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The amount of psi can also be calculated from the amount
of the milliamp signal, or one can calculate the amount of the milliamp
signal when the amount of psi is known. Each of these signals has an
offset and a span that must be taken into account for the calculation.
The offset for the milliamp signal is 4 mA, and the span is 16 mA. The
span is determined by subtracting the lower value from the higher value
(20 mA - 4 mA). The offset for the psi signal is 3 psi, and the span
is 12 psi (15 psi - 3 psi). The formula for converting between these
two systems is shown in the following equation. The milliamp signal is
identified as A (A The formula can be further simplified in terms of an unknown pressure or an unknown milliamp signal. The following equations show these two simplifications. P Or A ## ExerciseUse the graph in this graph or the formula to determine the amount of air pressure that one would expect to find at the valve for the following electrical signals: 4 mA, 8 mA, 12 mA, 17.5 mA, and 20 mA. One needs to know these values if one is troubleshooting the valve and amplifier, or if one were trying to complete a field calibration to ensure the valve moved its full range 0-100% as it received the electrical signal. ## SolutionThe graph shows that the air pressure ranges from 3-15 psi and the electrical signal ranges from 4-20 mA. Some of the values that line up on the graph will allow one to use the graph to get accurate conversion. For instance, the amount of air pressure for a 4 mA signal is 3 psi. The amount of air pressure for 8 mA is 6 psi. The amount of air pressure for 12 mA signal is 9 psi, and the amount of air pressure for the 20 mA signal is 15 psi. The amount of air pressure for 17.5 mA will not show up accurately on the graph, so one may use the formula to calculate the amount of pressure. P P P |

Applications That Use 3-15 psi | Motor (Electric)-Driven Valves |