications in which measurements are taken continuously and over long periods of time. Typical use cases include:
continuous CO and CO2 measurement in large combustion plants,
measurement of NO in energy and industrial plants,
integration in CEMS in accordance with relevant standards.
Electrochemical sensors score points for flexibility and compactness. They are preferably used in mobile emission measuring instruments that need to record several components simultaneously during service and commissioning operations. Typical applications include:
commissioning and maintenance of boiler and burner systems,
campaign measurements on CHP plants or industrial plants,
comparative measurements and plausibility checks.
When NDIR, when electrochemical?
The decision for a measurement principle depends on several factors:
Measurement task: continuous monitoring vs. recurring checks,
required accuracy and long-term stability,
number and type of components to be measured,
available installation space and ambient conditions.
In many systems, both technologies are combined: electrochemical sensors complement NDIR measurement cells to cover additional components or support mobile scenarios. For users, it is important to know which components are measured using which principle and what maintenance requirements result from this.
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