Patent Publication Number: US-2011049342-A1

Title: Gas concentration measurement device and method thereof

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas concentration measurement device and a method thereof, particularly to a technology, which uses a virtual dual-channel infrared approach to calibrate voltage drift and measure carbon dioxide concentrations. 
     2. Description of the Related Art 
     A research of ASHRAE (America Society of Heating, Refrigeration and Air Conditioning Engineers) shows that aeration is required when the carbon dioxide concentration exceeds 1000 ppm. A statistic report indicates that the carbon dioxide concentration is normally over this value in 90% buildings. When a person inhales too much carbon dioxide, his brain is slowed down, and his reaction is blunted. Meanwhile, he will feel dozy and fatigued. Therefore, some buildings are equipped with gas sensors to detect the concentration of carbon dioxide. 
     Most of the commercially-available carbon dioxide sensors or analyzers adopt an NDIR (Non-Dispersive Infra-Red) method to detect carbon dioxide. The molecules of carbon dioxide absorb special wavelengths of infrared light, and the absorbency is proportional to the concentration of carbon dioxide. For example, carbon dioxide has the highest absorbability in the infrared light with a wavelength of 4.26 μm. Such a characteristic is used to detect carbon dioxide. The conventional single-channel carbon dioxide detector comprises a gas sampling tube. An infrared source is arranged in one end of the gas sampling tube, and an infrared sensor is arranged in the other end. The carbon dioxide molecules inside the gas sampling tube absorb the radiant energy of the infrared light having a wavelength of 4.26 μm. The relationship between the carbon dioxide concentration and the absorbency of infrared light can be learned from the Beer&#39;s law. Thus, the carbon dioxide concentration can be worked out from the infrared absorbency. However, long-term operation causes the aging of the single-channel carbon dioxide detector and the voltage drift thereof, and the voltage drift further induces measurement errors and incorrect results. A U.S. Pat. No. 5,347,474 disclosed a single-channel gas measurement method able to compensate for concentration drift, wherein a carbon dioxide monitor is self-calibrated in a quiescent period. In the quiescent period, the carbon dioxide concentration is within 300-500 ppm. The carbon dioxide monitor is calibrated via fitting and extrapolating the previous data of the quiescent period. However, the prior art use the concentration value to do the compensate, it can only compensate one kind of drift at the same time, zero drift or span drift. And the prior art needs many data and complicated mathematical analysis. Further, the prior art is time-consuming and hard to effectively promote measurement accuracy. 
     A U.S. Pat. No. 6,114,700 disclosed a single-channel gas measurement device and a method thereof, which are an NDIR device and a method thereof, wherein a gas is sampled to detect the infrared absorbency thereof. In the prior art, parallel light sources having different temperature coefficients are arranged in one end of a sample tank to increase the temperature of the tube. An NDIR detector is arranged on another end of the sample tank to monitor the concentration of carbon dioxide. A controller is arranged in the servo loop to control the persistently output light. The prior art measures the concentration of carbon dioxide in an optical method. The prior art performs an NDIR gas analysis on the gas sample and undertakes temperature compensation and signal conversion to automatically detect gas concentrations. Then, the result is presented. However, the prior art has great errors in the low concentration range and the high concentration range because no nonlinear compensation circuit is installed in the device. Therefore, the prior art is only suitable to detect the gas in a specified concentration range. 
     The typical approach to overcome the aging-induced voltage drift is to adopt a dual-channel gas detector. Refer to  FIG. 1 . The conventional dual-channel gas concentration measurement device comprises a gas sampling tube  11 . An infrared source  111  is arranged in one end of the gas sampling tube  11 , and two infrared sensors  112  and  113  are arranged in the other end of the gas sampling tube  11 . The two infrared sensors  112  and  113  are dual-channel thermopiles and respectively connected with two amplifiers  12  and  13 . The two amplifiers  12  and  13  are connected with a converter  14 . The infrared sensor  112  is a measurement channel. because there are some CO2 molecules in the tube  11 , which will absorb the special infrared light (usually it&#39;s 4.26 um), the infrared sensor  112  receives above absorbed infrared light, thereof rising to generate a measurement voltage signal. The amplifier  12  receives and amplifies the measurement voltage signal. The converter  14  receives the amplified measurement voltage signal and converts the amplified measurement voltage signal into a digital infrared signal, this infrared signal can be one voltage signal or counter signal, but it&#39;s not gas concentration value yet. The infrared sensor  113  is a reference channel. The infrared sensor  113  receives the infrared light which is not absorbed by the CO2 molecules, so the infrared sensor  113  generates a reference voltage signal. The amplifier  13  receives the reference voltage signal and amplifies the reference voltage signal into an amplified reference voltage signal. The converter  14  receives the amplified reference voltage signal and converts the amplified reference voltage signal into a reference infrared signal, this reference infrared signal can be one voltage signal or digital signal, but it&#39;s not gas concentration value yet. The converter  14  is connected with a microprocessor  15 . The microprocessor  15  calibrates the measurement signal with the reference infrared signal. Thereby, the infrared signal drift is corrected, and the carbon dioxide concentration is accurately measured. However, the abovementioned dual-channel thermopile device is a complicated optical-vacuum structure. Therefore, the prior art is expensive and unsuitable for homes, offices, etc. 
