Pressure washers - useless, or less than useless?

Reading car forums, the overwhelming opinion is that a pressure washer is needed to wash one's car properly. So I went and bought a cheap Halfords one, complete with snow foam attachment. The results were disappointing to say the least.
I know it's not a mega powerful one, but I was amazed at how low the pressure was out of the nozzle. I was certain my hose gave a more powerful flow, so I did an experiment and timed how long it took to fill a bucket, and therefore what the flow rate is from my tap. It amounts to 1,000 litres per hour, nearly three times the flow rate of the PW. Some of the best pressure washers only appear to give half of that.
Am I missing something here? Is pressure a function of the flow rate, and if so, what's the point of a pressure washer if it gives less than mains pressure?
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On 01/04/2012 15:28, Hugo Nebula wrote:

There's certainly no need for them to shift vast volumes of water to be effective. I think the point is that they generate (or should do) a very high pressure by pumping water through a very small orifice. Certainly my Karcher blasts out a very effective jet, but the actual flow rate is way less than provided by a mains-fed tap.
David
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[Default] On Sun, 01 Apr 2012 16:03:42 +0100, a certain chimpanzee,
and wrote:

So would a hose with a small nozzle not achieve the same effect?
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On Sun, 01 Apr 2012 16:13:06 +0100

No, because you can't generate more pressure than you start with without some extra force, such as a pump.
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On 01/04/2012 16:13, Hugo Nebula wrote:

There is a limit to the flow rate through a small nozzle unless you apply a much larger pressure (ie pump) to the back side of it. People make terrible mistakes with leaking high pressure hydraulics systems.
A small amount of water carrying a large amount of kinetic energy can do a lot of damage. I would not use one on my car as I don't want to grit blast the surface or strip the paint. YMMV Fine for use on dirty patios though and stone doesn't suffer much from additional scratches.
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Martin Brown wrote:

That's why you vary the distance to the target and the nozzle dimensions.
They are not sandblasters.
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On 02/04/2012 02:15, The Natural Philosopher wrote:

Indeed, but it isn't a tool I would like to use on good car paintwork.

That depends how much sand and grit there is stuck on the car already. Mine picks up a lot of salt and mud (and this time of year muck) from local roads. Using a pressure washer would grind it into the paintwork.
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Martin Brown wrote

More fool you.

Nope.
Nope, it blasts it off.
Using your silly line, there isnt any way to clean a car like that, because it just 'grind it into the paintwork'.
The whole point of cleaning it is to get it off instead.
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On Mon, 02 Apr 2012 09:44:55 +0100, Martin Brown

That's why I use the fan setting first, to wash off the grit and loose bits.
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Hugo Nebula wrote

Nope, the pressure is much lower than with a decent pressure washer.
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Hugo Nebula wrote:

No.
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They, as the name suggests, provide extra pressure and not flow rate, so I would expect an open hose to provide a higher flow rate, otherwise they would be sucking water out of your mains.
I use a middle of the road Karcher and it is marvellous, fetches paving slabs and concrete back to a nice crisp finish and clears most of the dirt off the car. If you have it on a pencil jet and it cuts the rubber trim around the windows then it is OK. A problem I had when I first used mine years ago. Quite good at taking paint off painted bumpers too if there are any stone chips.
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writes

Those are mainly what I use mine for, plus with not-so-great drainage in my back garden, I can clean it all without flooding the garden because it uses less water than a hose. The patio looks like new at the moment, I don't think anything else would bring it up that clean.
I'm assuming the best way to compare tap versus jetwasher is the bar measurement rather than flow rate. I think my washer is about 120bar, that's way more than a tap. You can stop a tap running with your thumb over it, you ain't stopping a jet from these machines.
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On 01/04/2012 16:34, Mentalguy2k8 wrote:

Have a look at the FAQ. The way to compare them is with the pressure *and* flowrate - its the product of the two that does the work...

You would have a hard job stopping most cold mains taps with your thumb!
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Bill wrote:

add in getting winter lichen off the garden furniture..not bad at house paintwork and the windows, too.
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The Natural Philosopher wrote

And blasting off the wasp nests too.
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Pressure washers are a positive displacement pump. They deliver a fixed amount of water. http://en.wikipedia.org/wiki/Positive_displacement_pump#Positive_displacement_pump
The pressure you get depends on how small the outlet orifice is.
So to get a higher pressure you adjust the outlet down. If the outlet is wide open, you just get a miserable dribble. (But it is the same volume of water.
There is a relief/bypass valve inside opens if over pressure ocurrs. eg when the operating valve on the lance is closed.
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Really? I thought a problem with them for this purpose is the pressure can take the paint off.
I do use one for washing the car, but the main purpose is to pump the water from the water butt. I never point a high pressure jet straight at the paintwork, or elsewhere where it might cause damage. I use a soft brush attachment for washing, and a misting spray for rinsing.

The flow rate from most is quite low.

No.
To a first approximation, pressure x flow rate will be proportional to the power consumption, but that ignores that some have efficient induction motors, and others apparently have less efficient (and probably shorter life) universal motors.

They are much higher pressure than mains.
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Andrew Gabriel wrote:

Not for blasting the dirt (and paint) off, but for applying the snow-foam ...

http://youtu.be/RG9djNcu23M

http://youtu.be/RG9djNcu23M

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On 01/04/2012 17:41, Andrew Gabriel wrote:

Urban myth.
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