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 concentrations 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 concentration, causes a cross flow. This flow is converted into an electric signal by a microflow sensor (4).

OXYMAT 61, mode of operation

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 61.

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.

Special characteristics

• Four freely-parameterizable measuring ranges, also with zero offset, all measuring ranges linear
• 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 calibration
• 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
• Simple handling using menu-based operation
• Low long-term drift
• Two operation levels with separate access code to prevent unintentional and unauthorized inputs
• Automatic range calibration can be parameterized
• Operation based on NAMUR Recommendation
• Monitoring of sample gas (option)
• Customer-specific analyzer options such as e.g.:
  • Customer acceptance
  • TAG labels
  • Drift recording
• Simple handling using a digital membrane keyboard and menu-based operation
• Short response time
• Reference gas supply external (N₂, O₂ or air, approx. 3000 hPa) or via an integrated reference gas pump (ambient air, approx. 100 hPa)
• Monitoring of reference gas with reference gas connection 3000 to 4000 hPa
• Different smallest spans, depending on version 2.0% or 5.0% O₂
• Internal pressure sensor to correct sample gas variations.

Correction of zero error / Cross interferences

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 the table 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).

Reference gases