Paddleducks
Other Marine Models => Live steam => Topic started by: megatron on November 05, 2012, 08:32:19 AM
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Hello everyone
I am going to try and build a boiler control system which will sense the pressure and the water level and control the gas burner and feed pump. The problem is I have no idea how to detect the water level. I have seen sensors attached to the sight glass which is the type I would prefer to make. Does anyone know how these sensors work and what components they use?
Thanks
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This was posted on RC Groups,
"The book "Scale Model Ship Propulsion", by Tom Gorman, shows an electronic gas pressure regulation system which uses both a water level sensor and a boiler pressure sensor. The water level sensor is an optical device which clamps to the glass tube of the water level sight gage. The system was available from Cheddar Models Ltd.," http://www.rcgroups.com/forums/showthread.php?t=1384622
Regards,
Gerald.
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Thanks for reply steamboatmodel,
I have a model fitted with a cheddar ABC system but I was wanting to design one myself that uses off the shelf components which is cheap to make and doesn't involve programming microprocessors. The Cheddar water sensor is fairly simple with only 4 components but I haven't been able to identify what these components are. Controlling the pressure should be fairly easy to do as it only involves measuring the temperature and I have built thermostats before for my homebrew boiler.
I would love to hear from anyone who has built or is building one of these systems.
Stew
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If you are sucsessful please post it. I would like one. Geoff
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I understand that the actual sensor used. Is of the type used on commercial coffee machines. I could be wrong. If you wish please pm me your phone number as i think i can help you more. John.
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John .That is a very good lead. I will be having word with my son. He contracts to a major super market chain. (refrigeration & electronics) Geoff
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Be sure to report your findings on here.
Regards,
Gerald.
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I understand that the actual sensor used. Is of the type used on commercial coffee machines. I could be wrong. If you wish please pm me your phone number as i think i can help you more. John.
I've sent you a PM
megatron
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Have just constructed such a system, hoping to test it tomorrow. It's based on the system shown in Stan Bray's book, 'Model Marine Steam.' It's attributed to Roy Amesbury. I've made my sensors like his, a brass bush containing a bronze electrode in a teflon sleeve.
I've measured the sensor resistances, wet and dry, in order to develop my own circuit. This uses a couple of dual op-amps, LM358N. I got mine on ebay, pack of 5 for about six quid. Much simpler to make.
I've put the finishing touches to it only a couple of hours ago, and operated it with a 1K-ohm resistor to simulate the sensors. It seems OK. I'm knackered this evening, working in brass monkey conditions in my shed! Otherwise I'd have done a pukka trial already.
Spent the earlier part of the day silver soldering a couple of minor leaks in my boiler, and pickling it to clean the grot off, so am in good shape to put it all together tomorrow . . . in our nice warm kitchen!
I'll let you know how I get on. If it works I'm happy to share the details with the forum if anyone's interested.
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Hi Rayspan, what resistance did you get for the sensor, 1K seems very low?
Are you going to use this to operate a servo or just light an Led?
I've just about finished my design which controls the boiler level and gas supply and monitors tank level. I've built an optical sensor to detect the water in the sightglass but don't know how reliable it will be. A sensor like the one you've built will be more reliable but it means altering the boiler.
Stew
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Hi again megatron. Good question! Thereby hangs a tale!
First thing I did after making sensors was to measure resistance for each, both wet & dry. Findings were:
Dry - greater than 10 Megohms. Wet - less than 1 K-ohm. I also noticed that the wet resistance reading was steadily, and very slowly, falling to 800 ohms and less.
I decided it was a typically galvanic type of action, in mains water which probably has a non-neutral pH value.
Checking the article attached, I found brass copper and bronze are all next to one another in the galvanic table,
just above stainless steel, so why worry?
Fair enough, I designed my circuit and simulated the wet probe with a 1K-ohm resistor. Circuit switched nicely
so attached it to boiler electrodes and met total failure!
Puzzled, I repeated resistance measurement again and was horrified to find wet resistance over 3K and steadily increasing.
Intuition nudged me to reverse the probes of the resistance meter, to find a reassuring 800 ohms, falling steadily once more. I convinced myself by rechecking several times with consistent results.
I'd built my circuit assuming the same orientation Roy Amesbury shows in his circuit, !.E. bronze cathode, copper anode. The conclusion was it should be the other way around. A few quick flicks of the soldering iron
and I swapped the probe ends. This time - SUCCESS.
I've just coaxed my lady wife to operate the rig with a jug of water while I videoed it. Should be available on you tube tomorrow somewhen. I'll post its URL when that happens.
Circuit attached. I'd be glad of comments, critical or otherwise!
