1 Overview

The TGS4160 carbon dioxide sensor is a solid-state electrochemical gas sensor produced by FIGARO. This kind of carbon dioxide sensor not only has the characteristics of small size, long life, good selectivity and stability, but also has the characteristics of resistance to high humidity and low temperature. It can be widely used in automatic ventilation systems or long-term monitoring of CO2 gas. occasion. However, due to the long warm-up time of TGS4160 (generally 2 hours), the device is more suitable for long-term continuous operation at room temperature. In addition, in order to facilitate the use of customers, FI-GARO company also specially designed the sensor processing module AM-4 with temperature compensation. The module is controlled by a microprocessor. The output signal level of CO2 gas concentration is 0.0-3.0V, which is equivalent to a concentration of 0-3000ppm. It has a relay switch control port, which can output high and low. The threshold signal is used for external control. The main technical parameters of TGS4160 sensor are as follows:

●Measuring range: 0～5000ppm;

●Service life: 2000 days;

●Heater voltage: 5.0±0.2VDC;

●Heater current: 250mA;

● Heater power consumption: 1.25W;

●Internal thermistor (for compensation): 100kΩ±5%;

●Working temperature: -10～＋50℃

●Operating humidity: 5～95％RH;

●Product size: The maximum outer diameter is Φ24mm, the height is 24mm, and the lead length is 5.8mm.

2 Internal structure

TGS4160 carbon dioxide sensor is a hybrid CO2 sensor with a thermistor. The element is filled with cationic solid electrolyte between two electrodes. Its cathode is made of lithium carbonate and gold-plated material, while the anode is just gold-plated material. The base lining of the sensitive element is reinforced with terephthalate polyethylene and glass fiber, and then a stainless steel mesh is used for cylindrical packaging. The inner layer of the element adopts a 100-mesh double-layer stainless steel mesh sleeve on the nickel-plated copper ring, and is fixed with the base lining with a high-strength resin adhesive. The outer cover is covered with a layer of 60-mesh stainless steel mesh. In order to reduce the influence of interfering gas, TGS4160 is also filled with adsorbent material (zeolite) between the inner and outer stainless steel meshes. The 6 pins of the sensor are connected to the inside through a 0.1mm foil wire. Its equivalent internal structure is shown in Figure 1. In the figure, the anode is connected to the third pin S (+) of the sensor, the cathode is connected to the fourth pin S (-) of the sensor, the Pt heater is connected to the first and sixth pins of the sensor, and the internal thermistor is connected to the first pin of the sensor. The 2, 5 feet are connected. The function of the internal thermistor is to detect the ambient temperature through the resistor, so as to compensate the temperature of the sensor, so as to make the corrected measurement value more accurate.

3 working principle

The TGS4160 CO2 sensor is an electrochemical gas sensitive element. When the element is exposed to the CO2 gas environment, an electrochemical reaction will occur. The reaction formula is as follows:

Cathode reaction equation:

4Li＋＋2CO2＋O2＋4e-=2Li2CO3

Anode reaction equation:

4Na＋＋O2＋4e－=2Na2O

The general chemical reaction equation:

Li2CO3＋2Na＋=Na2O＋2Li＋＋CO2

As a result of the electrochemical reaction, according to the Neist equation (Nernst), the process will generate the following electric potential (EMF):

EMF=Ec-(RF)/(2F)ln(PCO2)

In the formula: PCO2 is the partial pressure of CO2; Ec is the constant; R is the gas constant; T is the temperature value (K); F is the Faraday constant.

It can be seen from the above formula that the concentration of CO2 can be measured by monitoring the potential EMF generated between the two electrodes of S (+) and S (-). In order to keep the sensor at the most sensitive temperature, it is generally necessary to provide heating voltage to the heater for heating, but the change of the heating voltage will directly affect the stability of the sensor, so the heating voltage must be stable, and its range should be 5.0± Within 0.2VDC. In order to ensure the accurate measurement of CO2, in addition to ensuring the stability of the heating voltage and temperature compensation for changes in ambient temperature, it is more important to measure the potential value ΔEMF that changes between the two electrodes, rather than the absolute potential value EMF, because ΔEMF and CO2 There is a better linear relationship between concentration changes. Although the absolute value of EMF increases with the increase of ambient temperature, ΔEMF remains constant, and it is basically not affected by temperature in the temperature range of -10°C to +50°C.

figure 2

The ΔEMF value can be obtained by the following formula:

ΔEMF=EMF1-EMF2

Among them, EMF1 is the EMF value in CO2 of 350 ppm; EMF2 is the EMF value of CO2 measured.

Under the conditions of a temperature of 20℃±2℃, a humidity of 65±5%RH, a heating voltage of 5.0±0.05 VDC, and a preheating time of 7 days or more than 7 days, the sensor is measured at a concentration of 350 ppm The EMF value of ΔEMF is 220~490mV, and the value of ΔEMF in the CO2 concentration of 350~3500ppm is 44~72mV. Therefore, in the actual measurement application circuit, according to the characteristics of the sensor, in addition to the use of high input impedance (≥100GΩ) , In addition to the operational amplifier with low bias current (≤1pA), the measured signal must be processed. There are usually two options for processing this signal: one is to use the FIC98646 special processor module of Figaro (FIGARO) company, and the other is to select other types of single-chip microcomputers and process the signal through their own programming.

4 basic applications

Using the TGS4160 sensor and amplifying it through an op amp with high input impedance and low bias current, and doing some simple calculations, the signal can be measured in the CO2 concentration range of 300 to 5000 ppm, and the signal is 0 to several hundred. The millivolt voltage signal can be used for high-precision A/D sampling. If you use the AM-4CO2 sensor module provided by Figaro (FIGARO), it can be directly used in automatic ventilation systems or CO2 gas monitoring. The module has an A/D converter inside, and the data has been sampled and processed. The output voltage signal has a linear relationship with the CO2 concentration value. The output voltage signal is 0-3.0V, which is equivalent to a CO2 concentration of 0-3000ppm. In addition, the module also provides relay transfer control signals. When the CO2 concentration is higher than the set value, the output switch control signal is high level 5V, which can make the red LED light up; otherwise, it will switch the control signal to low level 0V to make the green LED light up . However, the set value of this module is divided into files, rather than continuously adjustable. There are four levels (which can be realized by jumpers on the circuit board). Table 1 and Table 2 respectively show the concentration value of the threshold switch signal and the jumper connection method. The practical circuit schematic diagram of the AM-4 module is shown in Figure 2.

Table 1 Threshold signal concentration

Control signal threshold

Grade switch

â… 800ppm720ppm

â…¡1000ppm900ppm

â…¢1500ppm1350ppm

â…£2000ppm1800ppm

Table 2 Jumper connection

Connection method

Grade JP5JP6JP7JP8

â…  Continuously disconnected

â…¡ Continuously disconnected

â…¢ Connected and disconnected

â…£disconnected

If you think it is inconvenient to use the AM-4 module or think the price is too high, you can also design your own circuit and write your own program for processing.

5 concluding remarks

The TGS4160 CO2 sensor is particularly suitable for continuous monitoring of CO2. It does not need to be powered off and has good stability. But the TGS4160 sensor is not suitable for portable or hand-held CO2 measuring instruments. Because the warm-up time is too long, it cannot be measured immediately, and the power of the sensor is also relatively large. In addition, exposure of the sensor to certain gases (such as chlorine) will reduce sensitivity, because zeolite can filter out certain interfering gases (such as ethanol). Therefore, it can be placed in a desiccant when not in use, and sealed with a special bag.

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