The smart transmitter should not be calibrated just like the conventional analog transmitter calibrate. Analog transmitters are calibrated by 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.
For smart transmitters calibration is divided into three parts:
• Sensor trim
• Range setting (re-ranging)
• Current trim
All sensors drift over time, it can be due to excessive pressure, temperature, vibration, contamination, or other factors. Sensor reading may also be offset due to the mounting position.
Sensor trimming is used to correct digital reading as seen in the local LCD screen display and obtained through digital communication. For example, if the stress is 0 bar but the reading of the transmitter displays 0.03 bar, the sensor trim is adjusted back to 0 bar.
For sensor trim, a technician should apply actual physical input signal to the transmitter. For that calibration can be done in the field or at the lab. Typically there are three forms of sensor trim:
• Zero sensor trim: Zero trim requires the physical input applied to be zero, this is often used with pressure transmitters
• Lower sensor trim & Upper sensor trim: For more accuracy sensors should be trimmed at two points, lower range value and upper range value. This is where lower and upper sensor trim is used. 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.
Range setting is referred to as setting the range between 4mA and 20mA. This scale is usually referred to as “calibrated range” or “calibration range”. At what input shall the transmitter analog output be 4 mA it is referred as Lower Range Value (LRV) and at what input shall it be 20 mA; Upper Range Value (URV).
Transmitter range setting is done without applying input, and therefore can be done remotely from the control room.
Range setting is only applicable to the transmitter with 4-20 mA analog output. That is, for 4-20 mA/HART transmitters, not for pure digital solutions like FOUNDATION Fieldbus (FF) or WirelessHART transmitters.
Current trim is used to correct the transmitter analog output drift. For instance, if the analog output current is 4.13 mA when it should be 4.00 mA, then current trim is used to adjust it to 4 mA.
Current trim is used to match the transmitter analog output current to the current input of the analog input (AI) card channel on the DCS.
Current trim is only applicable to the transmitter with 4-20 mA analog output. That is, for 4-20 mA/HART transmitters, not for FOUNDATION fieldbus (FF) or WirelessHART transmitters, the reason being pure digital transmitters have no 4-20 mA analog output