Correct way to read the sight glass

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DannyTheElfman

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Hey everyone,

I'm sorry but I have a really dumb question. What is the correct way to read a sight glass on an Accucraft Loco? On my Countess the sight glass sits higher up than the boiler. I've drawn a diagram below to illustrate. But in the example the boiler is half full, but the water in the glass looks less than a third full. Is this correct or would the sight glass always mirror what is happening in the boiler, even if they sit at different heights?
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You can see here the the boiler sits below the top of the sight glass. So presumably even if the boiler were full the glass would only read about 3/4?
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I understand the sight glasses are actually pretty poor even at the best of times. Mine is definitely doing it's job, the longer I use the loco the more the water goes down, and a top up increases the water level in the glass. So it's definitely doing something correctly. I just don't know how to read it correctly.

Many thanks
Dan
 
It is like you showed in the drawing. With communicating vessels, the water level will be at the same height in both.

So the part of the sight glass that is higher than the boiler isn't useful for sighting.
 
Best thing to do to help is to get some way of putting stripes at 45’ to the glass. White metal sheet with bkack felt tip lines works quite well. This makes reading it much easier due to complex ways that water and glass work. Though that will not always help if the water in the glass is way out of kilter to the boiler level as it tends to be part of the time.
 
Yes, the water in the boiler and the sight glass should be the same height. Bear in mind that it is really for guidance only, as the level in the glass will go up and down with the movement of the loco. TBH I have only 1 engine with a sight glass, as the ones I have the gas is primed to run out before the water does
 
For decades we managed to run our tiny dragons without a sight glass and just learnt to get on with it, adding a few squirts of water if possible every few minutes to keep us in a good place. Early Roundhouse loco with "pop" safety valves were particularly helpful in that the safety valve would rapidly lift and then close when water was getting low, a feature lost now "pop" valves are a thing of the past on RH locos. And, of course, if the loco stopped with no pressure but the gas or meths still lit we knew we had gone too far ... put it out, and let it cool down!

I have several locos with gauge glasses and I adopt a simple approach to them: I don't believe anything they tell me and I have just learn to manage the loco.

I have two that show nothing in the glass when the boiler is full to the brim. I have one that shows a full glass even when the boiler is empty. Various chemical attempts to make them work have failed. Can I be bothered to take them apart and risk breaking the components to make them work properly? No, they will stay as they are.
 
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The sketch you have drawn is correct, although the section of glass above the boiler is technically surplus to requirements.

The water level in the boiler should always be above the fire tube, in a 1:1 loco a fusible plug is fitted to the crown of the boiler, should the water level drop below the top/crown of the firebox the fusible will melt a term known as dropping a plug, this save the boiler from becoming a poetical explosive device. Water in the sight glass should never under any circumstances be out of view.

The reason models historic and even new, aren't fitted with a sight glass is because they are generally designed so that you run out of gas before you run out of water, this in turn means the fire tube should never not have water covering it.

I have fired in a line where the terminus approach is on a steep downward grade, this line requires the driver to confirm with the fireman if it is okay to proceed down grade lum first in to the station, one one, and only one, occasion I need to advise the driver I wasn't happy to proceed and we stopped while I put more water in to the boiler.

A sight glass is always required on a coal fired boiler because the heat source is more than like to be present when the water level has dropped, personally I would never let the water drop below 1/3 of the sight glass, but that is a person preference as a 1:1 scale driver/fireman.

Hope this helps.


David
 
Thank you everyone! I'm learning not to put too much trust in the sight glass water level. It's good for an initial fill seemingly but as many say it's a little sporadic at the best of times. I'll keep it topped up because the Countess seems to be a heavy water user and gets through a full boiler before the gas runs out, I'm keen to not cause any damage!
 
Dan's diagram is a good one to contemplate where a gauge glass is fitted - on the Accucraft 7/8" Decauville that I have detailed in another post on this forum the glass incorrectly shows what could be assumed to be a safe water level when half a glass is actually exposing the top of the burner tube.

This is another design fault with this locomotive - the bottom nut (as referred to even on full size engines) should have been set level with the top of the burner tube, not below it, and the glass extended could have been as far above the boiler as per Dan's Countess. Worst case with water to the top of the Countess' glass will be no steam space in the boiler - on a real locomotive this will lead to priming with potential hydraulic damage to pistons (even with the cocks open), Westinghouse pump, turbo, etc.

David's comments about the fusible plug in the crown sheet on full size locomotives are correct and allowing the exposure of the crown sheet is to be avoided at all costs (considered the ultimate sin).

Despite having a blow-through cock on the Decauville it's water level was often not accurate due to a meniscus sometimes forming in the glass. This has been largely overcome by permanently inserting a thin copper rod inside the glass - I also intend to fit a zebra pattern backing behind (as often done on full size boilers (locomotive and others).

Tony's comments about not relying on these glasses are quite valid - particularly if your gas tank gives the ability to exceed the boiler capacity as it does on the 7/8" Decauville when the underfloor tank is fitted.

See other info on this loco, if interested, at 7/8" Decauville sorted with Radio Control on this forum.

The Klinger type gauge glass used by Roundhouse on my Davenport is a somewhat more reliable indicator than the one on either the Decauville or my Bowande Falk.
 
DannyTheElfman said:
even if they sit at different heights?
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The reason for the different physical heights is that you should only care about the water level above the flue.

Incidentally, Accucraft locos are a little more tolerant of low water levels than Asters. I hear you can distort an Aster by dropping the water below the top of the flue. From personal experience, I can tell you an Accucraft will probably survive without damage.
 
Fred2179G said:
The reason for the different physical heights is that you should only care about the water level above the flue.

Incidentally, Accucraft locos are a little more tolerant of low water levels than Asters. I hear you can distort an Aster by dropping the water below the top of the flue. From personal experience, I can tell you an Accucraft will probably survive without damage.
Click to expand...

The word "probably" in Fred's comment is an indication as to why you should avoid this if at all possible - repeated occurrences may very well lead to a boiler failure, particularly with a second-hand locomotive where its history in relation to running dry is not known.

Fortunately in this scale the consequences are not likely to be catastrophic - unlike one instance in the US where the hostler climbed into the cab of one of the engines under his charge in the roundhouse and put the injector on without looking at the water level in the gauge glass. That this is what happened is known - one of the fitters working on the engine actually survived the explosion when the crown sheet let go, launched the boiler in the air and took out the roof of the roundhouse.............

The second fitter and the hostler were killed immediately and much of the roundhouse was destroyed along with large amounts of damage to the surrounding engines.

From memory of the article detailing this it was in 1942 at Bellefontaine, Ohio. - cannot find the scan of the article at the moment.

A colleague of mine, who I fired for on a regular basis, used to drive steam during the sugar cane season at Qunaba, Queensland. He avoided driving one particular Bundaberg Fowler as its regular driver had a habit of dropping the fusible plug due to low water level when topping a grade. Russ always felt the Bundy was likely to let go the crown sheet one day due to repeated treatment like this.

Fortunately it never did.................
 
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