making a capacitor from a plastic container

I have been looking for a water level sensor. Many have probes in the water. My well water has chemicals that attack the probes. (attack or attach to ) I think I will build a oscillator that uses the capacitors to make a frequency that I can measure. I might need to divide by 1024 to get a frequency I can measure with a small micro.

Hi,

I am left only with wondering of the water characteristics might change with refill to refill. Maybe include a "calibrate" button to press once the container is refilled.

calibration is probably a must. i am going to try something a bit different with a plastic tube sealed one end with the electrodes inside that touching the sides, i am hoping that the level will have more affect than the composition.
if that works then that would be ideal for a well .
has to be a fairly easy answer as we once had a oil tank with a cap type level sensors the oil company installed and it was connected to the web. they never messed with it once installed and we had it 3-4 years.

ronsimpson said:

I have been looking for a water level sensor. Many have probes in the water. My well water has chemicals that attack the probes. (attack or attach to ) I think I will build a oscillator that uses the capacitors to make a frequency that I can measure. I might need to divide by 1024 to get a frequency I can measure with a small micro.

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use a 30f and you can divide by 4095 you might be accused of overkill tho also 30f4013 has CTMU

ok quick and dirty test with 2 PCB boards on sides of container, i havnt taken measurements because i cant get in the kitchen for long enough until after dinner
but quick test revealed that both pcb's on same side gave higher reading (seems right) but i get a lower reading but more variation with depth with a pcb on each side!
i will wait until tape gets here for proper test but is looking good! main concern is the fact that a ultra sonic fogger will be in the water and when its on i am expecting it to really affect the readings, also the water changes alot withing a couple of days of the fogger being in it. There seems to be alot of rod shaped bacteria build up on the fogger and the water clouds more than a plain container of water, the first problem of the fogger being on is easy to solve as i will use the micro that switches the fogger on and offf to do the reading for the capacitor, that way i can get it to do the reading when the fogger isnt active! the bacteria problem i wont know about until i try it for real with the tape, 18f chips have the CTMU as part of the ADC port (port A) wich is 10 bit, i might use a 30f because the CTMU is part of the 12 bit ADC on portB, so i will have more resolution. But i will try both and see if there is any difference, so next on reading list over next few nights is CTMU in the datasheets and see what they can do

If you use distilled water you should not get any bacteria or deposit build ups.

To increase capacitance the area of overlap of the two copper patches (e.g. pcbs) should be maximised and the spacing between them should be minimised. Here's one approach, using a pair of flat-sided thin-walled plastic containers such as margerine tubs, with the water in between the two containers, one patch inside the inner container, the other patch outside the outer container. Neither patch is in contact with water:-

ah i see what LG is on about now!! it's parents evening at his school tonight so i might not get chance today, but it would help if i posted some pictures for him to show exactly what he is trying to do and why.
You are giving good answer's but i am afraid you dont have all the information, very easy to rectify with a picture, then the problem of a container within a container approach will become clear.
actually having read this thread and knowing exactly what this will be for i think its pretty clever
the problem as i see it isnt going to be the amount of capacitance but the amount of change, the volume is a narrow range, for example the container could hold 1 litre or 10 litre that isnt important, the device sits inside the container normally on say a small ledge the top of it has to be no more than 3cm from the top of the water (it wont work with any more than that) 2.5cm is optimum, but the level from the top of the device cannot fall below 9mm or damage occurs to the device. i will get a picture posted for clarity.
i hope that helps a little.

In that case a cap could consist of a horizontal pcb on top of a float, with a fixed pcb just above and parallel to the first pcb.

This is fascinating (the transformer bridge is a particularly cleaver technique).
As a complete aside, these people make sensors based upon measuring the capacatance of fermenting solutions - in this case they're interested in the composition (how much microbe vs how much water).
https://www.aberinstruments.co.uk/
Just proves that it does have practical application.

rumpfy said:

A transformer ratio bridge might do the job for you.
Take a transformer which has a single winding and an accurately centre tapped winding.
The single winding goes to the detector amplifier.
The centre tap of the other winding goes to the output of an oscillator (say 1000 Hz). Each end of that winding is connected to an admittance to ground. the capacitive part of the admittance is formed by a capacitor to a water bath. The water is connected to ground and also the other output of the oscillator is ground. The capacitor can be formed for example by a piece of single insulated wire of the underground telephone cable type. For your BPO stuff, this is about 350 to 450 pF per meter from memory.
The real part of the admittance is a simple pot with its centre connected to the centre tap, and each end connected to each end of the winding. With the admittances balanced, the nett current in the detector winding is zero. Ideally the detector is phase sensitive and responds only to capacitance unbalance at each side of the winding. As the water level rises and falls, the bridge will move out of balance; the output voltage can be calibrated, or, you decide the capacitance value at the 'out of water' point. Use a chemists measuring cylinder for example and suspend the insulated wire as a loop from top to bottom.
These kind of capacitance bridges are used extensively in the manufacture of single insulated conductor for making multi-pair telephone cable. Generally, the measuring tube is guarded and the active element is 10 to 50 cm long.
You can guard the bridge by using coaxial cable to connect to the known and unknown admittances. The outer conductor of the coaxial cable is connected the output of the oscillator.
Hope this helps.
pr.

