Patent Publication Number: US-2019187113-A1

Title: System and method for automatically adjusting gas sensor settings and parameters

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. Non-Provisional Patent Application No. 13/018,039 filed-on Jan. 31, 2011 entitled SYSTEM AND METHOD FOR AUTOMATICALLY ADJUSTING GAS SENSOR SETTINGS AND PARAMETERS which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates to gas detection systems in general, and more particularly to a gas detection system platform in which a plurality of different sensor types can be used with a single transmitter design, and in which the plurality of different sensor types can be automatically recognized, adjusted, calibrated, and monitored with reduced user input. 
     BACKGROUND OF THE DISCLOSURE 
     Environmental sensing systems may include a variety of sensors to determine the presence and concentrations of hazardous (i.e., combustible) and/or toxic gases in industrial and other workspaces. Depending on the environment, it may be desirable to provide sensing information for a wide variety of different gas types and concentrations. Thus, a typical system can include a large number of different types of sensors, where each type of sensor is adept at sensing a particular gas in a desired concentration range. 
     In order for a particular sensor to detect a desired gas in a particular concentration range, and to transmit that information in a form readily understood by a remote transmitter, the sensor&#39;s output signals must be properly conditioned. Currently, sensor signal conditioning is accomplished by using discreet components (i.e. resistors, capacitors, operational amplifiers, etc.) to filter and amplify a specific sensor&#39;s output prior to performing a conversion to the digital domain for further processing. In one example, signals from electrochemical sensors are typically conditioned using the well-known potentiostat circuit. The drawback to using discrete components, however, is that the arrangement of such components is often specific to a particular type of sensor, and also to a particular gas being sensed as well as a desired concentration range. Thus, developing conditioning circuits for a wide range of gases and ranges requires changing the values of these components to achieve optimum analogue signal conditioning. This, in turn, requires a wide variety of conditioning circuits to cover ranges of potential interest. As a result, a large number of sensors of differing types, and of differing concentration range capacities, are manufactured and stocked to meet the associated wide variety of field applications. Moreover, most sensors operate in combination with an associated transmitter unit. Due to the specialized nature of the described sensors, such transmitters often only work with a single sensor type. As such, current systems require that a similarly large number of different transmitters are also manufactured and stocked. 
     In addition, when current remote transmitter and sensors are installed and/or replaced, they are individually adjusted to ensure they are appropriately calibrated, and also to ensure that they are in proper working condition. Currently, for remote transmitter and sensor applications this adjustment/verification process is a two-person effort in which one person stands at the sensor location reading a digital voltage meter, and a second person at the transmitter adjusting a manual potentiometer to achieve a desired output voltage for supplying the sensor. When this operation takes placed in a hazardous area, it can require that the area be declassified so that the transmitter can be opened to access the manual potentiometer. Much the same is true for integral transmitter and sensor applications, in which one person reads a digital voltage meter and adjusts a manual potentiometer at the transmitter to achieve a desired output voltage for supplying the sensor. This can also undesirably involve declassifying the associated hazardous area to open the transmitter to access the manual potentiometer. 
     It will further be appreciated that sensors undergo sensitivity losses over time. Present systems are not able to provide automatic recognition and adjustment of sensors to compensate for such losses in sensitivity. This, in turn, can lead to premature disposal of sensors that drop below a desired sensitivity threshold. Since such sensors ostensibly would continue to function desirably if their loss in sensitivity could be compensated for, current systems produce unnecessary waste. 
     Accordingly, there is a need for an improved environmental sensing system that; enables a single, transmitter to recognize and accept a plurality of different sensor types, automatically adjusts installed sensors to reduce or eliminate the need for manual adjustment, automatically calibrates sensors to enable a single sensor to accommodate a variety of different sensing ranges, enables a sensor to be calibrated at a single value and then be used at a variety of values, and enables automatic adjustments to extend sensor lifetime. 
     In addition, a type of environmental sensing system includes a transmitter portion connected to an associated sensor portion by a cable. The transmitter portion transmits information received from the sensor portion to a wireless network, for example. The sensor portion may be located in a hazardous and/or combustible environment remote from the transmitter portion. Further, the transmitter and sensor portions each include a gland arrangement having multiple holes through which wires extend. 
     It is frequently desirable to “hot swap” the sensor during use, i.e. replace the sensor without declassifying the hazardous area, in the event that the sensor has lost sensitivity, for example. However, removing the sensor may cause generation of a spark or an electrical are in the connection between the transmitter and sensor circuitry. These sparks could ignite a potentially explosive atmosphere. 
     SUMMARY OF THE DISCLOSURE 
     An environmental sensing system solving one or more of the aforementioned problems is disclosed. Specifically, a system is disclosed including: (1) an automatic sensor excitation voltage adjustment feature, (2) a multi-range concentration feature, and (3) a single calibration feature. The automatic sensor excitation voltage adjustment feature may include a transmitter having an associated microprocessor that provides an initial voltage to an associated sensor. The sensor also may have an associated microprocessor, and as the voltage changes, a correction signal may be relayed from the sensor microprocessor to the transmitter microprocessor. The correction signal may be used by the transmitter microprocessor to adjust the voltage being applied to the sensor to a desired value. The multi-range concentration sensor feature may include an amplifier associated with the sensor/microprocessor to create gain settings which can then be used to optimize sensor resolution by changing a gain value associated with the sensor. This, in turn, may enable a single sensor to be used for a variety of different concentration ranges, as desired by a user. The single calibration feature enables it sensor to be calibrated at a single gas concentration value, and thereafter be used for a variety of different concentration range applications. 
