Abstract:
A filter monitoring device which detects variances in pressure of fluid flowing through a filter element in a filter assembly includes (1) an indicator for conveying data of changes in the differential pressure of the fluid in the filter assembly and (2) a temperature sensor for transmitting data showing any changes in temperature of the fluid being filtered. A microcontroller converts data received about changes in differential pressure and temperature sensing means to digital format and transmits the data along two differential digital lines to a driver and output connector.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to an electronic apparatus for monitoring the differential pressure of a fluid across a filter element and the temperature of such fluid. In particular, the present invention relates to an apparatus that quickly, accurately and continuously measures differential pressure as it develops within a filtration system along with the temperature of the system. 
         [0002]    The apparatus, by monitoring the differential pressure without coming into contact with the fluid in the system, has an increased useful sensor life. The apparatus also monitors temperature changes in the fluid to render the differential pressure monitoring functional at temperature extremes and to inform about the overheating of the filtration system. 
         [0003]    The assignee of the present invention is the assignee of U.S. Pat. No. 5,702,592. The present invention is an improvement over the filter monitoring device disclosed in U.S. Pat. No. 5,702,592 which is incorporated herein by reference. 
       SUMMARY OF THE INVENTION 
       [0004]    Many fluid systems, such as hydraulic transmissions and lubricating systems, have filter systems to remove particulate contaminates which are present in the circulating fluids. The contaminates in the fluid system may come from an internal or external source. These undesired contaminates affect the quality of the system performance since all moving components in contact with the fluid are damaged by wear from the contaminates flowing through the system. When the viscosity of the fluid is low the fluid flows easily; however, when the viscosity is high the fluid moves slower or flows with difficulty. The high viscous fluid requires a greater amount of power to move the fluid due to the higher resistance of the fluid to flow. The high fluid viscosity causes a pressure drop across valves and lines in the filtration system and in the fluid system itself. On the other hand, if a fluid has too low a viscosity, there is increased leakage across seals and excessive wear to the moving components in contact with the fluid when an oil or fluid film between the moving parts is interrupted or broken down. 
         [0005]    The filter monitoring device of U.S. Pat. No. 5,702,592 provides a continuous monitoring of a pressure differential across a filter medium as the pressure differential changes within the filtration system. It continuously monitors the pressure differential within a filtration system and, and in certain embodiments, further monitors the temperature of the fluid within the filtration system. 
         [0006]    The filter monitoring device of U.S. Pat. No. 5,702,592 senses and responds to any change in the differential pressure within the filter system. 
         [0007]    The monitor of the present invention will hereinafter be referred to as “IFI,” meaning Intelligent Filter Indicator. As is the case of the monitor of U.S. Pat. No. 5,702,592, the IFI is a non-intrusive sensor, measuring both the differential pressure across a filter element and the fluid temperature. The device uses a sensor piston containing a magnet. As the filter clogs, differential pressure across it increases causing the piston to move. As the magnet moves closer to a Hall Effect sensor, the sensor detects a change in flux density resulting in an increase in voltage/current output. To account for the effect of temperature changes, the circuitry incorporates a thermistor, which provides a voltage/current output based on changing resistance. Both pressure and temperature indicator outputs are calibrated and presented at the output connector as a DC signal ranging between 0.5 V and 10.5 V. The IFI includes state of the art electronics providing linear outputs corresponding to differential pressure and fluid temperature. 
         [0008]    The IFI of the present invention transmits data in a form which complies with industry standard ANSI TIA/EIA-422 entitled Electrical Characteristics of Balanced Voltage Digital Interface Circuits published by the ANSI Telecommunication Industry Association/Electronic Industries Association (TIA/EIA) which is hereinafter referred to as “RS422” and is incorporated herein by reference. 
         [0009]    A signal processor can read the voltage output and convert the data to a corrected or normalized signal as follows: 
         [0010]    The corrected DPo at a given temp Ti is: Dpo=Dpi (Uo/Ui) 
         [0011]    Where: Dpi=actual differential pressure (psid) at temperature Ti
       Dpo=standard reference differential pressure (psid) based on contaminant load at To.   Uo=absolute viscosity (centipoise) at standard temperature To   Ui=absolute viscosity (centipoise) at temperature Ti       
 
