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Oil, gas and petrochemical process industries


    Pressure sensors

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    Talib Al_Munawri
    Director-General
    Director-General

    Posts : 337
    Join date : 2009-07-03
    Age : 30
    Location : Sultanate of Oman

    Pressure sensors

    Post by Talib Al_Munawri on Fri Aug 21, 2009 5:25 am



    WHAT IS A GAGE PRESSURE SENSOR ?

    A gage pressure sensor is a bonded foil strain gage-based pressure sensor
    designed to measure applied pressure referenced to sealed atmospheric
    pressure. The atmospheric pressure in a gage pressure sensor, which is sealed
    to prevent moisture and other air particles from entering the sensor, always
    reflects the atmospheric pressure on the day of manufacture, as opposed to the
    current or other date.




    A gage pressure sensor operates in the same way as a bonded foil strain gagebased
    pressure sensor. As pressure is applied to a diaphragm, pressure is
    transmitted to a gauged element via a mechanical connection, such as a bolt. As
    the gauged element is deflected, strain gages, which are bonded to the element,
    reflect this as a change in resistance. The bridge circuit identifies the resulting
    change in electrical resistance as a change in applied pressure to the sensor.
    (Consult the Pressure Reference Chart to compare how the various pressure
    sensors reference pressure.)


    WHAT IS AN ABSOLUTE PRESSURE SENSOR ?

    An absolute pressure sensor is a bonded foil strain age-based pressure sensor
    designed to measure applied pressure referenced to sealed vacuum, or absolute
    zero pressure.




    An absolute pressure sensor operates in the same way as a bonded foil strain
    gage-based pressure sensor. As pressure is applied to a diaphragm, pressure is
    transmitted to a gauged element via a mechanical connection, such as a bolt. As
    the gauged element is deflected, strain gages, which are bonded to the element,
    reflect this as a change in resistance. The bridge circuit identifies the resulting
    change in electrical resistance as a change in applied pressure to the sensor.
    (Consult the Pressure Reference Chart to compare how the various pressure
    sensors reference pressure.)


    WHEN SHOULD I USE AN ABSOLUTE PRESSURE SENSOR RATHER THAN A GAGE PRESSURE
    SENSOR ?


    The primary consideration concerns whether the atmosphere plays a role in the
    application for which the pressure sensor is being used. If any part of the
    application is exposed to the atmosphere, such as an automotive emission
    system or in level measurement in an open tank, a gage pressure sensor should
    be used. But if the atmosphere has no effect on the application, either type of
    pressure sensor can be used. However, when measuring extremely high
    pressures, such as those found in hydraulic applications, an absolute pressure
    sensor is typically used. (Consult the Pressure Reference Chart to compare how
    the various pressure sensors reference pressure.)




    WHAT IS A DIFFERENTIAL PRESSURE SENSOR ?


    A differential pressure sensor designed to detect a change in pressure as the
    difference between two applied pressures. A differential pressure sensor
    employs a dual diaphragm with each diaphragm linked to a common central
    gauged element (foil based sensors) or silicon substrate (silicon based
    sensors). If both applied pressures are equal, both diaphragms are balanced,
    and the reading shows zero; but if either diaphragm see a different pressure
    from the other this differential pressure is transmitted to the gauged element.
    As the gauged element or silicon substrate is deflected so the gages register a
    change in resistance. The bridge circuit identifies the resulting change in
    electrical resistance as a change in applied pressure to the sensor.
    Differential Pressure Sensor
    Differential pressure sensors are commonly used to measure the rate of flow in a
    pipe. By monitoring the pressure drop on either side of an orifice plate, the mass
    flow rate of the fluid in the pipe can be determined.
    (Consult the Pressure Reference Chart to compare how the various pressure
    sensors reference pressure.)


    WHAT IS A VACUUM PRESSURE SENSOR ?


    Vacuum can be measured in two ways. By a) a Gage Pressure Sensor measuring
    pressure below atmospheric pressure (i.e. referenced to atmospheric pressure) or
    b) by an Absolute Pressure Sensor measuring pressure greater than absolute zero
    but less than atmospheric pressure. For user convenience the Gage Pressure
    Sensor designed for vacuum applications is usually scaled to report a decrease in
    pressure below atmospheric pressure as an increase in positive voltage.
    Current atmospheric pressure, a vacuum pressure sensor actually reports 0 psi; but
    at absolute zero pressure, a vacuum pressure sensor reports the value of the
    current atmospheric pressure as psi of vacuum.


    Pressure Reference for Vacuum Pressure Sensor
    Because vacuum pressure sensors are measuring small pressure changes (max of
    15 psi) the output can be scaled inches of water, inches of mercury or in psi.
    (Consult the Pressure Reference Chart to compare how the various pressure
    sensors reference pressure.)
    Sensotec Note:
    Sensotec uses the term vacuum gages to define gage pressure sensors scaled in
    psi of vacuum or inches of water of vacuum. Absolute gages that are scaled to 15
    psi are just considered absolute gages.




    WHAT IS A BAROMETRIC PRESSURE SENSOR ?

    A barometric pressure sensor is an absolute pressure sensor that is used to
    measure barometric pressure. The full scale output can be calibrated in a number
    of different ways but is typically 16 to 32 or 0-30 inches of mercury.
    (Consult the Pressure Reference Chart to compare how the various pressure
    sensors reference pressure.)
    Pressure Reference for Barometric Pressure Sensor

    ]]]

    Arctic3
    Member
    Member

    Posts : 1
    Join date : 2011-05-06
    Age : 51

    Why is PSIA used on my SCADA System?

    Post by Arctic3 on Fri May 06, 2011 10:24 pm

    I have recently become responsible for a SCADA instrumentation system on an offshore rig and have the following query:
    there is a closed loop cooling system circulating treated fresh water to various pieces of equipment. It has a pressure sensor at the pump which is located approximately 100 feet below the main deck in a pontoon pump room and a pressure sensor at main deck level, where the cooled equipment is located. On the SCADA system, the pump pressure is displayed in PSIG, while the deck pressure is displayed in PSIA. However the conversion factors for the PSIA display seem wrong. A standard 0 - 100 PSIG, 4 - 20 mA sensor is used in each location and both are working. But the main deck one, which reads in PSIA on the SCADA System, is scaled 4 - 20 mA = 0 - 295.7 PSIA. Surely the scaling should be 4 mA = 14.7 PSI and 20 mA = 114.7 PSI as the 0 - 100 PSI sensor will actually equate to 0+14.7 PSI to 100+14.7 PSI, after the 14.7 PSI correction for gauge to absolute pressure is applied? Can anyone explain this strange scaling?

      Current date/time is Tue Aug 22, 2017 9:51 pm