Dishwashers & water pressure

When dishwashers turn on their incoming water supply, is the length of time usually determined by a timer, or do some or all machines actually measure how much water has entered? I'm asking because a friend's house is on well water. Based on my observations, the variation in pressure points to something wrong with her pump or pressure tank (not sure - I'm no authority on this). She thinks dishwashers should be able to compensate, and I think she should address the pressure question first, or concurrently with the dishwashwer purchase.
There's no money riding (yet) on who's right. Just curiosity so far.
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JoeSpareBedroom wrote:

There's a float switch in the bottom. At least ever one I've looked at. Lower incoming water flow should just mean that the unit will take longer to fill.
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Rick Brandt wrote:

That;s what I would think too. However, few days ago that question came up and a poster from an appliance repair website responded that in fact time is used to measure the water flow. He said the float was there just as a safeguard in case of overflowing.
Seems very strange to me, as since the float is already there and can directly measure what it is you are trying to do, if any reasonable person was designing these things, you would think the level would be the primary cut off, with time as the backup.
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snipped-for-privacy@optonline.net wrote:

If there are washers that work that way then they are poorly designed. With that design the wash cycle would still come on if the water source were turned off completely. I would guess that running the pump and heater with zero water in the unit would quickly cause damage (to something).
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I suppose the float and timer could serve as redundant safety devices for one another, if the machine was programmed that way.
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all my 4 dishwashers over the years first fill with water and use a float switch. see: http://home.howstuffworks.com/dishwasher.htm
JoeSpareBedroom wrote:

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My experience is limited to KitchenAid and they had the timer give the water solenoid so many seconds of power. There is a float switch which will interrupt power to the water solenoid if a certain level is reached but I never bothered to investigate whether this is normally attained or just a safety stop.
This differs from, say, a washing machine where the timer actually comes to a stop when the fill happens and does not resume until the selected level is reached. So on a dishwasher if water pressure is low or zero or you have some obstruction things will proceed with inadequate water in the tub.
I'm speaking partly in past tense because ours is 12 years old and has a electro-mechanical timer so things may have changed. Some years back we had a situation where it was intermittently filling incompletely. Tech came out and replaced the timer. It didn't cure it. (I don't fault him-- we may have given him some misleading symptomology based on what we thought at the time.) I finally was able to catch it in the act--timer sending power to the solenoid but no water flow) and found that the solenoid was electrically open (dead) sometimes when it got hot. Fortunately I'd saved the solenoid off the previous KitchenAid and with slight alterations in the way the tubing and hose hooked up was able to fix it at minimal further expense.
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In typed:

It depends. Usually the minimum flow rate or pressure is given in the manuals. IIRC at below about 30 ppsi there may not be enough water brought into the machine for the washing cycle. As long as there is -some- water, it isn't likely to result in much more than improperly cleaned dishes and some noisy cycles if it draws air into the washing pump. Anything in excess of 40 pounds should be plenty if the flow is unrestricted by pipe corrosion, etc..
A good way to check, AFTER it's installed is the float. Usually there is an overflow float near the front, close to the door on one side or the other, which is supposed to turn off the water if it gets too full, before it overflows. Open the door during the wash cycle and see if the level's near the float limit. If not, it's probbly not washing at top efficiency. Ours lifts about an extra 1/8" before you feel it trigger the shutoff so ours is pretty close. More t han a quarter inch or so and the water pressure's lower than it shoujld be to work well according to some things I've seen on appliance repair sites. Also, if you're not getting enough water and the machine doesn't also heat the water (only some models do that), the wash water may not get hot enough to work well. It's supposed to get to 120 degrees with 140 being the "normal" wash temp settings.
All that said, it's really more important to go after the water pressure if there's a problem with flow rate or pressure, either one. They might need the money for the dishwasher in order to get a good shower soon enough.
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