         [0015]    Since viscosity is a known function of temperature {i.e. Ui=f (Ti)}, it can be represented by an equation or tabulated in a data base for small increments of temperature. This establishes a method of interpreting the “thermal lockout’ condition. Therefore, the corrected output signal Dpo in VDC (by virtue of a microprocessor) is not biased by low temperature fluid viscosity effects and will reflect the percent life remaining in the filter element based on contaminant capacity only. The IFI of the present invention has a differential pressure range of 0 to 150 psid and a temperature range of −55° C. to +150° C.; however, this could vary between specific models and sizes of IFI devices. 
         [0016]    The IFI of the present invention with its digital output has numerous advantages over the filter monitoring device of U.S. Pat. No. 5,702,592 which utilized an analog output. These include the following: it provides for direct conversion of output voltage to temperature resulting in higher accuracy; it allows for operation in noisy environments and over long cable lengths both for the pressure differential output and the temperature output; it allows for easy conversion from voltage to actual differential pressure values in contrast to the prior art differential pressure voltage output and temperature voltage output which were non-linear and required tables for conversion. Additionally, the power consumption is much lower for the IFI, namely, 50 mAdc(maximum) as compared with the analog device of U.S. Pat. No. 5,702,592 which utilizes power up to 100 mAdc. The transmission rate of the IFI is software controlled and can be changed to accommodate various applications. 
         [0017]    It is an object of the present invention to provide a fluid monitoring device to monitor fluid pressure and temperature and changes therein on a non-intrusive basis and provide linear outputs corresponding to differential pressure and fluid temperature. 
         [0018]    Other objects and advantages of the present invention will become apparent to those skilled in the art upon a review of the following detailed description of the preferred embodiments and the accompanying drawings. 
     
    
     
