Pressure and differential pressure detectors and transmitter for use in hostile environment

A differential pressure detector for sensing the pressure of a fluid at two points, and suitable for use in a hostile environment in which pressures may reach 500 psig and temperatures may reach 500.degree. F., comprising a housing formed of material suitable for withstanding the temperatures and pressures of said hostile environment;

• pressure bellows enclosed within said housing;
• first and second ports in said housing for permitting fluid communication of said bellows with said two points so that said bellows moves in relation to the differential pressure therebetween;
• a differential transformer within said housing and having a primary winding, an A secondary winding, a B secondary winding, and a movable core disposed to move with said bellows;
• signal supply means disposed remote from said housing to supply an AC signal to the primary winding of said differential transformer;
• and a signal detecting and conditioning circuit disposed remote from said housing and including demodulating means coupled to said A secondary winding and said B secondary winding to produce an X demodulated signal and a Y demodulated signal respectively therefrom, summing means for providing a sum signal X+Y from said X and Y demodulated signals as a voltage analog of the temperature of said differential transformer, means for deriving a correction signal as a difference value of said sum signal X+Y and a reference voltage Ref, subtracting means for providing a difference signal X-Y from said X and Y demodulated signals as a voltage analog of said differential pressure, multiplying means for multiplying said difference signal X-Y by said correction voltage to produce a resulting temperature-compensated differential-pressure analog signal, and signal conditioning means for converting said temperature-compensated analog signal to an output signal suitable for use with an electrically-driven indicator device.

A pressure or differential pressure detector for sensing the pressure of a fluid and suitable for use in an environment in which the temperature can vary, comprising a housing formed of material suitable for withstanding conditions of said environment;

• pressure transducer means disposed within said housing for converting said pressure into a corresponding mechanical displacement;
• at least one port in said housing permitting fluid communication with said pressure transducer means;
• a differential transformer disposed within said housing and having a primary winding, an A secondary winding with first and second leads, a B secondary winding with first and second leads, and a movable core arranged to move with the mechanical displacement of said presure transducer means;
• signal supply means disposed remote from said housing to supply an AC signal to the primary winding of said differential transformer;
• and a signal detecting and conditioning circuit disposed remote from said housing and including first and second operational amplifiers each having first and second inputs and an output, first, second, and third resistors and first, second, and third diodes, the first resistor being connected between the first lead of a respective one of said secondary windings and the first input of the associated operational amplifier, the first and second diodes being connected back-to-back at one end to said second lead, and at the other end through said second resistor to said second input, and to said first input, respectively, of the associated operational amplifier, and said third resistor and said third diode being coupled in series between said output and said second input of the associated operational amplifier, so that the first and second operational amplifiers provide a temperature stabilized X demodulated signal and a temperature stabilized Y demodulated signal, respectively;
• means for combining said X and Y demodulated signals to form a sum signal X+Y and a difference signal X-Y as temperature and pressure analogs, respectively;
• multiplying means for producing a temperature-compensated analog signal in response to said sum and difference signals;
• and output means providing an output signal, based on said temperature-compensated analog signal, for use with an electrically-driven indicator device.

A differential pressure detector for sensing the differential pressure of a fluid between two points comprising a housing;

• pressure-sensitive bellows means enclosed in said housing;
• first and second ports in said housing for permitting fluid communication of said bellows means with said two points so that said bellows means moves in relation to said differential pressure;
• a differential transformer within said housing and having a primary winding, an A secondary winding, a B secondary winding, and a movable core disposed to move with said bellows means;
• signal supply means disposed remote from said housing to supply an AC signal to the primary winding of said differential transformer;
• and a signal detecting and conditioning circuit disposed remote from said housing and including demodulating means coupled to said A and B secondary windings to produce an X demodulated signal and a Y demodulated signal respectively therefrom, adding means producing a sum signal from said X and Y demodulated signals as a temperature analog signal, subtracting means producing a difference signal from said X and Y demodulated signals as a differential pressure analog signal, and multiplying means producing a temperature-compensated differential pressure signal based on said temperature analog signal and said differential pressure analog signal;
• wherein said bellows means includes an expandable longitudinal bellows, a shaft extending axially through said bellows to move therewith, and a helical spring biasing said shaft, said spring being generally cylindrical and having a coil member of generally rectangular cross-section so that the biasing force of said spring can be matched to said longitudinal bellows by machining the outer cylindrical surface of the spring.

A differential pressure detector for sensing the differential pressure of a fluid between two points and within a predetermined range comprising a housing;

• pressure-sensitive bellows means enclosed in said housing;
• first and second ports in said housing for permitting fluid communication of said bellows with said two points so that said bellows moves in relation to said differential pressure;
• a differential transformer with said housing and having a primary winding, an A secondary winding, a B secondary winding, and a movable core disposed to move with said bellows;
• signal supply means remote form said housing to supply an AC signal to the primary winding of said differential transformer;
• and a signal detecting and conditioning circuit disposed remote from said housing and including demodulating means coupled to said A and B secondary windings to produce an X demodulated signal and a Y demodulated signal respectively therefrom, adding means producing a sum signal from said X and Y demodulated signals as a temperature analog signal, subtracting means producing a difference signal from said X and Y demodulated signals as a differential pressure analog signal, and multiplying means producing a temperature-compensated differential pressure signal based on said temperature analog signal and said differential pressure analog signal;
• wherein said bellows means includes a longitudinal bellows member expandable in the axial direction thereof and having an outer surface in fluid communication with one of said first and second ports, a shaft connected to one end of said bellows member extending axially through the other end of said bellows member and coupled to the movable core of said differential transformer, upper and lower seal members disposed on said shaft between said bellows member and said differential transformer to permit fluid communication of the other of said first and second ports with an inner surface of said bellows when said differential pressure is within said predetermined range, and a sealing moulding affixed to said shaft between said upper and lower seal members to block said fluid communication between said other port and said inner surface when said differential pressure is above or below said predetermined range.