Principle of operation

The measuring principle is based on the different thermal conductivity of gases.

The temperature of a heated resistor surrounded by gas is determined by the thermal conductivity of the gas. Four such resistors are connected as a bridge.

Sample gas flows around two of them, reference gas surrounds the other two. A constant DC voltage heats the resistors above the temperature of the measurement block.

The different thermal conductivities of the sample and reference gases result in different temperatures of the resistors. A change in the composition of the sample gas thus also causes a change in the resistance values.

The electrical equilibrium of the measuring bridge is disrupted, and a voltage is generated in the bridge diagonal. This is a measure of the concentration of the measured component.

Note

The sample gases must be fed into the analyzers free of oil, grease, and dust. The formation of condensation in the sample chambers (dew point of sample gas < ambient temperature) must be avoided. Therefore, gas prepared for the respective task must be provided in most applications.

CALOMAT 62, principle of operation, example of a non-flow-type reference chamber

Important features

• Four freely-programmable measuring ranges, also with suppressed zero, all ranges linear
• Smallest spans down to 1 % H₂ (with suppressed zero: 99 to 100 % H₂) possible
• Measuring range identification
• Electrically isolated measured-value output 0/2/4 to 20 mA (also inverted)
• Automatic or manual measuring range switchover selectable; remote switching is also possible
• Measured value can be saved during adjustment
• Time constants are selectable within wide ranges (static/dynamic noise suppression); i.e. the response time of the analyzer can be adapted to the respective task
• Short response time
• Low long-term drift
• Measuring point switchover for up to 6 measuring points (parameterizable)
• Measuring point identification
• External pressure sensor can be connected – for correction of variations in sample gas pressure
• Possibility for correcting the influence of residual gases (correction of cross-interference)
• Automatic measuring range calibration can be programmed
• Operation based on the NAMUR recommendation
• Two operator input levels with their own authorization codes to prevent unintentional and unauthorized interventions
• Simple handling using a numerical membrane keyboard and operator prompting
• Customer-specific device versions, such as:
  • Customer acceptance
  • TAG labels
  • Drift recording
  • Clean for O₂ service

Spans

The smallest and largest possible spans depend on both the measured component (gas type) and the respective application (see ordering data).

Cross-interferences

Information on the sample gas composition is required in order to determine the cross-interference of residual gases with several interfering components.

The zero offsets in % H₂ which result from 1 % residual gas (interfering gas) are listed in the following table; the specified values are approximate values.

It should be noted that the influence of interfering gas is not linear to its concentration. Information on the sample gas composition is required in order to determine the cross-interference of residual gases with several interfering components.

Effect of 1 % gas component with nitrogen as the residual gas, expressed in % H₂

Moreover, it must be noted that - in addition to a zero offset - the gradient of the characteristic can also be affected by the residual gas. However, this effect is negligible in the case of variations in the interfering gas concentration below 10 %.

Taking these facts into consideration and due to the fact that the cross-interference analyzers cause further measuring inaccuracies, a larger error in measurement occurs than with digital gas mixtures despite correction of cross-interference.

Specification for the interface cable

Bus terminating resistors

Pins 3-7 and 8-9 of the first and last connectors of a bus cable must be bridged (see graphic).

Note

It is advisable to install a repeater on the device side in the case of a cable length of more than 500 m or with high interferences.

Up to four components can be corrected via the ELAN bus, correction of cross-interference can be carried out for one or two components via the analog input.

Bus cable with plug connections, example