For a pneumatic transmission system for instrumentation purposes, if a pneumatic instantaneous flow indicator has a calibrated scale of 0 to 1000 gpm, what would be the actual measured instantaneous flow rate if the transmitted pneumatic signal pressure to the indicator is 9 psig, assuming the industry standard of 3 to 15 psig is used for instrument air?
• Linear scaling of instrument signals (3–15 psig range) • Relationship between signal span (12 psi) and flow span (0–1000 gpm) • How to find the percentage of span represented by a given signal pressure
• First, find what fraction of the 3–15 psig range the 9 psig signal represents. Are you measuring from 0, or from the lower range value? • Once you have the percentage of span, how can you apply that percentage to the 0–1000 gpm flow range? • Check that at 3 psig you obtain 0 gpm and at 15 psig you obtain 1000 gpm using your method. Does your approach satisfy both end points?
• Subtract the lower range value (3 psig) before calculating the fraction of span. • Use the correct signal span: 15 psig − 3 psig = 12 psig. • Multiply the fraction of span by the full-scale flow range (1000 gpm) to get the indicated flow.
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