     A system is disclosed for recognizing and adjusting sensor voltage by using digital potentiometers, preferably without human intervention and without the need to declassify a hazardous area. The system may include a gas detector/transmitter power supply circuit comprising an adjustable power supply with a pair of digital potentiometers. One potentiometer can be used for coarse voltage adjustment, and the second potentiometer can be used for fine voltage adjustment. An output voltage from this power supply circuit is referred to as V adjust , and is used to power a sensor associated with the transmitter. This arrangement enables a single transmitter design to be used with a multiplicity of different sensor types and ranges, as the power supply circuit is able to automatically adjust the sensor excitation voltage (V adjust ) to a specific value associated with the particular sensor being used. It can also compensate for voltage variations due to environmental changes and voltage drop in the intervening cable. The disclosed system enables sensors to be replaced under power, without declassifying the associated area. In addition, the disclosed system can reduce the overall cost of ownership by enabling replacement of only the sensor kernel at sensor end of life, as opposed to current systems which require replacement of an entire sensor unit. 
     A system is disclosed for detecting the presence of a gas, comprising a transmitter portion, a sensor portion, and a variable voltage supply associated with the transmitter portion and the sensor portion. The transmitter portion may receive information from the sensor portion regarding a received voltage. The transmitter portion may also adjust the supplied voltage in response to the information received from the sensor portion. 
     A method is disclosed for adjusting sensor excitation voltage. The method may include providing, at a transmitter portion, an operating voltage to a sensor portion, receiving, at the transmitter, a signal from the sensor portion indicating a voltage required for operation of the sensor portion, and adjusting, at the transmitter, the operating voltage provided to the sensor portion. 
     A universal gas sensor/transmitter unit is disclosed. The unit may include a sensor portion including a sensor kernel and a processor configured to read a received excitation voltage. The unit may also include a transmitter portion having a recess configured to engage the external surface geometry of the sensor. The transmitter may include a transmitter processor and at least one power supply circuit for providing adjustable power to the sensor when the enclosure is engaged with a recess of the transmitter. The adjustable, power supply circuit may include at least one potentiometer controllable by the processor to adjust a power supplied to the sensor portion. 
     A method for calibrating a sensor is disclosed. The method may include setting an original zero offset and a spanning of a sensor at a first gain setting; obtaining a zero offset at a second gain setting; obtaining a ratio of the original zero offset to the zero offset at the second gain setting; and scaling a calibration factor by the ratio to enable operation of the sensor in an operating range associated with the second gain setting. 
     A method for providing replacement guidance for a sensor is also disclosed. The method may include determining a reduction of sensitivity for a sensor, trending said reduction of sensitivity over time, and adjusting a gain parameter associated with the sensor to compensate for the reduction of sensitivity. 
     A method for adjusting an operating range for a sensor is disclosed. The method may include providing an amplifier associated with the sensor, providing a table of gain settings for the amplifier, and selecting again setting from the table to optimize a resolution in an analog to digital converter associated with the sensor. Selecting a gain setting may adjust the sensor to one of a plurality of pre-determined operating ranges. 
     In addition, a system for detecting the presence of a gas is disclosed. The system includes a sensor portion for sensing a target gas and providing signals indicative of the gas, wherein the sensor portion is replaceable. The system also includes a transmitter portion for transmitting information received from the sensor portion to a network. Further, the system includes a barrier circuit for providing intrinsically safe power and communication signals to the sensor portion. 
