|noel shelley||03/03/2021 21:57:30|
|484 forum posts|
The amount of light that causes a trigger can be set, so that it is the change of light intensity that will change and activate a servo or relay. A capacitive circuit could be used, the presance of water would alter the tuned circuit and open or close the bypass valve. Noel
|Michael Gilligan||03/03/2021 22:04:03|
17658 forum posts
I’m prepared to be proved wrong [please form an orderly queue] ... but I believe I gave you the answer, Duncan
|duncan webster||03/03/2021 22:23:00|
3139 forum posts
That would be neat, but what would I make it from bearing in mind it will be 4mm diameter or less.
|Nick Clarke 3||04/03/2021 09:36:45|
1156 forum posts
I seem to remember an article in ME by the later Roy Amsbury which used sensors screwed into the boiler which when covered by water indicated to high and too low - It was when I first began taking ME as a teenager so early 70s.
Of course you wouldn't have a visual indication then.
If I get chance later on I'll see if I can dig it out.
|John Haine||04/03/2021 09:36:47|
|3780 forum posts|
What is the tube diameter? I use these a lot in clocks, have a 10mm gap, you may be able to get bigger. 5V in through 270R resistor, logic level out.
|duncan webster||04/03/2021 11:31:15|
3139 forum posts
I've got that article, but I'm trying to avoid probes and continuity.
I've got one of those sensors linked by John, I'll give it a whirl later, but it's #2 son and #1 grand daughter's birthdays, so today will be taken over shortly
|Martin Kyte||04/03/2021 11:48:47|
2313 forum posts
Often water guages have diagonal striped cards/plates fitted behind them. The water in the tube reverses the stripes. For an electronic guage sensor if the stripe was half and half vertically it should appear to swap sides changing from say white to black which should be detectable with an optical sensor.
Alternatively use one of these.
2886 forum posts
The simplest method works best, have a look at...
work on the acronym 'KISS' . Sometimes projects are over engineered where 'simples is as simples does' to closely paraphrase a line from a well known movie.
|Ian P||04/03/2021 12:31:56|
2485 forum posts
I was going to look up the density of Acetal but looked at the ball you linked to first. Not surprisingly one line of text below the detailed specification states 'These balls will not float'. TBH that is not strictly true as they would float on Mercury at least.
|Michael Gilligan||04/03/2021 13:17:07|
17658 forum posts
So why use anything other than the original design [which has no added parts] ?
|Grizzly bear||04/03/2021 18:46:41|
|278 forum posts|
"That would be neat, but what would I make it from bearing in mind it will be 4mm diameter or less."
Make it from anything that floats.
If your liquid is water, oil based liquid will float, I was thinking polystyrene or similar.
You are a man of many talents, think outside of the box, or in your case, tube.
If I'm talking rubbish, I haven't read all your posting.
7040 forum posts
How about sonar? A small piezoelectric transducer bounces a ping off the water/steam interface.
I think better to ping and detect at the bottom because I suppose the speed of sound in steam varies with pressure, which would cause the 'level' to appear to alter depending on what the boiler was doing. I think the speed of sound in water doesn't alter with pressure but could be wrong.
By alternating pings from top and bottom. it might be possible to measure water level and pressure with the same device.
Experiment required. Can an ultrasonic echo be bounced off the surface at all? If echo location works, what's the minimum distance it will work over and how accurately can distance be measured? The speed of sound in water is about 1450m/s, so pinging 10mm of water would take about 13uS, a bit fast for an Arduino.
|John Olsen||04/03/2021 20:33:57|
|1156 forum posts|
Is this for a plant in a small boat? I wonder if in fact it would be simpler to arrange a variable stroke pump and then adjust the setting over a number of trials until the sweet spot is found where the boiler level is maintained correctly without needing continuous attention. You might think that would not be possible with the speed of the engine possibly varying, but in my not so small steam launch, I have found that (purely by chance) one of the two pumps will maintain the level correctly in the boiler for long periods without having to either turn the pump off or turn the second pump on. When I say long periods, last Monday a group of us made a trip in Whanganui that took about five hours, and the pump settings were not changed for most of that time...I had the two on briefly at the start to get the level up to a good point, then left them alone for most of the trip.
Another approach that has been used in small but man carrying launches is to have a float in the hot well controlling the pump bypass. The requirement here is that no water should be being lost anywhere else in the system, through glands or excessive whistle blowing etc. So there is a fixed amount of water in the system, and if it is in the hotwell then it is not in the boiler and vice versa. This wouldn't work for me at present, since water is escaping somehow into the bilges slowly, so every couple of hours a bucket or so from the makeup needs to go into the hotwell. I think it is escaping from the pump glands slowly.
