Various Infrared Gas Analyzers and Features

HORIBA currently markets a range of infrared gas analyzers optimized for different applications. There are eight different methods of analyzers using NDIR, classified correspondingly to their operating principle (as of 2021). The infrared gas analyzers are classified into two major categories based on the mechanism of the modulation section which is a part of the feature of NDIR. Each method is summarized in the tables below (Tables 3 and 4). This section describes the features, structure, and operating principle of typical methods (method 1, 2, 4, 6, and 7).

<Optical Intermittence Modulation Method>

Table 3: List of HORIBA's Infrared Gas Analyzer Methods Using NDIR (Optical intermittence modulation method)

<Cross-Modulation Method>

Table 4: List of HORIBA's Infrared Gas Analyzer Methods Using NDIR (Cross-modulation method)

<Optical Intermittence Modulation Method>

Method Using a Chopper (rotating type) for the Modulation Mechanism (1-10 Hz)

Method 1: Dual-beam method (with condenser microphone)

Figure 11: Structure and operating principle of a dual-beam (with condenser microphone) analyzer

Features, structure and operating principle (Figure 11)

This is the method described in the previous infrared gas analyzers (Structure and operating principle of infrared gas analyzer).
It features fast response and high sensitivity.

The order of sensitivity of methods using dual infrared light source is generally as follows;

Cross-modulation method (with dual-beam)　　 …method 6
Dual-beam method (with condenser microphone)…method 1
Dual-beam method (with flow sensor)　　　　　 …method 2

Method 2: Dual-Beam Method (with flow sensor)

Figure 12: Structure and principle of operation of a double-beam (flow sensor) analyzer

Features, structure and operating principle (Figure 12)

Features

A combination of a block for the light collection and a flow sensor achieves external influence reduction (especially vibration), high sensitivity, and miniaturization for the infrared analyzer.

Structure and Operating Principle

Each infrared radiation absorbed by a sample cell and a reference cell is alternately collected in the block for the light collection by a chopper of the rotating half-moon plate, and collected infrared radiation transmitted by an optical filter to enter the main detector for the measured component. Infrared absorption occurs in the front and rear chambers inside the detector corresponding to each amount of entered infrared radiation, and this increases the temperature of the each chamber.

At the same time, the flow of the enclosed gas caused by the temperature difference between the two chambers is generated and passes through the flow sensor. Since the flow rate measured by the flow sensor is proportional to the gas concentration, it is sent to the signal processing as a gas concentration detection signal.

The direction of the gas passing through the flow sensor switches in synchronization with the movement of the chopper. Specific operations within the detector for measured component are as follows.

Infrared radiation from comparison cell enters -> enclosed gas from front chamber flows into rear chamber -> chopper rotates -> infrared radiation from sample cell enters -> enclosed gas from rear chamber flows into front chamber -> chopper rotates -> infrared radiation from comparison cell enters -> repeat .......

This sequence of operations corresponds to the movement of the diaphragm of a condenser microphone. The condenser microphone measures pressure difference, while flow sensor measures flow rate. Also, the operating principle of the compensation detector for the interfering component is the same as that of the main detector for the measured component.

Method 4: Single-Beam Method (with pyroelectric sensor)

Figure 13: Structure and operating principle of a single-beam (with pyroelectric sensor) analyzer

Features, Structure, and Operating Principle (Figure 13)

Features

The infrared radiation detector using a pyroelectric sensor does not require enclosed gas like a pneumatic detector. Therefore, miniaturization is the biggest advantage, but the sensitivity is lower than that of the pneumatic detector.

Structure and Operating Principle

This method uses pyroelectric sensors of the detector for infrared radiation absorbed by the sample gas with a chopper as the modulation mechanism. To detect each measured component in the sample gas as a temperature change, a set of optical filters and a pyroelectric sensor is used for each measured component. The pyroelectric sensors detect changes in infrared absorption by each measured component, and the concentration of each measured component is calculated based on the detection and comparison signals.

<Cross-Modulation>

Gas switching method using a solenoid valve for the modulation mechanism (1 Hz)

Method 6: Cross-Modulation Method (with dual-beam)

Figure 14-1: Structure of a Cross-Modulation Method (with dual-beam) Analyzer

Features, Structure and Operating Principle

Features

Fluid modulation method is also called cross modulation method. This method has a very small drift and provides a stable output signal over the long term. Furthermore, this method obtains the diaphragm of the condenser microphone of the detection sensor moves left and right, doubling amount of the detected signal than using a chopper, thereby improving noise immunity.
Another feature is that, this method does not require the position adjustment which chopper needs for the analyzer's maintenance. However, reference gas must flow constantly because no enclosed gas is used in the gas cell. A solenoid valve system is also required to flow sample gas and reference gas to the gas cell alternately.

Structure and Operating Principle (Figure 14-1 and 14-2)

Unlike conventional modulation using a chopper, this method uses a solenoid valve unit to switch at regular intervals to alternately introduce sample gas and reference gas into the same gas cell, so the solenoid valve unit performs the modulation mechanism. An example of an analyzer structure for this method is shown in Figure 14-1.

While modulation by the chopper changes the amount of infrared light source supplied to the sample and reference cells, the cross-modulation method changes the gas flowing to the sample and reference cells. Except for the modulation mechanism, the detection function of measured component and the compensation function for interfering component, which are necessary to detect the concentration of measured component, are the same as those of the infrared gas analyzer described so far, so this section focuses on the operation of the modulation mechanism (Figure 14-2).

Figure 14-2: Principle of Modulation Operation of Cross-Modulation Method

The solenoid valve unit allows the sample gas to flow into the left gas cell and the reference gas to flow into the right gas cell simultaneously. If there is measured gas component in the sample gas, the diaphragm of the condenser microphone will expand to the left side (toward the sample cell) (Figure 14-2, left figure).

Next, the solenoid valve unit is switched and the sample gas flows into the right gas cell and the reference gas flows into the left gas cell simultaneously.

If there is measured gas component in the sample gas, the diaphragm of the condenser microphone will expand to the right side (toward the sample cell) (Figure 14-2, right figure).

This operation is repeated at a regular cycle to modulate the detection signal of the condenser microphone. By swinging the diaphragm of the condenser microphone in the detector to the left and right sides, this method obtains twice the amount of detected signal than using a chopper, thereby improving noise immunity. In addition, a mechanism of flowing a sample and reference gas through each gas cell for measurement makes a result of stable measurement over time by reducing the influence of degradation of infrared light sources and contamination of gas cells for detection signals.

Method 7: Cross-Modulation Method (single-beam)

Figure 15: Structure and operating principle of a cross-modulation (single-beam) analyzer

Features, Structure and Operating Principle (Figure 15)

Cross-modulation method (single-beam) performs the operation of cross-modulation method (dual-beam) in one gas cell. The cyclic switching of solenoid valve unit causes one gas cell to switch to sample cell and reference cell functions, and the concentration of measured component is measured by the two detected signals by these cell functions.

In this method, condenser microphone is connected to only one chamber, so the diaphragm does not swing from side to side and moves only one way. When the switch is turned reference cell, the diaphragm returns to a flat condition. Otherwise, it has the same characteristics as cross-modulation method (dual-beam).

Related Products

Non-dispersive infrared absorption (NDIR) analyzers are used in a variety of fields because they can continuously measure a variety of concentration of measured components. For example, it is used for monitoring exhaust gas, process gas, and atmospheric conditions, and for measuring and controlling gases of processes in semiconductor manufacturing.

In addition to gas measurement, NDIR analyzers are also used for water and liquid analysis, continuous measurement, and elemental analysis of solid materials.