# Pressure washers constant pressure

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)

## Answers and Replies

berkeman
Mentor
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)

Try googling Ground Effect...

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:
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:
CWatters
Science Advisor
Homework Helper
Gold Member
How close is close?

Are pressure washers constant pressure (even when blocked)?

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.

How close is close?

Are pressure washers constant pressure (even when blocked)?
Close is say, an inch from the wall

CWatters
Science Advisor
Homework Helper
Gold Member
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.

Nidum
Science Advisor
Gold Member
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 .