For a pneumatic transmission system for instrumentation purposes, if a pneumatic temperature indicator has a calibrated scale of -40 to +40°F, what would be the actual measured temperature 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 relationship between instrument air pressure (3–15 psig) and the measured variable range (-40°F to +40°F) • How to find the percentage of span represented by a given pressure within the 3–15 psig range • Converting that percentage of span into a temperature value between -40°F and +40°F
• First, determine where 9 psig lies between 3 and 15 psig as a fraction or percentage of the total pressure span. • Apply that same fraction or percentage to the temperature span from -40°F to +40°F. • Then add the result to the lower temperature range value to find the indicated temperature.
• Confirm the total pressure span (15 – 3) and the total temperature span (+40 – (–40)). • Be sure to subtract the lower range pressure (3 psig) from 9 psig before taking any ratio. • After calculating the temperature, check that the value is within the -40°F to +40°F range and matches one of the choices.
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