     Accordingly, the present invention proposes a gas concentration measurement device and a method thereof to effectively overcome the abovementioned problems. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide a gas concentration measurement device and a method thereof, which adopts a simple single-channel infrared measurement structure, and which calibrates a measured infrared signal with a reference infrared signal, whereby is achieved a high precision gas concentration measurement, and whereby is overcome the problem that the conventional dual-channel infrared measurement structure has complicated structure and high cost. 
     Another objective of the present invention is to provide a gas concentration measurement device and a method thereof, which realizes a virtual dual-channel infrared scheme via adopting the voltage value obtained in a normal carbon dioxide concentration (400 ppm) as the initial reference infrared signal value to compensate for the infrared signal drift caused by sensor aging, whereby is promoted the accuracy and reliability of measurement. 
     A further objective of the present invention is to provide a gas concentration measurement device and a method thereof, which can apply to homes, offices, schools, shops, etc., and which can measure a wide range of different gas concentration, whereby is attained a very high commercial potential. 
     To achieve the abovementioned objectives, the present invention proposes a gas concentration measurement device, which applies to measuring carbon dioxide concentrations, wherein an infrared beam, which is emitted by an infrared emitter of a single-channel infrared gas detection module, passes through the gas sample in a gas tube and reaches an infrared sensor. The infrared sensor generates a voltage signal corresponding to the concentration of the gas sample according to the detected infrared intensity. An amplifier amplifies the voltage signal into an amplified voltage signal and transmits the amplified voltage signal to a digital-to-analog converter. The digital-to-analog converter converts the amplified voltage signal into a infrared signal value and transmits the infrared signal value to a processing module. The processing module calibrates the infrared signal value with a reference infrared signal value and outputs a concentration value of the gas sample. 
     The present invention also proposes a gas concentration measurement method, which can compensate for infrared signal drift, and which comprises steps: presetting an initial reference infrared signal value corresponding to a normal concentration of the detected gas in the atmosphere; using a single-channel infrared gas detection module to obtain a voltage signal of the detected gas, converting the voltage signal into an infrared signal, calibrating the infrared signal value with the reference infrared signal value, and outputting a concentration value of the detected gas. The method of the present invention may further comprise steps: periodically or non-periodically updating the reference infrared signal value; and calibrating the infrared signal value with the updated reference infrared signal value and outputting a concentration of the detected gas. 
     Below, the embodiments are described in detail in cooperation with the attached drawings to make easily understood the objectives, technical contents, characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing a conventional dual-channel gas concentration measurement device; 
         FIG. 2  is a diagram schematically showing a gas concentration measurement device according to the present invention; 
         FIG. 3  is a flowchart of a gas concentration measurement method according to the present invention; 
         FIG. 4  is a diagram showing the curves obtained via measuring the carbon dioxide concentration inside a building for many days before and after calibration; 
         FIG. 5  is a diagram showing the curves of the relationships between voltage value and the carbon dioxide concentration; and 
         FIG. 6  is a diagram showing the curves of the concentration-error relationships of the prior arts and the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Below, the technical contents are described in detail in accompany with the drawings, wherein identical elements are designated with identical numerals. 
     Refer to  FIG. 2  a diagram schematically showing a gas concentration measurement device according to the present invention. The present invention adopts a virtual dual-channel approach to measure the concentration of carbon dioxide. The device of the present invention comprises a single-channel infrared gas detection module  21  detecting the voltage signal of the carbon dioxide sample. The single-channel infrared gas detection module  21  includes an infrared emitter  211 , a gas tube  212  and an infrared detector  213 . The infrared detector  213  is a thermopile infrared sensor. One terminal of an infrared controller  22  is coupled to the infrared emitter  211 , and another terminal of the infrared controller  22  is coupled to a processing module  23 . The infrared controller  22  controls the infrared emitter  211  to emit infrared light. The infrared light passes through the carbon dioxide sample  27  contained in the gas tube  212  and reaches the infrared detector  213 . The molecules of the carbon dioxide sample absorb the infrared light having a specified wavelength. The infrared detector  213  generates a voltage signal of the carbon dioxide sample  27  according to the detected intensity of the infrared light. A filter  214  is arranged in the single-channel infrared gas detection module  21 . The filter  214  only allows a narrow infrared band that carbon dioxide can absorb to pass. Thereby, the infrared detector  213  is dedicated to detect carbon dioxide and generates the voltage signal of the carbon dioxide sample  27  according to the detected intensity of the narrow infrared band incident thereto. The infrared detector  213  is coupled to an amplifier  24 . The amplifier  24  amplifies the voltage signal into an amplified voltage signal and transmits the amplified voltage signal to a converter  25 . The converter  25  is a digital-to-analog converter converting the amplified voltage signal into a voltage value (or other infrared signal value which is not CO2 concentration still) and transmits the voltage value to a processing module  23 . The processing module  23  calibrates the infrared signal value with a reference infrared signal value according to Equation (1): 
         V   seAcor   =V   seA *( V   seBold   /V   seBnew )  (1)
 