       IN THE DRAWINGS 
         [0019]      FIG. 1  is a sectional view of the IFI monitor device. 
           [0020]      FIG. 2  is an end view taken from the inlet end on the left of  FIG. 1 . 
           [0021]      FIG. 3  is an end view of the electrical connector at the right end of  FIG. 1 . 
           [0022]      FIG. 4  is a sectional view taken through line  4 - 4  of  FIG. 1 . 
           [0023]      FIG. 5  is a block diagram of the IFI monitor. 
           [0024]      FIG. 6  is a wiring diagram showing input to and output from the electrical connector. 
           [0025]      FIG. 7  is a schematic view showing the relationship of HI and LO signals transmitted through two signal wires. 
           [0026]      FIG. 8  is a schematic view showing digital output. 
           [0027]      FIG. 9  is a chart showing conversion of the temperature output to a digital value and then to a voltage value. 
           [0028]      FIG. 10  is a chart showing output voltage code for differential pressure. 
           [0029]      FIG. 11  is a chart showing Hall effect sensor output vs. applied differential pressure. 
           [0030]      FIG. 12  is a chart showing Hall effect sensor output vs. pressure modified by opamp. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    Referring to  FIGS. 1-4  there is shown the IFI monitoring component  10  of the present invention which includes a housing  12  extending from a first end  14  to a second end  16  which defines an electrical connector  58  having a plurality of engagement pins  18 ,  19 . The housing  12  has a first interior chamber  20  extending from the first end  14 , which is open, to a transversely extending wall  22 . Positioned in the first interior chamber  20  is a piston assembly  24  which is axially movable in the first chamber  20  in response to fluid pressure exerted upon it. The piston assembly  24  is retained in the first chamber  20  by a fitting  26  having a central high pressure sensing port  28  lying on the axis of the first chamber  20 . A compression spring  30  has one end bottomed against the transverse wall  22  and the opposing end engaging a radial shoulder  32  of the piston assembly  24 . The spring  30  yieldingly urges the piston assembly  24  toward the fitting  26 . 
         [0032]    The fitting  26  is held in a fixed position on the housing by virtue of engagement with an internal step  34  of the housing  12  on its side facing the piston assembly  24  and engagement by a retaining ring  36  on the opposing side. The retaining ring  36  is snapped into an inwardly facing groove  38  and has a concavity  39  facing the first end  14 . 
         [0033]    The piston assembly  24  has an internal cavity  40  at its end adjacent the fitting  26 , such cavity  40  being in communication with the high pressure inlet sensing port  28 . A first-O ring  42  is positioned in an inwardly facing annular groove of the piston assembly  24  and prevents fluidized pressure which has been introduced through the high pressure sensing port  28  to the cavity  40  from escaping. 
         [0034]    The leading end of the piston assembly  24  has a reduced diameter extending from a position forwardly of the radial shoulder  32  to its forward-most end  43  facing the transverse wall  22 . Such reduced diameter portion is sized to fit in the opening defined by the compression spring  30 . That portion of the piston assembly  24  extending rearwardly from the forward-most end  43  has a hollowed out section in which it is positioned a magnet  44 . High pressure fluid introduced through the sensing port  28  enters the cavity  40  and drives the piston assembly  24  and the magnet  44  carried thereby toward the transverse wall  22  and toward electronic assembly mounted on the opposite side of the transverse wall  22  therefrom. 
         [0035]    The housing  12  is also provided with a low pressure sensing port  46  in the area of the housing between the transverse wall  22  and the radial shoulder  32  of the piston assembly  24 . The housing  12  also has outwardly facing O-rings  47  and retaining rings  48 , one set of which is slightly spaced from the first end  14  and the other set of which is on the opposite side of the low sensing port  46  in the area of the transverse wall  22 . 
         [0036]    Forwardly of the transverse wall  22  the housing  12  becomes enlarged and defines a second chamber  50  in which is positioned a sensor board  52  having (1) a magnetic position sensor  53  for detecting the Hall effect resulting from movement of the magnet  44  toward or away therefrom in response to high pressure introduced to the cavity  40  through the high pressure sensing port  28  and (2) a temperature sensor  83 . The sensing board  52  is spaced a short distance (on the order of two millimeters) from the transverse wall  22 . Potting compound is positioned in the space. 
