The measuring cell consists of a cylindrical (pipe-shaped) ZrO2 membrane. The sample gas (low O2 content) flows at a constant rate through the inside of the membrane, which is regulated at 650 °C. The exterior of the sensor is exposed to the ambient air (approx. 21 % O2).
Both sides of the ZrO2 membrane are coated with thin platinum films that act as electrodes. This forms a solid, electrochemical cell. The amount of oxygen atoms ionized depends on the oxygen concentration at the electrodes.
The differences in concentration at each side means that a differential partial pressure prevails. Since ZrO2 conducts ions at 650 °C, ionic migration takes place in the direction of the lower partial pressure.
An oxygen gradient arises across the width of the ZrO2 membrane, which, according to equation (1), results in an electrical potential difference between the platinum electrodes.
Defects in the crystal lattice, caused by contamination of the ZrO2 material with Y2O3 and/or CaO (introduced originally to prevent cracks forming in ceramic material) make it easier for O2 ions to diffuse in the ZrO2 grid.
Catalytically active ZrO2 sensor (CAZ)
The electrode material is made of platinum (Pt). This type of sensor has a higher cross-sensitivity when flammable accompanying gas components are present.
Catalytically inactive ZrO2 sensor (CIZ)
The catalytically inactive sensor has the same general design as the CAZ. The contacts and electrode surface inside the pipe are made of a specially developed material which largely prevents catalytic oxidation except of H2, CO and CH4.
OXYMAT 64, principle of operation
Measuring effect
U = UA + RT/4F (In [O2,air] - In [O2] (equation 1)
U measuring effect
UA asymmetric voltage (voltage, at [O2] = [O2,air]
T ceramic temperature
[O2,air] O2 concentration in the air
[O2] O2 concentration in sample gas
Note
The sample gas must be fed into the analyzer free of dust. Condensation should be avoided. Therefore, gas modified for the measuring tasks is necessary in most application cases.
Calibration of the calibration point is carried out as with the other analyzers of Series 6 after a maximum of 14 days by connecting the calibration gas O2 in residual N2 at concentrations of approx. 60 to 90% of the master measuring range.
Contrary to the other analyzers of Series 6, the zero point calibration cannot be carried out using pure nitrogen, but with a "small" concentration of oxygen in nitrogen appropriate to the selected measuring range (e.g.: measuring range 0 … 10 vpm; calibration gas approx. 2 ppm O2 in residual N2).
Essential characteristics
- Four measurement ranges freely parameterizable, all measurement ranges linear
- Galvanically isolated measurement value output 0/2/4 through 20 mA (also inverted) and as per NAMUR
- Autoranging selectable; possibility of remote switching
- 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 the sample gas (via pressure switch)
- Customer-specific analyzer options such as:
- Customer acceptance
- TAG labels
- Drift recording
- Simple handling using a numerical membrane keyboard and operator prompting
- Smallest span 0 to 10 vpm O2
- Largest span 0 to 100 % (testing with ambient air)
- Internal pressure sensor for correction of the influence of sample gas pressure fluctuations
Influence of interfering gas
Catalytically active sensor (CAZ)
Very large cross-interference of all combustible accompanying gases. Thus not suitable for use with combustible accompanying gases!
Catalytically inactive sensor (CIZ)
There is only a slight cross-interference in the case of accompanying gases with a concentration in the range of the O2 concentration. H2, CO and CH4 still have a noticeable effect in the case of flammable accompanying gas components.
|
Measured component / interfering gas |
Diagonal gas offset |
|---|---|
|
78 vpm O2/140 vpm CO |
-6.1 vpm |
|
10 vpm O2/10 vpm CO |
-0.6 vpm |
|
74 vpm O2/ 25 vpm CH4 |
-0.3 vpm |
|
25 vpm O2/ 357 vpm CH4 |
-1.1 vpm |
|
25 vpm O2 / 70 vpm H2 |
-3 vpm |
|
5 vpm O2 / 9.6 vpm H2 |
-0.55 vpm |
|
170 vpm O2 / 930 vpm C2H4 |
-118 vpm |
Examples of typical diagonal gas offsets on a catalytically inactive sensor
The listed deviations depend on the exemplar and can deviate up to ± 0.2 vpm. The actual deviation must be determined individually or the error will be eliminated through a corresponding calibration measure (displacement of the diagonal gas offset).