1. Archigas TCD3000 WLD

Advantages:

• Very high reaction speed (30 ms)
• High long-term stability
• Low maintenance requirements
• Robust against moisture in the process gas
• No position dependency
• Very large, dynamic measuring range from a few ppm to 100 vol.%
• Use at high pressure
• Very low flow dependency
• Less sample preparation
• High detection limit

Disadvantages:

• No selectivity
• Pressure dependence (but can be compensated)

2. Other WLD

Advantages:

• High long-term stability
• Low maintenance requirements
• Dynamic measurement possible
• Linear measurement behavior

Disadvantages:

• Often flow-dependent
• Sample preparation required
• Pressure-dependent (but can be compensated)
• Average reaction speed (1 s – 120 s)
• Not robust against moisture in the process gas

3. Catalytic (pellistor)

Advantages:

• Low price
• Sensitive to sample gas
• Higher selectivity

Disadvantages:

• Only small measuring ranges
• Non-linear measuring behavior
• Low reaction speed (10-80 s)
• Irreversible damage due to contact with e.g. halogens or combustion reactions without oxygen
• High maintenance requirements
• Not robust against moisture

4. Semiconductor

Advantages:

• Low price
• Good detection limit
• Low maintenance requirements
• High sensitivity
• Very good selectivity

Disadvantages:

• No concentration measurement possible
• Average reaction times

5. Electrochemical

Advantages:

• Very low price
• Low energy consumption (< 2 W)
• High sensitivity
• High selectivity

Disadvantages:

• Slow reaction speed
• Low long-term stability
• High maintenance requirements
• Not robust against moisture in the process gas
• Position dependent

The gas analyzers are designed for the measurement of binary and quasi-binary gas mixtures (e.g. H₂ in air, CH₄ in CO₂ etc.)

A gas mixture consisting of two gases: measuring gas and background gas

A quasi-binary gas mixture consists of more than two gases (e.g. air) – whereby the background gas mixture consists of several gases that are present in stable, low concentrations or have similar thermal conductivities.

Cross sensitivity describes the undesirable reaction of a sensor to gases or other influences such as pressure or temperature that are not the target of the measurement. This distorts the measurement result and affects the accuracy of the measurement. Cross sensitivity represents a challenge in the accurate measurement of gas concentrations. By carefully selecting and applying measurement technologies and using suitable calibration and compensation methods, the effect of cross sensitivity can be reduced in order to achieve more reliable and accurate measurement results.

Target gas: hydrogen (H₂) Background gas: oxygen (O₂) Third gas: nitrogen (N₂)
 Unknown parameters: concentration, pressure and humidity

Explanation:

 This binary gas mixture consists of the target gas hydrogen (H₂) and the background gas oxygen (O₂). It is assumed that this mixture will be used in various applications such as fuel cells or electrolysis processes.

Problem of cross sensitivity: If nitrogen (N₂) is also present in this binary mixture in an unknown concentration, this can affect the measurement. The cross-sensitivity of the sensor to nitrogen could lead to the displayed concentration of hydrogen being distorted and displayed as a negative value. The reason for this is the lower thermal conductivity of nitrogen compared to oxygen.

Other unknowns: The pressure of the gas mixture and the humidity are unknown, which further complicates the measurement. Both pressure and humidity can affect the accuracy of the measurement.

Minimization strategies

1. Calibration of the sensor: If the nitrogen content in the gas mixture is constant, regular calibration of the sensor with a known gas mixture containing nitrogen can minimize the effects of cross-sensitivity.
2. Adjusting the measuring range: If nitrogen is introduced as part of the process, the device can be calibrated so that 4 mA corresponds to pure nitrogen and 20 mA, for example, 4 vol.% H₂ in O₂. In this way, H₂ in O₂ and O₂ in N₂ can be determined with one device.
3. Compensation algorithms: If one of the interference components is known from other measuring instruments, the influence of this component on the measurement result can be corrected using mathematical algorithms. Thermal conductivity is very suitable for calculation because the cross-influences are reliably reproducible.

The pressure of a gas mixture influences the measurement of thermal conductivity. This pressure influence depends on the type of gas. For gases with lower thermal conductivities such as oxygen, the dependency is lower. For gases with high thermal conductivity such as hydrogen, the dependency is higher. This influence is greatest at low pressure, especially in the range of 1 to 2 bar.

As the pressure increases, the density of the gas increases, resulting in a more stable thermal conductivity. From a pressure of around 7 bar, the influence of pressure on the thermal conductivity measurement is less and becomes smaller and more linear as the pressure increases. This means that at high pressure, the measurements are more accurate and less susceptible to fluctuations.

By using compensation algorithms, available on request for Archigas devices, this influence can be effectively eliminated, enabling precise measurements even under variable pressure conditions.