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i kind of dismissed this because it went over my head, and i thought easy with a CTMU, but after rethinking i am going to give CTMU a go first but also find out more about this method (unravel what your saying in a way i can understand) what put me off was accurate center tap, but surely that cant be so hard hmmmm much reading ahead

ronsimpson said:

I have been looking for a water level sensor. Many have probes in the water. My well water has chemicals that attack the probes. (attack or attach to ) I think I will build a oscillator that uses the capacitors to make a frequency that I can measure. I might need to divide by 1024 to get a frequency I can measure with a small micro.

Click to expand...

what about using AC? wouldnt help stop the chemicals attaching as the + and - keep swapping, we have not long ago in chemistry at school done a bit on electrolysis, with anodes and cathodes and ions and that stuff, for producing hydrogen we used DC so i am thinking AC would make a difference? especially if it was run at a higher frequency than 50Hz but i couldnt swear to it

High frequency can help but it depends on your probe material.

ok got some pics
this is the commercial unit they charge over £100 for this!!!!



in the second picture you can see the fog coming out the end of the pipe, the great thing with this is the fog is so fine it actually feels dry! but is very fine water vapor so is brilliant for upping the humidity in the incubator




the only bit i cant seem to find online to buy is the connector for the pipe that fits onto the lid! it swivels as well, so if anyone knows what they are called, the fan dosnt sit over the hole its at the other end but works really well

and the box i have is slightly smaller

Personalty I would just put a float switch in it and be done with it.
But whats the fun in that.

4pyros said:

Personalty I would just put a float switch in it and be done with it.
But whats the fun in that.

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yeah you do have a valid point, but as normal just about all my projects have a heavy lean on learning new things, and for a while now i have wanted to use the pics CTMU, and this seemed a good way to do it. i can see why people would think i am trying to make life hard but what i am trying to do is get a project working that uses new techniques i haven't tried before. I learn better if i try something out and especially if i find it interesting, i also like to do stuff in a different way

Have you considered using inductance instead of capacitance for which the change will be quite small and dependent on the purity of the water*. Just mount a movable slug to a float and have it rise it a coil. I think it would be far more sensitive and more reproducible. Plus, you will learn a new technique.

*BTW, a comment was made somewhere in this thread about distilled water being sterile and not supporting microbial growth. While distilled water may be sterile when first distilled, even if sterile, it does not stay that way very long once open to the air. Moreover, it is a misconception to think that microorganisms can't grow in it. Distilled water can be loaded with organic and non-ionic chemicals that are food for microbes.

John

jpanhalt said:

Have you considered using inductance instead of capacitance for which the change will be quite small and dependent on the purity of the water*. Just mount a movable slug to a float and have it rise it a coil. I think it would be far more sensitive and more reproducible. Plus, you will learn a new technique.

*BTW, a comment was made somewhere in this thread about distilled water being sterile and not supporting microbial growth. While distilled water may be sterile when first distilled, even if sterile, it does not stay that way very long once open to the air. Moreover, it is a misconception to think that microorganisms can't grow in it. Distilled water can be loaded with organic and non-ionic chemicals that are food for microbes.

John

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dad pointed out to me that they use RO water in the lab to grow pure cultures of some microbes

OK LG. When you are ready, sing out and I'll give you more stuff to think about. Might be able to use up some of the 4000 2n3906? U have. But dont be scared of transformers. Most engineers in my experience are just frightened to start.
The way you are going with bits of plastic buckets and the like will be interesting for you and will show you what's wrong with the approach. You'll find instability of readings and errors due to stray capacitance will combine to dog your results. You've seen enough already to show that capacitance measurement can be made to work. All you have to do is get the actual design to be stable against drift.
The use of probes for oil level measurement (your post #23) is simplified by the low water absorption of mineral oils and their low dielectric loss and relatively stable dielectric constant. My suggestion to use a polyethylene insulated telephone cable conductor is based on the stability of polyethylene. The d/D ratio for telephone cable singles is about 1.7 and the dielectric constant for solid poly is 2.12 and about 1.6 for foam type insulations. So with this info, you can work out the coaxial capacitance per metre of insulated wire. The dielectric constant of water is high (80) so it doesn't affect the result. Most of the capacitance is due to the d/D of the insulated wire. The dielectric loss of the water doesn't matter either, (to a first approximation) cos when you use a phase detector, the conductivity in the unknown admittance is not seen at the detected output; you only see the quadrature current due to the capacitance difference between the unknown and the standard. The test frequency of say 1000 Hz provides the reference phase, and the phase detector is driven in quadrature. The reference phase can be used to drive the shields around the connecting wires and measuring setup, so the system is VERY easy to guard. With the right design of reference oscillator, you get an in phase signal and a quadrature signal. The quad signal drives the detector.

inductance now there is a thought! i have 2 of these units i have to build for now, one for the incubator and one for the hatcher so i might try cap on one and inductance on the other see which works better!
at the moment i top the container up in the morning and before bed, but i would like to do auto top up with aquarium ar pump pumping air on demand into a demi john with other tube into container. Its all bling in a way but you never know i might hit on something and even if i dont at least i took a problem and applied a solution and learnt a new skill or way of doing something! reading books i love but you cant beat making something work! and maybe one day i might be an inventor!
ok give me some info and i will see what parts i have and what i need to get hold of, i have foggers for two units but i found a place that sells them without the transformers and they are only £3 (still alot of money for me but worth it)
it will take me a while to get the third running anyway! and dad is interested in the transformer one for different reason's, he is thinking it might be useful for the hydroponic poly tunnel