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Measuring static and dynamic water pressure (C) Daniel FriedmanWater Pressure & Flow Measurement FAQs

FAQs on how to measure municipal or well water water pressure at a building:

Frequently-asked questions (and answers) about how to measure water pressure & flow in a building.

This article series describes how to measure water pressure and water flow in buildings in order to diagnose bad pressure or bad water flow.

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Q&A on How to Measure Building Water Pressure

water pressure test gauge (C) Daniel FriedmanRecent questions & answers about how to measure water pressure in a building, posted originally

at WATER PRESSURE MEASUREMENT, the home page for this topic.

On 2016-07-11 by (mod) re: start checking water flow or pressure obstructions by cleaning faucets & shower heads

Start by removing shower heads and faucet strainers and cleaning them of dirt and debris.

On 2016-07-11 by alice chapman

I live on the edge of an estate on top of a hill. We have a bit of a problem on a very hot period when there has been no rainfall but since last year when the whole area had big problem with burst pipes and for about a day there was no water.

So since that we do not have enough pressure to have a shower (in our perfectly good, fairly new electric showe)in the mornings before work, in the early evenings and weekends during the day (because of people using more water at weekends etc. We can only have a hot shower at about 9pm onwards. What can we do?


On 2015-12-23 by (mod) re: good water pressure in some building areas but not others

Jerry

If you have good water pressure in some building areas and not in others I suspect a more local problem (clogged shower head, clogged valve or pipe elbow or pipe) rather than a system problem. I would not do anything expensive before properly diagnosing the problem .

Follow your hot water pipes: tell me and your plumber where you see the point of change: from good water flow to poor water flow. The trouble is between that point and the fixture(s).

On 2015-12-22 by jerry peck

Hi...firstly I am not a plumber or close...just I own a lovely Edwardian property in Somerset and for the last 3 years we have had issues with the water supply...I have a 10 year old combi boiler (BAXI) turn the tap on cold in the bath and a trickle emerges....

turn the hot on and they both turn to a trickle so 4 young kiddies cant have a bath,. The shower works fine but turn ANY other tap on and it halves the pressure and goes cold.

Have had plumbers and water board look but no real solution.... water board don't want to know as they test at the meter in the road and its fine they say. water does have to travel uphill to our house....is there any reasonably price solution at all...chap last night said to put 2 big tanks in the loft and a pump?

It still has to go through the combi boiler though? Im getting at wits end... Thanks Jerry

On 2015-07-10 by (mod) re: flow rate out of a pipe of a given diameter

Nick, flow rate out of an aperature of any diameter is a function of the combination of the aperture diameter (possibly also shape), and the water pressure supply, and also the length, diameter, and number of bends or obstructions in the entire water piping system between the water source and the aperture outlet.
So I would be doubtful of a single "right" answer.

Engineering calculators at a minimum specify the pipe grade, diameter, and pipe length in water flow rate calculations.

The engineer may also consider the maximum allowable water flow velocity in the piping.

6mm is about 1/8" ID pipe size or DN 6.

If water velocity is limited to 1 m/second, through a DN6 pipe you can flow about 0.61 gpm or using another standard, about 0.138544 cubic meters per hour.

On 2015-07-10 by Nick

If the aperture of a port is 6mm dimeter what is the maximum flow of water and at what pressure would pass through it

Question: Does closing a water supply pipe stop valve part way reduce water pressure in the building?

this may sound silly but by reducing the flow via the stop tap do you reduce the pressure - Allan

Reply: No. But it will reduce the water flow rate. Here is the difference between water flow rate and water pressure

Does closing a stop valve reduce water pressure? Well yes if you mean flow rate and no if you mean static water pressure in the system.

No honest question is silly - thanks for asking.

Properly speaking, no, reducing the water flow rate does not reduce the STATIC pressure in the system in that the water pressure is determined by the water pump cut-off pressure setting on the pressure control switch if you have a private well, or by the street pressure and /or local water regulator pressure if you have a municipal water supply.

So if you are not running water - it's turned off - and you measure the water pressure using the methods we describe above, the water pressure will be the same regardless of whether or not a stop valve is partly closed.

But lots of people use the term "water pressure" to mean the DYNAMIC water pressure - how fast water comes out of the faucet. We call that the flow rate.

