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Instrumentation standard signals are essential in the field of industrial automation and control systems, serving as the means by which measurements and control commands are transmitted between devices. These signals can be categorized into two primary types: *analog signals* and *pneumatic signals*.
## *Analog Signals*
*4-20 mA Current Signal*
The most widely used standard in modern industrial instrumentation is the *4-20 mA DC current signal*. This analog signal represents measurements or control commands proportionately. In this system:
- A current of *4 mA* typically indicates *0%* of the measurement scale.
- A current of *20 mA* indicates *100%* of the scale.
- Any current value between 4 mA and 20 mA corresponds to a percentage between 0% and 100% of the measurement range.
This range is referred to as a "live zero" because it starts at a non-zero value, allowing for easy differentiation between a valid zero signal and a fault condition, such as a broken wire or a malfunctioning device.
*Calculation Example*
To calculate the equivalent current for a specific percentage of the signal range, the following formula can be used:
Formula of Instrumentation:
X = Y% ( SPAN ) + MIN
Where:
MIN = MIN VALUE
MAX = MAX VALUE
Y% = PERCENT VALUE
SPAN = ( MAX -MIN )
Percent = 0% (min) to 100% (max) value
Example,
4 to 20mA
What is 50%
X = Y% ( SPAN ) + MIN
X= 50%( max-min) + 4
X=( .50 ( 20-4) ) + 4
X= 12 mA
Y% = 50/100
Y% = .50
Example:
4 to 20mA
What is percent of 12mA
X = Y% ( SPAN ) + MIN
12= Y%( max-min) + 4
Y% = (12-4) / (20-4)
Y% = .50
Y% = ,50 (100)
Percent = 50%
Example:
3 to 15 PSI
What is 50%
X = Y% ( SPAN ) + MIN
X= 50%( max-min) + 3
X=( .50 ( 15-3) ) + 3
X= 9 PSI
Table Sample:
3psi = 4ma = 0%
6psi = 8ma = 25%
9psi = 12ma = 50%
12psi = 16ma = 75%
15psi = 20ma = 100%
## *Pneumatic Signals*
*3-15 PSI Signal*
In pneumatic systems, the *3-15 PSI (Pounds per Square Inch)* signal is commonly used. This signal operates similarly to the 4-20 mA current signal:
- *3 PSI* represents *0%* of the measurement scale.
- *15 PSI* represents *100%*.
- Values between 3 PSI and 15 PSI correspond to percentages in between.
Pneumatic signals are particularly favored in process industries where safety is a concern, as they can be less prone to electrical hazards.
## *Standards and Identification*
The *ANSI/ISA-5.1-2009* standard outlines the symbols and identification methods used in instrumentation. This standard aims to promote uniformity in the field of industrial automation, ensuring that signals and their representations are consistent across various systems and applications.
In summary, instrumentation standard signals like the 4-20 mA and 3-15 PSI are crucial for effective communication in control systems, allowing for accurate monitoring and control of industrial processes.
