4000 psi sounds like a lot. That's what some pressure washers can go up to. Just how much pressure is that? What are some real world examples that would help explain that kind of pressure?

  • Standard Scuba-Tank cylinders are around 3,000 PSI... It takes about 1/4 solid aluminum to hold back that much pressure, and they compress about 90 minutes of life-giving air into a small tank on your back. – abelenky Apr 17 '15 at 16:20
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    4000 psi is enough to cut your arm off with water. That's pretty much it. – Cole Johnson Apr 17 '15 at 17:44
  • Paint stripping comes to mind... and it does it pretty effectively. Use it on your car with the right nozzle held too close and you'll abrade the paint. – Fiasco Labs Apr 18 '15 at 2:31

4000 PSI is too much or a shitload of pressure. I would never use it on high on anything that I was cleaning at a home. Even brick work could get damaged of up close 4000 PSI. Probably the only thing I would use it for is cleaning my driveway but even then I would probably use a fan nozzle. 4000 PSI would just rip the paint off a home - one with a really good paint job.

If you want to really clean instead of blasting the crap out of something the key isn't just PSI, it is also a factor of the waterflow rate. This is measured on most powerwashers as GPM or gallons per minute. You multiply GPM and the PSI to get the cleaning units (CUs) of your powerwasher. This is the most important factor for cleaning, not PSI. A lot of the big box stores sell their cheaper powerwashers with high PSI because that is what the consumer is used to looking for. These high PSI, low GPM units will not clean as well and are more likely to damage the surface you're cleaning.

Note: The chart below references usage based on staying an appropriate distance for the material and PSI. Obviously if you just look at the first column and you spray your car with a 2500 PSI powerwasher from point blank, get ready for a new pain job.

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Things I have used a powerwasher for:

  • decks (be careful with composite)
  • sidewalks
  • driveways
  • non-painted fences
  • gutters
  • garage floors
  • houses wood (it isn't my first choice or even second. Also if I am painting I need 4-5 days of summer after powerwashing)
  • brick (but very risky and must take your time and stand back and properly presoap. You can cut mortar loose)
  • my trailers
  • greasy tools (non-electric all metal)

Things I try not to powerwash:

  • houses (vinyl no way, brick is a risk, even wood I will stand back a bit)
  • cars
  • windows (you will destroy the caulking)
  • roofs
  • painted fences
  • anything inside
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    This is a great answer -- it answers the real question that seems was asked -- what 4000psi means in the context of a pressure washer. Knowing how 4000 psi compares to a scuba tank or the bottom of the ocean doesn't really help determine whether or not that pressure is suitable for pressure washing a deck. – Johnny Apr 17 '15 at 18:04
  • "Even brick work could get damaged of up close 4000 PSI." Followed by a chart showing 4000PSI as entirely suitable for brickwork, twice. Decks too. The only thing said chart says 4000 PSI is not suitable for is automobile and trailer/boat washing...a chart you actually agree with might be more persuasive here. – Ecnerwal Apr 17 '15 at 18:45
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    @Ecnerwal - I said up close. Have you used a 4000 PSI power washer up close or within 3-4 feet on anything? I have one and I have to warn my guys every time. This chart is assuming that you will stand the appropriate distance for the PSI and materials being washed. I could wash a car at 4000 PSI if I stood 25 feet away. How productive this is compared to a regular hose/sprayer is debatable. – DMoore Apr 17 '15 at 18:49
  • I just use the hose and a sponge on the car, so I'm not debating that and don't really think it's debatable. I've seen folks waste hours with a PW doing things that a stiff brush and a bucket of bleachwater would have done a much faster job on - but they had the pressure washer and they were bound and determined that it was easier...;^) – Ecnerwal Apr 17 '15 at 18:53
  • It's gonna need a paint job and a lot of screen doors... – Mazura Apr 18 '15 at 21:22

224 x 10^6 Hundredweight per acre.

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    made me chuckle – hometoast Apr 17 '15 at 17:57
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    What the heck - I asked the internet to confirm the figure for a standard elephant in the Elephant•Furlong•Fortnight system for a jocular reply in this vein, and all it will give me is some upstart Firkin•Furlong•Fortnight system - history has been rewritten, our standard Elephant is not merely out-moded, it has become an un-unit, erased from the pages of history as though it never was. Notably the Electronic Frontier Foundation was prominent in results when searching for the EFF unit system. Are THEY behind this dastardly un-deed? – Ecnerwal Apr 17 '15 at 18:28
  • US or international acre? – DMoore Apr 18 '15 at 16:51

Take a moderately average car, such as a Honda Civic - roughly 3000 lbs (depending on options, year, etc)

Balance it on a US Quarter (0.955 inch diameter, 0.716 square inches)

Or:

Stick to quarters. Per US Mint specs, a quarter (at least a new, unworn one) has a mass of 5.67 grams, and so long as we are on Earth that's equivalent to 0.01250021026674 lbs. So a stack of 299,213 quarters (or $57,303.25 in quarters) would exert 4000 PSI.

