Thermal conductivity gas analyzer

Thermal conductivity of gas

The thermal conductivity gas analyzer is an instrument for analyzing the gas composition by measuring the thermal conductivity of mixed gases according to the different thermal conductivity of various substances. It is well known that there are three basic ways of heat transfer, namely heat convection, heat radiation and heat conduction. In the heat conduction gas analyzer, the heat exchange formed by the heat conduction is fully utilized, and the heat loss caused by the heat convection and the heat radiation is suppressed as much as possible.

The thermal conductivity indicates the thermal conductivity of the material, and the relationship between the thermal conductivity of the material can be described by Fourier law. As shown in Figure 6-1, there is temperature difference in a substance, and the setting temperature gradually decreases along the ox direction. Take two points a and b in the direction of ox, the spacing is △x. Ta and Tb are the absolute temperatures of two points a and b respectively. The change rate of temperature along the direction of ox is called the temperature gradient of a point along the direction of ox. A small area △s is taken between a and b in the vertical direction of ox. Through the experiment, it can be seen that in the time △t, the heat transfer from a point at high temperature through a small area △s is proportional to the time △t and the temperature gradient △T/△x, and it is also related to the nature of the substance. The equation is:

The formula (6-1) represents the relation between the heat transfer and the relevant parameters, which is called the Fourier law. The negative sign in the formula indicates the heat transfer to the direction of temperature decrease, the proportional coefficient λ is called the thermal conductivity of heat transfer medium (also called thermal conductivity).
Thermal conductivity is one of the important physical properties of matter, which characterizes the ability of matter to conduct heat. The thermal conductivity of different materials is also different, and varies with the composition, pressure, density, temperature and humidity.
In the formula (6-1) can obtain:

Relative thermal conductivity of gases

The absolute value of the thermal conductivity of gas is very small, and the difference is not very big within the same order of magnitude, so the concept of "relative thermal conductivity" is usually adopted in engineering. The so-called relative thermal conductivity (also called relative thermal conductivity) means the ratio of the thermal conductivity of various gases to the thermal conductivity of air under the same conditions. If λ0 and λΑ0 are used to represent the thermal conductivity of a gas and air at 0°C, then λ0/λΑ0 is the relative thermal conductivity of the gas at 0°C, and λ100/λΑ100 is the relative thermal conductivity of the gas at 100°C.

Relationship between the thermal conductivity of gas and temperature and pressure

he thermal conductivity of gas varies with temperature.The relation is:
λt＝λ0﹙1 +βt﹚
In formula λt——t(℃) thermal conductivity of gas
λ0——0℃ thermal conductivity of gas
β—— Temperature coefficient of thermal conductivity
temperature of gas
The thermal conductivity of gas also changes with the pressure, because the density of gas at different pressure is also different, which will inevitably lead to different thermal conductivity, but the change of thermal conductivity is not obvious at atmospheric pressure or pressure.
The temperature coefficients of thermal conductivity, relative thermal conductivity and thermal conductivity of common gases are shown in Table 6-1.

Thermal conductivity of mixed gas

All the components except the components to be measured in the mixed gas are called background gas, and the components which have influence on the analysis in the background gas are called interference components.
The volume fraction of each component in the mixed gas is C1, C2, C3,…、Cn..thermal conductivity is λ1, λ2, λ3,…、λn The content and thermal conductivity of the component to be measured are C1 and λ1. The following two conditions must be met to measure with a thermal conductivity analyzer.
①  
The thermal conductivity of each component of the background gas must be approximately equal or very close. As: 
λ2≈λ3≈λ4…≈λn
②The thermal conductivity of the component to be measured is obviously different from that of the background gas, and the larger the difference, the better the thermal conductivity.
λ1》λ2 or λ1《λ2
When the above two conditions are satisfied:

 λ in formula——thermal conductivity of mixed gas

Thermal conductivity of the i component in a mixed gas

Ci——Volume fraction of the i component in the mixed gas

The formula (6-5) shows that the content of the component C1 can be obtained by measuring the thermal conductivity λ of the mixed gas.

