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Specialized Level Sensors

05/12/2009 6:30 PM

I am in need of a sensor for level detection for a test apparatus I am in the process of building. I need to measure the level of a liquid from the outside of the containing vessel. Actually, the primary vessel containing the liquid is placed into a plastic holding bin, a secondary container really. Ideally, I would like a device that could be positioned outside the secondary container, detect through both the primary and secondary containing vessels as well as liquid residue left on the inner walls of the primary container. There may even be a small air gap. Are there any devices out that to do such monitoring?

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#1

Re: Sensor suggestion needed

05/12/2009 7:26 PM

The cheaper thing I can think about is a couple of sight glasses, one connected by a small pipe or tube to the bottom of the inner vessel, and the other to the bottom of the outer one. Or maybe a tube or hose with a shutoff valve, that may be opened from time to time to check if there was a leakage between them. If you need data logging, register, computer linked stuff or input in a controller to act in your process like closing and opening valves, signaling alarms, etc, so I'd suggest simple resistance based ones (a pair of electrodes that, when in contact with the fluid, close contact and warn controller). If fire harzard environment, maybe a ultrassonic stuff or magnetically coupled floater must be used. So... what are the constraints my friend? Whats the fluid nature? pressure, temperature, size of vessel, work cycle, harzard assessment, maintenance requirements, $$$ constraints?

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#2

Re: Sensor suggestion needed

05/12/2009 7:28 PM

plastic holding bin

????? primary vessel

More information, more help.

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#3

Re: Sensor suggestion needed

05/12/2009 7:53 PM

OK, more info...I have a 1 gallon polypro container filled with an alkaline detergent and some other much nastier chemicals, this is the primary. It is placed into a LDPE rotomolded secondary containment vessel that has a pocket to recieve the primary nice and tight. I use a peristaltic pump to move the fluid from the primary container to various places that need not be mentioned. The fluid is constantly being pumped and the container must be replaced frequently, therefore no "hard" plumbing can be associated with level detection. I would like to place a device, possibly a capacitive sensor, on the outside of the seconadary vessel, that would detect through both and signal when the level gets to a preset low point. We need to use as much of the chemical as possible before changing out the gallon container. We used to use a float switch, but it left too much in the container, an no, we cannot transfer the chemical into the new container due to some sort of law prohibiting it. Does that help any?

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#4
In reply to #3

Re: Sensor suggestion needed

05/12/2009 9:04 PM

I maybe looking up at the moon and saying that all we need to do is go up! But if you had a laser activating (diode, capacitor, resistor etc.) and a laser which would shine through both your containers at a set height, and when it's path becomes clear the laser reciever can actuate an alarm or message to notify you of the emptying nature of the vessel. If the fluid in the vessel is clear, then you may need to change the angle of the reciever/transmitter to cater for it's refractive index, the calibration for this could be done with an empty vessel.

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#7
In reply to #4

Re: Sensor suggestion needed

05/12/2009 10:45 PM

I do not know much about lasers, but the secondary containment unit is a very dark color and I'm not sure any light can penetrate it. Please enlighten me if I am absolutely clueless about what you suggest being possible.

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#8
In reply to #3

Re: Sensor suggestion needed

05/12/2009 10:52 PM

might it be easier to monitor the fluid after the pumping?..since your " pumping path " is known..ie; the amount of fluid in the pump[ing system supply lines, the units and the flow rate of each unit,..

possibly set up a timed trial: known quanity of fluid , pumed for x time.. = flow rate..

if i knew that after 20 minutes i'd used 95 % of my gal, ...might i not also swap out after that know time,

..

i'm sure you don't want to cavitate your system..

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#9
In reply to #8

Re: Sensor suggestion needed

05/12/2009 11:10 PM

One way I currently monitor in another application is to use a flowmeter that I calibrate for the specific fluid based on a volume value per pulse. I set up flow-rate parameters for both high and low. If no fluid is passing through, no pulses get recorded and the system times out and gives the error. It is a pretty good way to signal for a low level alarm. but this too is a pretty expensive way to do it, especially with the quantity that I need. I'm really looking for a "cheap and easy" method,but one that is reliable too. Also cavitation is not an issue with the peristaltic pumps that I use.

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#10
In reply to #9

Re: Sensor suggestion needed

05/13/2009 2:17 AM

ok.. that's my 2 cents worth..

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#21
In reply to #3

Re: Sensor suggestion needed

05/14/2009 8:35 AM

How about a small weight scale with data logging for the primary liquid level detection function. You should be able to get one of these without having to spend too much and it would provide a pretty simple and accurate to keep track of the level of the liquid. With the proper baseline of the empty containers you'd even be able to track residual buildup on the inside of the secondary containment. Cole Parmer may offer something.

