Abstract:
A gas sensor with instant response uses one or more oscillators while no chemical reactions or other material modifications are involved. Sensor can be used in any application to measure a percent range of gas concentrations, or mass of the absorbed gas.

Description:
[0001]    This application is a continuation-in-part application of U.S. patent application Ser. No. 11/524,698, which claims priority to U.S. Provisional Patent Application Ser. No. 60/719,548, both of which are hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to gas sensors, and more particularly to gas sensors utilizing oscillators and environmental sensors. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0003]      FIG. 1  illustrates an apparatus for measuring a frequency difference between oscillators; 
           [0004]      FIG. 2  illustrates a graph of sensitivity of a sensor; 
           [0005]      FIG. 3  illustrates another apparatus for measuring a frequency difference between oscillators; 
           [0006]      FIG. 4  illustrates a graph of sensitivity of a sensor; 
           [0007]      FIG. 5  illustrates a circuit for outputting results of embodiments of the present invention; 
           [0008]      FIG. 6  illustrates a sensor utilizing a nanowire or nanotube; 
           [0009]      FIG. 7  illustrates an exemplary application of embodiments of the present invention; and 
           [0010]      FIGS. 8-11  illustrate embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Hydrogen is known as the element with the smallest atomic mass. In a gas mixture in thermodynamic equilibrium, molecules have a mean energy of ˜3/2 kT, whether they are molecules of hydrogen, nitrogen, oxygen, etc. The momentum of a molecule is my, where m is molecular mass, and v is the mean molecular velocity equal to (8 kT/πm) 1/2 . So, the momentum of a gas molecule at a given temperature will depend on its mass as (m) 1/2 . The difference in momentum and size (effective diameter) of molecules leads to the difference in other macroscopic parameters of gases, such as viscosity and diffusion rate. 
         [0012]    During oscillations in a gas environment, a vibrating object such as tuning fork tines impart momentum to gas molecules resulting in mechanical energy loss in the tines. This loss causes a change in the resonant oscillation frequency of the fork, and the frequency shift will depend on a momentum that the tines impart to gas molecules. This means that in a gas that contains light molecules, such as hydrogen, the losses due to interaction with the gas molecules will be less than in an environment without hydrogen. Hence, the frequency of oscillations will be higher in an environment having a presence of hydrogen. 
         [0013]    In a tuning fork quartz oscillator, the fork tines symmetrically vibrate in an anti-phase flexure mode, wherein the tines move in opposite directions against each other at any moment in time. The speed at which the tines oscillate can be estimated as follows. The amplitude of the tine deflection is approximately 60 nm/V. If the driving voltage on the tines is about 1V, then at a frequency of 32768 Hz, the tines will have a characteristic speed of ˜2 mm/sec. That is much less than the speed of gas molecules (hundreds of meters per second), and it is possible to consider a quasi-static case for this interaction. Therefore, it is mostly the macroscopic characteristics of gases that will affect tuning fork oscillation frequency. 
         [0014]    The described tuning fork sensor may not be selective to a particular gas, and other light gases (e.g., helium) may interfere with the gas to be served. To avoid interference, it is possible to use gas-permeable membranes (e.g., Pd (palladium) membranes when sensing hydrogen) to improve selectivity. 
         [0015]    The frequency change in the tuning fork resonator is usually small, so a differential frequency detection method may be used for the detection of small frequency deviations. Along with frequency, the quality factor Q and the electric impedance of the tuning fork resonator changes as the oscillation energy is dissipated in gas environment. 
         [0016]      FIG. 1  illustrates an apparatus  100  with a plurality of ECS-327SMO type oscillators  101  . . .  102 , which may be used for gas detection. The tuning fork oscillator cans&#39; tops  103  . . .  104  may be sanded off for access of gas, and the oscillators&#39; outputs  110 ,  111  connected to D and CLK inputs of a D flip-flop trigger  105 . The oscillator (OSC 1 )  101  may be located in an environment suspected of containing a particular gas. The reference oscillator (OSC 2 )  102  may be used to account for changes in gas composition (such as humidity changes) and for temperature compensation. The frequency of the oscillator  101  will increase with the gas concentration. The frequency difference at the flip-flop output  106  is thus proportional to the gas concentration. 
         [0017]    By way of example, the sensor  100  may be characterized using hydrogen gas mixed with air at volume concentrations of 0 to 16% at room temperature. The hydrogen-air mixture may be prepared using two 100 sccm mass flow controllers. The interval between the frequency beats may be measured using a Tektronix CDC250 counter. The response of the sensor  100  to hydrogen is quite linear in all ranges of concentrations. As can be seen in  FIG. 2 , a 9% change in the differential frequency may be observed when 16% H 2  concentration is achieved in the chamber of oscillator  101 . 