Or:

Wikipedia claims that steel ranges in density from 485-503 lbs/cubic foot. Let's call it 495. there are 12x12x12 cubic inches in a cubic foot, so a cubic inch of steel weighs 0.286458333 lbs.

So a 1 inch square solid bar of steel would be 13,963.64 inches tall to exert 4000 PSI (if you could get it to stand on end) - that's a bit more than 0.22 mile tall.

Or:

A 97.6 lb lady wearing high heels that are 5/32" square, balancing on one heel.

  • 1
    Pascals are so not intuitive units, though. And it's just as easy to be off by a factor of 10 or 100 by fudging mm and cm (squared, for the factor of 100) as it is to screw up inches and feet. BT, DT. (responding to a deleted comment that it would have been easier in metric/SI) – Ecnerwal Apr 17 '15 at 19:02
  • Having 1 atmosphere ~= 100 kPa makes Pa rather useful for back of a brain figurings – Russell McMahon Apr 18 '15 at 1:20
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    I've had to repair floors after your 97.6 lb lady did that. – Comintern Apr 18 '15 at 22:12
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    Of course any 0.22 miles tall bar of steel would exert 4000 PSI, no matter what its cross section is – Hagen von Eitzen Apr 18 '15 at 22:48

As mentioned above, if you could fit two tons on a 1" square, that's what it would be.

Since you mentioned pressure washers, I'll throw in a few other observations. Many electric models will do no more than 2000 PSI. Some gas-engine models can exceed 4,000 PSI. Pressures as low as 1000 PSI can still cause damage or personal injury, so be careful. If you're going to use one for cleaning a deck, siding, or something else around your home, make sure your water system can supply the washer. (And be careful with water if you use an electric pressure washer.)

4000 PSI is exactly how much pressure it is. PSI = Pounds per Square Inch. I'm not really sure what kind of example you are looking for. PSI is a general unit that depends on context to determine if a given value is too much or not enough.

Since you ask for examples though:

  • A typical air compressor for powering air tools runs up to 140 PSI.
  • Water pressure in your house is about 40-70 PSI.
  • Hydraulic cylinders in machinery can operate up to 30,000 PSI or more.

Those are examples of fluids exerting pressure on their container.

But PSI is also used to measure compressive strength. For example:

  • "5000 PSI" concrete attains a compressive strength of 1500 PSI after 1 day of curing, 2500 PSI after 3 days of curing, 3500 PSI after 1 week of curing, and 5000 PSI after 28 days of curing.
  • Dry maple wood can attain up to 56,000 kPa (8122 PSI) of compressive strength parallel to the grain.
  • about 60000-100000 psi in a waterjet cutter to cut through steel. a direct injection engine squirts fuel at about 2000psi into the cylinder to atomise the fuel. I do know at that pressure (with the size of aperture) the jet will blow through your hand. – Hightower Apr 17 '15 at 7:43
  • "up to 30,000 PSI or more" – KSFT Apr 21 '15 at 12:42

Additional examples that might make it more intuitive to understand from a scuba diving perspective.

Air pressure is commonly measured in bar. Air pressure (at sea level) is about 14.7 PSI or 1 bar. So the pressure exerted by all the air around you is 14.7 pounds per square inch.

As a general rule of thumb the pressure in bars increases by 1 for every 10m that you go under water. So at 10m the pressure is 2 bar, 20 is 3 bar etc.

So to illustrate your question on 4000psi. That would be equivalent to the pressure 2,700m deep in the ocean! So yes that is quite a lot of pressure

Another practical example a typical high pressure home water heater is rated at 6 bar or 88psi or 50 meters of ocean depth. your example of 4000 is 45x greater than water pressure from your high pressure water heater.

Wolfram Alpha says 4000 psi is:

  • ~~ 0.33 × pressure at the bottom of the Mariana Trench (~~ 8338 dbar )
  • ~~ 0.5 × pressure at the bottom of the Pacific Ocean (depth 5.5 km) (~~ 6×10^7 Pa )
  • ~~ pressure of a typical aluminum scuba tank (~~ 20 MPa )
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    Nope. Unless you are overfilling them, typical aluminum tanks are 3000 PSI, 3300 for a few, and much above that is HP steel tank territory. Above water firefighter SCBA may go as high as 4500 with fiber-wrapped tanks, and all sorts of people who ought to know better exceed the rated pressures...but I've never seen 4000 PSI-rated Aluminum tank (I also don't think "laymen", or likely half of scuba divers actually grasp what scuba tank pressures are...) – Ecnerwal Apr 17 '15 at 18:09

I know the question was in regards to pressure washers, but to help visualize the amount of force involved from a human perspective, let's just model the human body roughly as a 2 foot by 6 foot rectangle laying on the ground. That puts your surface area at 24in x 72in = 1728 square inches. If the atmosphere were pushing down on you with 4000 pounds on every one of those 1728 square inches, it would be like a 6,912,000 pound elephant sitting on top of you...

But really, you don't have a feel for the situation until you account for the surface to which the pressure is being applied. For instance, the components of the pressure washer which are exposed to the 4000 psi have a much smaller surface area and thus do not experience near the kind of force in my example above.

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