     A system for detecting the presence of a gas is disclosed, whereby the system may be combined into a network having a common transmitter portion, with one or more barrier circuits, receiving information from a plurality of sensor portions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       By way of example, a specific embodiment of the disclosed device will now be described, with reference to the accompanying drawings, in which: 
         FIG. 1  is an illustration of an exemplary transmitter with a single sensor combination; 
         FIG. 2  is an illustration of an exemplary transmitter portion of  FIG. 1  associated with multiple detector head portion with a plurality of different sensor portion types; 
         FIG. 3  is a cross section view of an exemplary detector head portion assembly which contains a sensor portion assembly for use with the transmitter portion of  FIG. 1 ; 
         FIGS. 4A-4C  are exploded views of respective sensor portion assemblies for use with the detector head portion  FIG. 3  for use in the transmitter portion of  FIG. 1 ; 
         FIG. 5  is an exploded view of the transmitter portion of  FIG. 1 ; 
         FIG. 6  is a system diagram of the transmitter/sensor combination of  FIG. 1 ; 
         FIG. 7  is system board level diagram of the transmitter/sensor combination of  FIG. 1 ; 
         FIG. 8  is a block diagram for the sensor portion of  FIG. 4 ; 
         FIG. 9  is a block diagram of an exemplary power supply arrangement for the transmitter/sensor combination of  FIG. 1 ; 
         FIG. 10  is a circuit diagram of an exemplary adjustable power supply for rise with the transmitter/sensor combination of  FIG. 1   
         FIG. 11  is an exemplary circuit is shown for providing a reference voltage used by the transmitter processor; 
         FIG. 12  is a Schematic of an exemplary gas transmitter/sensor processor; 
         FIG. 13  is a schematic of an exemplary sensor processor; 
         FIG. 14  is a flowchart illustrating an exemplary embodiment of the disclosed method; 
         FIG. 15  is a block diagram for the arrangement shown in  FIG 2 ; 
         FIG. 16  is a transmitter portion block diagram; 
         FIG. 17  is a sensor portion block diagram; 
         FIG. 18  is a schematic of a transmitter power barrier circuit for providing an intrinsically safe (IS) power signal; and 
         FIG. 19  is a schematic of a transmitter communications barrier circuit for providing an IS communication signal. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosed system and method takes advantage of advances in microelectronics and performs final signal conditioning of a sensor using amplification built into microprocessors. This amplification can be software controlled to be arranged in either a differential or additive mode. Additionally, the level of gain can be adjusted in discrete levels, thus allows a wide range of input signals to be accommodated in a single circuit without resorting to changing in discrete components. 
     Several applications are realizable. First, a single sensor can be built for a specific gas, and the range of the sensor can be optimized for a particular application. For example, one sensor can be provided in either a 0-10 ppm or 0-100 ppm range simply by changing software parameters. Secondly, variations in sensor sensitivity can be accommodated over a wider range, leading to greater manufacturing throughput. Previously, sensor kernels had to be screened to ensure their sensitivity could be accommodated by a particular fixed circuit design. Now a wider variability in sensitivity can be utilized, leading to less manufacturing waste. Lastly, as sensors are subjected to normal use, their sensitivities degrade. With prior designs, once the sensor&#39;s sensitivity had degraded to the point the fixed circuit cannot compensate for this degradation, the sensor had reached the end of its useful life. The current system and method can be used to compensate for sensor sensitivity degradation beyond the traditional limit by adjusting gain upward. This has the advantage of reducing life cycle cost for a gas detector by reducing the total number of sensor replacements. 
     As previously noted, current systems utilize fixed circuit designs for each range of a target gas. In practice, sensors need to be calibrated on a recurring basis to ensure accuracy. Calibration is performed at 2 points: one with no gas present (zero), and one point within the sensor&#39;s range (span), where the span is typically 25-75% full scale. In an installation with sensors of various ranges, this requires the presence of multiple calibration gasses at different concentrations. 
     With line disclosed system and method, the calibration variables are scaled to different ranges within the sensor. This enables calibration using one concentration of calibration gas and then adjusting the range of the sensor for the particular application. This has the advantage of enabling calibration of different range sensors using one common calibration gas. As will be appreciated, this reduces the number of different concentration calibration gasses required and/or it eliminates the need to use a gas concentration that is more widely available than another for a specialized application. 
     A gas sensor/transmitter combination is disclosed that recognizes and adjusts sensor voltage by using digital potentiometers, preferably without human intervention and without declassifying a hazardous area. A gas sensor/transmitter power supply circuit includes an adjustable power supply with two digital potentiometers. One potentiometer is for coarse voltage adjustment and the second potentiometer is for fine voltage adjustment. The output voltage of this power supply circuit is called V adjust  and aids in powering the sensor associated with the transmitter. 
     Referring now to  FIG. 1 , a transmitter/sensor combination  1  is shown comprising a detector head portion  2  and a transmitter portion  4 . The transmitter portion  4  may be configured to transmit information received from the detector head portion  2  to a wireless network  6  via a wireless link  8 . The wireless link  8  can be any of a variety of protocols, including, ISA 100.1 Ia, wireless HART and the like. The wireless network  6  may distribute information received from the transmitter portion  4  to one or more local or remote alarms, and one or more local or remote monitoring stations via intranet, Internet, Wi-Fi, or other network arrangement. It will be appreciated that although  FIG. 1  illustrates a wireless connection to network  6 , that the invention is not so limited. Thus, the connection could be hard wired, protocols including Modbus, HART, foundation fieldbus, Profibus and the like. 
     Referring to  FIG. 3 , detector head portion  2  includes a sensor portion  200 . As will be understood, the primary purpose of the sensor portion  200  is to sense a target gas and provide signals representative of the concentration of that gas to the transmitter portion  4 . The primary purpose of the transmitter portion  4  is to collect information from the detector head portion  2  and to transfer that data upstream. Upstream devices may include controllers, gateways, converters and similar devices. 
     In addition to remote transmission of sensor data, the transmitter portion  4  may include a local display  10  for providing local indication of sensor operation. In some embodiments, the transmitter portion  4  may be configured to accept a plurality of detector head portions to provide an expanded area coverage.  FIG. 2  shows a transmitter portion  4  hardwired to a plurality of detector head portions  2 A- 2 C representing a plurality of different sensor types that can be used with the transmitter portion  4 . Indeed, although the description will proceed in relation to a transmitter portion  4  associated with a single detector head portion  2 , it will be appreciated that the transmitter portion  4  may be associated with, and accept signals from, a plurality of detector head portions  2 A- 2 C at once. 
