Alternative to Float Switches - The Planted Tank Forum
 
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post #1 of 14 (permalink) Old 09-12-2007, 11:54 PM Thread Starter
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Alternative to Float Switches

I'm trying to come up with a viable alternative to a float sensor as a water level sensor in an automated tank. I've designed everything to be under tank thus far and don't want a plastic switch in the tank.

I've been around and have found these as resources. I'm not good enough with electronics to start this project all on my own but if you've any input that'd be great. I'm thinking about capacitance based sensors as described here:

"Capacitance Level Sensors (also called RF)
Capacitance level sensors excel in sensing the presence of a wide variety of solids, aqueous and organic liquids, and slurries. The technique is frequently referred to as RF for the radio frequency signals applied to the capacitance circuit. The sensors can be designed to sense material with dielectric constants as low as 1.1 (coke and fly ash) and as high as 88 (water) or more. Sludges and slurries such as dehydrated cake and sewage slurry (dielectric constant  50) and liquid chemicals such as quicklime (dielectric constant  90) can also be sensed. Dual-probe capacitance level sensors can also be used to sense the interface between two immiscible liquids with substantially different dielectric constants, providing a solid state alternative to the aforementioned magnetic float switch for the “oil-water interface” application.

Since capacitance level sensors are electronic devices, phase modulation and the use of higher frequencies makes the sensor suitable for applications in which dielectric constants are similar. The sensor contains no moving parts, is rugged, simple to use, easy to clean, and can be designed for high temperature and pressure applications. A danger exists from build up and discharge of a high-voltage static charge that results from the rubbing and movement of low dielectric materials, but this danger can be eliminated with proper design and grounding.

Appropriate choice of probe materials reduces or eliminates problems caused by abrasion and corrosion. Point level sensing of adhesives and high-viscosity materials such as oil and grease can result in the build up of material on the probe; however, this can be minimized by using a self-tuning sensor. For liquids prone to foaming and applications prone to splashing or turbulence, capacitance level sensors can be designed with splashguards or stilling wells, among other devices.

A significant limitation for capacitance probes is in tall bins used for storing bulk solids. The requirement for a conductive probe that extends to the bottom of the measured range is problematic. Long conductive cable probes (20 to 50 meters long) suspended into the bin or silo, or subject to tremendous mechanical tension due to the weight of the bulk powder in the silo and the friction applied to the cable. Such installations will frequently result in a cable breakage."

from http://en.wikipedia.org/wiki/Level_sensor

and have been meaning to get a post onto this board:

http://tech.groups.yahoo.com/group/sensorforum/
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post #2 of 14 (permalink) Old 09-13-2007, 05:21 PM
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The only successful one I've heard of is Scolley's below-the-tank pressure sensor. I'd heard of capacitive sensors but never played around with them. I imagine they'd be tricky to make work from outside the tank, since the thick glass, and glass's dielectric constant, would mean the water would have only a slight effect on the measured capacitance.

One alternative with a float switch is to use one with a low-liquid level float. This type of float has a low specific gravity, so only a little of it sticks down below the water surface, and is thus visible. 'doesn't help if you want to go with the open-top tank look, though.
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post #3 of 14 (permalink) Old 09-13-2007, 10:04 PM Thread Starter
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I'm not going open tank so that's not the issue. My thought was to mount a capaciatance sensor in the hood to read water proximity. I'd think the fluorescent lights may do something to accuracy but I'm no electronics expert.
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post #4 of 14 (permalink) Old 09-16-2007, 12:29 PM
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We use capacitance level sensors on the aircraft to judge fuel level. (I'm a fuel systems technician) The way the probe work is as follows:

The probe has to stand nearly vertical but not completely (about 80 degrees or so) and go from not submerged to fully submerged to judge fuel level. In other words you've have to stick the entire probe in your tank.

The probes are constricted of an inner tube and an outer tube evenly spaced the entire length of the probe. As fuel level rises, a larger portion of both tubes is covered causing current transmission between the two. IE 1 inch of fuel covering both tubes will allow X amount of energy to be transferred from inner to outer. 2 inches will cause twice that energy. 10 inches will cause 10X the energy. Etc. I'm not sure on the exact energy transfer but our probes go from reading 0.00 pF to over 250.00 pF. But of course the bigs ones are over 5 feet long. The current transfer is low enough that it will not cause any sort of combustion with JP-8 jet fuel. I'm not sure on how it would effect living organisms. Especially the micro bacteria living in the water.

The probes use a sophisticated computer system to detect the pF of each probe and uses calculations to judge the approximate fuel level. We have 124 probes on the jet so our system is quite complex. Remember I said approximate. If the probe gets contaminated in any way (IE any scum or algae in your tank) it will fail to read correctly. We had a lot of problems with the jet's "venting fuel" in other words, becoming over filled and spilling over due to dirty probes. To solve this, we installed a HIGH LEVEL FLOAT SWITCH.

In my opinion, your best bet would be to stick with the switch and avoid all the complicated capacitance probes (unless you just want to be technical) since we're not worried about the water level in our tanks as long as they don't over fill. I apologize for the long definition but I believe I described them well enough to allow you to make a decision. (Plus I feel good now that I have contributed to the knowledge of the site )

John
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post #5 of 14 (permalink) Old 09-17-2007, 05:54 PM Thread Starter
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Indeed you have, thanks for the insight, I guess from that story that capacitance isn't going to solve the absolute level issue.
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post #6 of 14 (permalink) Old 09-17-2007, 07:48 PM
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There are ultrasonic fluid level sensors that can be mounted above the water. They estimate fluid levels by measuring the time it takes for a sound wave to return to the sensor. There are several different varieties, but one can be found here:

http://www.globalspec.com/FeaturedPr...Sensor/22714/0

Figuring out how to make it talk with the rest of your system is your problem.
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post #7 of 14 (permalink) Old 09-18-2007, 03:58 AM Thread Starter
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The ultrasonic sensors generally speaking seem pretty expensive ($200 to 500). Having said that, somewhere along the line I've seen a moderately priced top-off system that used an ultrasonic sensor so there must be a cheap one somewhere.