         [0037]    Signals from the sensor board  52  are transmitted through an electronic assembly  54 , which includes a temperature sensor  55 , to a control board assembly  56  and then to an electrical connector assembly  58  adapted to plug into an element (not shown) for transmitting data via pins  19  to an external monitor  64  such as a computer or other receiver and to receive power from a source of power by means of pins  18  (see  FIG. 6 ). The control board assembly  56  includes a microcontroller. Following assembly of the above components and the wiring therefore, the remainder of the second chamber  50  is filled with potting compound to rigidify the positions of the components mounted in the chamber  50  and prevent damage from shocks or impacts. Additionally, the second chamber  50 , is hermetically sealed. 
         [0038]    Referring to  FIG. 5  there is shown a block diagram of the IFI monitor component  10 . The block diagram of  FIG. 5  shows the magnet  44  moving toward and away from the Hall effect sensor  52 . Signals from the Hall effect sensor are delivered to an operational amplifier U 1  and then to an analog to digital converter of the microcontroller of electronic assembly  54 . The temperature sensor  55  also sends an output in analog form to the electronic assembly  54  and its microcontroller which converts such data from analog to digital. The microcontroller transmits the digital data to the board assembly  56  in a format that is consistent with the requirements of RS422. The board assembly  56  includes a differential line driver  57  which delivers data in compliance with RS422 to the output connector  58  for transmission to the external monitor  64  as shown in  FIG. 6 . 
         [0039]    Referring to  FIG. 6  there is shown a wiring diagram for power into the IFI monitor component  10  and the transmission of data therefrom. The data being transmitted therefrom is in a format consistent with the requirements of RS422. Power from a power source  60  is transmitted through a pair of shielded wires  61 ,  62  at +28 Vdc and −28 Vdc to pins  18 . The external power source could be in the range of 18 to 32 Vdc. Another pair of pins  19  provide an outlet connection for delivering data consistent with the requirements of RS422 to an external monitor  64 . One of the pins  19  delivers the high level digital output (5-0 Vdc) through wire  65  to the external monitor  64  and the other pin  19  delivers the low level digital output (0-5 Vdc) through wire  66  to the external monitor  64 . The wire  65  is hereinafter designated as the HI signal wire and the wire  66  is hereinafter designated as the LO signal wire. Shielding is provided for both the HI signal wire  65  and the LO signal wire  66 . 
         [0040]    As previously mentioned the digital output of the IFI monitoring component  10  conforms to RS422 standard. This standard requires a point to point connection between the IFI component  10  and the external monitor  64  via two wires, namely, the HI signal wire  65  and the LO signal wire  66 . The signal transmitted by each of the wires  65 ,  66  swings between 0 Vdc and 5 Vdc. The HI and the LO signals transmitted by the HI signal wire  65  and the LO signal wire  66  are mirror images of one another (See  FIG. 7 ). When HI signal is 5 Vdc, the LO signal is 0 Vdc and visa-versa (Differential signals). 
         [0041]    The differential signal method was chosen to minimize noise effects of the environment and provide long transmission lines to be employed. In order to operate properly, the RS422 output of the IFI monitor component  10  must have a termination resistor  67  with a rated minimum of 120 Ohm positioned between the IFI monitor component  10  and the external monitor  64  (See  FIG. 6 .). The external monitoring apparatus  64  will convert this differential signal to a single ended one swinging between 0 and 5 Vdc. The IFI monitor component  10  does not read or accept any transmissions from the external monitor  64 . 
         [0042]      FIG. 8  show the transmission protocol for the IFI monitor component  10 . 
         [0043]    The data sent on the HI and LO signals via wires  65  and  66  ( FIG. 7 ) comprises of 4 bytes (8 bits each) representing the temperature and differential pressure as measured by the IFI monitor component  10 . Each byte in a frame contains two BCD (Binary Coded Decimal). 
         [0044]    The IFI monitor component  10  sends 4 bytes of data every one second (a frame of four bytes)—each byte is preceded with a START Bit and ends with a STOP Bit to allow the external monitor  64  to begin decoding the data sent from the IFI monitor component  10 . A PARITY bit is also sent prior to the STOP bit. The external monitor  64  can check this PARITY bit to determine if a valid byte has been received or not. The PARITY bit is set to “1” if the total number of “1”s in the transmitted byte is ODD and set to “0” if the total number of “1”s in the transmitted byte is EVEN. This is defined as ODD Parity. The byte transfer rate for the RS422 output of the IFI monitor component  10  is set to a fixed 9600 bits per second (Baud). 
         [0045]    Decoding the Output of the IFI monitor component  10  is set forth as follows in Table I. Each byte in a frame contains two BCD numbers (Binary coded Decimal)—each BCD number consists of four bits. These four bits digitally represent a numerical value between 0 and 9. Table 1 shows the various numbers associated with each 4 bit patterns: 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 BIT Pattern 
                 Corresponding BCD number 
               