     wherein V seAcor  is the calibrated infrared signal value, V seA  the detected infrared signal value of the carbon dioxide sample  27 , V seBold  the reference infrared signal value, and V seBnew  the updated reference infrared signal value, whereby is calibrated the concentration drift caused by long-term usage and generated a correct concentration of the carbon dioxide sample  27 . A display device  26  is coupled to the processing module  23  and presents the concentration. Thus, the present invention uses a simple single-channel infrared measurement structure to realize a high-precision virtual dual-channel gas measurement. 
     The atmosphere has about 350 ppm to 450 ppm carbon dioxide, which is the concentration most suitable for human beings naturally. In the present invention, an initial reference digital value is preset before a gas sample is measured. The initial reference digital value is obtained via measuring the gas concentration in the atmosphere. Refer to  FIG. 3  a flowchart of a gas concentration measurement method able to compensate for infrared signal drift according to the present invention. If a gas inside a building is to be detected in daytime, Step S 31  is undertaken. In Step S 31 , the single-channel infrared gas detection module  21  is used to measure the voltage signal of the gas sample. Inside the building, the concentration of carbon dioxide increases with the personnel and the activities thereof. When the carbon dioxide concentration increases, for example, from 400 ppm to 1000 ppm, the detected voltage signal is weakened. In Step S 32 , the voltage signal is converted into an infrared signal value. In Step S 33 , the infrared signal value is calibrated with the reference infrared signal values to generate the concentration of the gas sample. Repeating the measurement and calibration of from Step S 31  to Step S 33  can promote the correctness of gas concentration measurement. Long-term usage will cause aging and infrared signal drift of the single-channel infrared gas detection module  21 . Therefore, the reference infrared signal value should be updated periodically or non-periodically (Step S 34 ). At night, the personnel and the activities inside a building decrease. Therefore, the carbon dioxide concentration inside the building also decreases, for example, from 1000 ppm to 400 ppm or less, and approximates the carbon dioxide concentration of the atmosphere. Thus, the update is preferably undertaken at night. For updating the reference infrared signal value, the single-channel infrared gas detection module  21  is used to detect the voltage value of carbon dioxide concentration at night according to Step S 31 . Then, the detected voltage value directly replaces the reference infrared signal value used in Step S 33 . Thus, the updated reference infrared signal value is used to calibrate the drifted infrared signal value into the correct infrared signal value. 
     Another method to update this reference infrared signal value is to use one standard CO2 gas by pumping into the gas tube. In this way, the pumped standard CO2 can be any value in the range of 0 ppm˜3000 ppm, and the preset reference infrared signal need be set according to this future pumped CO2 gas. 
     Refer to  FIG. 4  for the curves obtained via measuring the carbon dioxide concentration inside a building for many days before and after calibration. From  FIG. 4 , it is known that periodically or non-periodically updating the reference voltage value can effectively correct the drifted infrared signal value. 
     Refer to Table.1 to further understand how the virtual dual-channel gas concentration measurement device of the present invention uses Equation (1) to calibrate the infrared signal drift caused by long-term usage and achieve the precision of a dual-channel infrared gas concentration measurement device. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Vsegz V2Ch 
                   
