Thermal Conductivity Detector Method (TCD)

Table of Contents

• What is the Thermal Conductivity Detector Method (TCD) ?
• Structure and Operating Principles of Gas Analyzer Using TCD
• Structure and Operating Principle of a Hydrogen (H₂) Gas Analyzer
• Reduction of Factors Influencing the Measurement
• Related Products

Gas analyzers using TCD detect changes in the thermal conductivity of a sample gas using a thermal wire sensor (thermal sensor) to measure the concentration of the sample gas. The thermal sensor detects changes in the thermal conductivity of the gas as changes in electrical resistance. To perform this detection with high accuracy, a bridge circuit that combines four thermal sensors is used. (Figure 1: Bridge circuit of TCD)

Figure 1: Basic structure (bridge circuit) and principle of operation of a gas analyzer using TCD

The TCD's bridge circuit features four thermal sensors (electrical resistances) with identical specifications integrated into the detector. Two of these sensors are situated in each of the two sample cells, while the remaining two are located in the two reference cells. This configuration forms an electrical circuit comprising two resistances that vary identically within the sample cells and two resistances that remain constant within the reference cells.

The bridge voltage ("E" in Figure 1) and output voltage ("V" in Figure 1) by a measurement function in the bridge circuit are actually located in the signal processing section.

As the concentration of the sample gas drawn into the sample cell fluctuates, the thermal conductivity of the gas in the cell changes accordingly. Consequently, the surface temperature of the thermal sensor within the cell is also affected. Changing temperature in a thermal sensor is detected as changing electrical resistance. Since the reference cell is filled with nitrogen (N₂), the electrical resistance detected in the reference cell is always constant. By combining these four electrical resistances and signal processing, the change in the concentration of the sample gas is detected as a change in the output voltage of the bridge circuit ("V" in Figure 1). Under certain conditions, this output voltage is proportional to the concentration of highly thermally conductive gas in the sample gas, so measuring the output voltage of the bridge circuit measures the concentration of highly thermally conductive gas.

Structure and Operating Principle of a Hydrogen (H₂) Gas Analyzer

Hydrogen (H₂) has the highest thermal conductivity of all gases, so the TCD can be used to measure the concentration of hydrogen with high accuracy. （Table 1: Thermal conductivity of gas）　
This section describes the hydrogen (H₂) gas analyzer, which uses TCD to measure hydrogen as a measured component in the sample gas.

Figure 2 shows an example of the structure of a detector for a hydrogen gas analyzer using TCD. Two sets of sample and refernce cells are incorporated into a stainless steel detector. Each cell incorporates a thermal sensor (electrical resistance). 

Figure 2: Structure and operating principle of the detector of hydrogen (H₂) gas analyzer

Sample gas is drawn into two sample cells, and the sample gas diffuses within each sample cell, causing changes in thermal conductivity. For example, hydrogen has the highest thermal conductivity, so as the concentration of hydrogen in the sample gas decreases and the concentration of other gases increases, the overall thermal conductivity of the sample gas decreases. This change in the thermal conductivity of the sample cell changes the surface temperature of the thermal sensor, resutling in a change of its electrical resistance.

Since two reference cells are filled with nitrogen (N₂), the thermal conductivities in the reference cells are constant, so the electrical resistances of thermal sensors are always constant. The output voltage of the bridge circuit composed of these four electrical resistances is detected by the signal processing. Under certain conditions, this output voltage is proportional to the concentration of hydrogen in the sample gas so that the concentration of hydrogen can be measured.

Reduction of the Influence for the Change of the Temperature in the Detector

The thermal conductivity of the gas is influenced by temperature. Under the same pressure, the thermal conductivity increases as the temperature of the gas increases. Therefore, temperature changes on the inner surfaces of the sample and reference cells will influence the measurement. To reduce this influence, the temperature control, which ensures that the temperatures of the inner surfaces of sample cell and reference cell are constant with high accuracy, is critical for TCD analyzers.

Reduction of the Influence of the Changing Flow Rate of Gas into the Sample Cell

The change in the flow rate of the sample gas in contact with the surface of the thermal sensor influences the measurement. The electrical resistance of the thermal sensor decreases when the flow rate is fast, and increases when it is slow. To reduce the influence of this flow rate, the introduction and discharge of the sample gas into and out of the sample cell and the cell volume is optimized to ensure that the sample gas is constantly diffused in the cell at an appropriate flow rate.