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#5

Re: Sensor suggestion needed

05/12/2009 9:34 PM

The best solution I can think of is load cells. These measure the weight of the vessel and its contents but you can easily tare out the vessel weight to get only the contents.

You've got nasty chemicals in there so putting a pressure type level sensor might not be a good idea since you'll have to cut through the outer lining and inner lining to sense the liquid's pressure. Plus, if your liquid tends to stick to things, your pressure (ergo: level) readings will change and introduce an error. Ditto for other sensors that need to penetrate into the vessel.

regards,

Vulcan

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#6
In reply to #5

Re: Sensor suggestion needed

05/12/2009 10:42 PM

I've actually thought about the load cells, however that would be very costly. You would need signal conditioners and such and it would require frequent calibration. How about a microswitch with a spring loaded plate. Get the spring rate correct for the weight that I would desire to signal a nearly empty vessel. Could be cheap and pretty easy to do.

Right now, the best thing that I have gotten to work is a capacitive sensor mounted to the suction tube near the vessel. I calibrated it to detect air in the line, which would signify that my fluid has run out. As soon as this is detected, the system shuts off and sets am alarm strobe/siren. This actually works quite well and allows only a very little bit of air into the line.

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#12
In reply to #6

Re: Sensor suggestion needed

05/13/2009 3:10 AM

"Something for nothing" doesn't work.

The second paragraph is a change of scope. Instead of measuring level, now "absence of liquid" is being detected.

Before solving the problem, how about specifying what the process is, and how it is supposed to operate first? The correct instrumentation is as a result of this procedure, not in spite of it.

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#13
In reply to #12

Re: Sensor suggestion needed

05/13/2009 8:22 AM

The process is that you pull liquid from a container, pretty simple. Ideally, you want to be notified when the container is as near to empty as possible, hence "low level". Monitoring the presence of liquid within the suction tube is one way to know that you are out of chemical. As soon as air bubbles are detected, the system shuts off and issues an alarm. It is OK to be completely out for a few seconds of pumping. It is non-invasive and not subject to any material compatibility issues. The scope hasn't changed, there are multiple ways to go about solving it.

Also, I am not looking for "something for nothing". There are some bright minds on this forum and I'm just looking for alternative ways to do this at a good cost.

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#14
In reply to #6

Re: Sensor suggestion needed

05/13/2009 8:23 AM

Gave you both a GA for agreeing with me

Quit trying to measure directly and switch to measuring weight.

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#15
In reply to #14

Re: Sensor suggestion needed

05/13/2009 8:27 AM

I am going to look into this today. The more I think about the simplicity of the concept, the more I like it.

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#16
In reply to #6

Re: Sensor suggestion needed

05/13/2009 10:14 AM

...would be very costly.

You're right about that, but if you don't need high accuracy, you can buy cheaper ones.

...would need signal conditioners...

There are brands and models that put out 4-20mA directly so no transmitters or converters are needed. Even if you use the mV output type, using an ordinary mV input device will work (just have a good quality power supply to provide the excitation voltage).

...require frequent calibration.

If you're installation is properly done, and the level of accuracy is not very high, I don't see why you need to calibrate more frequently than once every three months or maybe even more.

It seems, however, that you're not really interested in knowing the level but more of knowing when the tank is almost empty. If that is the case then a simple level switch will suffice. I suspect, however, that once you install a load cell, you'll find that you needed it all along.

regards,

Vulcan

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#17
In reply to #16

Re: Sensor suggestion needed

05/13/2009 10:43 AM

You are correct and I guess I didn't really make that clear. I do not need to know what the current level is but rather when it has run out or is about to run out, so yes a device that is "switch-like" in nature is desirable. The controller that I use for monitoring is not a data aquisition module of any sort and is not programmed to recieve an analog signal. I could ask the software engineer if the chip can be programmed to accept analog inputs. Right now, all our detecting devices are NPN/PNP or NO/NC.

Might you be able to direct me to a place that I could source the transducers?

Thank you

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#18
In reply to #17

Re: Sensor suggestion needed

05/14/2009 5:49 AM

Uhm, transducers for the level switch or load cells?

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#20
In reply to #18

Re: Sensor suggestion needed

05/14/2009 8:06 AM

That's what I meant, my bad.

