How does the washing machine know when the proper water level is reached in the washing tub?
I’m curious because I want to know if I should put my clothes in while it is filling or if I am fooling the washer into thinking it has more water in it than it actually has.
You’d think after 28 years I’d know how to do laundry. 
I had a big argument over this with a girlfriend some years ago because i wait for the tub to fill before I put the clothes in and she puts them in first.
There is no “correct” answer. Water level is water level and that’s what stops the water flow. A bit more or less water is going to make no difference.
According to Mr. S, some washers are on a timer. This caused problems with the washer his family used to use in the house we’re living in now, because the water pressure was low and the timer would shut off the water before the level was adequate.
Of course, he could be full of caca. It’s happened before.
I vote for caca. The timer idea doesn’t make a lot of sense, because the timer would have to “know” how much laundry you put in the washer and how fast the water was flowing in.
FWIW, a washing machine repair guy once told me that you should always put the clothes in before turning on the water, to prevent overflowing.
Hmmm, maybe it’s not so simple. 
In home ec, I was taught to put in detergent, allow the washer to agitate to dissolve the detergent, THEN put the (pre-treated) clothes in. Now, I put in the detergent as the water is filling, then put the clothes in the washer after the detergent has dissolved, but as the washer is still filling.
Incidentally, when setting a water level, remember that you can’t get clean clothes from filthy water. In other words, don’t be too stingy with the water, or you’ll have to rewash the clothes, thus defeating any savings in water, energy, or money.
Can’t vouch for every single washer out there, but I know (cause I took one apart) that some rely on a rubber diaphramed pressure switch on the bottom of the outside wash basin. They know the basin is full by the weight of the water+clothes inside the machine.
My reasoning was the same as Lynn’s. Great minds think alike.
While this really makes little difference one way or the other, what I do is dissolve the detergent well, then put the clothes in, let it agitate just a few seconds and then stop the washer and let the clothes soak for a while. I want to make sure the detergent is well dissolved at this point. Then I restart the washer and let it finish.
The idea that if you put the clothes in later the washer may overflow I think is not warranted. The clothes probably only raise water lever by a negligible amount.
Also the timer thing seems quite difficult to believe as you would need a fixed water pressure or the washer would not fill or would overflow. I do not think any washer would use this system.
Lynn, you might want to rethink that idea.
Low-water usage machines are actually quite good.
Well, in Lynn’s defense, “more water is always better” isn’t quite what she said. She said that too little water is bad, which is true by definition (otherwise it wouldn’t be “too little”).
Lets throw another wrench in this.
I have a front loader. Lets say I am washing a big load of towels or jeans. It fills with water and starts washing. After a few moments of tumbling and the water gets sucked up by the said clothing/towels, the machine knows to add more. How in the hell does it know to do that?
I love watching that machine. It has a window, you know…
Sheesh, you people. I’d like to introduce ya’ll to a nifty website called “How Stuff Works.”
And most manufacturers recommend that you put the clothes in first.
http://www.ebuyguru.com/BuyingGuide.asp?categoryID=25&articleID=97
The detergent is on the bottom, so it has first contact with the water and can start to dissolve. By the time the tub is full, it will be completely dissolved. But you shouldn’t wait until the tub is full to put the clothes in, and here’s why.
Also because…
From http://www.maytag.com , the Ideas & Advice article “Fabric Damage”, which would not make a link.
To the OP, you’d think there would be a flow-meter, similar to fuel pumps, that can guage the volume of water going by and adjust accordingly (small, med. or large loads.) But I see DDG posted a description that would seem to work, too. So, I guess that’s that.
To my hijack: I wanted to say something about the “new” front-loaders that are coming out. IIRC, they use something like 1/3 the “normal” amount of water pre wash. I say “new” because, also IIRC, they’ve been standard use in Europe for some time.
Uh, that was “…they use something like 1/3 the “normal” amount of water per wash.”