mechanical fill level sensors?

I lack mech engr experience and would be grateful for answers in layman-ese :-)
Let's say we have an *unpressurized* open vessel with a capacity of
approximately 250ML, filling with water from a pump that gets water from a reservoir. The water in the vessel is heated to boiling temperature by a heating element.
Are there mechanical devices exist that could detect when such a small vessel was full and then do something that would cause the pump to stop pumping?
The device(s) would have to be robust--not wear out with daily use for years, and be able to withstand the bumps things get during shipping and daily use in a home. Not a lab setting.
Are there any of these devices that could also detect that the vessel was down to "n" ML of water and then do something that would cause the fill pump to begin pumping? Or would this be a separate device?
Thanks Liam
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> Let's say we have an *unpressurized* open vessel with a capacity of

Yes. Probably the easiest is a float switch, but you could also do it with a pressure switch, an ultrasonic level sensor, a radar tube, etc. Check out www.omega.com and look in the "Flow & Level" handbook for ideas.

If your set points are always the same (full and "n") then you could use two binary sensors, like float switches. If you want continuos readout, you'd need something variable like an ultrasonic, pressure, or radar...radar probably wouldn't work in a vessel this small though. The last time I used it it was on a ~500 gallon tub.
Tom.
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Thanks for the suggestion of omega.com and Flow and Level section, Tom. I'm off to a good start. Liam
Tom Sanderson wrote:

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There are definitely multiple choices for your application. 250 ml is a relatively small volume, but can still be monitored with float valves. Some of the other sensors might be more appropriate depending upon the available space. I would start by looking at the optical sensors because they require less space and are highly reliable. I know Honeywell makes some, but there are multiple manufacturers of such items.
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Dear Liam:
wrote:

I concur with "Smitty Two". The application cries for a pair of "capacitive proximity sensors", if the "vessel" were non-conductive. Better still, a light beam could be used to probe the presence or absence of liquid, if the "vessel" were optically clear (this option can be really cheap, and extremely precise).
David A. Smith
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David, I am not sure how to reply to the underlying question, is the vessel non-conductive? Do you mean a heat conductor or an electricity conductor? The vessel is made of stainless steel or brass (something that can retain heat) and it holds water. There's a heating element on the outside rather than inside of the vessel, to avoid calcium and mineral buildup. 'Cheap' and 'precise' are excellent features. Are these devices fragile in any way? Can they take the heat? Liam
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Dear Liam:

Electricity conductor. Capacitive proximity sensors cannot "see" through a conductor. http://www.dwyer-inst.com/htdocs/pdffiles/cat/level/ps_cat.pdf ... says 212 oF

It'll still buildup, just not on the element. The calcium (etc.) doesn't vaporize, so it can't go anywhere else.

Fragile, no. Take heat... maybe not. That can be overcome by having a second "chamber" in parallel, acting as a sight glass. I have used a fitting and teflon (or any other kind of) tubing to remote the fluid level from the hot area. Any clear tubing can be used for a cheap optical sensor or two.
If Danaher Controls hasn't trashed their engineering department (like they have done to so many other aquisitions), Gems had some good sensors... http://www.gemssensors.com/TOCproducts.asp
http://www.xs4all.nl/~bernard/optosens.html
http://www.dwyer-inst.com/htdocs/pdffiles/cat/level/ols_cat.pdf ... 200 oF
I've seen a sensor that had a body that was designed to straddle a clear tube, with an LED on one side, and a detecor on the other. All you'd need to do is strap it on... no penetrations required. Can't remember who it was that made it... want to say SMC, but that is probably not right. I'd check out offerings by Banner Controls, if optical seems the way to go.
David A. Smith
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Your own experience will bring to mind at least one device that serves that purpose on a bigger scale, and with cold water: the water cistern hooked to your sit down lavatory. This consists of a float that stops the water flowing through a valve at a given level. Make it smaller, and heat resistant.
Brian Whatcott
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Yes. They're called level-sensors. :-)