               
                   
                   
               
             
             
               
                   
                 0000 
                 0 
               
               
                   
                 0001 
                 1 
               
               
                   
                 0010 
                 2 
               
               
                   
                 0011 
                 3 
               
               
                   
                 0100 
                 4 
               
               
                   
                 0101 
                 5 
               
               
                   
                 0110 
                 6 
               
               
                   
                 0111 
                 7 
               
               
                   
                 1000 
                 8 
               
               
                   
                 1001 
                 9 
               
               
                   
                   
               
             
          
         
       
     
         [0046]    The IFI monitor  10  sends 8 BCD numbers in each packet. These numbers are designated as follows:
       8 BCD numbers received represent Temperature and Differential Pressure as measured by the IFI Monitor  10 .   4 BCD numbers for temperature are transmitted first and 4 BCD numbers for pressure are transmitted next.   Frame Transfer Sequence
           1 st  transmitted byte contains:
               BCD number for 1 st  digit (LSD) of temperature in bits  3  (MSB) to  0  (LSB)   BCD number for 2 nd  digit of temperature, in bits  7  (MSB) to  4  (LSB)   
               2 nd  transmitted byte contains:
               BCD number for 3 rd  digit of temperature in bits  3  (MSB) to  0  (LSB)   BCD number for 4 th  digit (MSD) of temperature, in bits  7  (MSB) to  4  (LSB)   
               3 rd  transmitted byte contains:
               BCD number for 1st digit (LSD) of pressure, in bits  3  (MSB) to  0  (LSB)   BCD number for 2nd digit of pressure, in bits  7  (MSB) to  4  (LSB)   
               4 th  transmitted byte contains:
               BCD number for 3rd digit of pressure, in bits  3  (MSB) to  0  (LSB)   BCD number for 4th digit (MSD) of pressure, in Bits  7  (MSB) to  4  (LSB)   
               
               
 
         [0062]    Decoding the output of the IFI monitor component  10  is accomplished in two steps: 
         [0063]    Decoding-Step one: Assemble the BCD numbers, convert to numeric format, and divide the result by 100: 
         [0064]    For example if the received BCD numbers for the temperature channel, in the order received, are 
         [0065]    Two BCD numbers in byte  1  of temperature data: 59 
         [0066]    Two BCD numbers in byte  2  of temperature data: 06 
         [0067]    Combine byte  2  with byte  1 , yielding 0659. Convert to numeric format, then divide by 100 resulting in 6.59 (numeric value). 
         [0068]    The same applies to the pressure channel. If the received BCD numbers are 
         [0069]    Two BCD numbers in byte  1  of pressure data: 3.7 
         [0070]    Two BCD numbers in byte  2  of pressure data: 10 
         [0071]    Combine byte  2  with byte  1 , yielding 1037. Convert to numeric format, then divide by 100 to obtain 10.37 (numeric value). 
         [0072]    Decoding, Step  2 : 
         [0073]    Calculate actual pressure and temperature values: 
         [0074]    For the pressure channel, the IFI monitor component  10  sends a numeric value between 0.5 and 10.5 (when decoded). 
         [0075]    The actual pressure (In PSID) can be calculated from the following formula: 
         [0000]      dP=(dPVdc−0.05)*15 
         [0076]    Where dPVdc is the numeric value obtained by decoding the Pressure channel bytes. 
         [0077]    For the temperature channel, the IFI monitor component  10  also sends a numeric value between 0.5 and 10.5 (when decoded). The actual temperature (In degrees Celsius) can then be calculated from the following formula 
         [0000]        T =( T Vdc−4−/165−0.5)*16.5 
         [0078]    Where TVdc is the numeric value obtained by decoding the temperature channel bytes. 
         [0079]    A Complete Example: 
         [0080]    The information shown is the external monitor&#39;s  64  memory content after receiving one frame from the IFI monitor  10 : 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 M    L 
                   
                   
               
               
                   
                 S     S 
                   
                 BCD Equivalent 
               
               
                   
                 B     B 
                 Parsed 
                 (Bytes Decoded) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 1 st  byte 
                 0 1 0 1 1 0 0 1 → 
                 0 1 0 1 1 0 0 1 → 
                 5 9 
               
               
                 2 nd  byte 
                 0 0 0 0 0 1 1 0 → 
                 0 0 0 0 0 1 1 0 → 
                 0 6 → 0659 → 6.59 V 
               
               
                   
                   
                   
                 (Temp) (Divide by 100 
               
               
                   
                   
                   
                 to obtain Temp value) 
               
               
                 3rd byte 
                 0 0 1 1 0 1 1 1→ 
                 0 0 1 1 0 1 1 1 → 
                 3 7 
               
               
                 4 th  byte 
                 0 0 0 1 0 0 0 0 → 
                 0 0 0 1 0 0 0 0 → 
                 1 0 → 1037 → 
               
               
                   
                   
                   
                 10.37 V (dP) 
               
               
                   
                   
                   
                 (Divide by 100 to 
               
               
                   
                   
                   
                 obtain Pressue Value) 
               
               
                   
               
               
                 To Calculate Use: T = (TVdc − 400/165 − 0.5) * 16.5 
               
               
                 T = (6.59 − 400/165 − 0.5) * 16.5 = 60.48° C. 
               
