For a pneumatic transmission system for instrumentation purposes, if a pneumatic pressure indicator has a calibrated scale of 0 to 5000 psig, what would be the actual measured pressure if the transmitted pneumatic signal pressure to the indicator is 6 psig, assuming the industry standard of 3 to 15 psig is used for instrument air?
• 3-15 psig pneumatic signal range represents the full calibrated range of the indicator (0–5000 psig) • You must first find what 1 psig of signal corresponds to on the indicator scale • This is a linear relationship problem: use proportion based on span, not on the total scale including the lower offset
• What is the span of the pneumatic signal (from 3 to 15 psig), and what is the span of the indicator (from 0 to 5000 psig)? • If 3 psig corresponds to 0 psig on the indicator and 15 psig corresponds to 5000 psig, how much indicator pressure does each 1 psig change in signal represent? • Once you know psig per psig (indicator per signal), how many psig above 3 psig is a 6 psig signal, and what indicator pressure does that represent?
• Be sure you subtract the lower range value (3 psig) from the signal before scaling; don’t treat 6/15 as the fraction. • Confirm you’re using the signal span (15 − 3) and not the full 0–15 range directly. • After you compute the answer, check that your result is between 0 and 5000 psig and corresponds to a signal between 3 and 15 psig.
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