And yes, if you partly close a stop tap or valve, you will get slower water flow at any faucet or plumbing fixture that is downstream from that valve. That's because we're now supplying water through a smaller opening, so at the same starting pressure or static pressure, the volume of water that flows through that valve will be less per minute if the valve is partly closed than it would be if the valve were fully open.

Question: how does adding plumbing fixtures reduce the water flow rate & what is the difference between upstairs & downstairs water pressure?

(July 4, 2011) mike in p.s. said:

I own a condo with the following: 2-Tub/Shower Combo's, 2-Toilets, Dishwasher, Kitchen Sink, 4-Bathroom Sinks, 1-Patio Faucet. My unit is upstairs with an identical unit below (same fixture count). Each unit HAD it's own 3/4" feed with seperate shut-off vales about 30' away at the common meter box. HERE'S the problem.

The downstairs unit had a leaf in their line undeer the slab and aftere two days of attempting to locate and repair the line the plumbers deceided to abandon the downstairs fee and tee into our 3/4" feed so now both units share one feed, escentially doubling the number of fixtures on one 3/4" line.

Additionally am I correct in thinking that because I am upstairs and if both units are using water, the downstairs unit gets more pressure? And ideas would be helpful!

Reply:

Mike:
I think it's reasonable to guess that doubling the number of fixtures served by your water feed line will reduce the flow rate you'll see upstairs and that the first floor probably sees a bit better pressure

- in particular and in fact ONLY when multiple fixtures are running at once on both floors; otherwise you wouldn't see an effect. Of course with occupants in two condos you have no control over when your neighbors run water.

If your pressure upstairs is unacceptable it may be possible to improve things with a separate pressure tank and booster pump. we provide

an ARTICLE INDEX to RELATED ARTICLES that includes a live link

to - WATER PRESSURE PROBLEM DIAGNOSIS TABLE

see the detailed article: WATER PRESSURE BOOSTER PUMP

Question: how do I get more water pressure?

(Oct 19, 2012) sly said:

How do you increase the water pressure in your building building, because the water flow rate is low.

Reply:

Sly: you might want to install an auxiliary water pressure tank and booster pump. See WATER PRESSURE BOOSTER PUMP

Question: pressure of water per foot of height

(Mar 3, 2014) Rob Cunningham said:

What is the water pressure per foot, I know it is 2.??? per foot but I don't remember the the formula.

Reply:

Rob,

Here are some water weight & pressure basics:

At 30-feet of water (say diving under the ocean) at sea level, you've equaled 1 ATM of pressure.

The Weight per US gallon of water is based on 7.48 gallons per cubic foot.
The actual weight of water varies by temperature as its density varies by that factor. At 60F a gallon of water weighs 62.37 gallons per cubic foot. To be clear, if we had a 12"x12"x12" cube of water the pressure at its base would be that figure.

1 Gallon (U.S.) = 3.785x10-3 m3 = 3.785 dm3 (liter) = 0.13368 ft3 = 4.951x10-3 yd3 = 0.8327 Imp. gal (UK)

Details of this calculation showing how to convert the weight of water to pressures per foot in pipes of various diameters or inside the well casing are given in the article above.

Question: water piping size

(Nov 14, 2011) Anonymous said:

what size of waterline from meter pit to house at 600'

Reply:

Anon typical residential water piping uses 1" to 1.5" ID piping but the size that you need to use depends on several factors including required flow rate, delivery pressure, elevations, distances, and the required number of elbows or bends - so your plumber may have her own recommendations.

Question:

(Jan 4, 2012) need of water presure pump said:

I am planning to install a water misting

(July 15, 2014) SUMOD said:

Hai,in a 3/4 inch 40m lenth pipe is connected with pressure pump and it is closed at the end and the pressure is 7 bar once the valve will opened the pressure is dropping to 4 bar.how to calculate the flow rate.

Reader Comments:

12/3/2014 garyasale said:

Hello guys, I do not mean to split hairs here, or rather a few oxygen and hydrogen molecules, but if water weighs 62.4 LBS/cu.ft, and there are 7.48 gallons/cu.ft., then water must weigh 8.34 LBS/gal. not 8.25 as indicated in your article.

Also, in your example regarding pressure: I read in your article that a container 12" x 12" x 12" (a cu.ft. of water has a pressure of 62.4 pounds/sq.ft which doing the math would be .433 psi.