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#24
In reply to #16

Re: Sensor suggestion needed

05/14/2009 3:31 PM

All I can see is (1) pulling a near? vacuum between primary and"B"

2 measure (vacuum) full and measure empty. 3 then add,alarm,bells & whistles .......second thought duh,,, this won't work unless kept at same ambient temp MMMmmmmmmmm

judge

unfortunatly I am haveing a lot of trouble withe the "Reply Editor,,,Tutorial available?

regards

Joe (judge)

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#32
In reply to #6

Re: Sensor suggestion needed

11/12/2010 2:20 AM

There are several devices that are used in signal conditioners have different task to perform. Like amplifiers help in increasing the voltage level to match with the analog to digital converter thus help in increasing the measurement resolution and sensitivity. Isolated signal conditioning devices pass the signal from its source to the measurement device without a physical connection by using a transformer, optical or capacitive coupling techniques.

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#11
In reply to #5

Re: Sensor suggestion needed

05/13/2009 3:07 AM

Quite.

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#19
In reply to #5

Re: Sensor suggestion needed

05/14/2009 7:30 AM

How about the a system that they use to fill propane cylinders. Using a beam scale and knowing the empty and full weights of the tank you attach a limit switch to open when the beam moves downward (switch is shut until the tank gets light enough to cause the beam to move downward). The beam will have counter weights that you can set to cause the beam to move at some weight slightly more than the empty weight of the vessel.

Just an Idea. Relatively inexpensive too.

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#22

Re: Specialized Level Sensors

05/14/2009 1:38 PM

I build houseboats and it took many hours of searching to find such sensors;

I guess you are a lucky man, because here is where your search ends...

Feriellosales.com has what you need and that guy is cool too!

Good luck!

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#23
In reply to #22

Re: Specialized Level Sensors

05/14/2009 2:10 PM

Thanks. I called and spoke to Dennis over there. I gave him some info and he informed me that his sensors would not work for the application. The possible air gap between the 2 containers would throw it off. Too high a dialectric. Thanks though.

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#25

Re: Specialized Level Sensors

05/14/2009 6:13 PM

if there is away to mount the sensor unit milltronics is use by the water wastewater industry to set the on an off of pumps as the level of liquid rises and falls inside a chamber, you can look into that

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#26

Re: Specialized Level Sensors

05/14/2009 9:06 PM

I know this may seem rather basic. Not knowing the configuration of your tanks, have you considered a simple sight glass? Are there any restrictions for temperature or chemical composition? The sight glasses could be placed through both walls. Again a set of drawings would make it much easier to answer your question. It may also be a matter of simple hydraulics in combination with an external sight glass if you know the volume of the primary vessel, you could calibrate the sight glass or glasses. Hope this helps or at least sparks an idea, Joe

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#27
In reply to #26

Re: Specialized Level Sensors

05/15/2009 12:43 AM

The chemicals are stored and pumped from a remote location. The low level signal needs to be sent to the equipment which could be anywhere between 25 and 100 feet away. There need to be a visual and possibly an audible alarm set when the level is low and nobody is in the remote location to see it.

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#28
In reply to #27

Re: Specialized Level Sensors

05/15/2009 5:27 AM

My apologies. I went back and read #3 You are concerned about the gallon container running out. I suggest you consider installing a simple volume flow meter with an alarm set to go off in time for you to replace the chemical and simplify the problem.

Unless i totally missed the design and intent of your system, this seems to be the best engineered solution. If the concept makes sense. I'll help dig out a source for the devices, Joe

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#29
In reply to #28

Re: Specialized Level Sensors

05/15/2009 12:09 PM

Actually, we manufacture our own flow meters. This is an option and a good one which i've used in a similar manner. The only problem is, is that it lets you know you're out of fluid when it's too late. Once fluid stops moving past the sensing area, there is air in the line. Although the impact is small as far as a slight change in the delivered volume for one or two cycles, some of the chemicals we use, whe they come in contact with air, their properties change signficantly. I'm really leaning towards Vulcan's thought of using load cells to measure the weight or options similar. I've got a coulle on order to test next week.

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#31
In reply to #29

Re: Specialized Level Sensors

05/15/2009 2:52 PM

If you wanted to keep with the flow meter, you could include a totalizer that would set off an alarm at a predetermined total volume. This would fit in with what you have now. Just a thought on keeping the cost low and utilizing what you have already to save some money. I still think that using the weight is the best course of action.

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#30

Re: Specialized Level Sensors

05/15/2009 12:49 PM

You are clearly on the right track. If you have the ability to have a larger volume tank to fill the gallon size additive tank you can contact MICRO MOTION in Boulder CO, who makes a flow meter that you can tie into your PLC and dose your system. They can assist you with anything you need. They can even provide you with the technology to account for air bubbles in the line.

Long way from glass ware!

I hope this ends your search, Joe

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