                   
                   
                   
                   
                   
               
               
                   
                   
                 Vseg 
                   
                 Vsegz 
                 Vsez after 
                 Cnt0 
                   
                 CntSh 
                   
                   
                 CntV2Ch 
               
               
                   
                   
                 Vse after 
                 Vsez 
                 Vsez After 
                 aging cor. 
                 Cnt before 
                 CntAA 
                 Cnt cor. 
                 CntSp 
                   
                 Cnt cor. 
               
               
                 Symbol 
                 Vse 
                 aging [mV] 
                 Vsez 
                 aging 
                 by V2Ch 
                 aging 
                 Cnt after 
                 by shift 
                 Cnt cor. 
                 Cnt2Ch 
                 by V2Ch 
               
               
                 Cnt True 
                 Vse 
                 −1% 
                 Normal- 
                 −1% 
                 −0.1% 
                 Calc. from 
                 aging no 
                 103 ppm 
                 by span 
                 Cnt cor. 
                 −0.1% 
               
               
                 [ppm] 
                 [mV] 
                 aging 
                 ized 
                 aging 
                 residual 
                 Equation07 
                 cor. 
                 shift 
                 113% 
                 by 2Ch 
                 residual 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 0 
                 1.6908 
                 1.6739 
                 1.0000 
                 0.9900 
                 0.9990 
                 0 
                 46 
                 −56 
                 41 
                 0 
                 2 
               
               
                 400 
                 1.6030 
                 1.5870 
                 0.9481 
                 0.9386 
                 0.9471 
                 400 
                 503 
                 400 
                 443 
                 400 
                 410 
               
               
                 1000 
                 1.5204 
                 1.5052 
                 0.8992 
                 0.8902 
                 0.8983 
                 1009 
                 1144 
                 1041 
                 1009 
                 1009 
                 1023 
               
               
                 1000 
                 1.4207 
                 1.4065 
                 0.8403 
                 0.8319 
                 0.8394 
                 2038 
                 2215 
                 2113 
                 1955 
                 2038 
                 2055 
               
               
                 3000 
                 1.3499 
                 1.3364 
                 0.7984 
                 0.7904 
                 0.7976 
                 3020 
                 3237 
                 3135 
                 2856 
                 3020 
                 3041 
               
               
                 4000 
                 1.2986 
                 1.2856 
                 0.7680 
                 0.7604 
                 0.7673 
                 3910 
                 4166 
                 4063 
                 3676 
                 3910 
                 3935 
               
               
                 6000 
                 1.2099 
                 1.1978 
                 0.7156 
                 0.7084 
                 0.7148 
                 5991 
                 6349 
                 6247 
                 5602 
                 5991 
                 6025 
               
