How to Measure Gallons per Minute

Measuring gallons per minutes is easy. The calculations required to perform this depends on the object for which the flow rate must be determined. Essentially, a gallon per minute is a unit of flow rate in physical science. Flow through circular pipes, orifice plates and venturi metre is required to be calculated for engineering applications. Typically GPM is determined using Bernoulli’s equation. The steps involved in obtaining the GPM value are relatively simple.

Things required :

- Calculator or Microsoft Excel spreadsheet
- Physical data for the instrument for which GPM is to be calculated
- Application data for the instrument
- Graph paper


  • 1

    Firstly determine the flow measurement application. For calculating the GPM using differential pressure in a pipe section, consider getting the physical data for the pipe. Once you obtained the data, you should determine the internet and external pressure. Internal pressure is the pressure inside the pipe while external pressure is the environment pressure. Any liquid through the pipes moves due to pressure differential and if you can determine that difference, it becomes very easy to find the flow rate using Bernoulli’s equation.

  • 2

    Pressure differential can be determined through dividing the elevation by external pressure/ statistic head pressure. Subtract this value from the initial pressure value to find the pressure different over the length of the selected section of pipe. Now consider calculating the flow rate in gallons per minute by dividing the differential pressure by the static pressure. Use the D’Arcy Weisbach equation to obtain the tabular data for pressure variations inside the pipe.

  • 3

    Calculating pressure differential through an orifice plate is also extremely easy. Determine the initial and final pressure before and after the orifice disk. When water or any other liquid moves through the orifice plate, the velocity increases considerably while the pressure drops significantly. Calculate the pressure differential using Bernoulli’s equation to calculate the actual flow through the head pipe. If you final answer is not converted in gallons per minute units, then you should use unit conversion equations to achieve this.

  • 4

    It is recommended to use engineering books to become familiar with the equations and quantities used for flow rate calculations in pipe shaped objects. Remember to study as much as you can to help you understand how to apply these measurements and their overall significance.

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