Hand Pump Operation
Country: United States
Date: March 2006
Hi, I am a Housewife.
I usually use the Big Tide from Costco which has a soap dispenser.
When it is new the soap dispenser works well. As time passes the
soap dispenser dispenses only a small amount of soap even if we press hardly.
So I got irritated and opened the regular lid and got the soap.The
next day instead of using the regular lid, I used the soap dispenser and
it worked properly.
I know that the compressed air inside was released when I opened
the regular lid and that is why the soap is dispensing properly.
I want to know the science behind it.
Most likely what is happening is that dispensing soap lowers the pressure inside
the soap container, and the container is well enough sealed that the pressure
takes a long time to equalize. Eventually, the pressure inside gets low enough
that the dispenser cannot pump very well against the pressure difference.
I would guess the problem will go away if you loosen the lid or poke
a tiny hole in
I must first apologize as I am not sure of the configuration of the "Big
Tide" bottle. However, I am going to assume that it is similar to other
large laundry detergent containers in that you place it on its side and
you press a rubbery button on the spout which releases the detergent.
I believe what you seeing with your bottle can be described as follows:
These bottles typically do not contain compressed air. Gravity is what
causes the detergent to flow out of the bottle. Initially, there is a
small pocket of air at the top of the bottle since the bottle is not
totally filled at the factory. If you start dispensing detergent
without opening up the vent on the bottle, this pocket of air will
increase in volume. From the ideal gas law, we know that PV (initial) =
PV (final). Pressure and volume of the gas in the initial state must
equal pressure and volume in the final state. Since volume final is
getting bigger, pressure final must get smaller.
As the air pressure inside the bottle becomes less than the air pressure
outside the bottle (vacuum), the pressure outside the bottle, working
through the spout starts slowing down the amount of detergent coming
out. The longer you dispense, the slower and slower it flows.
When you opened the bottle up you allowed more air into the bottle
restoring the pressure to its original state. When you went back to use
the dispenser again, it worked fine.
Most of these bottles have a small cap on the top of the bottle when it
is setting on its side. If you loosen this cap, the soap should always
When you start to empty the liquid soap, you create a vacuum in the
space above the liquid soap. The atmosphere "pushes" the soap back
into the container when you open the dispenser plunger. When you
open the vent (that is the lid) every so often, you equalize the
pressure inside the soap dispenser and the atmospheric pressure, so
the soap dispenser works properly, until you create another lower
pressure inside the space above the soap level. The cure is as you
have discovered -- open the cap every so often-- and the soap flows
When the soap comes out of the dispenser, it creates a vacuum inside
the container since there is nothing to replace the soap. As more
soap is taken out, the bigger the vacuum becomes. Eventually, there
will be so much of a vacuum that it will prevent any soap from coming
out at all. When you opened the container, you relieved the vacuum by
replacing the volume of the soap you had removed with air. If you
open the cap opposite to the dispenser side a little as you use the
soap, you will let air in to replace the amount you dispense and you
should not have the slow-down problem. Just make sure that some air
can come into the container.
Hope this helps.
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Update: June 2012