The trouble about experimenting with this is that you'd need redundant sensors while developing a solution if only just to be sure that the sensor doesn't die easily in this application.

My other idea is a conductivity sensor that just shorts out if its wet. I'm thinking of something that polls at a given interval, possibly only while the filling solenoid is open/powered. You could have a high level sensor that's always on that would be your safety with the timed/interval version deeper to monitor level. Such a sensor would hardly be visible and should be pretty darn fail safe if only due to simplicity. It would also work quite well with controllers since it would be so simple, either on or off, rather than a distance or digital type signal.
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post #8 of 14 (permalink) Old 09-21-2007, 08:17 PM
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A sump tank under your display tank would maintain the water at a constant level and allow you to hide everything by connecting it to the sump instead of the main tank. The only think that would be visible in the main tank would be the overflow, and depending on the design they can be easily hidden.


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post #9 of 14 (permalink) Old 09-22-2007, 03:54 PM
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I don't know the ins-and-outs of the electronics involved, but resistance water level sensors are frequently used in Leibert computer room A/C units. There are 3 conductors to a probe, all different lengths, that are submerged in the water. The longest probe is supplied with 24 VAC, and the other two shorter ones go into the humidification control. As the circuit between the lowest & highest conductor is made the "high water level" alarm sounds. When the circuit between the lowest and middle conductor is lost, the controller opens the reservoir refill solenoid valve.

On second thought, I don't know how good this would be for an aquarium as the potential for "stray voltage" into the water column is pretty high. Oh, well, just a passing idea...

Tommy

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post #10 of 14 (permalink) Old 09-24-2007, 11:48 PM
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Quote:
Originally Posted by guaiac_boy View Post
There are ultrasonic fluid level sensors that can be mounted above the water. They estimate fluid levels by measuring the time it takes for a sound wave to return to the sensor. There are several different varieties, but one can be found here:

http://www.globalspec.com/FeaturedPr...Sensor/22714/0

Figuring out how to make it talk with the rest of your system is your problem.
Ooo, that's nifty. I never knew they made such a thing. One question, do you think the ultrasonic signal would screw with the fish in the tank? I know mine act up any time I make an "odd" noise in the house, such as the vacuum.

Any way you look at it, I believe you're going to spend a bunch of $$$ and end up with something that you'll never get to work as you want it to work. It all seems super complicated. I do like the previously mentioned idea of a sump system though. You'd be able to hide all your float switches and such in there. Out of site, out of mind.

John
55g long , 48" X 13"
New to planted tanks, Learning as I go.
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post #11 of 14 (permalink) Old 09-30-2007, 03:35 PM
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Quote:
Originally Posted by Flopster843 View Post
It all seems super complicated. I do like the previously mentioned idea of a sump system though. You'd be able to hide all your float switches and such in there. Out of site, out of mind.
My setup with a sump has the water changes completely automated and there are no floatswitches used, just plumbing and overflows. The only electronics for the waterchanger are a solenoid valve plugged into a timer for the water supply. It adds new clean water to the sump for a preset amount of time, and the excess water overflows into a house drain pipe.

In my opinion the more electronics you put into it the more things there are to eventually fail. Even with mine I've had a timer fail, and had the 24Vdc power cord for the solenoid valve fail.

Just remember when you design your system to design it so that if things fail they do it in a safe state.

With mine the worstcase thing that can happen is if the solenoid valve got stuck on so that fresh cold tapwater was entering the tank nonstop. The tank temp would eventually drop far enough to kill the tropical fish, but atleast there would not be a flood that destroyed the house.


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post #12 of 14 (permalink) Old 09-30-2007, 04:41 PM
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Quote:
Originally Posted by Flopster843 View Post
It all seems super complicated. I do like the previously mentioned idea of a sump system though. You'd be able to hide all your float switches and such in there. Out of site, out of mind.
My setup with a sump has the water changes completely automated and there are no floatswitches used, just plumbing and overflows. The only electronics for the waterchanger are a solenoid valve plugged into a timer for the water supply. It adds new clean water to the sump for a preset amount of time, and the excess water overflows into a house drain pipe.

In my opinion the more electronics you put into it the more things there are to eventually fail. Even with mine I've had a timer fail, and had the 24Vdc power cord for the solenoid valve fail.

Just remember when you design your system to design it so that if things fail they do it in a safe state.

With mine the worstcase thing that can happen is if the solenoid valve got stuck on so that fresh cold tapwater was entering the tank nonstop. The tank temp would eventually drop far enough to kill the tropical fish, but atleast there would not be a flood that destroyed the house.


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post #13 of 14 (permalink) Old 10-01-2007, 02:11 AM Thread Starter
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Problem is, I've no sump... I agree about failsafe design, and yes electronics are not the best idea. However, properly designed with the necessary redundancy in the sensor I think that the issues should be reduced to a minimum.

Thanks for the insight and suggestions, just working on the right design.
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post #14 of 14 (permalink) Old 10-01-2007, 04:07 AM
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I have been in the industrial world for many years, and this is the best of the best for quick and simple level switches.
I have used many of these reed switches in all sorts of applications.
Trust me, this is the answer to your problems for automation. I have been involved as an industrial controls/ calibration technician/ field engineer for more then 25 years. This is the simplest fix and a very reliable answer as well.

Maximo
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