Composition and working principle of instrument

The composition of the heat-conducting gas analyzer can be divided into two parts: the heat-conducting detector and the circuit. The thermal conductivity detector (commonly called transmitter) is composed of a thermal conductivity cell and a measurement bridge, the thermal conductivity cell as the bridge arm of the measurement bridge is connected in the bridge, so the two are inseparable. The circuit part includes voltage stabilizing power supply, constant temperature controller, signal amplifying circuit, linearization circuit and output circuit.

Working principle of heat conduction cell

Because the thermal conductivity of the gas is very small, its variation is smaller, so it is difficult to measure accurately by direct method. The thermal conductivity change of the mixed gas is converted into the change of the resistance value of the thermal element by the indirect method, and the change of the resistance value is easy to be accurately measured.

Figure 6-2 is the working principle of the heat conducting cell, a resistance wire with larger resistivity and larger temperature coefficient is tensioned and suspended at the center of a cylindrical metal shell with good heat conducting performance, the two ends of the shell are provided with an inlet and an outlet of gas, the cylinder is filled with the gas to be measured, and the resistance wire is heated by a constant current.

Since the current passed through the resistance wire is constant, the heat generated in unit time on the resistance is also constant. When the sample gas to be tested passes through the cell at a slow speed, the heat on the resistance wire is transmitted to the cell wall by the gas in a heat conduction way. When the heat transfer rate of the gas is equal to the heating rate of the current on the resistance wire (this state is called thermal equilibrium), the temperature of the resistance wire will be stable at a certain value, this equilibrium temperature determines the resistance of the resistance wire. If the concentration of the component to be measured in the mixed gas changes, the thermal conductivity of the mixed gas changes, the thermal conductivity rate of the gas and the equilibrium temperature of the resistance wire will also change, eventually resulting in the resistance of the resistance wire corresponding change, thus realizing the conversion between the thermal conductivity of the gas and the resistance wire resistance value.

The relation between the resistance of the wire and the thermal conductivity of the gas mixture is given by the following formula (the derivation is omitted)

In the formula, the resistance of Rn, R0-hot wire at tn(°C)(the temperature of hot wire in thermal equilibrium) and at 0°C is obtained.
a——Resistance temperature coefficient of hot wire
tc——Temperature of the air cell wall of the thermal conductivity cell
I——Current flowing through the heating wire
λ——Thermal conductivity of mixed gas
K——Gage constant, which is a constant related to the structure of the thermal conduction cell
The formula (6-6) shows that Rn and λ are single-valued functions when K, tc, and I are constant.
The hot filament material uses a plurality of platinum wires (or platinum iridium wires), the platinum wires have strong corrosion resistance, large resistance temperature coefficient and high stability. The platinum wire can be exposed and directly contacted with the sample gas to improve the response speed of the analysis. However, the platinum wire is easy to be eroded and deteriorated in the reducing gas, which causes the change of resistance value and in some cases also plays the role of catalyst. For this reason, glass film is usually used to cover the surface of the platinum wire. The heat-sensitive element covered with glass film has the advantages of strong corrosion resistance (hydrogen in chlorine can be measured) and easy cleaning, but the existence of glass film delays the time of reaching thermal equilibrium between gas and platinum wire, so the dynamic characteristics of the element are slightly poor.
The material used for manufacturing the heat conducting tank body is copper. In order to prevent the corrosion of the gas, a layer of gold or nickel can be plated on the inner wall and the gas path of the heat conducting pool, and the stainless steel can also be used for manufacturing.

Structure formation of heat conduction cell

The structure of the heat conduction cell is straight-through, convection, diffusion, convection diffusion and so on, as shown in Figure 6-3

（1）Straight-through
The measuring chamber is parallel with the main gas path, and the gas of the main gas path is distributed to the measuring chamber. The structure has fast reaction speed and small hysteresis, but is easily affected by the fluctuation of gas flow rate.
（2）Convection
The measuring chamber is connected with the main gas path inlet in parallel, and a small part of the gas to be measured enters the measuring chamber (circulating pipe). The gas is heated in the circulating tube, which causes heat convection, and pushes the gas to return from the lower part of the circulating tube to the main gas path according to the arrow direction. The advantage is that the gas flow fluctuation has little effect on the measurement, but its reaction speed is slow and the lag is large.

