In contrast to almost all other gases, oxygen is paramagnetic. This property is utilized as the measuring principle by the OXYMAT 61 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 contents meet in a magnetic field, a pressure difference is produced between them.

In the case of OXYMAT 61, 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 chamber (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 content, causes a cross flow. This flow is converted into an electric signal by a microflow sensor (4).

OXYMAT 61, principle of operation

The microflow sensor consists of two nickel-plated grids heated to approximately 120 °C, which, along with two supplementary resistors, form a Wheatstone bridge. The pulsating flow results in a change in the resistance of the Ni grids. This leads to an offset in the bridge which is dependent on the oxygen concentration of 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 microflow 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's operating position.

The sample chamber is directly in the sample path and has a small volume, and the microflow sensor is a low-lag sensor. This results in a very short response time for the OXYMAT 61.

Note

The sample gases must be fed into the analyzers free of dust. Condensation should be prevented from occurring in the sample chambers. Therefore, gas modified for the measuring tasks is necessary in most application cases.

Essential characteristics

• Four freely parameterizable measuring ranges, also with suppressed zero point, all measuring ranges linear
• Galvanically isolated measured-value output 0/2/4 to 20 mA (also inverted)
• Autoranging possible; remote switching is also possible
• Storage of measured values possible during adjustments
• Wide range of selectable time constants (static/dynamic noise suppression); i.e. the response time of the device can be adapted to the respective measuring task
• Easy handling thanks to menu-driven operation
• Low long-term drift
• Two control levels with their own authorization codes for the prevention of accidental and unauthorized operator interventions
• Automatic, parameterizable measuring range calibration
• Operation based on the NAMUR recommendation
• Monitoring of sample gas (option)
• Customer-specific analyzer options such as:
  • Customer acceptance
  • TAG labels
  • Drift recording
• Simple handling using a numerical membrane keyboard and operator prompting
• Short response time
• Reference gas supply either externally (N₂, O₂ or air, approx. 3 000 hPa) or via built-in reference gas pump (ambient air, approx. 1 100 hPa abs.)
• Monitoring of reference gas with reference gas connection; only on version with built-in reference gas pump
• Different smallest measuring ranges, depending on version 2.0 % or 5.0 % O₂
• Internal pressure sensor for correction of fluctuations in the sample gas pressure

Correction of zero error / cross-sensitivities

Table 1: Zero error due to diamagnetism or paramagnetism of some accompanying gases with nitrogen as the reference gas at 60 °C and 1 000 hPa absolute (according to IEC 1207/3)

Conversion to other temperatures:

The deviations from the zero point listed in Table 1 must be multiplied by 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 deviation from zero point.

Reference gases