Using Water Flow Meters and Pressure Gauges During Nozzle Evaluation

Are you evaluating new firefighting nozzles for your crew? Are you completing NFPA (National Fire Protection Association) tests on your nozzles?  

When you do either of these activities, you know that having the best, most accurate information is necessary. You’ve probably heard someone mention the use of a pressure gauge and water flow meter for evaluation and testing.  

Are you wondering if they are necessary? We think they are – here's why! 

Getting the Whole Picture 

When you evaluate or test nozzles, you can use formulas to calculate the flow rates and friction loss you are experiencing. However, these formulas are not foolproof. They are subject to human error and miscalculation if any of your information is incorrect. They often need to know flow or pressure, and the only way to know your exact flow or nozzle pressure is to use a flow meter or pressure gauge.  

Ultimately, without these two tools you are not able to confirm that your nozzle is performing the way it is supposed to be.  

Let’s look at flow meters and pressure gauges separately.  

Flow Meters 

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A flow meter is a device that tells you the flow moving through your hose line. A flow meter is particularly important for a couple of scenarios. 

First, when you evaluate an automatic nozzle. With an automatic nozzle, pressure stays relatively stable (+/- 15 psi) throughout the flow range. This means the only reliable way to determine your flow is to use a flow meter. You will want to do so to ensure your automatic nozzle is maintaining pressure throughout the expected flow range. 

Our second example is with a fixed gallonage nozzle. A flow meter is particularly helpful when you need to verify that your nozzle is flowing within the +/- 10% of its flow rating. For this, you would bring the nozzle to its rated base pressure and verify with the flow meter that it is operating within range. 

These scenarios are not the only times that a flow meter is useful, but they are two specific examples of when a flow meter is necessary to evaluate your nozzle. You might have noticed that each one mentions a pressure reading. 

Pressure Gauges 

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A pressure gauge is a device that allows you to measure the pressure at a certain point in your hose lay or at the nozzle. Like the flow meter, the pressure gauge is important if you want to verify your nozzles are performing as expected.  

Pressure gauges themselves remove the need for any complicated formulas. You can simply read a gauge face or in the case of a Bluetooth pressure gauge, find the reading on an app. This process simplifies your work and helps you perform many of the tests for an NFPA test or that you’ll want to do during an evaluation, including the checks mentioned above. 

Using the Right Pressure Gauge 

It is important to remember that there are different types of pressure gauges. NFPA 1962:2018 tests use base nozzle pressure, which is a side wall pressure reading. This differs from a pitot pressure reading which is taken from the center of the stream.  

Side Wall Pressure Gauge 

A side wall pressure gauge can come in different formats, but the important part is the area where it reads pressure is at the outside wall of the gauge. This is important because pressure varies in different places in your water stream. The stream's center will have a higher pressure (velocity) than the sides because it has a higher velocity. The water toward the sidewall encounters more friction loss because it is working against the liner of the hose which slows it down, lowering the pressure (velocity).  

Pitot Pressure Gauge 

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Your handheld pitot is a gauge that takes a pressure reading anywhere its placed in the water stream exiting the nozzle. Some in-line gauges also use a pitot reading. You will want a center reading for smooth bore nozzle exit pressure and from that reading you can calculate or reference a chart to find the flow rate at that pressure. 

GPM = 29.7 X D² X √NP 

Flow Meter and Pressure Gauge Placement 

Placement of your flow meter and pressure gauge is important during testing and evaluation. Incorrect placement can lead to faulty readings and make you believe your nozzle is functioning properly when it isn’t, or vice versa.  

Let’s examine where to place each device.  

Flow Meter Placement 

Your flow meter should be placed in an area with limited turbulence for the best reading. Typically, in between two sections of hose where it can lay flat on the ground works well.  

We do not recommend gating the inlet of your flow meter if you are using it with a gate valve. This creates turbulence that can cause an inaccurate reading. If you need to use it with a gate valve, gate the outlet of the water flow system instead.  

Pressure Gauge Placement 

The placement of your pressure gauge depends on your needs and the readings you are interested in. Here are a couple of placement scenarios.  

If you are interested in calculating the friction loss of your hose line, you can place a pressure gauge at the discharge outlet and at the inlet of your nozzle. The difference in pressure will allow you very easily calculate the friction loss throughout the lay.  

If you need to know the base pressure of your nozzle, your pressure gauge should be coupled directly between your hose line and the nozzle itself. This tells you the base nozzle pressure at that point in time. 

If you need to check discharge pressures, you can attach a pressure gauge to a discharge outlet elbow. This may be necessary to verify that your on-board pressure gauge is functioning properly.  

Nozzle Evaluations 

As you can see, the flow meter and pressure gauge have a variety of uses when evaluating your firefighting equipment. For nozzles, it is valuable to use these two elements together for a complete picture of the nozzle’s function or to complete your annual NFPA 1962 evaluation.  

Don’t forget, NFPA is consolidating a wide range of standards and guidelines! NFPA 1962 will be consolidated into NFPA 1930 in the near future.