It can be even simpler (i.e. no valve) given that all it has to do is to do is to switch a pump off and on at pre-determined levels. Obviously, a taller vessel will provide more accuracy... and the boiling liquid may cause some perturbation in the level resulting in phantom switching if the levels between off and on are too close. A side-tube (copper) with a magnetic float would solve most of the problems with the action of boiling.
A third magnetic switch can be placed at the level of the heating elements in case the pump fails to deliver.
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Dear Bernd Felsche:


...
I wonder if the "vessel" isn't insulated, if he couldn't just measure the temperature at the places he wants to do switching at...
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

To retain heat, the vessel would be made of brass or stainless steel with a heating element beneath it. Would the heat radiate to all portions of the vessel surface? Regards Liam
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Dear Liam:

The metal doesn't "retain heat", rather it conducts it well.

Only if you insulate the walls of the vessel, or make the vessel very thick-walled.
Are you boiling the water, or simply heating it?
Is there a microprocessor in this device for another purpose? I ask, because a fixed heater increases temperature on a small volume of water much faster than it does a large volume of water. Depending on a few things, you might get a single temperature probe to do everything.
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

A single temp probe to do everything is attractive. But here are the constraints which seem to me to work against that approach, and they might also work against the magnetic switch/float approach too:
Vertical space is very limited. The vessel dispenses its water by gravity and the device which receives water from the vessel has a pre-defined location. And so, given those space constraints, the water-heating vessel has only about 3cm height. Therefore I was thinking 10cm x 10cm x 3cm would be its dimensions since the minimum volume the vessel can contain is ~250ml.
Water is being heated only to ~98C not to a rolling boil. But it must reach this temperature very quickly. ~1000W heating element is projected, more or less.
The thickness of the vessel walls is ~11mm. An overarching design goal is temperature stability. We need to keep the water at 98C +/- 1C. There would be a thermocouple to monitor water temperature and control the heating element.
Regards Liam
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Dear Liam:

...
Single temp probe level control is out if the vessel is to be maintained at 98 oC for some time. Evaporation will be quite hgih.
This small height will prevent insertion of the temperature probe in any useful location.
I'd recommend you tap off to a separate open-top tube, and sense level on/in the tube. Any sensor will be fried by the 1000 watts, should the water supply fail for any reason.
There are hot water heaters that get installed directly to the cold water tap of a shower. No hot water tank required. Other than low dispensed volume, and low flow rate, could this be made to work for you?
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

I have heard of that type of heater. But shower temperatures are much lower. We could eliminate the heating tank if a flash-heater is able to heat a much smaller amount of water, say 50ml, to 98C in ~5 seconds. Is that feasible? The entire 50ml would have to reach 98C, not the latter portion of it. The resulting 'dose' of water would have to be evenly heated to 98C. Do these little heaters have their own self-contained heating vessel?
How would the pump be told to cut off if such a flash-heater were positioned between the fresh-water reservoir-pump and the device which receives water from the now-expendable heating vessel? Or would the pump be set to move a defined volume and then cut off automatically?
How does the flash-heater 'know' when to turn on?
Regards Liam
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Dear Liam:

That is simply a function of "thermostat" setting, and heater size / flow rate.

Yes. You could have the heated delivery tube be at ~120 oC at the beginning, and 90 oC at the end.

Yes. Just imagine your "vessel" stretched long and thin (into a tube), with heater wire wrapped around.

The pump and heater are triggered on when water is requested.

It is your product...

On all the time, on only when the pump is running... you choose.
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