（3）Diffusion
A measuring chamber is arranged at the upper part of the main gas path, and the gas to be measured enters the measuring chamber through diffusion action. The advantages of this structure are less affected by the fluctuation of gas flow rate, suitable for gases with lighter mass that are easy to diffuse, but have larger hysteresis for gases with smaller diffusion coefficient.
（4）Convection diffusion 
A branch pipe is added to form a flow separation on the basis of the diffusion type to reduce the lag. When the sample gas flows from the main gas path, a part of the gas enters the measuring chamber in a diffusion mode, and is heated by the resistance wire to form an ascending gas flow. Due to the restriction of the throttle hole, only a part of the airflow enters the branch pipe through the throttle hole, is cooled and moved downwards, and finally is discharged into the main air path. The power of the gas flow superheating guide tank has both convection and diffusion, so it is called convection diffusion. The structure can not generate the gas reverse flow phenomenon, but also avoid the gas accumulation in the diffusion chamber, thereby ensuring the sample gas to have a certain flow rate. The heat conduction cell is insensitive to the change of pressure and flow rate of sample gas, and the lag time is shorter than that of diffusion. Due to the advantages, the convection diffusion type heat conduction cell is widely applied.

Measuring bridge

From the introduction above, we can see that the function of the thermal conductivity cell is to change the concentration of the components in the mixed gas into the resistance of the resistance wire value change, the use of bridge to measure the resistance is very convenient, and the sensitivity and accuracy are relatively high, so the various types of thermal conductivity gas analyzer almost adopt the bridge as the measurement link.
In the measurement bridge, in order to reduce the current fluctuation of the bridge or the influence of the change of the external conditions, the measurement bridge arm and the reference bridge arm are usually arranged, the measurement arm is the thermal conductive cell of the sample gas flow, the reference arm is the thermal conductive cell of the package reference gas (or the through reference gas), and the two have identical structural dimensions. The reference arm is placed on the bridge arm adjacent to the measuring arm and acts as follows.
①The heat loss of the measuring arm through the flow and radiation is almost the same as that of the reference arm, and the two offset each other, the change of the resistance of the hot wire is mainly determined by the heat conduction, that is, the change of the gas heat conduction ability.
②When the temperature change of the thermal conduction cell arm is caused by the temperature change of the environment, the reference arm and the measuring arm change in the same direction, which is mutually offset and beneficial to weaken the influence of the temperature change on the measuring result.
③By changing the reference gas concentration, the lower limit concentration of the bridge detection is changed, which is convenient to change the measurement range of the instrument.
In the bridge structure and bridge arm configuration mode, there are several forms such as single-arm series-connected unbalanced bridge, single-arm parallel-connected unbalanced bridge and double-arm series-parallel unbalanced bridge. Figure 6-4 is the structure of the double-arm series-parallel type unbalanced bridge which is commonly used at present. It adopts two measuring heat conducting cell and two reference heat conducting cell. In the figure, Rm is the resistance of the measuring arm, Rs is the resistance of the reference arm. The two measuring arms and the two reference arms are arranged at intervals to form a double-arm series structure, and the sample gas flows through the two heat conducting pools in series in turn.
The output of the bridge in the initial state is:

The above formula is the relation between △Rm and △Uo, and is also the expression of the measurement sensitivity of this kind of bridge. Compared with the single-arm bridge with the same structure, the measurement sensitivity has doubled.

Figure 6-5 is a combined heat conduction cell used in a double-arm series-parallel type unbalanced bridge, two measuring heat conduction cells and two reference heat conduction cells, the leads of which are respectively connected into the four arms of the measuring bridge, and each heat conduction cell adopts a convection diffusion type structure.
The four heat conducting pools are made of a metal material with good heat conducting performance, so that the temperature of the measuring pool and the reference pool can be at the same temperature, and when the ambient temperature changes, the influence on the four pool walls is equal, thus reducing the measurement error. The temperature control device can be used to keep the temperature of the whole heat conduction pool constant in the situation of high measurement precision.