         [0018]      FIG. 3  illustrates a sensor  300  where two oscillators  303 ,  304  are used to detect a gas (e.g., hydrogen) in a gas chamber  301 . Sensor  300  has the reference oscillator  304  sealed (the can&#39;s top is not sanded off) to protect from access to the gas to be sensed and/or measured. Along with the open oscillator  303  they can be placed into the measured gas environment next to each other. As noted above, a gas-permeable membrane  302  may be used. In this embodiment, cross-compensation can be performed for the temperature, but not for humidity changes. 
         [0019]    The concentration of the gas can then be calculated as follows: 
         [0000]        f   1   =f   10   +kC   H2 , 
         [0000]        f   2   =f   10   +f   12 , 
         [0000]      Δ f=f   1   −f   2   =f   10   +kC   H2   −f   10   +f   12   =kC   H2   +f   12    
         [0000]    where f 10  is the frequency of oscillator OSC 1   303  without the gas, f 12  is the difference between the frequencies of OSC 1   303  and OSC 2   304  without the gas, k is the proportionality factor, and C H2  is the concentration of the gas. The last expression can be recalculated as follows: 
         [0000]        C   H2 =(Δ f−f   12 )/ k    
         [0020]    By way of example, test results for the sensor  300  for sensing hydrogen with sealed and open can oscillators are shown in  FIG. 4 . The sensor  300  was tested at room temperature, the flow rate of nitrogen was 200 sccm, and the flow rate of hydrogen changed from 0 to 10 sccm. The sensor  300  shows a near linear response to H 2  concentration changes. 
         [0021]    Referring to  FIG. 5 , since a difference in frequencies of two oscillators can be as small as several Hz, it may be more convenient to measure time intervals between two frequency beating pulses. In this case, a separate high-frequency oscillator (not shown) may be used to fill the time intervals with pulses at a fixed frequency f 0 . For precision measurements, oven-controlled crystal oscillators (OCXO) may be used to generate such pulses. The time interval between the frequency beatings is 
         [0000]        T= 1 /Δf= 1/( f   1   −f   2 )=1/( kC   H2   +f   12 ) 
         [0022]    If the OCXO stabilized generator has a frequency of f 0 , then the number of pulses at the output N of pulse counter  501  is 
         [0000]        N=f   0   T=f   0   /Δf=f   0 /( kC   H2   +f   12 ) 
         [0023]    In another embodiment, a device for measuring oscillation parameters of the tuning fork detects changes in Q factor and an impedance of the tuning fork as hydrogen will change the energy that is dissipated by the tuning fork tines. The energy dissipation in the tuning fork can described as follows. 
         [0024]    If a mechanical system such as a tuning fork has a mechanical resistance R M , the quality factor Q at a resonant frequency f 0  is 
         [0000]      Q˜f 0 /R M    
         [0025]    The mechanical resistance R M  is a function of the gas viscosity V, and thus R M  can be described with the following series 
         [0000]        R   M   =R   M0 (1 +c   1   V+c   2   V   2 + . . . ) 
         [0000]    where c1, c2, . . . , are the proportionality coefficients. For media with low viscosity, such as a gas, this can be rewritten as 
         [0000]        R   M   =R   M0 (1 +c   1   V ) 
         [0000]    where R M0  is the mechanical resistance in vacuum and does not depend on a gas viscosity. 
         [0026]    Hydrogen has approximately two times lower viscosity than air (8.4×10 −6  Pa*s vs. 17.4×10 −6  Pa*s at 0° C.), and, hence, the mechanical resistance will decrease at higher relative hydrogen concentrations. Thus: 
         [0000]        R   M   =R   M0 ( a−bC   H2 ) 
         [0000]    where a and b are functions of viscosities of hydrogen and a balanced gas (such as air), and C H2  is a relative concentration of hydrogen in the gas mixture. Then the concentration can be defined as 
         [0000]      C H2 ˜(a−f 0 /QR M0 )/b 
         [0000]    where Q can be measured experimentally. The quality factor can be easily found when the quartz tuning fork is a part of an electrical circuit, since, by definition 
         [0000]    
       
      
       Q=f 
       0 
       /Δf  
      
     
         [0027]    Measuring Δf may be performed by conventional methods used in electronics (frequency sweeping around f 0 , measuring amplitude attenuation of oscillation pulses (damping factor), etc.). Since the electric impedance is also a function of the quality factor 
         [0000]      |Z(ω)| 2 ˜(1/Q 2 −1)+(ω/ω 0 ) 2 +(ω 0 /ω) 2    
         [0000]    it may be used for determination of hydrogen concentration as well. 