     As will be appreciated, the detector head portion  2  (or detector head portions, where multiple sensors are used with a single transmitter) may be any of a variety of known sensor types, a non-limiting exemplary listing of such types including an JR gas sensor, a catalytic bead sensor, an electro-chemical sensor, a photo-ionization sensor, and a metal-oxide sensor. 
     In practical application, particular detector head portions  2  may be used to detect a wide variety of toxic gases, an exemplary listing of which includes, but is not limited to, Ammonia, Arsine, Boron Trichloride, Boron Trifluoride, Bromine, Carbon Dioxide, Carbon Monoxide, Chlorine, Chlorine Dioxide, Diborane, Fluorine, Germane, Hydrogen, Hydrogen Bromide, Hydrogen Chloride, Hydrogen Cyanide, Hydrogen Fluoride, Hydrogen Sulfide, Methanol, Methyl Mercaptan, Methyl Iodide, Nitric Oxide, Nitrogen Dioxide, Nitrogen Trifluoride, Oxygen, Ozone, Phosphine, Silane, Silicon Tetrafluoride, Sulfur Dioxide, Tetraethyloxysilane (TEOS), and Tungsten Hexafluoride. 
     In addition, particular detector head portions  2  may be used to detect a wide variety of combustible gases, a non-limiting exemplary listing of which includes Acetone, Benzene, Butadiene, Butane, Ethane, Ethanol, Ethylene, Hexane, Hydrogen, Isobutanol, Isopropyl Alcohol, Methane, Methanol, Methyl Ethyl Ketone (MEK), Pentane, Propane, Propylene, Toluene, and Xylene. 
     A benefit of the disclosed arrangement is that a single detector head portion  2  may quickly accept any of a variety of sensor portions  200 . Thus, as shown in  FIGS. 4A-4C , the sensor portion  200  may include internal sensing components such as a sensor kernel  12 , mounted within an upper sensor enclosure  22 . And although different sensor portions  200  may include different sensor kernels  12 , as well as additional processing components, all of the different sensor kernels  12  will be fit within upper and lower sensor enclosures  22 ,  14 , thus allowing the sensor portion  200  to be of a single size and shape for all applications. 
     The lower sensor enclosure  14  may be arranged to allow simple installation of a particular sensor kernel  12  and associated components. This can make it possible to replace a sensor kernel  12  without requiring the remaining components of the sensor portion  12  to be replaced. 
     Thus arranged, to engage the detector head portion  2  with the transmitter portion  4 , the upper region  16  of the detector head portion  2  is inserted into a recess (not shown) in the transmitter portion  4 , and the end cap  18  of the detector head portion  2  engages the recess and locks the sensor portion  200  to the detector head portion  2 . The end cap  18  may have one or more recesses or other geometry suitable for receiving an o-ring, gasket or the like to seal the sensor portion  200  to the detector head portion  2 . This sealing arrangement protects the internal sensor and components from potentially harsh exterior environments. A sensor portion  200  can include self-aligning features (e.g., keyed interaction with the transmitter) that can further facilitate-quick installation and replacement of sensor portions  200 . Retaining features, such as external threads and the like, can also be provided to ensure firm engagement of the sensor portion  200  with the detector head portion  2 . 
       FIGS. 4A-4C  show a plurality of sensor portions  200  used for sensing different gas types. As can be seen, each of the sensor portions  200  includes a lower sensor enclosure  14 , an upper sensor enclosure  22 , a sensor kernel  12 , a contact board  24 , a sensor printed circuit board (PCB)  26 , and an interface PCB  28 . As can be seen, the sensor kernel  12  has a different size/geometry for each of the different sensor portions  200 . Such differences can be accommodated by the lower sensor enclosure  14  which can have an internal geometry configured to receive the specific sensor kernel  12 , but which has a common external configuration so that it can be received by the upper sensor enclosure  22 . These differences also may be accommodated by the contact board  24 , which may include receptacles  25  (see  FIG. 4A ) to plug in the specific sensor kernel  12 . This allows the sensor portion  200  to be of a single size and shape for all applications. 
     As can be seen, a variety of different sized/shaped sensor kernels can be accommodated without impacting the external arrangement of the sensor portion  200 . Thus, each of the sensor portions  200  of  FIGS. 4A-C  can fit to the detector head portion  2  in exactly the same physical manner. 
     The sensor PCB  26  may be unique to each sensor kernel  12 , and as such it may include a sensor processor  30 , as well as a conditioning circuit  32  that performs conditioning of the signals received from the sensor kernel  12 . For example, the conditioning components  32  may convert the signal from the sensor kernel in μA per ppm to a voltage level useable by the sensor processor&#39;s analog to digital converter. The interface PCB  28  provides an interface between the sensor PCB  26  and the detector head portion  2 . The interface PCB  28  may include a pin arrangement  34  common to all sensor portions  200  that is configured to be received by the detector head portion  2 . 
