# Pressure washers constant pressure

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1. Aug 12, 2015

### Spiralout

My GF and I got into a petty argument about pressure washers. She believes that the constant back-pressure you feel holding a pressure wand increases the closer you hold it to a wall (for example). Where as I believe that if the wand had a constant psi, with outward pressure, wanting to force your arms back while holding it, it would make no difference how close you held it to the wall. I believe that the set pressure coming out would be the only pressure you would feel. She believes the pressure would increase the closer you held the nozzle to a solid surface. Can anyone squash this bug for us?? Sincerely, Irritated boyfriend (who hopes he's not wrong)

2. Aug 12, 2015

### Staff: Mentor

Try googling Ground Effect...

3. Aug 13, 2015

### Spiralout

Thanks for the reply, but i'm not sure if that helps? Couldnt find anything other than aerodynamics of plane wings basically??

Last edited: Aug 13, 2015
4. Aug 13, 2015

### Spiralout

Again im looking for the answer: will the pressure that is forcing my arms back, increase, the closer i hold the pressure washer to a wall. Not, will the wall feel more pressure the closer I hold the pressure washer. Which obviously is yes, it would.

Last edited: Aug 13, 2015
5. Aug 13, 2015

### CWatters

How close is close?

Are pressure washers constant pressure (even when blocked)?

6. Aug 16, 2015

### Spiralout

If you blocked the opening then pressure could build up and blow a hose. You always have to turn off the pressure. Anyway, I guess nobody knows. Maybe I asked it wrong. Whatever.

7. Aug 16, 2015

### Spiralout

Close is say, an inch from the wall

8. Aug 17, 2015

### CWatters

I doubt there is any effect until you are very close to the wall. How close is tricky to say. At what point would the flow of water be affected?I wouldn't expect an effect at 1 inch. At some point water is going to bounce off the wall and start hitting the area surrounding the nozzle. I suspect that might happen before any effect due to the flow of water being blocked or slowed.

9. Aug 17, 2015

### Nidum

You will start to get increased reaction back into jet when annular flow area between jet and wall becomes of the same order as jet area .

So for a 10mm dia jet :

Area = 10 * 10 * .0.7854 = 78 mm^2

If h is the stand off distance between jet and wall :

Annulus area = 10 * 3.142 * h = 31 * h

h = 78/31 = 2.5 mm

Exact calculation is a bit more complex but expect some detectable reaction from h = 10mm . Reaction will increase as h decreases and there should be a marked increase in reaction for h = 2,5 mm and values down to zero .