For an analog electronic transmission system for instrumentation purposes, if an analog pressure indicator has a calibrated scale of 0 to 30 psig, what would be the actual measured pressure if the transmitted analog signal current to the indicator is 12 mA, assuming the industry standard of 4 to 20 mA is used for instrument current?
• 4–20 mA analog signal range represents the full measurement span • How to find the percentage of span represented by a given mA value • Using that percentage of span on a 0–30 psig scale to find the indicated pressure
• What portion of the 4–20 mA range does 12 mA represent? Is it below, at, or above the midpoint of the span? • Once you know the percentage of the 4–20 mA span, how do you apply that same percentage to a 0–30 psig scale? • Check if your final pressure value makes sense: should it be closer to 0, 15, or 30 psig based on where 12 mA lies between 4 and 20 mA?
• First calculate the span current: 20 mA minus 4 mA • Find the fraction of span: (measured current – 4 mA) ÷ span current • Multiply that fraction by 30 psig (the full-scale range) to get the indicated pressure
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