how does this valve work?

^^^^^^^^^^^^^^^^^^ I don't think your dentist would know about it in any case. I don;'t think they study plumbing in dental school. Here is one way you could do it: One switch is in the hanger for the bucket, and it turns the water on when the weight of a bucket is added. The second, in series with it, is below the bucket, and turns the water off when the weight of the bucket indicates it is full. The first switch is normally open--the second is normally closed.

You are simply looking at a toilet valve upside down. The force of the float is replaced with the weight of the cup. Randy
Years ago, a standard part of the peripheral stuff around a dentist's chair was a small round platform with a water spout over it. On it was a paper or plastic cup in a small metal holder (to give it some weight). When the cup was full, it just sat there waiting. Pick it up and there was a small squirt of water from the spout into the cup as an unseen spring pushed the platform up. When you put the empty cup back on the platform, it sank down a litle because of the weight of the metal holder and the water started. As the cup filled, the platform moved down a little more and as the water level neared the top, the water went off.
How'd they do that? It's an off-on-off valve, where "on" corresponds to platform in the middle, neither empty nor full, but there's a visible snap action to it -- the water flow starts up, doesn't change as the cup fills, then suddenly stops. The snap action is the hard part and I haven't figured it out.
I'd like to build something that works the same way, holding a gallon bucket on a hook. Hang the bucket on the hook and the water starts and fills the bucket, automatically shutting off when it's full.
I've tried searching the USPTO database but haven't had any luck, possibly because I don't know the right terms to search for. When I asked my dentist about it, he said he hadn't seen one like that for years.
Help!
Tove

I am thinking that the valve must have been shaped like a spool valve with more than one face seating against the valve seat according to position. As a kid I remember that as you lifted the cup off the location it would give a spurt and then stop if you lifted slowly. The table supporting the cup was definitely supported by springs. Randy
You are simply looking at a toilet valve upside down. The force of the float is replaced with the weight of the cup. Randy
Years ago, a standard part of the peripheral stuff around a dentist's chair was a small round platform with a water spout over it. On it was a paper or plastic cup in a small metal holder (to give it some weight). When the cup was full, it just sat there waiting. Pick it up and there was a small squirt of water from the spout into the cup as an unseen spring pushed the platform up. When you put the empty cup back on the platform, it sank down a litle because of the weight of the metal holder and the water started. As the cup filled, the platform moved down a little more and as the water level neared the top, the water went off.
How'd they do that? It's an off-on-off valve, where "on" corresponds to platform in the middle, neither empty nor full, but there's a visible snap action to it -- the water flow starts up, doesn't change as the cup fills, then suddenly stops. The snap action is the hard part and I haven't figured it out.
I'd like to build something that works the same way, holding a gallon bucket on a hook. Hang the bucket on the hook and the water starts and fills the bucket, automatically shutting off when it's full.
I've tried searching the USPTO database but haven't had any luck, possibly because I don't know the right terms to search for. When I asked my dentist about it, he said he hadn't seen one like that for years.
Help!
Tove

That sounds right. A spool with a narrow section in the middle could be made to allow water to flow only when the weight on the table was greater than a small value (less than the weight of the empty cup and holder) but less than the weight of a full cup.
Chris

A spool valve that would actuate with the weight of an empty paper cup seems like a bit of a reach. I wonder if it worked like a little toilet. Weight of empty cup actuates a little "flush" valve, which fills the cup with prescribed volume, with abrupt start and fairly abrupt stop when the reservoir is drained. Removing the cup actuates a little reservoir refill valve, which results in a spurt until the flush valve re-seats.

Or it's an internally piloted valve, like most hydraulic and pneumatic solenoid valves. The weight of the cup actuates a small valve that uses the water pressure to shift a larger valve.
Ned Simmons

The key of course is to engineer hysteresis into the valve. Like a toilet fill valve. If you have a leaky flapper the water dribbles out of the tank slowly over time, punctuated by a few seconds of filling every half hour or so. The level in the tank is best represented by a sawtooth pattern.
I would say that the best way to approximate this is to build a balance, where the weight of a full bucket of water is counterballanced by a spring force. But something's got to snap overcenter to allow the balance to drop, and actuate the filling valve. There will have to be three settings, the spring contant that tells you when the bucket's full, a setting that says start filling when the level gets below a certain level, and a setting that says stop fillling when the level gets above some other level.
If the OP really wanted to build a mechanism to do this, I would suggest against a mechanism, but rather would use an electrically operated solenoid valve, and use electrical sensors to measure the water level in the bucket. By mounting the sensors at different heights inside the bucket (hang them down off the hook that holds the bucket handle) you can engineer in whatever hysteresis you want.
Water level drops below the lower one, valve is signaled on. When it rises above the top on, it is tripped off.
Jim

The OP (and my memory) said that the paper or plastic cup was in a metal cup holder which gave it enough weight to be more easily detected when it was empty.
IIRC those round sinks next t the dental chairs were colloquially called "spit sinks" and usually had a constantly running little stream of water squirting in tangentially to help wash stuff down the drain. It was sort of nasty to see your blood spiraling around inside them for a few seconds after you spit in them.
For those too young to have used one, they looked like this, though I'm not sure this one had an automatic valve:
My dentist was bitching to me a couple of years ago that the state made him spend multi-kilobucks on a waste water filtering system to keep the tiny amounts of old mercury amalgam fillings he might drill out of someone's tooth from polluting the world. That seems a bit far out to me.
Don't let me get started or I'll srart reminiscing about my dentist uncle Leo's foot treadle powered dental drill and those shoe store X-ray machines (outlawed everywhere now) which let you see the bones in your toes and how well they fit inside the shoes you were considering buying.
Thanks for the mammaries...
Jeff

According to Don Foreman : The empty paper cup was fitted into a heavy holder which was necessary to trip the flow. Lift the cup out of the holder and the water starts to flow and to overflow the holder until you put the empty cup back in and it refills. You are *supposed* to lift the cup and holder together.
Enjoy, DoN.

What fun is that? I think the fun is in the ingenuity of a mechanism that requires no electronics or even electricity.

I'm absolutely all ears on this. It would be a fun project.
Jim

A toilet tank float valve would be a possibility with some modification and a spring holding the bucket.
John

Ned mentioned use of a pilot valve. That had occurred to me too. That doesn't explain the spurt when the cup is withdrawn, but I like the concept. It's a feedback control system with loop gain while using no electronics or electricity. The power source is the water department. Hysteresis could be incorporated in the linkage between pilot valve and main valve. The pilot valve could be quite small and easily responsive to the small weight change of a cup of water.

Definitely. I've putzed around with electronics enough that I could build something involving a sensor to measure weight and a solenoid valve, but that doesn't do much for me. It'd work, but it wouldn't be as much fun as something that's nice and shiny with a little brass and copper for contrast.
The hard aspect IMHO is the "snap off" part where the water's flowing full speed and then something slams shut and it's over. It'd involve some sort of a toggle action, and since I have no idea how to design something like that to specs, a lot of trial and error.
Tove

One possible reason for there being no patent is that it's based on prior art--classical Greek temples often used fonts and such that opertated on similar principles to "wow" their followers. Try searching for classical Greek hydraulics...
--Glenn Lyford