Instrumentation Calculation

 Instrumentation Calculation

1. Calculating the equivalent milliamp value for any given percentage of signal

Current (mA) = (16 mA)(x/100)+4

Where “x” is desired percentage of signal

 Example: calculate the mA for 40 % of signal

Current (mA)      =          (16 mA)(x/100)+4

                                    =          (16)(40/100)+4

                                    =          10.4 mA

2. Calculating the equivalent measured Variable for any given percentage of signal

           

Measured Variable=span(x/100) +LRV

Span = (URV) - (LRV)

Where “x” is desired percentage of signal

                        URV is Upper Range of value

                        LRV is Lower range of value

Example: A pressure gauge is ranged -1 to 1 bar; calculate the Measured Variable for 40 % of signal

Span                            =          (URV) - (LRV)

                              =          2 Bar

Measured Variable         =          span(x/100) +LRV

                                    =          (2)(40/100)+(-1)

                                                =          -0.2 Bar

3. Calculating the equivalent percentage for any given mA or measured value

           

(i)             Percentage %               =          ((A-4)/16)(100)

           

(ii)            Percentage %               =          ((measured variable – LRV)/span)(100)

Where   A is the output current(mA)

            LRV lower range value

            Span = URV-LRV

Example: An electronic Pressure transmitter is ranged -1 to 5 Bar and has a 4-20 mA

Output signal.

(i).Calculate the Percentage output by this transmitter if the measured Pressure is 1 Bar

(ii).Calculate the Percentage output by this transmitter if the measured output current is 13mA

(i) Percentage %                        =          ((measured variable – LRV)/span)(100)

                                                            ((1-(-1))/6)(100)

                                                           

                                                            =          33.33 %

(i)             Percentage %               =          ((A-4)/16)(100)

=          ((13-4)/16)(100)

                                                            =          56.25

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