Principle 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 contents 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 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).

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 flow, 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 6.

Vibrations frequently occur at the place of installation 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 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.

OXYMAT 6, principle of operation

Advantages of the function-based application of reference gas

• The zero point can be defined specific to the application. It is then also possible to set "physically" suppressed zero points. For example, it is possible when using pure oxygen as the zero gas to set a measuring range of 99.5 to 100 % O₂ with a resolution of 50 vpm.
• The sensor (microflow sensor) is located outside the sample gas. Through use of an appropriate material in the gas path this also allows measurements in highly corrosive gases.
• Pressure variations in the sample gas can be compensated better since the reference gas is subjected to the same fluctuations.
• No influences on the thermal conductivity of the sample gas since the sensor is positioned on the reference gas side.
• The same gas is used for the serial gas calibration and as the reference gas. As a result of the low consumption of reference gas (3 to 10 ml/min), one calibration cylinder can be used for both gases.
• No measuring effect is generated in the absence of oxygen. The measured signal need not therefore be set electronically to zero, and is thus extremely stable with regard to temperature and electronic influences.

Essential characteristics

• Four measuring ranges which can be freely configured, even with suppressed zero point, all measuring ranges are linear
• Measuring ranges with physically suppressed zero point possible
• Measuring range identification
• 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 analyzer can be matched to the respective measuring task
• Short response time
• Low long-term drift
• Measuring point switchover for up to 6 measuring points (programmable)
• Measuring point identification
• Internal pressure sensor for correction of pressure variations in sample gas range 500 to 2 000 hPa (abs.)
• 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 sample gas flow (option for version with hoses)
• Monitoring of sample gas and/or reference gas (option)
• Monitoring of reference gas with reference gas connection 3 000 to 5 000 hPa (abs.) (option)
• Automatic measuring range calibration can be configured
• Operation based on the NAMUR recommendation
• Two control levels with their own authorization codes for the prevention of accidental and unauthorized operator interventions
• Simple handling using a numerical membrane keyboard and operator prompting
• Customer-specific analyzer options such as:
  • Customer acceptance
  • TAG labels
  • Drift recording
  • Clean for O₂ service
  • Kalrez gaskets
• Analyzer unit with flow-type compensation branch: 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 chamber for use in presence of highly corrosive sample gases

Reference gases

Table 1: Reference gases for OXYMAT 6

Correction of zero point error / cross-sensitivities

Table 2: Zero point error due to diamagnetism or paramagnetism of some accompanying gases with reference to nitrogen at 60 °C und 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.