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P.S. I'm hoping to cobble up a feed pump driven by an electric motor. I have several high torque devices from my aeromodelling days. Just wondering how powerful it would have to be to inject feed water into boiler at say 60P.S.I.
If anyone can suggest a way of doing it like that, I'd be glad to hear from you.
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Hi Ray,
there's a chart on this website to calculate power needed for water pump.
http://www.engineeringtoolbox.com/pumping-water-horsepower-d_753.html
Looks like 0.03hp to pump 1gallon per minute at a head of 125ft which I think is about 60psi, you don't need anywhere near 1gallon/min so you're not talking about a very big pump at all. I'm sure I've read about people using car windscreen washer pumps but you would need a 12v battery. Could you not build a pump to be driven from your crankshaft?
Have you read Malcolm Beaks article in the downloads section, Water level control?
Stew
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Many thanks megatron, all looks right on target. Numbers are encouraging too, I quite fancy devising a neat little reciprocating pump to operate in the ranges indicated.
Yes, I like the idea of pumping with crankshaft. Maybe it's my ignorance, but what happens to boiler pressure when you start injecting cold water into an almost empty boiler? Surely, pressure will drop until it gets up steam again. In which case there won't be any power o/p to drive the pump will there?
12v isn't a problem, I still have several battery packs and requisite chargers from aeronautical days. Weight shouldn't be a problem either, I used to fly these things, so floating should be a doddle - compared to steam plant and engine anyway.
Water source is something else, if I can't just pump it up over the side, it will have to be stored on board. I should think outdoors water in a lake for instance, would be less inclined to scale up steam plant than mains tap water. I know rainwater is recommended, but there is so much pollution in the atmosphere it's difficult to weigh pro's over con's.
Advice please all you steam engineers out there?
I forgot to mention the LED's I used in my circuit. They are the big, 'superbright' ones, operating around 20mA. Should be easily visible from the bank with binoculars if not naked eye.
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I've just checked youtube, seems to be up & running.
http://www.youtube.com/watch?v=hhDXCVXVZyg
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Hi Rayspan,
I see you are using 2 sensors, how far apart are they? If 1 is near the top and 1 is near the bottom of your boiler then filling with cold water could be a problem. I use 1 sensor and a time delay so that when the water drops below the sensor the pump starts and when the level reaches the sensor again the pump continues for a set amount of time thereby keeping the water level just a couple of mm above or below the sensor. This way you are not pumping huge amounts of cold water into the boiler. Don't forget that the water temp at say 60psi is well above 100deg and pumping a small amount of cold water in has little effect.
Stew
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Can't say I've thought it through as much as I should have. I started out by buying a glass sight tube and inserted flanges for it top and bottom. It was only when I found how difficult it was to fit the darned thing and I'd broken the glass bit that I decided to fit probes instead, so yes they are at extremes of boiler ends.
So I think an electric pump will do it best.
I had also intended to latch up the triggers on first impulse, to prevent leds flashing on and off in rough water. Then I decided that would give advance indication that limit was approaching, so didn't bother.
It also occurs to me, that inserting multiple sensors would give almost an analogue of boiler level, not that I intend to do that here.
One thing I'm uncertain about, what value will probe resistance have in wet steam under pressure. Only one way to find out!
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Ray
If you just used the top sensor this circuit could help. Arrange the sensor to give you greater than 2.5v on pin 3 when the level is low, this will give you a low output from pin7. When pin 3 goes high pin 7 immediately goes low. When pin 3 goes low there will be a 10sec delay before pin 7 goes high.
You can alter R6 and C3 to alter the delay.
Stew
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Oops
There is an error in the schematic I posted above. Pin 4 should go directly to earth and not connect to the line between pin 5 and C3.
Stew
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Thanks megatron, I'll check it out.
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Being new to this forum and seeing the posts on water level sensors I thought the following infomation may be useful. I have been experimenting with water level sensors since the 80s and have written a number of articles on the subject in Model Engineer magazine. For a little light reading try Model Engineer 25 Nov 2005, 5 Jan 2007, 2 Feb 2007
(for corrections to circuit diagram). I also recommend the following articles also in M.E. by Martin Ranson titled "Automatic Boiler Controls for Steam Boats" 30 Dec 2011, 13 Jan 2012 and 10 Feb 2012.
Also letter by me in Post Bag section of M.E. 10 Feb 2012 and lots of posts on the subject on Model Boat Mayhem, although some may have been deleted during the great meltdown of the site.
M.E. is now in posession of an article, by myself, on electrode versus optical detection of boiler water levels-publication awaited
My electrode system uses an AC signal to eliminate corosion rather than DC.
