Discussion on the factors affecting the instability of the detection system of infrared carbon and sulfur analyzer

1 Introduction

The infrared carbon-sulfur analyzer is one of the important analytical instruments in the metallurgical and mechanical industries. It can quickly analyze carbon and sulfur in solid materials such as steel, iron, copper, alloys, carbon compounds, ores, cement, ceramics, glass, etc. The detection system is the heart of the carbon-sulfur analyzer. It is very important to ensure the stability of the detection system output (baseline). However, there are many factors that affect the instability of the detection system, and the technical requirements for operators and maintenance personnel are also relatively high. Analyzing and discussing the unstable factors that affect the detection system is of great significance to the maintenance of the instrument and equipment and to ensure the normal development of scientific research.

Over the years, the instability of the infrared carbon and sulfur analyzer detection system has been a problem that has troubled many users. After a long period of research and exploration, we have accumulated valuable experience in the maintenance of this type of equipment, laying a good foundation for improving equipment utilization, successfully completing scientific research and production tasks, and maintaining infrared carbon and sulfur analyzers.

2. Instrument working principle

The sample is introduced into the high-frequency furnace and oxidized by oxygen at high temperature in the combustion furnace, so that the carbon and sulfur in the sample are oxidized to C02, CO, and SO2. The generated oxidation products are carried by oxygen into the sulfur detection pool to measure sulfur after passing through the dust removal and water removal purification device. The mixed gas containing CO2, CO, SO2 and O2 enters the heating catalytic furnace together, and is catalytically converted into CO-》CO2, SO2-》SO3 by the catalytic furnace. After passing through the desulfurization reagent tube, this mixed gas is introduced into the carbon detection pool to measure carbon, and the residual gas is discharged to the outside through the analyzer. The output of the carbon and sulfur detectors is sent to the microcomputer system for data processing through pre-amplification and A/D conversion, and finally the percentage of carbon and sulfur is obtained. Figure 1 is the principle block diagram of the infrared carbon and sulfur analyzer.

Figure 1 Principle block diagram of infrared carbon and sulfur analyzer [page]

The signal of the material being measured is closely related to the output value of the working power supply in the detection system, the radiation power of the infrared source, the frequency of the chopper motor, the infrared light detector, the A/D analog-to-digital converter, and external interference factors. The reasons that affect the system are as follows.

3.1 Working power supply

The working power supply of the detection system is ±15V, 5.5V, 24V, and 5V. It is a prerequisite for the normal operation of the detection system. The reasons for the abnormal working condition and output value of this part of the circuit are:

(1) Aging of some components causes unstable output values ​​and large ripples. Under normal circumstances, the allowable fluctuation range of power supply output should be within ±10%. If it exceeds the range, it is considered abnormal.

(2) Some components are damaged and have no output or the output waveform is incorrect. The damaged components can be found by measuring the voltage of the main working points in the circuit and comparing them with the normal value.

3.2 Infrared light source

The infrared radiation emitted by the infrared light source is proportional to the light radiation power. Changes in light radiation power directly affect changes in the signal output size, and such changes will cause changes in the detector baseline, that is, the presence or size of infrared radiation, and the detector baseline will be reflected accordingly. There are several reasons for changes in the infrared light source radiation signal:

(1) As the light source ages, the light radiation weakens and the signal output becomes low. The baseline output values ​​of the carbon and sulfur detectors will also gradually decrease, and the instrument will alarm when it is below the normal range.

(2) The light source heating wire is broken or desoldering, and there is no signal output. Measure the resistance value of the light source heating wire to see if it is correct (the correct value is generally around 5Ω).

(3) The light source heating wire is poorly soldered and the power plug is oxidized, resulting in poor contact. Due to changes in contact resistance, the signal output fluctuates greatly, which is often ignored.

3.3 Chopper Motor

The chopper motor modulates the light signal into a signal with a certain frequency and sends it to the detector. This design is to chop the infrared light signal into a square wave signal to ensure the stability of the signal after being amplified by the circuit. Therefore, if the motor does not work properly, it will cause the detector signal output to be small or even no output. The main reasons are as follows:

(1) The chopper motor does not rotate. One reason is that the power supply fails, and the other reason is that the chopper motor shaft is stuck.

(2) Due to long-term rotation and wear of the motor, the shaft sleeve gap is large, the motor blades rotate unstably, and the blades even hit the wall of the detection pool and get blocked and stuck.

(3) The modulation frequency of the chopper motor deviates from the normal value, causing the light passing through the light hole to fluctuate and the detector's receiving signal to fluctuate or even be zero.

3.4 Infrared detector

The infrared detector is a key component of the gas analyzer. It converts optical signals into electrical signals through infrared radiation. Under normal working conditions, the ambient temperature should be kept stable to avoid interference with the signal. The following situations are likely to occur:

(1) The device is damaged and there is no output signal.

(2) The device has low aging sensitivity and high output signal noise.

(3) The device solder joints are oxidized, the solder joints are poorly connected, and the output signal is unstable.

3.5 Preamplifier

The preamplifier amplifies, filters, and DC-amplifies the weak signal output by the detector. Slight changes in each part of the system will cause changes in the amplifier. The most common problems are as follows.

(1) The performance of the micro signal amplifier has deteriorated. When checking, you should focus on the filter capacitor and whether the grounding is good.

(2) The zero adjustment potentiometer and gain adjustment potentiometer have poor contact. After using for a period of time, rotate the potentiometer left and right several times to ensure good contact.

(3) The gain of the four-op-amp amplifier is reduced, resulting in lower subsequent analysis results.

3.6 A/D conversion board

The voltage signal is collected by a 16-to-1 chip, then converted into a digital signal by an A/D chip and sent to a computer for processing. This process is prone to generate interference signals, which affects the working conditions observed in the system monitoring window, the sample detection integration process and calculation. The main reasons are:

(1) The components of the A/D converter board age and their quality deteriorates, causing an increase in interference signals and digital signal disorder.

(2) The instrument is not properly grounded, and external interference signals may cause missing collected data.

(3) The A/D conversion board has poor contact with the bus slot, resulting in loss of collected data.

4 Conclusion

Through the analysis of unstable influencing factors, it plays an important role in solving a series of problems caused by aging of electronic components and performance degradation, such as unstable signal output value or no output, large ripple, large output signal noise, omission or loss of collected data, and improving the utilization rate of equipment.