Convert 4-20ma to engineering units (PLC)


#1

When Analog Input (4-20ma, 1-5VDC) are utilized on PLC and RTU; Input Calibration and Scaling is utilized. There may be differences in implementation depending on the PLC/RTU manufacture and device, however the following steps are utilized:

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  • PLC/RTU Calibration
  • Engineering Unit Scaling
  • End to End Calibration
  • Offset
  • Unit Conversion / Formulae

A PLC/RTU Calibration – Scaling Analog Inputs to RTU Digital Units

This step compensates for variation in PLC tolerances. The specified input range for a PLC input might be a 4000 counts for a 4 ma input and 32000 count for a 20 ma input level. Difference in input circuit components or loop resistor tolerances may require PLC/RTU I/O Calibration.

This calibration may be a function in the PLC/RTU software, or with a user function programmed in a function block or ladder logic when the PLC/RTU does not have this function. This step ensures that the digital count and analog correspond correctly.

B Engineering Unit Scaling – Scaling 4-20ma to Engineering Units

This step assigns scaling from Engineering Units to RTU Units corresponding to the input Instrument range. Typically scaling is done by setting 2 X, Y pairs (LOW RAW, LO ENG) and (HI RAW and HI ENG). The LO ENG and HI ENG units corresponds to the 4 and 20 ma ranges of the Instrument.

For example, you for a 4-20ma pressure sensor that is ranged 0-100 PSIG. Low Eng is 0 PSIG corresponding to Low Raw of 4 ma (4000 RTU); HI ENG of 1000 PSIG correponsds to a HI RAW of 20ma. Note instruments may be field calibratable or factory set (“throw away sensors”). If the sensor is out of calibration, it is recalibrated when the sensors allow recalibration. Throw-away sensors. are replaced. It is not recommended that Engineering Units are adjusted to compensate for the instrument that are out of calibration.

C End to End Calibration

On some PLC/RTU devices, there a “End to End Calibration” function that effectively cascades step A and B into a single function. A End to End Calibration is typically a 2 point calibration. One must recognize this paradyn when it is used PLC.

Sometimes the PLC input or instrument calibration is so far out of calibration, it must be recognized as a problem and addressed rather than trying to using an end to end calibration function to compensate. The PLC Input Calibration (A) and Instrument Calibration must be verified independently prior to using an end to end calibration function.

D Instrument Offset Value

An offset value may be used to compensates for sensor installation offsets. This is typically used in level sensor applications, where there is offset of the zero position.

Note that offset value is entered after Engineering Unit Scaling.

E Unit Conversion / Formula/ Strapping Tables

This step converts from one set of units to another. For example, this may be used to convert to PSI to kPa. This function is achieved by using a scaling block or function in the PLC, RTU. Alternatively, a formulates or strapping tables can be used to derive a secondary measure for the primary measure such as tank volume calculations from a level measurement.