Tim (TAG)
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Hi Tag,
So what was the verdict, optical or electrode?
Is corrosion such a big issue?
Any chance of a preview of your latest article? My control system is almost finished just got to cross the Ts and dot the Is, it will be able to use either optical or electrode. It also controls boiler pressure and monitors feed tank level.
Stew
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Hi megatron
The answers to your questions are:
1) Electrode, it is the most reliable method of detection.
2)No, not for the time the electronics are switched on. Brass bushes in copper boilers are a much bigger issue as far as corrosion is concerned.
3)I am afraid not, M.E. now own the rights.
Regards
Tim
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Hi PD's....& welcome Tim... :coffee ...clearly your postings will make valuable reading for those interested or wanting to semi automatically control & maintain a reliable boiler water level................ :trophy .....Derek
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Megatron - many thanks for delay tip. Topping up is a much better idea than filling up. I'm planning to do a mock up over the weekend, having found an adequate motor and some hexfets to switch it with. All I need now is a worm gear and warm enough weather to get in the workshop to make a pump. Might even get it done by Xmas - have a good one!
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No problem Ray.
If you need any help with circuit get in touch.
Stew
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I will be looking forward to TAG's (Tim) article in M.E. I was thinking of cancelling my subscription, as I was getting feed up with how to build trains & more trains. I did ask my son about coffee machines didn't know much, expert they use electrodes.
One thought the sight glass is unreliable as it is to small, why not use a 1/2" tube?? Geoff
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Control system up & running - thanks again megatron.
I'm using a small, nominally 7.2V motor, (which last saw action at a few hundred feet up over Beaulieu heath) at 4.8V. Nominal gives it too high a speed. Plan is to reduce it further with a worm gear to drive the pump. I also needed to raise control circuit voltage to 7.2V because the n-channel mosfet requires minimum gate voltage of about 4.5V, and I was only getting 3 - and a bit.
The higher voltage has also crisped up the timing, delay is very close to 10 seconds.
My little tail is wagging furiously!
Now a proper rebuild without the birds nest of wires. Then the pump. Think I'll use two cylinders, each 12mm bore. Stroke t.b.d.
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Hi Rayspan,
if you replace the LM358 with one of these or similar ICL7621DCPA. It's a dual cmos rail to rail opamp. Output is just a few millivolts below supply voltage.
Stew
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Hi Rayspan,
I think your pump may be a bit OTT. 2 cyl 12mm dia? The pump on my Cheddar Proteus is a single cylinder 4mm bore x 11mm stroke running at engine speed and it's more than big enough.
Stew
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You know more about it than I do, so I'm sure you're right. It's just that I have those materials to hand, and I really fancy making something like that. Much tastier watching two pistons operating than one!
Anyway, I'll have to modify the plan before I start cutting metal. Do some arithmetic with the data you showed me and measure the speed I get from the motor when the gears get here. I think the delayed top-up is a brilliant idea, can't stop playing with it!
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Ray, have you got any readings from your sensors in steam yet?
Just a thought but as your sensor is close to the top of the boiler it might be better to delay the pump starting rather than stopping, ie allow the water to drop below the sensor before pump starts, and then stop as soon as the level reaches the sensor. This would prevent it from over filling the boiler.
Geoff, sorry I missed your earlier post.
I don't know if a larger sight glass would make any difference. The problem with the sight glass seems to be when you are using a large amount of steam the water in the gauge starts boiling violently. Perhaps the bottom feed for the glass could be routed along the side of the boiler and back again to the glass making the water in the glass slightly cooler than the water in the boiler, this might prevent it boiling.
Any thoughts?
Stew
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Stew, I have found surface tension in a 5mm out side diameter tube the problem with the unreliable readings. Even just filling the boiler. Laboratories would be a good supply for 1/2" glass tube, so not hard to find. Just scale appearance the only concern!!! Geoff
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Hi
You are starting to discover what I found when trying to "look" through 5mm glass tube, results not reliable!
Think blowdown first........it is all in my article but do not know when it will be published.
Tim
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I've heard about people using a fine copper wire in the sight glass to reduce surface tension, does this work?
I'm sure I've also read about someone adding glycol to the water but I can't remember if the outcome was good or bad.
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Tag
Do you have any resistance readings for the sensor in steam and hot water, as I need some values to complete my circuit.
Did you make your sensor like the one in Malcolm Beaks article or is yours a different design?
Stew
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Hi Megatron
I too have heard of putting a wire in the sight glass and by all accounts it works. I have not used this method as the opto coupler system "sees" it as water always present, the same using a fibre optic. Therefore method rejected. Not wild about putting additives in boiler feed water. Did once try a tanin based additive (as used on some preserved steam locos) but the brown colour confuses the opto.
