The measuring method is based on the different levels of thermal conductivity of gases. CALOMAT 7 modules work with a micromechanically produced Si chip, the measuring membrane of which is equipped with thin-film resistors.

The resistors contained in the diaphragm are regulated for constant temperature. The amperage required fluctuates in accordance with the thermal conductivity of the sample gas. This raw value determined in this way is processed further electronically to calculate the gas concentration.

The sensor is in a thermostatically controlled stainless steel enclosure in order to suppress the effect of the ambient temperature. To rule out flow influences, the sensor is mounted in a bore hole next to the flow channel.

Note

The sample gases must be fed into the analyzers free of dust. Condensation (dew point sample gas < ambient temperature) is to be avoided in the sample chambers. Therefore, the use of gas modified for the measuring tasks is necessary in most application cases.

CALOMAT 7, mode of operation

Essential characteristics

• Four measuring ranges which can be freely configured, even with suppressed zero point, all measuring ranges are linear
• Smallest spans down to 0.5% H₂ (with suppressed zero: 95 to 100% H₂ ) possible
• Autoranging or manual measurement range switchover possible; remote switching is also possible
• Storage of measured values possible during adjustments
• Time constants can be selected within wide ranges (static/dynamic noise suppression); i.e. the response time of the device can be adapted to the respective measuring task.
• Short response time
• Low long-term drift
• Measuring point switchover for up to 6 measuring points (programmable)
• Measuring range identification
• Measuring point identification
• External pressure sensor can be connected – for correction of variations in sample gas pressure
• Automatic measuring range calibration can be configured
• Operation based on the NAMUR recommendation

Cross-interferences

To determine the cross-interferences of accompanying gases with several interfering gas components, you must know the sample gas composition. The following table contains the zero offsets for the carrier gas N₂ as H₂ equivalent values with 10% interference gas

Zero offset in the system H₂ in N₂

If you are using accompanying gas concentrations ? 10%, you can use the corresponding multiples of the respective table value as an approximation. This procedure applies depending on the type of gas for an accompanying gas concentration range up to approx. 25%.

The thermal conductivity of most gas mixtures has a non-linear response. Even ambiguous results can occur in specific concentration ranges, e.g. with H₂ in He mixtures.

In addition to the zero offset, the accompanying gas also affect the characteristic curve. For most gases, however, the effect on the characteristic curve is negligible.