               
                 To Calculate Use: dP = (dPVdc − 05) * 15 
               
               
                 dP = (10.37 − 0.5) * 15 = 148 PSID 
               
             
          
         
       
     
         [0081]    Transmission Under Normal and Abnormal Conditions: 
         [0082]    Under normal operating conditions: 
         [0083]    The IFI monitor will transmit the numeric value of 0.50 for each channel that is at its MINIMUM: 
         [0084]    If dP=0 PSID, then transmitted pressure bytes are 0050 
         [0085]    If T≦−40° C., then transmitted temperature bytes are 0050 
         [0086]    The IFI monitor  10  will transmit the numeric value of 10.50 for each channel that is at its MAXIMUM: 
         [0087]    If dP≧150 PSID, then transmitted pressure bytes are 1050 
         [0088]    If T≧125° C., then transmitted temperature bytes are 1050 
         [0089]    Under abnormal conditions the IFI monitor component  10  will transmit the proper error codes depicted in Table 2. 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 IFI Transmitted Error Codes 
               
             
          
           
               
                 Possible 
                 Transmitted 
                 Transmitted 
                   
               
               
                 Abnormal 
                 Temp bytes 
                 Pressure Bytes 
               
               
                 Condition 
                 (TTTT) 
                 (PPPP) 
                 Comment 
               
               
                   
               
               
                 Magnetic 
                 Normal BCD 
                 AAAA 
                 Bit stream for PPPP (all four bytes) 
               
               
                 sensor output 
                 numbers 
                 (Hex) 
                 1010101010101010 
               
               
                 stuck low, 
                 between 0 and 9 
                   
                 Corresponding to HEX 
               
               
                 Temperature 
                   
                   
                 “AAAA” 
               
               
                 sensor OK 
               
               
                 Magnetic 
                 Normal BCD 
                 BBBB (HEX) 
                 Bit stream for PPPP (all four bytes) 
               
               
                 sensor output 
                 Numbers 
                   
                 1011101110111011 
               
               
                 stuck high, 
                 between 0 and 9 
                   
                 Corresponding to HEX 
               
               
                 Temperature 
                   
                   
                 “BBBB” 
               
               
                 channel OK 
               
               
                 Temperature 
                 AAAA (HEX) 
                 Normal BCD 
                 Bit stream for TTTT (all four bytes) 
               
               
                 sensor output 
                   
                 numbers 
                 1010101010101010 
               
               
                 stuck low, 
                   
                 between 0 and 9 
                 Corresponding to HEX 
               
               
                 Magnetic 
                   
                   
                 “AAAA” 
               
               
                 sensor OK 
               
               
                 Temperature 
                 BBBB(HEX) 
                 Normal BCD 
                 Bit stream for 
               
               
                 sensor output 
                   
                 numbers 
                 TTTT1011101110111011 
               
               
                 stuck high, 
                   
                 between 0 and 9 
                 Corresponding to HEX 
               
               
                 Magnetic 
                   
                   
                 “BBBB” 
               
               
                 sensor OK 
               
               
                 Both 
                 AAAA (HEX) or 
                 AAAA (HEX) or 
                 Bit stream for each 
               
               
                 Temperature 
                 BBBB (HEX) 
                 BBBB (HEX) 
                 channel same as above. 
               
               
                 and Magnetic 
               
               
                 sensor 
               
               
                 abnormal 
               
               
                   
               
             
          
         
       
     
