A mixture of gases with different thermal conductivities (abbreviated to TC) has a thermal conductivity that depends on the concentration of the components. The proportions of the individual gases can be determined by measuring the thermal conductivity.
The thermal conductivity detector (abbreviated TCD) is used today primarily for the measurement of noble gases (He, Ar, Ne, Kr, etc.) and homonuclear gases (H2, N2, etc.), which are not accessible to other simple and robust online analysis methods.
The principle is particularly applicable when the TC of the gas to be measured differs significantly from the TC of the accompanying gas and one of the three of the following three criteria is fulfilled:
Cross-sensitivity compensation extends the range of application of thermal conductivity measurement for non-(quasi-)binary gases.
Cross-sensitivity compensation means that the thermal conductivity measurement is combined with signals from other gas analysis methods. The determination of the thermal conductivity alone is sufficient to determine the composition of a two-component mixture. With a three-component mixture, there might be several gas compositions with the same thermal conductivity. A measurement with an additional sensor, which selectively determines one of these components, resolves this ambiguity. A compensation model, previously determined experimentally and verified by measurements, combines the signals from the thermal conductivity sensor and the additional sensor in the analyzer in real time. The signals can be measured using an integrated additional sensor, e.g. humidity, pressure, oxygen or infrared radiation as in the FTC400. Alternatively, signals can be fed into the device from outside using a 0 – 10 V input.
The error caused by fluctuating sample gas pressure is corrected
Measuring principles such as infrared absorption or the electrochemical O2 sensor determine amounts
of substance – their signals are proportional to the partial pressure. The thermal conductivity signal is
only slightly pressure-dependent, only below 800 hPa absolute, a stronger pressure dependence is
seen.
Using integrated pressure measurement and compensation models, the pressure dependence of the measurement signal can be compensated
online. The displayed concentration is then only very slightly dependent on the pressure.
The pressure is also available as a measured variable.
Selective humidity measurement with capacitive sensor
The gases to be determined in analytical technology are often loaded with humidity.
If the humidity is not a measured variable of interest and the humidity content is relatively constant, its content can be included in the calibration.
However, if the humidity measurement itself is of interest or if a varying humidity content leads to cross-sensitivities, the capacitive humidity sensor should be integrated. This enables compensation.
Selective electrochemical oxygen measurement
The oxygen sensor selectively determines the partial pressure of gaseous oxygen. The oxygen is in exchange with a liquid, slightly acidic electrolyte through a diffusion-open PTFE membrane. Electrochemical reactions at the cathode and anode provide the measuring electric current, which is proportional to the oxygen partial pressure. Due to the acidic electrolyte, it can also be used for acidic
gases, e.g. CO2. Measurement is also possible in the presence of hydrogen.
The conversion from partial pressure to volume percent depends on the absolute pressure of the sample gas. Therefore, an absolute pressure measurement is integrated in the analyzer, which ensures the correct determination of the concentration in Vol.% even if the absolute pressure varies.
Depending on the humidity and oxygen content of the sample gas, the lifespan of the sensor is typically 1 to 3 years. The sensor can be replaced by the customer.
Gas type-independent flow measurement for binary mixtures in the range from 0 l/h to 130 l/h
A closed valve or a plugged filter may accidentally interrupt flow through the analyzer. In this case, the gas measurement cannot monitor the process reliably. Flow measurement can help avoid these kind of potential errors while giving additional information about the process.
For flow measurement, the pressure drop across a vortex-free flow restrictor is determined in the gas path. The pressure drop is a measure of the gas flow but depends on the type of gas. However, as the composition of the gas is known from the thermal conductivity measurement, this dependency is balanced out by calculation.
This enables gas flow measurement independent of the gas type, showing how much process gas is actually flowing through the analyzer.
Selective measurement of infrared-active gases
The measuring principle of thermal conductivity is only suitable for (quasi-) binary gas mixtures. Molecules such as CO2, CH4, C2H6, CO, NO, SO2 and H2O absorb infrared radiation. The position of the absorption in the wavelength domain is a “fingerprint” and characteristic of a molecule.
The level of absorption is a measure of the amount of the gas in question. The selective measurement of up to three infrared-active gases in a mixture is made possible by a detector that uses three interference filters to measure the absorption at three different wavelengths. The selection of the interference filters thus determines which gases are analyzed.
The combination of an IR sensor with thermal conductivity measurement enables the complete determination of complex gas mixtures that also contain homonuclear or noble gases.
Protective measures against condensate and dust
Condensate in the sample gas usually leads immediately to the failure of the sensor element and thus to the failure of the unit. Dust, dirt, particles or chips can also destroy the sensor element.
A hydrophobic filter separates the sensor element from the sample stream. With pore sizes in the μm range, the filter is impermeable to condensate and dust. The gas exchange of the atoms and molecules of the sample gas takes place by diffusion with virtually no time delay.
If condensate enters a device with a device with a membrane, the measurement is impaired for a few minutes, but the sensor is very effectively protected against total damage. After the condensate
has evaporated, the device is ready to measure again. Loose soiling cannot reach the sensor element.
Protection against corrosive attacks
Only high-quality materials such as ceramic, stainless steel 316Ti and AISI 316L are exposed to the sample gas.
However, the sensor element itself and its electrical contact represent a point of attack. Corrosive attacks can damage the sensor element and thus lead to the failure of the device.
In order to protect the micromechanical sensor element Messkonzept has developed an effective protective coating made of an inert polymer. It reliably covers all parts that are susceptible to corrosion. A sensor element with a coating is very effectively protected against corrosion.
The cavities in the enclosure are tightly filled with small glass beads to minimize the free volume
The handling of flammable gases requires additional measures to ensure a high level of safety even in the event of a fault. If flammable gases are to be introduced into an analyzer with a housing, we fill the cavities with glass beads.
These beads, diameter of 0.6 mm, are shaken in so that all cavities inside the housing are tightly filled. Of course, before the analyzer is put into operation, the sample gas path must be checked for leaks.
In the unlikely event of a leak in the analyzer’s internal gas path, there is only a minimal free residual volume in the housing, which further increases safety.
Each of our devices can be easily switched between different measurement tasks
It is often desirable to perform different measuring tasks, with one measuring instrument.
The required gas pair can be selected comfortably through the display or via the digital interface. The composition and number of gas pairs, that the Multi Gas Mode covers is completely flexible and is set up
according to customer requirements.
Niedwiesenstr. 33
60431 Frankfurt am Main
Germany
Telephone: +49 (0) 69 53056444
Telefax: +49 (0) 69 53056445
Email: info@messkonzept.de