               
                 8000 
                 1.1490 
                 1.1375 
                 0.6796 
                 0.6728 
                 0.6789 
                 8073 
                 8562 
                 8459 
                 7554 
                 8073 
                 8120 
               
               
                   
               
            
           
         
       
     
     Table.1 shows the data obtained in simulated measurements performed by the present invention and the prior arts, wherein the carbon dioxide concentration (Symbol) ranges from 0 ppm to 8000 ppm and the initial voltage (V se ) ranges from 1.6908 mV to 1.1490 mV. There are several prior arts used to perform calibration for the uncalibrated measured carbon dioxide concentration (Cnt AA). For examples, a single-channel carbon dioxide concentration zero shift calibration method (Cnt Sh) subtracts 103 ppm from all the measured carbon dioxide concentrations (Cnt AA); a single-channel carbon dioxide concentration span calibration method (Cnt Sp) divides each of the measured carbon dioxide concentrations (Cnt AA) by 113%; and a dual-channel method (Cnt 2Ch) calibrates the measured carbon dioxide concentrations (Cnt AA) into the standard carbon dioxide concentrations (Cnt0). The virtual dual-channel gas concentration measurement method (Cnt V2Ch) of the present invention sets a reference voltage value to function as the reference voltage value of a virtual reference channel firstly and uses the reference voltage value to calibrate the detected voltage value. For example, 1.6030 mV, the voltage value for the normal carbon dioxide concentration in the atmosphere (400 ppm), is set to be the initial reference voltage value. The initial reference voltage value may be used as V seBold . As the single-channel infrared gas detection module  21  will have infrared signal drift after long-term usage, the user had better periodically or non-periodically perform measurement at night to obtain the voltage value of the gas sample (such as 1.5870 mV) to replace the initial reference voltage value and function as V seBnew . When the carbon dioxide concentration is 1000 ppm in daytime, the initial voltage value should be 1.5204 mV. Suppose that the voltage detected by the single-channel infrared gas detection module is 1.5052 mV (V seA ). According to Equation (1), 
         V   seAcor =1.5052*(1.6030/1.5870)=1.5204mV 
     Thereby, the voltage value is calibrated to be 1.5024 mV exactly equal to the initial voltage value. Therefore, the present invention can output the accurate concentration of carbon dioxide. 
     Refer to  FIG. 5  for the curves expressing the relationships between voltage value and the carbon dioxide concentration, wherein the curve of the standard voltage value (V sez ) coincides with the curve of the calibrated voltage value. Although the single-channel infrared gas detection module used by the present invention is also aged by long-term usage and has an error of about 0.1%, the method of the present invention can almost perfectly correct the error. 
     Refer to  FIG. 6  for the concentration-error relationships of the prior arts and the present invention.  FIG. 6  shows that the accuracy of the method of the present invention is slightly lower than that of the conventional dual-channel infrared carbon dioxide measurement method. However, the present invention can almost approach the accuracy of the conventional dual-channel method. Therefore, the present invention can solve the problems of complicated structure and high price of the conventional dual-channel infrared carbon dioxide measurement device. In other words, the present invention adopts a simple single-channel infrared measurement structure and performs the calibration of a measured voltage value with the reference voltage value to economically overcome the infrared signal drift caused by long-term usage. Thus, the present invention is less expensive and suitable for homes, office buildings, schools and shops. Further, the present invention has a wide detection range. Therefore, the present invention has considerable commercial potential. In a conventional single-channel carbon dioxide zero shift concentration calibration method (Cnt Sh), 103 ppm is subtracted from the detected carbon dioxide concentration. In a conventional single-channel carbon dioxide concentration span calibration method (Cnt Sp), the detected carbon dioxide concentration is divided by 113%. The two conventional calibration methods cannot accurately measure gas concentration because the post-calibration error increases with usage time and gas concentration. The prior arts are hard to overcome infrared signal drift and thus have poor accuracy and inferior reliability in gas concentration measurement. The present invention can solve the abovementioned problems. 
     The embodiments described above are only to demonstrate the technical contents and characteristics of the present invention to enable the persons skilled in the art to understand, make, and use the present invention. However, it is not intended to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.