     As arranged, in one embodiment the sensor portion  200  may constantly measure a local target gas concentration, supply voltage, and ambient temperature and report a temperature compensated gas concentration, when requested, to the transmitter portion  4 . 
       FIG. 5  shows the internal components of the transmitter portion  4 , which may include a display  10 , processor board  36 , relay/network board  38 , power supply board  40 , and intrinsic safely (IS) barrier  42 . One or more plug-in blocks  44  may also be included for providing a variety of connectivity functions for the transmitter portion  4 . The plug-in blocks  44  may be used to provide power, relays, remote acknowledge, communications and detector head connections. 
       FIG. 6  shows a logical arrangement of an exemplary transmitter/detector head/sensor combination  1  in accordance with one or more embodiments. In the illustrated embodiment, the transmitter portion  4  comprises a processor  46  that connects to the sensor portion  200  via digital communication  48 , arid it relays the output of the sensor portion  200  through a variety of communications means. A display  10  is provided to permit local monitoring of data as well as setting parameters and setting system parameters associated with process changes and calibration. An expansion port  50  is provided to enable methods of communication beyond the 4 to 20 milliamp signal and MODBUS. Memory  52  is provided to allow a history of process data, calibration data and expanded user information. Watchdog circuits  54  are provided to assure enhanced reliability. One or more additional circuits  56  can be provided for factory use to program and test the device during production. Interface/power supply  40  provides power to the transmitter portion  4  and the sensor portion  200 . 
     The inputs to the transmitter portion  4  can be HART, Serial communication from a host, serial communication from sensors, PC communication from on-board and off-board devices, SPI communication front or board and off-board devices and contact closures from magnetic switches located on the display  10 . The outputs from the transmitter portion  4  include LEDs on the display  10 , LCD on the display  10 , alarm relays, 4-20 milliamp current loops, MODBUS communication with external hosts, I 2 C communication to on-board and off-board devices, SPI communications to on-board and off-board devices, power for multiple sensors, and optional serial communications modules for external hosts. 
       FIG. 7  shows a board level diagram illustrating the interconnection between the transmitter portion  4  and the sensor portion  200 . The transmitter portion  4  may include display  10 , processor  46 , expansion modules  50 , terminal/relay board  38 , power supply board  40 , and IS barrier  42 . A connection  47  is provided between the processor  46  and the power supply board  40 . 
     The display  10  generally provides human interfaces, graphical LCD, magnetic switch inputs, and alarm status LEDs. The processor  46  controls functions of the transmitter and includes non-volatile memory  52 . The expansion modules may include capabilities for wired or wireless communications as previously described. Terminal/relay board  38  may provide standard connections including power, relay,  4  to 20 mA. RS485 MODBUS, and remote acknowledge. The power supply board  40  may convert 10-30 V DC to 3.3 V, 12V, may provide adjustable 2-9V sensor voltage, and may generate 4-20 mA loops. The IS barrier  42  may provide intrinsically safe connections to the detector head portion  2 . 
     The transmitter portion  4  may further include a terminal  58  to provide a connection to the detector head portion  2 . The terminal  58  may connect to digital communications  48  which itself can connect to a converter  60  for converting signals between RS485 and TTL levels. The process loop  48  connects to the interface PCB  28  of the sensor portion  200 . As previously described, the interface PCB  28  connects to sensor PCB  26  and kernel  12 . The sensor PCB  26  can include a sensor processor  30  and associated circuitry for providing sensor control, calculating gas concentrations, and performing temperature compensation and linearization. 
       FIG. 8  shows an exemplary block diagram for the sensor portion  200 . When requested by the transmitter processor  46 , the sensor portion  200  provides a digital output which represents a sensed gas input. The detector head portion  2  is connected to the transmitter via a cable  48 . The transmitter portion  4  provides intrinsically safe power to the detector head portion  2 , 3.3 V and V adjust , ground, and two IS communications signals. In general, the sensor portion  200  comprises a processor  30  in communication with conditioning circuitry  32 , kernel  12  and memory  62 . The memory  62  may include a variety of sensor specific information, including an excitation voltage value for the particular sensor with which the memory  62  is associated. In addition, the memory  62  may serve a data logging function, recording the sensor&#39;s historical exposure(s) to a target gas in order to develop a lifetime estimate for the sensor portion  200 . The memory  62  may also store date/time and other significant events associated with the sensor portion  200 . 
     In one embodiment, the sensor processor  30  may communicate with the transmitter processor  46  in a master/slave arrangement where the sensor is the slave. The sensor processor  30  may include as a peripheral an analog to digital converter (ADC and 2.5V reference for converting analog kernel voltages representing gas concentration to their digital equivalent. 
     As will be appreciated, different types of sensors kernels are used to detect different types of target gases. The different types of sensor kernels generate an analog output as either a current, a voltage or a bridge output. The amplitudes of these signals across full scale also vary. The input of the sensor processor A/D  30  requires a reference voltage input front 0 to 2.5 V. The individual sensor PCBs  26  for each type of sensor kernel  12  can provides conversion, amplification, filtering, and biasing, depending on the need of a particular sensor kernel. 