         [0028]      FIG. 6  illustrates an apparatus based on measurements of the frequency and/or the Q factor of a periodic movement (vibration) of a nanowire or a nanotube  601 , such as a carbon nanotube, in flexure mode. The measurement system  603  of the sensor will include a means to detect and quantify such oscillations. Nanotube  601  vibrates due to an applied external force (not shown), such as mechanical or electrostatic force. An electric probe  602  coupled to the nanotube  601  either by field electron emission to/from the nanotube  601 , or through the capacitance between the nanotube  601  and the probe  602 , or by any other equivalent coupling mechanism. The probe  602  forms a part of the measuring electric circuit  603  that can measure deviations in coupling parameters (such as capacitance) and determine the frequency of these deviations. 
         [0029]    As described above, the vibration frequency of the nanotube  601  or nanowire will depend on the viscosity of a surrounding gas, which, in turn, will depend on the concentration of the gas. 
         [0030]    By way of example, the sensitivity range of the sensor is 0 to 100% H 2 , with a detectivity limit of at least 1%, as can be seen from the sensitivity graphs shown above in  FIGS. 2 and 4 . The lower flammability level of hydrogen is 4%, and lower explosive limit (LEL) is 17%. 
         [0031]    A gas sensor based on the kinetic characteristics of a gas, such as its molecular mass, viscosity, diffusion, etc., depends not only the gas concentration but also the characteristics of the environment, such as pressure, humidity, and temperature. Embodiments of the present invention utilize a combination of sensors to more accurately calculate the gas concentration. 
         [0032]      FIG. 8  illustrates a sensor system  800  that comprises electro-mechanical oscillators in a configuration as described above with respect to  FIGS. 1-6  (e.g., apparatus  100 ), a temperature (T) sensor  802 , a pressure (P) sensor  804 , and a humidity (RH) sensor  803 . A Sensirion SHT-75 temperature/humidity sensor may be used for the T sensor  802  and the RH sensor  803 , and a ICS-1451 pressure sensor may be used for the P sensor  804 . 
         [0033]    An accurate signal for gas concentration may be calculated using an algorithm determined by circuitry in controller  801  that has the frequency difference, pressure, temperature, and gas humidity as variables. A multiple linear regression may be used, and the algorithm is applied to the data obtained while testing the sensor in environments with different temperature, humidity, pressure, and gas concentrations. The obtained coefficients are stored in the memory of the sensor controller  801  to further calculate the gas concentration using a linear or quadratic equation, as further described hereinafter. 
         [0034]    The response of different tuning fork oscillators from even the same manufacturer may be different for the same detected gas, and if the response of two tuning forks is similar for one gas, it may be different for the other gas. Since the oscillators are not specific to any gas, such difference in response, which may be caused by mechanical imperfections in oscillator packaging, quartz crystals, electronics, etc., may be used to create some of specificity to distinguish between the two or more gases. In this case, several oscillators may be measured with different gases, and then a multiple regression method may be used to calculate the relevant concentrations. 
         [0035]    This approach may also be used to non-selectively detect the mass of adsorbed gas contaminants if the sorbent is deposited on the open to the environment oscillator. For example, for activated carbon sorbent, the sorption capacity is quite high that results in a several percent increase in the sorbent mass. The sensors described herein may be able to detect the retaining capacity of the sorbent based on the frequency difference between the open and the sealed oscillator. The frequency of the open oscillator increases if more gas is absorbed in the sorbent, while the frequency of the sealed oscillator remains constant. 
         [0036]      FIG. 9  illustrates a sensor system  900  that comprises electro-mechanical oscillators  902  . . .  903  similar to oscillators  101  . . .  102 . A temperature (T) sensor  802 , a pressure (P) sensor  804 , and a humidity (RH) sensor  803  are similar to those as described above with respect to  FIG. 8 . A differential amp  904  determines the frequency difference from the outputs of oscillators  902  . . .  903 . An accurate signal for gas concentration may be calculated using an algorithm determined by circuitry in controller  901  that has the frequency difference, pressure, temperature, and gas humidity as variables, in a manner as similarly described with respect to  FIG. 8 . 
         [0037]      FIG. 10  illustrates a sensor system  1000  that comprises more than two electro-mechanical oscillators  1005 ,  1006 , . . .  1007  similar to oscillators  101  . . .  102 . One or more of the oscillators may be sealed in a hermetic packaging, while other oscillators may be exposed to one or more environments that may contain one or more gases to be sensed. A temperature (T) sensor  1002 , a pressure (P) sensor  1004 , and a humidity (RH) sensor  1003  are similar to those as described above with respect to  FIG. 8 . A plurality of differential amps  1008  . . .  1009  determine the frequency difference from the outputs of oscillators  1005 ,  1006 , . . .  1007 . An accurate signal for gas concentration may be calculated using an algorithm determined by circuitry in controller  1001  that has the frequency difference, pressure, temperature, and gas humidity as variables, in a manner as similarly described with respect to  FIG. 8 . 