Advances in thermal conductivity detectors

The inner volume of the thermal conductivity cell is of the order of milliliter, and the lower limit of measurement is in the order of 100ppm. With the progress of sensor technology, the micro thermal conductivity detector has been used in the thermal conductivity gas analyzer and thermal conductivity gas chromatograph produced abroad, the volume of the thermal conductivity cell is micro-upgraded, the thermal element is also micro, thus greatly improving the sensitivity of the inspection, the lower limit of measurement can reach the order of 10ppm, even the order of 1ppm, as shown in Figure 6-6, this kind of thin film resistance is made on the silicon wafer by using ultra-micro technology lithography of very thin platinum wire, from the figure, we can see that the structure of the thermal conductivity cell is diffusion.

Whole machine circuit
The circuit of CI2000-RQD heat conduction type hydrogen analyzer has been introduced in many books and teaching materials. The CI2000-RQD heat conduction type hydrogen analyzer produced by Chang Ai Electronics Company is taken as an example to briefly introduce the whole circuit of heat conduction type gas analyzer.

Microprocessor and digital processing technology are used in the circuit of CI2000-RQD. The whole circuit is shown in Figure 6-7. The heat conduction pool structure in the figure belongs to convection diffusion type, the measuring bridge power supply uses current source circuit. The measurement signal of Wheatstone bridge is sent to an amplifier which can be controlled by software to be amplified and filtered by a Butterworth low-pass filter, then the A/D conversion is controlled by a microprocessor, then the converted data is processed by the software to be digitized, including filtering, linear processing, scale conversion, error calculation and compensation of the influence of temperature and pressure etc., and finally output the signal.

Applications

The thermal conductivity gas analyzer is an effective method for measuring one component in two mixed gases (with a very large difference in thermal conductivity). The invention is mainly used for measuring H₂, and is also commonly used for measuring the content of CO₂, SO₂ and Ar, and has wide application range. Here are some typical applications:
Measurement of H₂ Content in Syngas from Ammonia Plant
H₂ purity measurement in hydrogenation plant
Measurement of CO₂ content in furnace flue gas
Measurement of SO₂ content in production process of sulfuric acid and phosphate fertilizer
Measurement of Ar content in air separation device
Measurement of O₂ in pure H₂ and H₂ in pure O₂ during the process of hydrogen production and oxygen electrolysis
Measurement of H₂ in Cl2 in Chlorine Production Process
Measurement of H₂ Content in Hydrocarbon Gas
Monitoring of H₂ and CO₂ Content in Hydrogen-cooled Generator Sets
Monitoring in pure gas production, such as He in N₂, Ar in O₂, etc.

Measurement error analysis

Thermal-conductive gas analyzer is a kind of analysis instrument with poor selectivity. Although various measures have been taken in the design and manufacture of the instrument, the operating conditions have been specified, and the influence of some interference factors has been suppressed or weakened to some extent, but the basic error of the analyzer is generally within ±2%. The main reason is the influence of background gas composition on the analysis results.
The thermal conductivity detector of industrial gas chromatograph and the detector of thermal conductivity gas analyzer are identical, but the measurement precision element is higher than the latter. The reason is that after the sample is separated by the chromatographic column, only the binary mixed gas of a single component and a carrier gas enters the thermal conductivity tank, but it is difficult to do this in the thermal conductivity gas analyzer. The background gas is often a mixture of multiple gases, which will have different degrees of influence on the thermal conductivity of the sample gas, when the composition of the background gas changes, the influence is greater.
The measurement error of the heat-conducting gas analyzer is composed of two parts: the basic error and the additional error. The basic error is determined by the measuring principle, the structure characteristic, the signal conversion accuracy of each link and the display instrument accuracy. That is, the error of the analyzer when it works under the specified conditions. The additional error is due to the adjustment of the instrument, improper use or the change of external conditions. The main factors of the additional error of the thermal-conductive gas analyzer are: the composition and accuracy of standard gas; interfering the presence of components, dust and droplets; the pressure, flow rate and temperature of sample gas; the changes in the current of the bridge.

