Sensor technologies and measuring principles

The gases to be measured, their expected concentrations and other factors such as cross-sensitivities to other gases, ambient conditions and explosion protection determine which sensor technology(ies) should be used. With our wide range of stationary transmitters, portable gas detection devices and various sensors, you are able to reliably monitor combustible and toxic gases, oxygen and volatile vapours.

Catalytic Combustion (CC)


Catalytic combustion, also called heat tone (HT), is a proven measuring principle for detecting flammable gases and vapours to the lower explosion limit (LEL). Two sensors (detector and reference) are connected via a "Wheatstone Bridge" circuit. A combustible gas or gas mixture burns on contact with the catalytic detector sensor and oxygen. The heat generated increases the electrical resistance. This produces a measurable current flow that is proportional to the concentration of the combustible gas.

  • Sum measurement of combustible gases and vapours
  • 0 - 100 % LEL
  • High measuring accuracy
  • Linear display behaviour

Thermal Conductivity (TC)


With thermal conductivity (TC), toxic and flammable gases can be measured in high concentrations of up to 100% by volume. The functional principle is similar to catalytic combustion, including Wheatstone's bridge circuit. The difference is that the gas measurement is not carried out via the flammability, which eliminates the dependence on the presence of oxygen. In addition to the gas to be monitored, a second gas such as air with a different thermal conductivity is required as a reference for the measurement.

  • Toxic and flammable gases
  • Wide measuring range (up to 100 vol.%)
  • Suitable for a variety of applications

Photoionization Detector (PID)


In a photoionisation detector (PID), air is sucked in and exposed to ultraviolet light in the sensor. The photons in the UV light cause certain molecules to decompose into positively charged ions and electrons. A measurable current flow is created between the electrodes in the measuring chamber, which the detector converts into a measured value proportional to the gas concentration. Target gases of photoionisation are volatile organic compounds (VOCs) such as solvents and petrol, diesel, heating oil or paraffin vapours. They are harmful to health even in very low concentrations. Photoinonisation detectors can monitor over 300 of these substances in groups or individually - many already in concentrations of less than 1 ppm.

  • Volatile organic substances
  • Short response times
  • Very high sensitivity
  • Many measuring gases

Zirconium Dioxide (ZD)


For oxygen measurement, this measuring principle is used with an electrochemical oxygen pump cell made of zirconium dioxide. At high temperatures (> 650 °C), zirconium dioxide behaves like an electrolyte for oxygen, firstly transporting oxygen ions and secondly generating measurable current in the event of a partial pressure difference between the two sides of the membrane. The measuring principle is insensitive to environmental influences and is suitable for measurements in the percentage as well as in the trace range (ppm).

  • Selective for oxygen
  • Very short response time
  • Insensitive to ambient conditions
  • Long lifetime

Infrared (IR)


The infrared measuring method uses the property of some gases to absorb light in certain wavelength ranges (bands), whereas this is not the case for the main natural components of air (nitrogen, oxygen and argon). Two infrared beams of different wavelengths (measuring and reference beam) are guided into the measuring chamber and finally hit two detectors (measuring and reference detector). If the measuring beam is weakened by the absorption of a gas present, the reduced intensity corresponds to the gas concentration. Gases that can be detected by infrared include all heteroatomic gases such as carbon dioxide and hydrocarbon compounds.

  • Combustible gases and CO2
  • Low cross-sensitivity
  • High accuracy
  • Long lifetime

Electrochemical (EC)


An electrochemical measuring cell is similar to a battery in the way it functions. The gas to be measured diffuses through a membrane into the sensor, which consists of three electrodes (working, reference and counter electrode) and a conductive electrolyte. The individual components are adapted to the gas to be measured. The reaction with the working electrode creates a flow of ions to the counter electrode. The measured current corresponds to the concentration of the gas to be monitored. The electrochemical measuring method is suitable for selective measurement of a specific gas.

  • Toxic gases, O2 and H2
  • Linear display behaviour
  • Very energy efficient
  • High sensitivity

Chemisorption (CS)


In chemisorption, the sensor element consists of a metal oxide semiconductor (e.g. tin dioxide) located in a measuring chamber with a flame arrester. Oxidation of the gas being monitored at the sensor element increases the electrical conductivity. The current flow is converted into an output signal corresponding to the gas concentration. The temperature of the sensor element is adjusted according to the gas to be measured. Chemisorption is suitable for detection of a wide range of combustible and toxic gases and is characterised by long-life sensors and low cost.

  • Flammable and toxic gases
  • Cost-effective
  • Different measuring ranges (vol.-%, LEL, ppm)
  • Long lifetime
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