BRANDCO FIELD RESOURCES
Static vs Residual Pressure in a Hydrant Flow Test
Static pressure tells you the pressure before test water is discharged. Residual pressure tells you the pressure remaining at the measurement location while the test is flowing. Neither reading, on its own, tells you how many gallons per minute the water system can supply.
A useful test pairs those pressures with measured flow and a clear description of the setup. The City of Lawrence’s flow-test guidance distinguishes the residual hydrant, where pressure is observed, from the flow hydrants, where water is discharged and flow is measured.
Static pressure: the starting condition
For a hydrant flow test, static pressure is the baseline recorded before the test discharge begins. “Static” does not mean the entire municipal system has stopped using water. Other customers, tanks, pumps, and controls may still affect the pressure you observe.
Record the location and time along with the number. A baseline from yesterday, a different elevation, or another measurement point is not automatically interchangeable with the static reading for today’s test. Keep the pressure instrument and measurement connection consistent between the static and residual observations.
Residual pressure: pressure during the test flow
Residual pressure belongs to a flowing condition. A report should identify both the pressure and the associated discharge rate. “Residual pressure was 60 psi” leaves out an essential part of the result if the report does not also say what water was flowing when that reading was taken.
For distribution-supply testing, the approved layout typically uses separate measurement and flow locations. Do not substitute pressure measured at a flowing outlet for a remote water-supply measurement without confirming that it matches the test’s purpose and method.
Flow rate: the missing third measurement
Flow rate is a volume per unit of time, commonly gallons per minute. It can be determined using an appropriate pitot arrangement, calibrated flow nozzle, or other suitable flow device. Pressure at a flow device may be converted to GPM using the relationship specified for that device.
This is different from taking static or residual system pressure and treating it as flow. The U.S. Fire Administration’s water-supply evaluation manual explains how pressure and measured discharge work together in evaluating a water supply. If a report includes an extrapolated available-flow value, label it as a calculation and retain the underlying measured data.
A simple pressure-drop example
Suppose an illustrative test records 80 psi static pressure, then 60 psi residual pressure while 900 GPM is discharged. The pressure drop is 20 psi. Relative to the starting pressure, that is 20 ÷ 80 × 100, or 25 percent.
That arithmetic describes this example; it is not a universal passing score or instruction to keep increasing discharge until a particular drop occurs. The required test condition and minimum operating pressure must come from the approved procedure. A different flow rate or changed supply condition would be a different test point.
How Brandco keeps the readings together
Place a Pressure Sender Mini on the compatible static/residual measurement connection and read it on the Universal Remote Display at the flow location. This lets the operator see the remote pressure while the flow measurement is taken, reducing dependence on a shouted or radio-relayed number.
With the Dual Wireless Water Pressure Gauge set, a second sender can connect to a compatible flow-device gauge port. Enter the manufacturer’s specified device information for calculated GPM. Brandco’s display settings guide identifies outlet diameter and coefficient as configuration inputs. A correct sensor connection and correct flow configuration matter more than the convenience of seeing both readings on one screen.
Make the result easy to interpret
Use distinct report fields for static pressure, residual pressure, measured discharge, and calculated available flow. Include units every time. Record the identification of the pressure hydrant and every flowing outlet so another tester can reconstruct the arrangement.
If the pressure changes while you are documenting the result, do not quietly combine a later pressure value with an earlier flow reading. Note the changing condition and follow the approved procedure for obtaining a usable test point. Wireless visibility helps you notice a change; it does not make inconsistent observations equivalent.
Frequently asked questions
Does higher static pressure mean more available fire flow?
Not necessarily. Available flow must be evaluated from the water supply’s behavior under the applicable test conditions.
Is residual pressure the same as pitot pressure?
No. Residual pressure describes the system measurement during discharge. Pitot pressure is used at the flowing stream as part of a flow-measurement method.
Does Brandco measure GPM directly at the static hydrant?
No. The Pressure Sender measures pressure. Calculated GPM requires a suitable flow-device connection and correct settings.
Related resources
How to perform a hydrant flow test · Plan the testing crew
References
Lawrence flow-test guidance · USFA Water Supply Systems, Volume II · Brandco display configuration