4-20mA Analog Current Signals & Relating It With Instrument Variables-1

4-20mA Analog Current Signals:

This is the most popular standard of analog signals compared to 0-10V, 0-5V & 0-20mA standards. The reason why this standard is popular other analog signal standards is described in our previous post (http://instrupc.blogspot.in/2014/06/why-we-preferrably-use-4-20ma-over-0_4.html). Typically, A 4mA signal value represents the 0% of scale & 20mA signal value represnts the 100% of scale. Any signal value in between 4mA to 20mA represents a commensurate percentage in between 0-100%.

For example, if we were to calibrate a temperature transmitter for a measurement range of 50-250°C, we could relate the current and measured temperature values on a graph like this:

Analog current signals are also used in control systems to command the positioning of final control element such as a control valve & variable speed motor drive. In these cases the value doesn't represent the process measurement(i.e. PV), but rather how the degree to which the final control element influence the process. Typically a 4mA signal value represents the command to fully close a valve or stop a motor whereas 20mA signal value represents the command to fully open the valve or to run a motor at maximum speed.

Thus, The industrial control systems use two differents 4-20mA signals. One to represent the process variable(PV) & another one to represent the manipulated variable(MV)

There is no reason to ever expect the two signals to be equal, they represent entirely different things. The relation between these two signals depends upon the type of controller. If the controller is "Reverse acting type" then as the PV value Increases, the MV value from the controller Decreases in proportion & vice versa. If the controller is "Direct acting type" then as the PV value Increases, the MV value from the controller Increases in proportion & vice versa. 

If the controller is switched to "Manual mode" by operator then the output signal(MV) will have no automatic relation to the PV signal at all, instead being entirely determined by the operator command. 

Relating 4-20mA Signals To Instrument variables:

Calculating the equivalent milliamp value for any given percentage of signal value is quite easy. Given the linear relationship between signal percentage and milliamps. The equation takes the form of standard slope intercept line equation y = mx + b. Here,

y = equivalent current in milliamps,

m = span of the 4-20mA range(16mA),

x = desired percentage of signal,

b = live zero(4mA)

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

This equation form is identical to the one used to calculate pneumatic instrument signal pressures (for example 3 to 15 PSI standard)

                                             Pressure = ( 12PSI) (x/100%) + ( 3PSI )

The same mathematical relationship holds for any linear measurement range. Gives a percentage of range "x" the measured variable equals to,

                                           

                                           Measured variable = ( Span ) ( x/100% ) + ( LRV )

Example calculation No.1 : Controller output To Valve

An electronic loop controller outputs a signal of 11.35mA to a direct responding control valve( where 4mA is shut & 20mA is wide open). How far shouls the control valve be open at this MV signal level?

Sol) We must convert the Milliamps signal value to percentage of valve. This means determining the value of the 8.55mA signal on the 4-20mA range.

                                 

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

                                                     ( 16 mA ) ( x/100% ) = Current - ( 4mA )

                                                         ( x/100% ) =  Current - ( 4mA)/( 16mA )

                                                               x = ( current - 4mA/16mA) 100%

Given current value is 8.55mA, Then    

                                                               x = ( 11.35mA - 4mA/16mA) 100% 

                                                               x = ( 0.459) 100%

                                                               

                                                                       x = 45.9% 

Therefore, The control valve should be 28.4% open for the MV signal is at a value of 8.55mA.

Example calculation No.2 : Flow Transmitter

A flow transmitter is ranged 0 to 350 gallons per minute, 4-20ma output, direct responding. Calculate the current signal value at a flow range of 226 GPM.

Sol) First, we convert the flow value of 226 GPM into a percentage of range. This is a simple matter of division, since the flow measurement range is zero based.

                                                            226 GPM/350 GPM =  0.645 = 64.5%

Next, we take this percentage value and covert into an milliamp value using the formula

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

The value of x = 64.5%,

                                                            ( 16 mA) ( 64.5%/100% ) + ( 4mA ) = Current

                                                                      

                                                                ( 16 mA ) ( 0.645 ) + ( 4mA) = Current

                                                                   

                                                                   10.32mA + 4mA = Current

                                                                      Current = 14.3mA

Therefore, The transmitter should output a PV signal of 14.3mA at the flowrate of 226 GPM.

Example calculation No.3: Temperature Transmitter

A pneumatic temperature transmitter is ranged 50° to 140° Fahrenheit and has a 3-15 PSI output signal. Calculate the pneumatic output pressure if the temperature is 79° Fahrenheit.

Sol)                               Measured variable = 79°F

                                     The temperature range span = 140° - 50° = 90°

                                     LRV = 50°

     First, We convert the temperature value of 79° into percentage of range.
 
                                   Measured variable = ( Span )( x/100% ) + ( LRV )

                                   Measured variable - ( LRV ) = ( Span ) ( x/100% )

                                   ( x/100% ) = Measured variable - ( LRV )/( Span )

                                      x = [Measured variable - ( LRV )/( Span )] 100%

                                                     x = [79° - 50°/90°]100%

                                                                x = 32.2%

Next, We take this percentage value and translate it into a pneumatic pressure value using the formula previously shown.

                                       ( 12PSI ) ( x/100% ) + 3PSI = Pressure

                                  ( 12PSI ) ( 32.2% / 100% ) + 3PSI = Pressure

                                       ( 12PSI ) ( 0.322 ) + 3PSI = Pressure
 
                                                 3.86PSI + 3PSI = 6.86PSI

Therefore, The transmitter should output a PV signal of 6.86PSI at a temperature of 79° F.



Thanx for reading.
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