     Non-volatile memory  62  can be provided for storage of sensor parameters and other variables that need to be sustained during the loss of power. Some parameters are used locally by the sensor processor  30 , but the majority are used by the transmitter processor  46 . 
     The sensor interface PCB  28  may provide connection to the detector head portion  2  via an  8  pin connector  34  ( FIGS. 4A-4C ). A variety of signals may be accommodated in the connector  34 , including ground, 3.3V, V adjust , transmit (TX), receive (RX), DIR and the like. 
     Referring now to  FIG. 9 , a block diagram is shown for an adjustable power supply circuit  64  for the transmitter/sensor combination  1 . The adjustable power supply circuit  64  may use an input voltage  66  of 10 to 30 VDC, and includes a step-down (buck) switching regulator with an adjustable output voltage, for example, from about 2V to about 9VDC. Specifically, the adjustable power supply circuit  64  includes an adjustable power supply with two digital potentiometers  68 ,  70 . One potentiometer  68  is for coarse voltage adjustment and the second potentiometer  70  is for fine voltage adjustment. As will be appreciated, output power  72  is adjusted by adjusting the potentiometers, and is provided to the sensor portion  2  accordingly. 
     Referring now to  FIG. 10 , an exemplary adjustable power supply (V adjust  Output Voltage) circuit is shown. Referring to  FIG. 11 , an exemplary circuit is shown for providing a 2.5V reference voltage used by the transmitter processor  46 . As previously noted, the transmitter processor compares the V adjust  voltage reading from the sensor portion to this reference voltage to determine the need to adjust the V adjust  voltage to the sensor portion. 
     Referring to  FIG. 12  a schematic of an exemplary gas transmitter/sensor processor is shown in which V adjust  output voltage is read through a voltage divider circuit and external reference voltage into the analog to digital (A/D) inputs. Referring to  FIG. 13 , a schematic of an exemplary sensor processor is shown in which V adjust  output voltage is read through a voltage divider circuit into an analog to digital (A/D). inputs. 
     Thus described, the disclosed system may automatically adjust the excitation voltage provided to a particular sensor portion to match the exact requirements of the sensor type. 
     Thus the specific voltage that a sensor requires may be different from a default voltage initially provided by the transmitter portion  4 . The sensor-specific voltage may be stored as a parameter in the sensor&#39;s nonvolatile memory  62  and can be accessed by the sensor processor  30  and the transmitter processor  46 . This parameter may be one of the parameters initially read by the transmitter portion  4  when it recognizes a new sensor portion  200  has been installed. The initial sensor voltage setting is read with an A/D converter on the processor board  36  of the transmitter portion  4 . Once set, the transmitter processor  46  reads this voltage from the sensor portion  200  and uses that value as the initial voltage supplied to the sensor portion  200  by the transmitter portion  4 . 
     To set this initial value, the transmitter processor  46  may set the first and second potentiometers  68 ,  70  to default values to provide the initial excitation voltage to the sensor portion  200 . The sensor processor  30  measures the exact value of voltage received, and determines whether it corresponds to the voltage being provided by the transmitter portion  4 . Both the transmitter and the sensor processors  46 ,  30  read the V adjust  output voltage through a voltage divider circuit into an analog to digital (A/D) input on the respective processor (see  FIGS. 12 and 13 ). The transmitter processor  46  uses an external reference voltage circuit for its measurements. The sensor processor  30  uses the internal voltage reference of the processor for its measurements. The sensor processor communicates to the transmitter processor the V adjust  voltage reading at the sensor portion  200 . The transmitter processor compares the V adjust  voltage reading at the sensor portion  200  to the voltage reading at the transmitter processor  46  and determines the need to adjust the V adjust  voltage to the sensor portion  200 . If the transmitter processor  46  determines that a voltage adjustment is required, it adjusts the first and/or second potentiometer  68 ,  70  to provide the requisite adjusted voltage to the sensor portion  200 . 
     In one embodiment, when a new sensor portion  200  is “plugged into” an associated detector head portion  2 , as part of an initialization process the sensor processor  30  communicates to the transmitter processor  46  that it requires an excitation voltage of, for example, 4.5 V. In response, the transmitter processor  46  adjusts the first and second potentiometers  68 ,  70  to provide 4.5 V to the sensor portion  200 . The sensor begins operating, the sensor processor  30  measures the voltage actually received from the transmitter portion  4 , and relays to the transmitter processor  46  the value of the actual received voltage. For example, although the transmitter portion may be configured to provide 4.5 V to the sensor portion, the actual voltage received by the sensor portion  200  may be 4.25 V, as measured at the sensor. When it receives this information from the sensor processor  30 , the transmitter processor  46  may increase the voltage until the sensor senses 4.5 V. 
     Thus, the disclosed adjustable power supply arrangement is an automatic feature that “tells” the transmitter portion what excitation voltage the sensor portion is receiving, and provides closed loop error correction to ensure a desired voltage is being provided to the sensor portion at all times. In one embodiment, the circuitry of the adjustable power supply arrangement is provided as part of the transmitter portion  4 , preferably as part of the processor board  36 . 