There is a much more sophisticated system used in full size practice where you have a closed end tube connected to a column at the desired controlled water level. The tube slopes upwards from the connection point. When the water level in the boiler is at the height of the connection point, the steam above it will condense due to heat loss from the tube. Water is not a good conductor, so the top end of the tube will cool down. When the water level is below the connection point, any water in the tube will drain out, it will be full of steam, and will be hot, since any condensate can run out and be replaced with steam. So a temperature sensor here can detect the change and be used to control the pump. This works very well in full size practice, but might be harder in a small size, especially if it is in a boat that bounces around a bit.
|Michael Gilligan||04/03/2021 20:49:01|
17658 forum posts
Astonishing [but interesting] ‘scope creep’ in this thread. considering Duncan’s actual question ^^^
|John Haine||04/03/2021 21:38:40|
|3780 forum posts|
A neat sensor is a transparent rod with a 45* cone on the end above the fluid with the point just below the desired level. When you look down the rod if the fluid is below the cone level there is total internal reflection so the top appears light, but once the cone is submerged most of the light escapes into the fluid and the top appears dark. Use an optical proximity sensor to detect the difference.
|Nigel Graham 2||04/03/2021 21:53:31|
|1265 forum posts|
Intriguing idea, Dave!
A flat water surface is highly reflective to sound incident perpendicularly to it; but some still gets through.
The speed of sound v, sometimes 'c' in the calculations) In a medium depends on density.
Water is practically incompressible so pressure does not affect c, but the temperature certainly does by changing density, so as the temperature in a boiler is a function of pressure, c will vary with steam pressure. However, from an operating point of view the temperature at working pressure will be what matters, so any error at lower pressure is probably not important. As long as the device indicates there is water in there...
Now, the limits on depth and range are going to depend very much on wavelength (L) hence frequency f. It needs wavelengths shorter than the distance.
You give 13µs for 10mm distance - is that one-way or two-way travel time? We are looking at MHz frequencies - I don't know what upper limit can be achieved, but how are you receiving the return? On an adjacent transducer or the same one, by reciprocity? We are looking at some very fast switching speeds as well as very high frequencies.....
It may be possible to ascertain pressure by sound-speed through the steam, but I don't think this would be very accurate. A strain-gauge sensor would be simpler and potentially more accurate.
Thinking of optical water-level sensors, suppose the normal gauge-frame were adjoined by an opaque metal column with a glass cap top and bottom, between an l.e.d. and suitable photo-voltaic cell. Pure water won't show much, but if tinted by the usual tannin-based boiler water-treatment, would that give enough light attenuation with depth to be detectable by the cell?
|Robert Atkinson 2||04/03/2021 21:54:21|
933 forum posts
Michael G. is correct, the sensor uses a directional source (Spot or slit) which is off-axis with the detector. Empty tube has two identical but opposing "lenses" so has little effect even if thick walled. When filled with water it acts as a cylindrical lens and spreads the light beam. This causes some light to hit the sensor. There needs to be some depth to the hole or slot on the source and detector to make them directional. A slot (must be parallel to tube axis) gives a stronger signal but less level resolution.
|duncan webster||04/03/2021 21:58:33|
3139 forum posts
Although not explicitly stated in the original post, this is for the gauge glass on a model boiler, so it is hot, potentially 160C+. I think this rules out Martin's link, which quotes a max temp of 80C. Otherwise that seems very similar to the one I linked in the original post. I think it also rules out plastic floats. The glass is 6mm OD, 3.8mm ID, so SOD's idea of sonar would need to be very small and tolerant of heat. It's for a test boiler, so demand can vary from nothing to max, and it might not be driving an engine, or the engine might not have a built in pump, so John's fixed pump displacement won't hack it. It won't have a hot well, so that's a non starter
I've just spent a fruitless hour trying to find the slotted opto I know I've got. I've obviously put it somewhere safe. I'll have to wait until my opto transistors arrive.
As to why not just copy the original, well I wouldn't learn much doing that, and even at my advanced years that is a big part of why I set about things.
I'm beginning to think that Michael's idea of cylindrical lens might be the answer, if the water makes it splay out like a fan it would cause a step change in intensity, same amount of light in a wider beam. I'm going to rig up an LED facing a tube and have a play, but tomorrow, tired out by birthday festivities.
I quite like Noel's capacitance idea, but it's beyond my electronic expertise. Are we talking relaxation oscillator and phase locked loop chip? If so I've just bought some of those for another project, but I can't find them either!
Edited By duncan webster on 04/03/2021 22:02:46
Edited By duncan webster on 04/03/2021 22:03:34
|Michael Gilligan||04/03/2021 22:08:55|
17658 forum posts
Using my arbitrarily chosen numbers: The empty tube splays it more than the water-filled tube; but yes, there would be a step-change in intensity.
If you can give me actual diameter, bore, and refractive index ... I can demonstrate.
Edit: Here’s a little taster
Edited By Michael Gilligan on 04/03/2021 22:26:26
|Martin Kyte||04/03/2021 22:25:41|
2313 forum posts
That's why I suggested the virtical half and half stripe. With the water the background changes side so should be able to be picked up with a reflective type sensor arrangement mounted off centre to the tube. I don't see why a transmissive sytem should not work though.
I would think that the outside of the tube is going to be somewhat less than 160 degrees but I agree it would challenge the Farnell sensor at 80 degree max.
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