Instrument Calibration Transmitter

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2012-05-18 09:35:00

Intelligent Device Management Tutorial: Calibration
In the early days of smart transmitters the concept of remote range setting (“remote calibration”) and re-ranging without applying input was revolutionary. It took years of education to be accepted and understood. Calibration can be carried out using a handheld communicator in the field, a laptop in the workshop, or from intelligent device management software as part of an asset management solution. Electronic Device Description Language (EDDL) is the technology used by device manufacturers to define how the system shall display the device information and functions to the technician. EDDL makes calibration of smart transmitters and other intelligent devices easier thanks to user guidance such as wizards and help, and unparalleled consistency of use.

Calibrating Intelligent Transmitters
The term "calibration" in the context of smart/intelligent transmitters is often misunderstood. In the days of analog transmitters calibration meant applying a physical input and turning the trim potentiometers to adjust the sensor so that the analog output current becomes correct according to the desired measurement range. Once smart transmitters appeared, this "calibration" process was divided into three parts:

• Sensor trim
• Range setting (re-ranging)
• Current trim
The reason for separating these functions is that the range can be changed without applying a physical input. This was a huge time and cost saver and one of the major reasons for the rapid adoption of smart transmitters. However, do not confuse "sensor trim" with "range setting". Both are part of calibration, but two very different things.

Sensor Trim
Over time all sensors drift. Depending on the type of sensor it may be due to extreme pressure or temperature, vibration, material fatigue, contamination, or other factors. Sensor reading may also be offset due to mounting position.

Sensor trim is used to correct the digital reading as seen in the device local indicator LCD and received over the digital communication. For instance, if pressure is 0 bar but transmitter reading shows 0.03 bar, then sensor trim is used adjust it back to 0 bar. Sensor trim can also be used to optimize performance over a smaller range than was originally trimmed in the factory.

Sensor trim requires the technician to apply a physical input to the transmitter. Therefore the technician must either do sensor trim in the field at the process location, or the transmitter has to be brought back into the workshop to perform sensor trim. This applies to 4-20 mA/HART, WirelessHART, FOUNDATION fieldbus, as well as PROFIBUS transmitters. Sensor trim in the field is easiest done using a handheld communicator which is supported by 4-20 mA/HART, WirelessHART, and FOUNDATION fieldbus.

Typically there are three forms of sensor trim:
• Zero sensor trim
• Lower sensor trim
• Upper sensor trim
Zero trim requires the physical input applied to be zero, this is often used with pressure transmitters www.eddl.org
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For best accuracy perform sensor trim in two points, close to lower range value and upper range value. This is where lower and upper sensor trim is used. A known physical input is applied to the transmitter to perform the sensor trim, the technician keys in the applied value (on a computer or handheld communicator) communicated to the transmitter, allowing the transmitter to correct itself. The physical input values applied for lower and upper sensor trim respectively are stored in the transmitter memory and are referred to as Lower Sensor Trim Point and Upper Sensor Trim Point respectively.

Sensor trim requires a very accurate input to be applied. The factory calibration equipment is usually more accurate than equipment at site. Moreover, transmitters these days are typically very stable. Therefore, sensor trim of brand new transmitters is rarely done at commissioning. Sensor trim is the...
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