How to Calculate Process Value to 4-20 mA Signal(How to do 4-20mA Conversions Easily)

Basic Fundamental: - In the world of process control, there are a myriad of different types of process inputs. Thermocouples and RTDs provide direct temperature reading while digital signals such as Modbus^® provide exacting control over process variables and display. Analog signals, where information about the process is transmitted via varying amounts of voltage or current, are the predominant type of input in industries requiring process control today. Of all possible analog signals that can be used to transmit process information, the 4-20 mA loop is, by far, the dominant standard in the industry.

As major as the 4-20 mA loop standard has become in the process control industry, many do not understand the fundamentals of its setup and use. Not knowing the basics could potentially cost you money when it comes time to make decisions about process display and control. Having a grasp on the history, workings, pros and cons of the 4-20 mA loop will help you to understand why it is the dominant standard for the industry and allow you to make informed decisions about your process control.

Type of Signal :-

Digital Signal :-  Digital Signal are Discrete not Continue. A digital signal carries the data in the form of binary because it signifies in the bits. 

There are many types of analog signals and loops in the industrial process world, some of which is.

v  4 to 20 mA

v  1 to 5 VDC

v  3 to 15 PSIG (pneumatic)

v  10 to 50 mA

v  -10 to 10 VDC

v  0 to 20 mA

v  0 to 5 VDC

What is the difference between Span and Range?

Range :-  The range of a signal consists of two values, such as 4 mA to 20 mA, or 1 VDC to 5 VDC.

Span :- The span of a signal is the difference between the two ends of the range, such as 16 mA and 4 VDC, respectively.

Span :-  Maximum Value - Minimum Value

 

 Analog instrumentation in an industry is based on the signal transmission of the process variable in a standard range of 4-20, 1-5 VDC. A process variable such as pressure, temperature, flow, level, displacement, in any range of measurement is converted to 4-20mA current signal for transmission to the control panel.

This is an analog signal standard, meaning that the electric current is used to proportionately represent measurements or command signals. Typically, a 4 milliamp current value represents 0% of scale, a 20 milliamp current value represents 100% of scale.

Controllers such as Mostly Smart Controller uses digital to analog converter to convert the voltage signal to digital numbers which are understandably by CPU. The measuring device has 250-ohm resistor. As for current signals, they are converted to voltage signals like this

4 ma * 250 ohm = 1 v
20ms * 250 ohm = 5v

mA Signal	Process Value in Percentage
4 mA	0
8 mA	25
12 mA	50
16 mA	75
20 mA	100

Why we use 4-20mA, not 0-20mA?

v  Live Zero i.e 4 mA. It is easy to detect that the signal wire is not broken. If “0” mA , unable to know that it is a signal or wire broken.

v  The 4 mA is used as “range zero”. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument. Field instrument would required 3 or 4 wire devices. Additional wiring costs come in to account.

v  Another reason for choosing 4 mA is when the transmitter were first introduced, it was working on pneumatic pressure signal for transmission. Later, electronics chips were introduced those require minimum of 3 mA of current to function. So, a margin of 4 mA is taken as reference.

v   Scale and Calculate : with 4-20 mA signal it is easy to scale and calculate the process parameter.

How to relate 4-20mA with process variable?

Pv (Present Value) mA  =        ___________________  x 16 + 4                         Full Span

 Example :- Given a temperature transmitter with a measurement range of −50 degrees to +200 degrees and a signal range of 4 to 20 milliamps, calculate the proper signal output at an applied temperature of + 25 degrees.

Solution :-

Given

Present Value Pv-  25 Degree

Instrument Range = -50 to 200 Degree

Signal Range = 4 to 20 Ma

Instrument Lover Range: - -50 Degree

Instrument Higher Range: -  200 Degree

We Know that

Full Span = Maximum Range-Lower Range

                = 200-(-50)

                = 250

Full Span = 250

Pv (Present Value)

mA  =              ___________________  x 16 + 4

                            Full Span

mA=                  25

                   --------- 16+4                                                                            

                           250

          =     0.25 x 16

          =      4 + 4

mA          =         8