David, This is also an attractive option. Let's assume we have a vertical flash-heating tube and beneath it the piston chamber, with the two tubes separated by a one-way valve; the valve could be as simple as a flexible washer screwed into the ceiling of the piston chamber so that it seals a ring of eight 3cm portholes which connect to the boiling tube; that's exactly how one of these vintage machines works, only with a kettle above the piston-chamber instead of a flash-heating tube:
| H20| | H20| | H20| heat | H20| | | | | ------ valve |....| |PSTN| |PSTN| piston |PSTN| chamber
Downward motion of the piston pulls the edges of the flexible washer down into the piston chamber, "unplugging" the port holes, and water is drawn down from the unpressurized vessel above, by suction and gravity, into the piston chamber.
The draw is never more than ~2.5 ounces at a time. That's the maximum volume of the piston chamber available for water when the piston retracts completely.
The draw from the retracting piston is instantaneous, so the water closest to the valve would not have a chance to heat up before it gets drawn down into the piston chamber. The water must therefore be at temperature when it reaches the valve. And the water must be kept at temperature once it reaches temperature. The flash-heater must have two states, super-heat to bring cold water to temperature quickly and low-heat to keep hot water hot.
How hot would the heating element have to get to bring 2.5 ounces of water in copper tube to 98c in ~10 seconds?
If 2.5 ounces could be brought to and kept at 98C by this heat-tube device, the device would solve the problem of how to get water from the reservoir into the kettle and from the kettle into the piston chamber. The fill pump would no longer be necessary. The fill sensors would no longer be necessary. It would be a continuous draw system with the water being heated while in transit, and all we would need is a temperature sensor and a feedback mechanism to control the two basic ON-states of the heating-element.
Regards Liam
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Dear Liam:

Why so big? Did you mean 3mm?

...
Why not a full "shot"? 250 ml is closer to 8 ounces...
...

Significantly hotter than 98 oC. This would be hard on the piston seals also, going from unheated tube, to heated tube, to a tube that is heated by the water (and expanding).

It wouldn't have to be required, even with a classical kettle.

Chew it around a while and see if you like the flavor. Lots of different ways to skin a cat (so to speak).
David A. Smith
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N:dlzc D:aol T:com (dlzc) wrote:

Yes, 3cm was a typo. The diameter of the circular arrangement of holes is itself only 35mm, and the individual holes are ~3mm.

A single espresso is only about 1 to 1.5 oz. A double 2+ oz.
"A double espresso is a 47-62.5 mL (1.5-2 ounce) extract that is prepared from 14-17 grams of coffee through which purified water of 88-95C has been forced at 9-10 atmospheres of pressure for a brew time of 22-28 seconds. The espresso should drip out of the porta-filter like warm honey, have a deep reddish-brown color, and a crema that makes up 10-30% of the beverage." http://www.coffeeresearch.org/espresso/definitions.htm
********************************************************************** With the flash-heater, we don't really need to heat more than a single shot (1-1.5oz) at a time to 98c. How quickly can that be done with a 1000W or 1250W heating tube? **********************************************************************
I was talking 250ml when considering the heating kettle approach. With the kettle, we'd need to have water enough on hand to make several doubles in a row without a downtime for recovery, plus some extra for a preliminary flush to bring the brass group up to temperature. You don't want to brew when the group is cold, otherwise the temperature of the water drops precipitously and the coffee is underextracted and tastes really sour. The brew water starts at 98C and loses a few degrees as it enters the piston chamber and flows to the brass group and moves down to the coffee. Temperature of the water as it reaches the ground coffee should be 88-95C. I prefer to the higher end of that range, 92-94C, though perhaps the user could be allowed to set the target temperature, if it wouldn't complicate the machine too much.

We could have this (rotated horizontally):
-------------------..........::O:::::::::::::::::: H20 H20 ---> H20 | H20 PISTON SPRING -------------------..........:::::::::::::::::::::
Key: ----- is the flash-heat tube ..... is unheated copper tubing | is the seal ::::: is the piston chamber O is the porthole to the group
The seal between the piston chamber and the unheated copper tube would be some distance away from the heating element. Shouldn't an EPDM seal be able to withstand the ambient temperatures if the seal is some centimeters distance away from the heating element? Can the flash device be insulated?

I don't understand how we'd get water from the large (e.g. 2 liter) reservoir into the 250ml heating kettle without some kind of pump. Please elaborate.
Regards Liam
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Dear Liam:
...

What is the water source? Isn't it some sort of pressurized, filtered water supply? Or are you doing a free-standing counter-top model, filled by jugs?
Keep in mind that a standard percolating coffee pot does exactly what I describe, moving a whole lot of water from a low location to a higher location at high temperature, by "flashing" small quantity of water to steam. http://en.wikipedia.org/wiki/Percolator
David A. Smith
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