     As will be appreciated, in addition to providing a correct initial voltage supply to the sensor portion  200 , the disclosed power supply circuit can also automatically compensate for power supply voltage changes that result from local and environmental temperature changes. 
     Upon initial installation, sensors are usually calibrated. This requires a zeroing, which sets the zero offset in the sensor, as well as a spanning of the sensor, usually at 50% of full scale. This gives the sensor fixed points which are then used in calculating gas concentration. The disclosed system allows a sensor to be calibrated at a single value or limited range, followed by a re-ranging of the sensor and scaling of the calibration data so recalibration is not required for operation of the sensor in different ranges. For example, the system may read zero offsets at a new gain setting, compare to a previous zero offset, and then scale calibration factors by the same ratio in order to operate at a desired range. 
     The disclosed system and method may be used to provide replacement guidance for a particular sensor portion  200 . Thus, during periodic sensor calibration operations, a corresponding loss of sensor sensitivity may be determined. This loss information can be trended over time to produce an end of life prediction. The trend information can also be used to adjust the gain parameters to extend the sensor&#39;s useful life. For example, the system may include a table of gain values for each range. A user may select from these gain values to obtain a desired operating range. 
     As previously noted, detection of different target gases requires the use of a variety of specific sensor types. In addition, to detect specific concentration ranges (e.g., 0-25 ppm, 0-50 ppm) of a target gas, specific signal conditioning is provided to enable a transmitter to process the received signals. With current devices, such signal conditioning is provided by a sensor-specific or transmitter-specific conditioning circuit. This requires a large number of application specific sensors/transmitters to be stocked. The disclosed system and method eliminate the need for such application-specific circuits. With the disclosed system and method, by adjusting the gain built into the microprocessor instead of using fixed components, a single circuit type can be provided for a particular target gas. Using a gain adjustment, the sensing range (e.g., 0-25 ppm, 0-50 ppm) can be adjusted. The result is that only a single sensor need be stocked for a particular gas. In one embodiment, a sensor can be shipped using a default range, and the end user can adjust the sensor to one of a variety of pre-determined ranges via a user interface. For example, the transmitter processor may have a pair of operational amplifiers that can be arranged in a staged mariner. Each amplifier may have a plurality of gain settings. In one non-limiting embodiment, each amplifier may have eight (8) gain settings. Thus, in combination, there would be 256 different combinations, but in practice many of the combinations could provide the same gain. A table of unique gain settings may be available to adjust the range. Based on the sensor&#39;s sensitivity and desired range, a gain value can be selected which optimizes the resolution in the A/D converter. These settings can be programmed into the sensor and made available to the user through a display menu. In some embodiments, discrete ranges would be made available, so a user would not have infinitely adjustable range scales. 
     Referring now to  FIG. 14 , a method according to one or more embodiments will be described. At step  100 , a sensor portion  200  is engaged with a transmitter portion  4 . At step  110 , the transmitter portion  4  reads a voltage value from memory  62  associated with the sensor portion  2 . At step  120 , the transmitter portion  4  provides an operating voltage to the sensor portion. At step  130 , the sensor portion  200  determines a value of the operating voltage received from the transmitter portion  4  and makes that value available to the transmitter portion  4 . At step  140 , the transmitter compares the value from the sensor portion  200  to the value in memory  62 . At step  150 , the transmitter portion  4  adjusts the operating voltage based on the comparison performed in step  140 . In some embodiments, this adjustment is performed using a variable voltage supply. The variable voltage supply may include at least one potentiometer. In some embodiments, multiple potentiometers can be used to provide coarse and fine voltage adjustment. 
     Some embodiments of the disclosed device may be implemented, for example, using a storage medium, a computer-readable medium or an article of manufacture which may store an instruction or a set of instructions that, if executed by a machine, may cause the machine to perform a method and/or operations in accordance with embodiments of the disclosure. Such a machine may include, for example, any suitable processing platform, computing platform, computing device, processing device, computing system, processing system, computer, processor, or the like, and may be implemented using any suitable combination of hardware and/or software. The computer-readable medium or article may include, for example, any suitable type of memory unit, memory device, memory article, memory medium, storage device, storage article, storage medium and/or storage unit, for example, memory (including non-transitory, memory), removable or non-removable media, erasable or non-erasable media, writeable or re-writeable media, digital or analog media, hard disk, floppy disk, Compact Disk Read Only Memory (CD-ROM), Compact Disk Recordable (CD-R), Compact Disk Rewriteable (CD-RW), optical disk, magnetic media, magneto-optical media, removable memory cards or disks, various types of Digital Versatile Disk (DVD), a tape, a cassette, or the like. The instructions may include any suitable type of code, such as source code, compiled code, interpreted code, executable code, static code, dynamic code, encrypted code, and the like, implemented using any suitable high-level, low-level, object-oriented, visual, compiled and/or interpreted programming language. 
