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# How do you convert PSI water pressure to gallons per minute?

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You cannot directly convert PSI to GPM. They are two different units of measure. PSI measures pressure, and GPM measures flow rate. However, if other variables are known, you can use Bernoulli's equation to indirectly make a conversion. The following assumes the flow is in a pipe and the pipe diameter is known. It also assumes a steady state incompressible flow.

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Credit: Greengirl 24 CC-BY 2.0

1. Ensure all units agree

Ensure all units to be used agree with each other. This means ensuring there are no variables given in metric and that everything is measured in inches, not feet.

2. Use Bernoulli's equation to determine fluid velocity

Use this equation, (g*h + (P/rho) + (V^2/2)) = constant, to solve for velocity. Upon isolating velocity, the equation will look like this: V = sqrt(2 * ((-P/rho) - g * h)) where P is the pressure, rho is the density of the fluid, g is gravity and h is the elevation above the reference plane.

3. Determine the flow rate

Using the equation Q = V * A, calculate the velocity. Q is the volumetric flow rate, V is the velocity of the fluid and A is the cross-sectional area of the pipe.

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One inch of water, as measured using a pressure gauge at four degrees Celsius, is equivalent to 0.036127291827 pounds per square inch (PSI). This conversion can be found at OnlineConversion.com.

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The term "absolute pressure" refers to the sum of the atmospheric pressure and gage pressure, expressed in the equation Pabsolute = Pgage + Patm. Absolute pressure is typically measured against a pure vacuum.