Influence of composition and precision of standard gas

Thermal-conductive gas analyzer, like other analytical instruments, needs to be calibrated regularly with standard gas, but the difference is that thermal-conductive gas analyzer requires more standard gas. In principle, the composition and content of the background gas in the standard gas should be the same as that of the measured gas, which is difficult to achieve in fact, but the thermal conductivity of the background gas in the standard gas should be consistent with that of the measured gas, otherwise the calibration results should be corrected. In addition, to ensure the accuracy of standard gas, the error must not be more than half of the basic error of the instrument.

Effects in the presence of interfering components in the sample gas

The existence of interference components in the sample gas is an important factor for generating additional errors. For example, when the CO2 content in flue gas is analyzed by the thermal conductivity CO2 analyzer, the SO2 in flue gas is the interference component, and its thermal conductivity is 1/2 of the thermal conductivity of CO2. If the SO2 content in flue gas is 1%, the error of the analysis result will be nearly 2%. It is necessary to understand the interference components in the background gas and their influence on the measurement. Table 6-2 shows the influence of the interference components in the measured gas on the zero point of hydrogen content measurement.

When the interference gas concentration is not 10%, the approximate results can still be obtained by using the data in the table above. The table is valid even if the interference gas concentration is as high as 25%. When the zero point is found, the linear deviation is also affected by the interference gas, because the thermal conductivity of most gases is non-linear. However, for the vast majority of gases, this effect is almost negligible when the concentration is low.
In practical work, the influence of interference gas on measurement results can be corrected by reference to table 6-2. When the content of the interfering components is small, a certain device or chemical reagent can be used to filter out the interfering components.

Influence of droplets and dust in sample gas

If the sample gas contains droplets, the evaporation in the thermal conduction pool will absorb a large amount of heat, which has a great influence on the analysis. Therefore, the dew point of the sample gas is required to be at least lower than the ambient temperature of 5°C, otherwise, the liquid removal and discharge measures shall be taken.
If the sample gas contains dust or oil stain, the surface of the resistance wire is stained and the wall of the pool is stained when passing through the heat conducting pool, thereby changing the heat transfer condition of the heat conducting pool and changing the characteristics of the instrument. Therefore, the sample gas should be fully filtered before entering the instrument.

Influence of sample gas flow, pressure and temperature variation

Different types of heat conduction pool have different requirements for the stability of the sample gas pressure and flow rate. The variation of sample pressure and flow rate has different effects on the flow, convection and convection heat transfer heat transfer cell. When the flow rate changes, the heat taken by the gas from the heat conduction pool will change, the change of the gas pressure will also make the heat taken by the gas unstable, and make the convective heat transfer unstable, and make the convective heat transfer unstable, causing analysis error.
The influence of sample gas temperature change on the thermal conductivity cell is obvious. After rough calculation, when the CO2 content is analyzed by the measuring bridge without temperature control device, the relative error of instrument indication value is about 5% for each change of 2%. Therefore, the thermal conductivity gas analyzer is equipped with a temperature control system, the temperature is generally 55~60°C, the temperature control accuracy is above ±0.1°C, some can reach ±0.03°C. The constant temperature device has a certain power limit, when the ambient temperature is too high or too low, exceeding the instrument's prescribed conditions, the constant temperature system will lose its function and introduce additional error. Therefore, the detectors of the thermal-conductive gas analyzer are usually installed in the analysis cabin with little environmental temperature change.

Influence of bridge working power supply stability
Whether the power supply voltage of the unbalanced bridge is stable or not has great influence on the analysis accuracy. Generally speaking, if the analysis precision is ±1%, the stability of bridge current must be kept at ±0.1%, so almost all the bridges of thermal conductivity analyzer adopt stable current (or stable voltage) power supply with high stability.