     Referring to  FIG. 15 , a block diagram for an alternate embodiment of a gas detection system  190  in accordance with the present invention is shown. The transmitter portion  4  may be used in conjunction with a one, two or n (where n is any positive integer) number of detector head portions  2 . In one embodiment, detector head portions  2 A- 2 C (see  FIG. 2 ) are configured as an IR gas sensor  205 , a catalytic bead detector  210  and an electro-chemical sensor  215 ; for detecting toxic gases, although it is understood that other sensor types may be used. The detector head with an IR gas sensor installed  2 A, a detector bead with a catalytic bead sensor installed  2 B and a detector head with a electro-chemical sensor installed  2 C are connected to the transmitter portion  4  by the cable  48 . The transmitter  4  and detector head portions  2 A- 2 C each include a gland arrangement having a hole through which the cable extends. 
     In use, the transmitter portion  4  and the detector head portions  2 A/ 2 C may be located in a hazardous and/or combustible environment. Alternatively, the detector head portions  2 A- 2 C may be located in a hazardous and/or combustible environment remote from the transmitter portion  4 . It is frequently desirable to “hot swap” one or more of the sensor portions  200  during rise, i.e. replace the sensor during use without significant interruption to the system, in the event that the sensor has lost sensitivity, for example. However, hot swapping the sensor may cause a spark or an electrical are to be generated in the circuitry for the transmitter portion  4  or sensor portions  205 ,  210 ,  215 . The spark may then escape through the hole in the gland arrangement and cause the hazardous location to ignite. 
     In order to reduce the likelihood of a spark occurring, an intrinsic safety (IS) barrier is utilized which includes circuitry for limiting current, voltage and power in accordance with industry standards for intrinsic safety. In conventional systems, an intrinsic safety barrier is needed for each sensor portion  205 ,  210 ,  215 . Further, IR gas sensors and catalytic bead detectors have higher voltage and current requirements than electro-chemical sensors. Therefore, it is more difficult to provide IS power for IR gas sensors and catalytic bead detectors than it is for electro-chemical sensors. 
     In accordance with the present invention, the IS barrier  42  described in connection with  FIGS. 5, 7 and 8  is adapted to provide IS power and communications to a plurality of detector head portions  2  which contain different sensor  200  types such as the IR gas sensor  205 , catalytic; bead sensor  210  and electro-chemical sensor  215 . Referring to  FIG. 16 , a transmitter system block diagram  80  for the transmitter portion  4  is shown. The transmitter system  80  includes a power supply  82  connected to input voltage  84 . The power supply  82  provides power to the transmitter processor  46  and associated peripheral circuits (denoted generally as reference numeral  86 ) as previously described herein. The IS barrier  42  then provides intrinsically safe power and communication signals  88  to a detector head portion  2  or a plurality of detector head portions  2 , such as detector head portions  2 A- 2 C. 
     Referring to  FIG. 17 , a sensor system block diagram  90  for the sensor portion  200  is shown. Although only one sensor portion  200  is shown, it is understood that a plurality of sensor kernels  12  of different types may be utilized. The sensor system  90  receives the intrinsically safe power and communication signals  88  from the transmitter portion  4 . The power and communication signals  88  are separated into a power signal  98  and a power and communication signals  160 . The power and communication signals  160  are provided to sensor circuitry  96  (which includes previously described sensor processor  30  and associated circuitry). The sensor system  90  includes a first IS barrier  92  for providing an intrinsically safe power signal to the sensor circuitry  96 . Further, many types of sensors, such as the electro-chemical sensor  2 C, generate voltages during use. In accordance with the present invention, the sensor system  90  also includes a second IS barrier  94  located between the sensor kernel  12  and sensor circuitry  96  for providing intrinsically safe power to the sensor circuitry  96 . The first  92  and second  94  IS barriers include a resistor or a plurality of resistors for providing intrinsically safe power. 
     Referring to  FIG. 18 , a schematic of a transmitter power barrier circuit  162  for providing an IS power signal is shown. The circuit  162  may be a conventional zener barrier circuit including a fuse  164 , a first resistor  166  for limiting a current surge, a second resistor  168  for limiting a continuous current and a first zener diode  170 . The circuit  162  also includes second  172  and third  174  zener diodes which serve as redundant zener diodes. 
     Referring to  FIG. 19 , a schematic of a transmitter communications barrier circuit  176  for providing an IS communication signal is shown. The circuit  176  may include, a conventional zener barrier circuit including a fuse  178 , a first resistor  180  for limiting a current surge, a second resistor  182  for limiting a continuous current and a first zener diode  184 . The circuit  176  also includes second  186  and third  188  zener diodes which serve as redundant zener diodes. 
     The present invention enables the use of a single barrier assembly to provide IS power and communication signals to a sensor or plurality of sensors each of a different type and having different voltage and current requirements. By way of example, a plurality of sensor portions  200  of different types may be used such as a detector head with an IR gas sensor installed  2 A, a detector head with a catalytic bead detector sensor installed  2 B and a detector head with an electro-chemical sensor installed  2 C. Further, the IR gas sensor  205  and catalytic bead sensor  210  have higher voltage and current requirements than the electro-chemical sensor  215 . 
     While certain embodiments of the disclosure have been described herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto