Water level sensor

[megatron]

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

[rayspan]

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!

[megatron]

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

[megatron]

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

[rayspan]

Thanks megatron, I'll check it out.

[TAG]

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)

[megatron]

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

[TAG]

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

[derekwarner_decoy]

Hi PD's....& welcome Tim... ...clearly your postings will make valuable reading for those interested or wanting to semi automatically control & maintain a reliable boiler water level................ .....Derek

[rayspan]

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!

[megatron]

No problem Ray.
If you need any help with circuit get in touch.
Stew

[R.G.Y.]

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

[rayspan]

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.

[megatron]

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

[megatron]

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