A pocket reference for the street: what each step looks like, how to read the gauges, and which photos to take so the test can be defended later. Open it on your phone, or print it.
These are illustrations, not photographs — simplified drawings to show positions and readings. Hydrant hardware varies; follow your water purveyor’s procedure, wear eye protection and keep people and traffic clear of the stream.
The six steps
1
Put the cap gauge on the residual hydrant
Use 2½-in. outlets where you can (the 4½-in. pumper outlet needs a correction). Thread the gauge onto a 2½-in. outlet, open the hydrant slowly, then crack the air bleed until water spits and close it. This hydrant stays closed to flow — it only reads pressure.
Watch for: A needle that creeps up after bleeding means trapped air. Bleed again.
Static: 72 psi
nothing flowing
2
Read static pressure
With every flow hydrant still closed, wait for a steady needle and write the number down. Read straight on, at eye level.
Watch for: Reading from the side shifts the needle against the scale (parallax). A needle that swings means someone is drawing water nearby.
3
Open the flow hydrant fully
Check the discharge area is clear of people, cars and soft ground. Open the hydrant all the way and let the stream clear and steady.
Watch for: Never throttle a hydrant with its main valve. A half-open valve makes the numbers meaningless.
4
Position the pitot
Hold the tip in the middle of the stream, facing the flow, about half the outlet diameter from the opening. Keep the air bleed on top and the tube out of the stream edges.
Watch for: Edge-of-stream readings are low. A bouncing needle means turbulence — reposition slightly.
Residual: 58 psi
residual hydrant
read at the same moment
Pitot: 24 psi
flowing outlet
5
Read residual and pitot together
One person reads the residual gauge while the other reads the pitot — at the same moment. Repeat for each flowing outlet. Add the flows in the calculator.
Watch for: Aim for pitot readings of 10–30 psi (NFPA 291): over 30, open another outlet. If the residual barely moves from static, open more outlets — NFPA 291 (2022 and later) wants a drop of at least 10%.
6
Close slowly and record
Shut the hydrant slowly to avoid water hammer. Read static again and compare it with your first reading. Check the barrel has drained (feel for suction at the open outlet), replace caps and chains, and write down the date, time, hydrant IDs and what you saw.
Watch for: A second static that differs a lot means something changed during the test. Note anything unusual: a pump cycling on, a tank filling, a hydrant that was hard to open.
Reading the gauges
Read the needle against the nearest tick, straight on. These three dials show the readings from the worked example in the guide: static 72 psi, residual 58 psi, pitot 24 psi. Gauges vary in range — check the scale before you read it.
Static: 72 psi
residual hydrant, no flow
Residual: 58 psi
residual hydrant, flowing
Pitot: 24 psi
at the flowing outlet
What does the outlet look like inside?
The coefficient c in the flow formula depends on how water enters the outlet. If you can see inside the barrel, match the shape; if you cannot, ask the purveyor which value they use.
c = 0.90
smooth, rounded entrance
c = 0.80
square, sharp edge
c = 0.70
tube projects into barrel
Photograph this for the record
The calculator gives a number; photographs make it defensible. Tick each shot as you take it (the ticks reset when you leave the page).
WHAT IS THIS LIST FOR?
A flow test is just numbers on a clipboard. Weeks later, an engineer, the water utility, a plan reviewer or the fire marshal may ask which hydrant, which outlets, what did the gauges really say, and was the test done correctly? These ten photos answer those questions without anyone having to go back out.
Proof of what you tested: shots 1-2 identify the exact hydrant.
Proof of the readings: shots 4-6 show the gauges themselves, so a misread digit can be caught later.
Proof of the setup: shots 3 and 7 show the outlet size and pitot position, the two things that change the calculated flow most.
Proof of no damage: shots 8 and 10 protect you if someone says the test eroded a lawn or left a hydrant open.
Proof of when: shot 9 ties every photo to this test, not a similar one.
It is a good-practice checklist, not an NFPA requirement. Take as many as the job and your client need; if you can only take three, take 2, 4 and 5.
Hydrant cap colours
NFPA 291 marks hydrant bonnets or caps by the flow available at 20 psi residual — the same number the calculator projects. Some utilities use their own scheme, so check locally.