Oxygen is highly paramagnetic. This outstanding property of paramagnetism is used as a physical measuring effect for oxygen analysis.

Oxygen molecules in an inhomogeneous magnetic field always move toward the higher field strength. This results in a higher oxygen concentration where the field strength is higher (higher oxygen partial pressure). If two gases with differing oxygen content are combined in a magnetic field, a (O₂ partial) pressure difference arises between them.

Since the measuring effect is always based on the difference of the oxygen content of the two gases, one refers to the sample and reference gases.

For measuring oxygen in the OXYMAT 7, the reference gas (N₂, O₂ or air) flows through two channels into the sample chamber (6). One of these partial flows enters the measuring chamber (7) in the area of the magnetic field. If the sample gas is O₂-free, the reference gas can flow out freely. If the sample gas does contain O₂, however, the oxygen molecules concentrate in the area of the magnetic field. The reference gas can then no longer flow off freely. An alternating pressure results between the two reference gas inlets. This pulsates in step with the magnetic field and depends on the oxygen concentration. This causes an alternating flow in the microflow sensor (4).

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 alternating flow results in a change in the resistance of the nickel-plated grids. The resulting offset in the bridge is a measure of the concentration of oxygen in the sample gas.

Because the microflow sensor is located in the reference gas flow, the measurement is not influenced by the thermal conductivity, the specific heat or the internal friction of the sample gas. Additionally, the microflow sensor is protected through this arrangement from corrosion caused by the sample gas.

Further information

The oscillating magnetic field (8) means that the basic flow at the microflow sensor is not detected. The measurement is, thus, independent of the module's operating position or the position of the sample chamber.

The sample chamber is directly in the sample path and has a small volume, and the microflow sensor is a low-lag sensor. As a result, extremely short response times are realized.

Vibrations at the installation site can interfere with the measured signal (e.g. large fluctuations in the output signal). This behavior can be compensated for by a second (optional) microflow sensor (10), which functions as a vibration sensor. Since large differences in density between the sample and reference gases further amplify the undesired influence of vibration, reference gas is channeled to both the compensation microflow sensor (10) and the sample microflow sensor (4).

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

Flowing reference gas prevents the microflow sensor from being damaged and maintains the measurement capability of the analysis module.

OXYMAT 7, principle of operation

Essential characteristics

Technical features

Depending on the reference gas, the physical zero point can be set between 0% and 100% oxygen.

• Smallest measuring spans (up to 0.5% O₂) possible
• Measuring ranges with physically suppressed zero points possible (e.g. 99.5% to 100%)
• Short response time
• Low long-term drift
• Also suitable for use with highly corrosive sample gases (material 1.4571 or Hastelloy C22)
• Monitoring of reference gas pressure with reference gas connection 3 000 to 5 000 hPa (abs.) (option)

Features

• Electrically isolated measured value output 0/4 to 20 mA (also inverted)
• Internal pressure sensor for correction of pressure variations in sample gas in the range from 500 to 2 500 hPa (absolute)
• External pressure sensor - only with piping as the gas path - can be connected for correction of variations in the sample gas pressure up to 3 000 hPa absolute (option)
• Monitoring of reference gas (option)
• Analysis part with flow-type compensation circuit as an order variant for reducing the vibration impact at the installation site
• For sample gas path with hoses: Connection cable to the pressure sensor with hoses
• Hardware adapted to application
• Customer-specific analyzer options such as:
  
  • Drift recording
  • Clean for O₂ service
  • Kalrez gaskets
• Sample chamber for use in presence of highly corrosive sample gases

Reference gases

Table 1: Reference gases for OXYMAT 7

Correction of zero point error/cross-sensitivities

Table 2: Zero point error due to diamagnetism or paramagnetism of some carrier 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 2 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).