Pressure sensors (including level and flow sensors) are vital to process control and safety in industrial processes. In both the mechanical and electromechanical classes of pressure sensors, the applied pressure is converted into a displacement through an elastic sensing element. A displacement sensor, such as a strain gauge or a differential transformer, is then used to convert the displacement into an electrical signal so it can be displayed on a pressure gauge. The three most commonly used sensing elements for both mechanical and electromechanical pressure sensors are the Bourdon tube, bellows, and diaphragm.
In most applications, pressure sensors and the indicating or recording equipment associated with them are located away from each other to ensure safety and convenience, as in nuclear power plants, where radiation hazards are involved, or in chemical processes, where corrosive or flammable fluids under dangerously high pressures are present.
Two methods are available for remotely transmitting pressure signals from the process to the control room: pneumatic transmission and electrical transmission. In pneumatic transmission, the motion of the elastic element in the pressure sensor is usually converted into a standard 3 to 15 psi pressure signal, which is piped to a remote location. Pneumatic pressure transmitters have two important shortcomings: (1) they have large response lags that can limit the maximum length of transmission, and (2) they require quality air supply, free of moisture and undesirable fluids such as lubricating oil. To overcome the disadvantages of pneumatic transmission, electric transmission systems were developed, in which the motion of the elastic element is typically converted into a 4-20 mA (or 10-50 mA) electrical signal.
Accuracy in Pressure Transmitters
Accuracy is an objective statement of how well a pressure transmitter may measure the value of a process parameter. Accuracy, uncertainty, and error refer to the difference between the actual value of the process and the value that is indicated by the sensor. The deterioration of accuracy is called calibration drift or calibration shift. The static performance or accuracy of pressure transmitters depends on how well the transmitter is calibrated and on how long it can maintain its calibration.
Pressure transmitters are usually designed so the relationship between input and output is predominantly linear. Therefore, the calibration curve for a pressure transmitter on rectangular coordinates (X-Y axis) is a straight line, represented by the following equation:
y = mx + b (1)
where m is the slope of the straight line, and b is its intercept. The slope is also referred to as gain, and the intercept is also referred to as zero, offset, or bias. A transmitter’s range is the minimum-to-maximum pressure that a transmitter is designed to measure (e.g., 0 to 2500 psi). The input range is expressed in terms of pressure (e.g., 0 to 2500 psi), and the output range is expressed in terms of an electrical signal (e.g., 4 to 20 mA or 1 to 5 V). The lowest pressure at which a transmitter is calibrated is called its zero and is synonymous with offset and bias. A transmitter is usually set up to indicate pressure over a portion of its full range (e.g., 500 to 1500 psi for a pressure transmitter that has a range of 0 to 2500 psi). This is called the calibrated range or the span of the transmitter.
The initial accuracy of a pressure transmitter after calibration is determined from the accuracy of the calibration standard on which it is based and the accuracy of the calibration process. The pressure source must be calibrated with traceability to national standards, such as National Institute of Standards and Technology (NIST) in the United States or Germany’s Deutsches Institut für Normung (DIN). The accuracy is usually expressed in terms of a percentage of span. The initial calibration of industrial pressure transmitters (including both absolute and differential pressure transmitters) is often referred to as a bench calibration.
In most applications, pressure sensors and the indicating or recording equipment associated with them are located away from each other to ensure safety and convenience, as in nuclear power plants, where radiation hazards are involved, or in chemical processes, where corrosive or flammable fluids under dangerously high pressures are present.
Two methods are available for remotely transmitting pressure signals from the process to the control room: pneumatic transmission and electrical transmission. In pneumatic transmission, the motion of the elastic element in the pressure sensor is usually converted into a standard 3 to 15 psi pressure signal, which is piped to a remote location. Pneumatic pressure transmitters have two important shortcomings: (1) they have large response lags that can limit the maximum length of transmission, and (2) they require quality air supply, free of moisture and undesirable fluids such as lubricating oil. To overcome the disadvantages of pneumatic transmission, electric transmission systems were developed, in which the motion of the elastic element is typically converted into a 4-20 mA (or 10-50 mA) electrical signal.
Accuracy in Pressure Transmitters
Accuracy is an objective statement of how well a pressure transmitter may measure the value of a process parameter. Accuracy, uncertainty, and error refer to the difference between the actual value of the process and the value that is indicated by the sensor. The deterioration of accuracy is called calibration drift or calibration shift. The static performance or accuracy of pressure transmitters depends on how well the transmitter is calibrated and on how long it can maintain its calibration.
Pressure transmitters are usually designed so the relationship between input and output is predominantly linear. Therefore, the calibration curve for a pressure transmitter on rectangular coordinates (X-Y axis) is a straight line, represented by the following equation:
y = mx + b (1)
where m is the slope of the straight line, and b is its intercept. The slope is also referred to as gain, and the intercept is also referred to as zero, offset, or bias. A transmitter’s range is the minimum-to-maximum pressure that a transmitter is designed to measure (e.g., 0 to 2500 psi). The input range is expressed in terms of pressure (e.g., 0 to 2500 psi), and the output range is expressed in terms of an electrical signal (e.g., 4 to 20 mA or 1 to 5 V). The lowest pressure at which a transmitter is calibrated is called its zero and is synonymous with offset and bias. A transmitter is usually set up to indicate pressure over a portion of its full range (e.g., 500 to 1500 psi for a pressure transmitter that has a range of 0 to 2500 psi). This is called the calibrated range or the span of the transmitter.
The initial accuracy of a pressure transmitter after calibration is determined from the accuracy of the calibration standard on which it is based and the accuracy of the calibration process. The pressure source must be calibrated with traceability to national standards, such as National Institute of Standards and Technology (NIST) in the United States or Germany’s Deutsches Institut für Normung (DIN). The accuracy is usually expressed in terms of a percentage of span. The initial calibration of industrial pressure transmitters (including both absolute and differential pressure transmitters) is often referred to as a bench calibration.
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