The parts of a brand-new pitot gauge, placed for easy view and assembly.

Pitot gauges make it easy to get accurate GPM measurements for fire hydrant flow tests

To ensure that fire hydrants can supply adequate water during an emergency, municipalities and other authorities require regular hydrant flow testing. Pitot gauges make it simple to quickly measure the flow of fire hydrants and convert readings in pounds per square inch (PSI) to gallons per minute (GPM).

Read on as we describe what a pitot gauge is, why hydrant flow testing is necessary, who conducts these tests, and how to use a pitot gauge during a flow test. Just need to buy one or some accessories? We've got you covered.

How does a pitot gauge work?

The handheld pitot gauge remains the fastest and easiest method for measuring both straight-tip and hydrant-flow GPM. By calculating the GPM, inspectors can uncover reduced flow rates. Heavy pipe-wall deposits and closed valves are often discovered when analyzing data collected by hand-held pitot gauges.

A pitot gauge consists of three components: a blade, handle, and pressure gauge. Once placed within an open fire hydrant’s water discharge, a narrow tube inside the blade directs water towards the gauge to enable a pressure reading. The captured PSI readings can be used to calculate the disparities in pressure between two points along a distribution system.

The blade, handle, and pressure gauge of a pitot gauge.

Each pitot gauge features a blade, a handle, and a pressure gauge.

The desirable unit of measure we seek from hydrant flow testing is a hydrant’s GPM. To convert from PSI to GPM, we refer to Bernoulli’s principle. Bernoulli, a Swiss mathematician, developed a formula to represent the relationship between velocity and pressure along a streamline. In practice, these conversions are often made using a theoretical discharge table, like the one we provide with our selection of pitot gauges.

Bernoulli’s equation, however, doesn’t account for friction produced when water flows through a pipe system. This friction represented mathematically is called the hydrant’s coefficient and it represents a pipe's wall roughness or abnormalities. The pipe's coefficient is required to convert to GPM. The Moody diagram or Colebrook equation are both resources that can help in estimating your pipe's coefficient. Once the coefficient is identified, you're ready to convert to GPM.

Who performs fire hydrant flow tests with pitot gauges?

City officials and professional contractors perform hydrant flow tests with Pitot gauges. The regulations they follow are often drafted by the National Fire Protection Association (NFPA), which provides guidelines for hydrant flow testing procedures. It is recommended that a city’s entire distribution system be tested every five years.

Besides peace of mind for emergencies, the tests enable municipalities to color-code their fire hydrants according to their strength of output. This allows fire departments to assess their water resource capabilities quickly when arriving on the scene of an emergency.

How to use a pitot gauge in 10 steps

Provided below are 10 high-level steps for performing a hydrant flow test with a pitot gauge. It is by no means a comprehensive step-by-step guide. Consider consulting your local fire department or inspector if you want detailed, local information.

  1. Determine the location of your test by selecting a group of hydrants in the same vicinity. Per NFPA 291, as many as eight hydrants may be required for systems with large water mains and a strong system.
  2. Mark the hydrant measuring pressure as the ‘residual’ hydrant. Both static pressure (when flow hydrants are closed) and residual pressure (when flow hydrants are open) are measured from this hydrant. The residual hydrant should be between the hydrant(s) to be flowed and the large mains that supply water to the area.
  3. Flush the residual hydrant to remove any sediment and attach a nozzle cap with a gauge to the hydrant’s outlet.
  4. Slowly release the main valve until air is vented. Take a static pressure reading.
  5. Measure the inside diameter of the outlet nozzle or hydrant outlet where flow occurs. A hydrant's inside diameter is usually 4”.
  6. Field personnel should slowly open each fire hydrant, one at a time, to avoid pressure surges.
  7. After the residual pressure read from the outlet cap stabilizes, take readings at each hydrant using a pitot gauge. Residual pressure and pitot gauge readings must be taken simultaneously. For accurate pitot gauge readings, the pitot tube should be held downstream and in the center of the nozzle.
  8. Record both the residual and pitot gauge readings.
  9. Slowly close each fire hydrant.
  10. Combine the PSI readings from the residual hydrant’s static and residual pressure, with the coefficient determined by measuring the inside diameter of the hydrants outlet nozzle, and calculate your GPM value using Bernoulli's principle.

Now that you know how to conduct a hydrant flow test with a Pitot gauge, check out this short clip to see one in action:

How to choose a pitot gauge

If you’re in the market for a new pitot gauge, keep the following tips in mind:

  • The pitot gauge will be held against powerful flows. Make sure the gauge is light but firm in your grip.
  • Look for a rotating pitot gauge on a quick disconnect fitting for easier reading and faster draining of the system.
  • For playpipe or hose nozzle readings, choose a kit with a notched blade in addition to the straight blade required for hydrant and pump testing.

For a compact, lightweight and accurate Pitot gauge, look no further than the Inspector's Choice Pitot Gauge Kit.

A foam-lined kit containing the inspector's choice pitot gauge and accessories.

The foam carrying case, a NIST-certified gauge, and ultra-lightweight components make this gauge ideal for hydrant flow testing.

The Sure-Grip handle is designed for a comfortable fit and weighs only 14 ounces. This inspector's choice kit delivers highly-accurate readings with a NIST-certified, Ashcroft 1009 Duralife Gauge at 1% full-range accuracy. Buy the Inspector's Choice Pitot gauge from QRFS.com and choose from eight gauge ranges up to 600 PSI at no extra charge.

The Inspector's Choice Pitot Kit is ready for any job. Measure discharge from hydrants, nozzles, and playpipes using the included straight and notched ultra-thin blades. The quick disconnect brass fitting enables 360-degree gauge rotation and easy system draining. Carry up to two gauges in the foam lined, lockable carrying case that comes with an extra roll of thread seal tape.

Need your gauge quickly? QRFS ships most orders the same day. Click the link to get started:

View the Inspector’s Choice Pitot Gauge Kit Today as well as our selection of components.

Questions about this article or our pitot gauges? Call us at +1 (888) 361-6662 or email [email protected].

This blog was originally posted on May 20, 2016 by Jason Hugo and Cameron Sharp of Quick Response Fire Supply, and updated on Jan. 7, 2019. Like what you read? Check out other articles at QRFS.com/blog