       Differential Pressure Measurements 
       [0090]    As previously mentioned, the IFI monitor  10  reads and interprets a differential pressure applied to its inputs and generates a suitable output corresponding to the applied differential pressure. A separate function also reads and interprets the temperature of the IFI monitor and generates a suitable output corresponding to the temperature. The differential pressure applied to the IFI monitor results in movement of the internal magnet  44  in the first internal chamber  20 . The higher the pressure, the further the magnet  44  will move inside the first internal chamber  20 . The electronic sensor  52  generates a voltage proportional to the proximity of the magnet  44  to the sensor.  FIG. 11  shows the output of the Hall effect sensor  52  vs. differential pressure applied to the IFI monitor  10 . 
         [0091]    The range of the output voltage is limited to 2.5 and 5 Vdc. In order increase the range and provide additional accuracy and resolution of the measured differential pressure, an Opamp (U 1 ) is utilized. The Opamp (U 1 ) increases the range by subtracting 2.5V from the output of the Hall effect sensor  52  and multiplying the result by 2. The resulting curve is shown in  FIG. 12 . 
         [0092]    The microcontroller of the control board assembly is tasked with interpreting the outputs of the Opamp (U 1 ) and the temperature sensor and generating suitable outputs for each. Both the temperature and opamp outputs are read and converted into digital format by the microcontroller before further processing. The analog to digital inputs of the microcontroller each employ 1024 steps to convert the inputs to digital format, i.e. a number between 0 and 1023 (ADC_output). (See  FIGS. 9 and 10 .) These outputs are then converted to a number between 0 and 5 Vdc using the following relationship: 
         [0000]      Vdc=(5/1024)*ADC_output   (1) 
         [0093]    During the IFI calibration process, an internal table of values is generated which correspond to the output of the opamp vs. applied differential pressure. Each index value in the table corresponds to a fixed step of differential pressure. The firmware then proceeds to compare the voltage value obtained from (1) to the internal table of values and finds the lowest value that corresponds to the one calculated from formula (1) above. The pressure is then calculated as: 
         [0000]      Diff pressure=(Index number of table corresponding to the lowest value to fit the measurement)*(table index step value)   (2) 
         [0094]    Once the value of the differential pressure is known, the microcontroller proceeds to generate the output code for this pressure value. Output codes are 0050 for zero pressure and 1050 for 150 PSID of pressure. The equation for the conversion is as follows: 
         [0000]      Pressure output code=Integer value of (Diff Pressure/15)+15)   (3) 
         [0095]    The result is a linear output for the pressure channel obtained from a nonlinear output of the Hall effect sensor  52 . The external monitor  64  used to read and decode the values sent from the IFI monitor  10  need to convert this code to a voltage by dividing the code by 100 to obtain the graph shown in  FIG. 10 . 
       Temperature Output: 
       [0096]    The microcontroller has a separate table of values for temperature output stored in its memory. This table is fixed for the IFI monitor  10  and is a linear table of values with a step of  1  degree centigrade. 
         [0097]    The procedure is the same as for the pressure channel. The output of the temperature is converted into a digital value between 0 and 1023 and then converted to a voltage value using formula (1) above. To find the corresponding temperature value, the external monitor  62  scans the temperature table to find the lowest entry that exceeds the input from the temperature sensor. It then uses the index value (1 degree C.) to find the actual temperature by using: 
         [0000]      Temp in degree C.=Index value*1 degree C.   (4) 
         [0098]    The temperature channel output is then converted to the required output using the following relationship: 
         [0000]      Temperature output Code=(Temperature in degree C.+40)*1000/165+50 
         [0099]    Code 0050 corresponds to −40° C. and code 1050 corresponds to 125° C. The external monitor  64  used to read and decode the values sent from the IFI monitor  10  need to convert this code to a voltage by dividing the code by 100 to obtain the graph shown in  FIG. 9 . 
         [0100]    Output of the IFI monitor  10  is a digital signal conforming to EIA RS422 standard. The output consists two differential digital lines which swing between 0 and 5 Vdc and contain the codes for the differential pressure and temperature outputs. Output conversion from internal digital signals to signals in compliance with RS422 is accomplished by the differential line driver  57 . 
       Input Voltage Regulator 
       [0101]    The input voltage to the IFI monitor  10  is a dc signal between 18 and 32V (28 Vdc nominal). A switching regulator  69  converts this input voltage to a regulated 5 Vdc supply for internal use by the various components of the IFI monitor  10 . 
         [0102]    The above detailed description of the present invention is given for explanatory purposes. It will be apparent to those skilled in the art that numerous changes and modifications can be made without departing from the scope of the invention. Accordingly, the whole of the foregoing description is to be construed in an illustrative and not a limitative sense, the scope of the invention being defined solely by the appended claims.