I am afraid I have never seen Malcolm Beek's electrode design and my grade in Paddleducks is to low to let me look at his design. I use the Roy Amsbury design.
As far as water resistance is concerned it is what you make it. I use air conditioning condensate with a dash of tap water to make it reliably conducting. Steam which is pure water has theoretically infinite resistance hence the ease of detecting the presence of water using electrodes.
My electrode electronics (AC) can detect water resistance up to 150 kohms.
Hope this helps Regards Tim
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Stew - ref. pump start delay, that's exactly what I've done. When top sensor goes dry, white led goes out and delay is triggered. After 10 seconds-ish, motor starts and runs until sensor goes wet, white led illuminates and pump stops. And so on, ad infinitium. Or at least until top up supply tank runs dry.
Ray.
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Ray
great minds think alike.
Is your little tail still wagging?
Stew
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Faster than ever!
Just completed calculating pump flow rates et.c. from equation you pointed at.
Conclusion: single cylinder 12mm diameter x 12mm stroke will deliver required flow rate into 60p.s.i. at 9.2 r.p.m!
Kind of proves your point I think! I've yet to measure motor speed. I'm guessing a couple of NiMH cells will do it, if not one,
and a much smaller pump of course.
Greatly appreciate your guidance.
Ray.
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I am following with interest although I know nothing about electronics. I still like to but my pennies worth in. The idea of Glycol in the boiler was to make the steam more visible out of the funnel, apparently not a good idea. As for 60psi in the boiler remember it is one square inch, so the smaller the piston bore the less force required. I am sure intelligent people like you know this, but some think 61 lb will be needed to put water in not so. Geoff
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Ray
You may want to put another sensor into the feed tank so that when the water level gets low it prevents the pump from switching on and possibly running dry.
Stew
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Megatron - there's some serious telepathy going on here! The first sensor system I built was neatly packaged in a small plastic 'black box' with no room for the motor delay circuit. I've since shoe-horned the whole caboodle into a slightly larger box with a new built circuit board. Contemplating the old one I thought, I can use that for the feed tank, and use it to stop the motor when empty!
Telepathy goes both ways - so I've had to restrain myself from telling you what you're going to get for Xmas!
Have a good one, and hopefully a somewhat less wet new year.
Ray.
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Ray
I hope your thinking something red, fast, made in Italy, and beginning with the letter F.
Merry Xmas
Stew
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Ray
I hope your thinking something red, fast, made in Italy, and beginning with the letter F.
Merry Xmas
Stew
I suppose it depends on your definition of fast, but I fail to see why anyone with any sense would really want a Fiat. ;D
Merry Christmas
Eddy
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Thankfully Eddy my definition of fast does NOT include Fiats.
Stew
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Thankfully Eddy my definition of fast does NOT include Fiats.
Stew
We certainly agree on that one Stew! :08
Eddy
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TAG
I've just read your article in ME which I found very interesting.
I am disappointed to hear that the Optek sensors gave variable results. I found exactly the same thing using home made sensors and was thinking about using the Optek ones to get over this problem.
You say you had to use a variable resistor on the pcb to calibrate these. You could calibrate the sensor on a test rig and instead of using a variable resistor on the pcb use a small 1/8w resistor in the lead to the sensor, all the sensors would then read the same, independant of the pcb, making it possible to replace the sensor without any further adjustments.
Ray
Have you got your system up and running yet?
Stew
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megatron
Pleased you enjoyed my article in ME.
I agree your system of calibration would be the way to get over the problem. I think it would be better to feed the output of the Optek sensor into an op amp in max gain mode such that the change from water to no water would cause a swing from 0v to +5v then the PIC would know exactly what to do!
You probably gathered from my article that I have abandoned my work on the optical system because of the bubble problem and am standardising on the electrode system. I have some prototype optical set ups that I could let you have to play with,pm me to discuss.
Regards
Tim
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Tim
Even if you use an opamp to supply the pic you still run into the same problem.
I use opamps in my system with the reference voltage set at half supply voltage (2.5v) and using various values for the resistors to the led and the phototransistor the best I could achieve was a swing of from 2 to 3v for high and low water. These values could easily change by over 0.5v from one sensor to another.
I would love to know what components Cheddar used in their sensors. I have had a look at one and there appears to be an ir led and some form of detector which has 4 conductors coming from it. This detector is wrapped in aluminium tape with a very fine slot cut in it, the tape also covers any indentification numbers which may be on it.
I'll email you to discuss trying one of your sensors.
Stew