         [0038]      FIG. 11  illustrates a sensor system  1100  that comprises electro-mechanical oscillators  1105  . . .  1106  similar to oscillators  101  . . .  102 . One or more of the oscillators (e.g.,  1105 ) may be sealed in a hermetic packaging. One or more of the oscillators may comprise a gas sorbent coating and exposed to a gas-containing environment. When the gas is absorbed by the sorbent, the mass of the sorbent increases by the mass of the absorbed gas. Since the resonance oscillation frequency of the tuning fork prongs depends on their dimensions and their mass, the added mass of the sorbent deposited on the prongs&#39; surface will result in lowering of the oscillation frequency of the tuning fork. A temperature (T) sensor  1102 , a pressure (P) sensor  1104 , and a humidity (RH) sensor  1103  are similar to those as described above with respect to  FIG. 8 . A differential amp  1108  determines the frequency difference from the outputs of oscillators  1105  . . .  1106 . An accurate signal for gas concentration may be calculated using an algorithm determined by circuitry in controller  1101  that has the frequency difference, pressure, temperature, and gas humidity as variables, in a manner as similarly described with respect to  FIG. 8 . 
         [0039]    The inputs to the controllers  801 - 1101  measure gas concentration (and thus also sense a gas) with algorithms as described as follows. The frequency difference (dF) measured by a controller, as well as data from the pressure sensor (p), humidity sensor (RH), and temperature (T) sensor are further used to calculate the sensor response to hydrogen or other sensed gas with concentration c. The frequency difference is generally a function (S) of these parameters: 
         [0000]        dF=S ( p,RH,T,c ) 
         [0040]    During sensor calibration, the environmental parameters and gas concentration are changed in a controlled manner, for example, using an environmental chamber for changing T and p, mass flow controller settings to change gas concentration c, and gas humidifier for changing RH. During the sensor calibration, the frequency difference dF and other above mentioned parameters are recorded. For small variations in these parameters, the dF linearly depends on changes in any parameter. For example, if the pressure changes from p0 to p, it can be written 
         [0000]        dF 1 =S ( p 0 ,RH,T,c )+( dS/dp )( p−p 0) 
         [0041]    For the change in concentration from c0 to c, it can be written in a similar way: 
         [0000]        dF 2 =S ( p,RH,T,c 0)+( dS/dc )( c−c 0) 
         [0000]    and so on for different values of the parameters. 
         [0042]    As a result of changes in all the parameters, there will be a set of “i” linear equations 
         [0000]        dFi=S +( dS/dp )Δ pi +( dS/dT )Δ Ti +( dS/dRH )Δ RHi +( dS/dc )Δ ci    
         [0000]    that are further solved by standard multiple linear regression method. 
         [0043]    Solving these equations will provide a set of calculated constants (dS/dx), which will be further used as linear coefficients for the sensitivity function S: 
         [0000]        dF=S 0 +S 1 *p+S 2 *T+S 3 *RH+S 4 *c    
         [0000]    where S1=(dS/dp) and so on. 
         [0044]    From this equation, the gas concentration c as a function of dF and other environmental parameters can be easily calculated: 
         [0000]        c =( dF−S 0 −S 1 *p−S 2 *T−S 3 *RH )/ S 4 
         [0045]    If, for example, the linear approximation for one of the parameters is not satisfactory, second order approximation can be used while using the same algorithm. For example, for pressure it will be 
         [0000]        dF 1 =S ( p 0 ,RH,T,c )+( dS/dp )( p−p 0)+(½)( d   2   S/dp   2 )( p−p 0) 2    
         [0000]    and the concentration will be calculated as 
         [0000]        c =( dF−S 0 −S 11 *p−S 12 *p 2 −S 2 *T−S 3 *RH )/ S 4 
         [0000]      where 
         [0000]        S 11=( dS/dp ) and  S 12=(½)*( d   2   S/dp   2 ) 
         [0046]    These parameters Si are stored in memory and are used by a controller to calculate gas concentration c. 
         [0047]    Same approach is used to calculate the change in sorbent mass m. 
         [0048]    Thus, the described sensors may be used as a leak detector for many applications. For example, referring to  FIG. 7 , in a fuel cell powered car, the sensor may be installed near the fuel cell reactor, near the passenger seats, or in the exhaust system. 
         [0049]    An LEL detector that uses the described sensor may be a portable handheld device, with a sensor incorporated in the device body, or placed at the end of an attachable sampling probe. The device may have indications of concentration on a display along with a sound alarm if the concentration of hydrogen reaches a certain critical level. Other applications include water electrolysers, hydrogen storage systems, industrial equipment, etc. 
         [0050]    Improvements can be made to stabilize the sensor response against temperature, humidity, atmospheric pressure, quartz aging, and other conditions of operation. 
         [0051]    A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.