Now we previously established that a cu.ft of water is 7.48 gallons weighing 8.34lbs.

Then I read that a volume(your example was a 6" well casing containing 1.46 gallons has a pressure of .426 psi. Y

our explanation suggests the difference is due to the difference in weight. My understanding is that the weight is not a factor, and to illustrate this point, I suggest you put a pressure gauge at the bottom of a pipe 12" in diameter, and one at the bottom of a 1" diameter. If both are 1 foot high, they would both read .433 psi.

There is .433 psi per foot elevation regardless of the size of the container and therefore regardless of the overall weight of the water.

A water tank one foot high which holds 300,000 gallons of water would be have a water weight of 2.5 million pounds would have a pressure of .433 psi.

Reply:

Thanks, Gary. We'll review the article text.

1 Gallon of water indeed = (weighs) 8.3454 Lbs.

1 cuFt of water indeed = (weighs) 62.42796 lbs {presumably distributed over a square foot or 144sq inches) or about .433 psi.

We've no intention to try a repeal of the basic laws of physics.

One goes from a cubic foot to the volume of a cylinder to do the same calculations.

What can indeed be confusing (we'll edit the article as needed) is that one linear foot of water in a 6-inch diameter x 12-inch tall well casing is not going to equal a cubic foot in total water volume.

The pressure exerted by any volume of a substance is a function of its height (or depth) as you discuss.

The weight of any total volume of water above a specific number of square inches (the area of a circle at the bottom of the well casing) is calculable.

Moving to your example, let's look at scuba diving.

At sea level the atmospheric pressure on earth, on the top of the water and on us = 14.7 psi.

The "container size" as you argue, is not a factor. If we ignore the very small variatioss over our height and assume we're standing, we're being pushed-in on all sides by air at 14.7 psi. (more or less).

Let's dive down to 32 feet below sea level in sea water. Water pressure is now 2 atmospheres or 29.4 psi (little enough to be safe from getting the bends even if we hold our breath a long time or swim around using a SCUBA tank).

Container size is not a factor in the example.

Thanks for the comments. We've reviewed and edited the original text to remove some confusion.

Question: clarify the meaning of dynamic water pressure

(Dec 9, 2014) Anonymous said:
you should clarify that the "dynamic water pressure" you reference above is not the same as the dynamic pressure one discusses in fluid dynamics (1/2*density*velocity^2)

Reply:

Thank you Anon, I'll review and edit the text.

By "dynamic water pressure" as used here here we simply mean that if you put a pressure gauge on a hose bib, laundry faucet, etc. while water is running elsewhere in the building the measured pressure is ambiguous. Similarly, in a pump and well system, if you make a pressure measurement at any time other than at the point that the well pump cuts on or off the results can be confusing if not ambiguous.

But I agree that we should add the engineering definition from fluid dynamics that you cite.

Thank you. Working together we're smarter.

Question: relationship between water pressure and water meter readings

(Jan 30, 2015) PAUL said:
Will a surge in the city water supply make the water meter turn faster?

Reply:

Paul

No, no, and then Yes:

No: If no water is running in your building the incoming pressure does not impact the rate at which you consume water.

No: if your building uses a water pressure regulator to maintain regular water pressure inside the building.

Yes, IF your installation has no water pressure regulator or if the water pressure regulator in your building is either not working or is set above the incoming water pressrue.

In that case, when you have water running or have a leak, since incoming pressure is greater you will presumably run water at a faster flow rate at individual fixtures when you turn them on.

Certainly when you run water if water is being delivered at higher pressure then the flow rate at your fixtures will increase and you'll see the water meter spin faster.

You could avoid this phenomenon if you have frequent pressure surges by double checking that your

If your well water pressure is always too low, perhaps below 30 psi,

see WATER PRESSURE LOSS DIAGNOSIS & REPAIR.
Also

see WATER PRESSURE VARIATION CAUSES.

Watch out: If your well water pressure is too high,

see WATER PUMP CONTROLS & SWITCHES and see WATER TANK SAFETY.

Watch out: measuring "flow rate" at any faucet or fixture served by a well pump system will be inaccurate and will reflect pump capacity, piping restrictions, fixture restrictions, and even actual well flow rate variations where pump protection tailpieces or similar devices are installed. Measuring flow rate at a fixture does not measure the well's true flue rate.


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