Mode of operation

In contrast to almost all other gases, oxygen is paramagnetic. This property is utilized as the measuring principle by the OXYMAT 6 gas analyzers.

Oxygen molecules in an inhomogeneous magnetic field are drawn in the direction of increased field strength due to their paramagnetism. When two gases with different oxygen concentrations meet in a magnetic field, a pressure difference is produced between them.

In the case of OXYMAT 6, one gas (1) is a reference gas (N₂, O₂ or air), the other is the sample gas (5). The reference gas is introduced into the sample cell (6) through two channels (3). One of these reference gas streams meets the sample gas within the area of a magnetic field (7). Because the two channels are connected, the pressure, which is proportional to the oxygen concentration, causes a cross flow. This flow is converted into an electric signal by a microflow sensor (4).

The microflow sensor consists of two nickel grids heated to approx. 120 ?C which form a Wheatstone bridge together with two supplementary resistors. The pulsating flow results in a change in the resistance of the Ni grids. This results in a bridge offset which depends on the oxygen concentration in the sample gas.

Because the microflow sensor is located in the reference gas stream, the measurement is not influenced by the thermal conductivity, the specific heat or the internal friction of the sample gas. This also provides a high degree of corrosion resistance because the flow sensor is not exposed to the direct influence of the sample gas.

By using a magnetic field with alternating strength (8), the effect of the background flow in the microflow sensor is not detected, and the measurement is thus independent of the instrument orientation.

The sample cell is directly in the sample path and has a small volume. The microflow sensor thus responds quickly, resulting in a very short response time for the OXYMAT 6.

Vibrations frequently occur at the place of measurement and may falsify the measured signal (noise). A further microflow sensor (10) through which no gas passes acts as a vibration sensor. Its signal is applied to the measured signal as compensation.

If the density of the sample gas deviates by more than 50% from that of the reference gas, the compensation microflow sensor (10) is flushed with reference gas just like the measuring sensor (4).

Note

The sample gas needs to be free of dust. Condensate in the cells must be avoided. That is why the most measuring tasks require an appropriate gas preparation.

OXYMAT 6, mode of operation

Essential characteristics

• Four freely parameterizable measuring ranges, also with zero offset, all measuring ranges linear
• Measuring ranges with physical zero offset possible
• Measuring range identification
• Electrically isolated signal output selectable as 0/2/4 to 20 mA (also inverted)
• Autoranging or manual range switching possible; remote switching is also possible
• Storage of measured values possible during adjustments
• Time constants selectable within wide limits (static/dynamic noise suppression); i.e. the response time of the analyzer can be matched to the respective application
• Short response time
• Low long-term drift
• Measuring-point selection for up to 6 measuring points (programmable)
• Measuring point identification
• Internal pressure sensor for correction of pressure variations in sample gas (range 500 to 2000 hPa absolute)
• External pressure sensor can be connected for correction of variations in sample gas pressure up to 3000 hPa absolute (option)
• Monitoring of sample gas flow (option for viton tubed version)
• Monitoring of sample gas and/or reference gas (option)
• Monitoring of reference gas with reference gas connection 2000 to 4000 hPa (option)
• Automatic range calibration can be parameterized
• Operation based on NAMUR Recommendation
• Two-stage access code to prevent unintentional and unauthorized inputs
• Simple handling using a numerical membrane keypad including operator prompting
• Customer-specific analyzer options such as e.g.:
  • Customer acceptance
  • Tag labels
  • Drift recording
  • Clean for O₂-Service
  • Kalrez gaskets
• Analyzer section with flow-type compensation circuit: a flow is passed through the compensation branch (option) to reduce the vibration dependency in the case of highly different densities of the sample and reference gases
• Sample cell for use in presence of highly corrosive sample gases.

Reference gases

Table 1 Reference gases for OXYMAT 6

Correction of zero error / Cross interferences

Table 2 Zero error due to diamagnetism or paramagnetism of residual gases with nitrogen as the reference gas at 60 °C and 1000 hPa absolute (according to IEC 1207/3)

Conversion to other temperatures:

The zero errors mentionned in Table 2 must be multiplied with a correction factor (k):

• with diamagnetic gases: k = 333 K / (? [°C] + 273 K)
• with paramagnetic gases: k = [333 K / (? [°C] + 273 K)]²

(all diamagnetic gases have a negative zero error).