This is Armchair advice, and I hope an experienced loco builder will comment.  My interest is in full-size boiler accidents rather than model steam and misleading water gauges are a common factor.   They can read high when the boiler is dry and about to melt, or low when it’s full to overflowing.

I believe something is wrong, and the Britannia’s gauge can’t be trusted. Needs to be fixed – expensive bad news if water doesn’t completely cover the top of the firebox.  Even if it doesn’t puncture, likely to be permanently weakened by severe overheating.

In theory there is very little to go wrong with a water gauge.  The lower pipe connects into the boiler below the water-line, and the upper pipe connects to the steam space.  As the pressure on both sides is equal, the water line in the glass is level with that in the boiler.    What could possibly go wrong?

In practice a great deal!  The gauge relies on steam and water pressure being equal on both sides of the glass, and it might not be.

• Valves not fully open.  (The ones used to disconnect the gauge if the glass breaks.  Some nasty accidents due to fitters putting handles on the wrong way round, leaving dirt in the valve, and driver error.)
• A part blockage on one side of the feed pipework, so pressure can’t equalise in the glass.   A gauge reading high when the regulator is open suggests the steam-side pipe is blocked.  Could be scale, dirt, or – in a new engine – flux, or excess solder in a joint.  Or the pipe diameter is just too small.
• Design is another potential issue.  The steam side should have it’s own private connection into the boiler.  In practice, many designers or builders, connect the gauge to another convenient external steam pipe, perhaps going to the whistle, injector, water pump or whatever.  It simplifies the build.  The problem is the water gauge then responds pressure in the external pipework, not the boiler pressure, and it could be much lower.  The pipes are colder than the boiler and whatever it is feeding could be leaking. External pipe pressure can drop much lower than boiler pressure, causing the water gauge to give a false level.

All three faults tend to show up when the engine is consuming steam.   Working hard causes the boiler pressure to drop and pull steam out of the whole system, causing pressure in external pipework to drop significantly.  Then gauges that seem OK at rest, go wrong when the engine is ruining.

Check valves, look for blockages throughout, and make sure the gauge feed pipes are of the correct diameter.  If the gauge pipe connects to external pipework or a manifold as per design, check whatever else is connected for leaks or empty space remote from the boiler.  If the build was simplified by ignoring the design and connecting to an external pipe instead of making a separate connection, rebuild in accordance with the design.

Dave

John, greetings, the water gauges valves on the 71/4 gauge Modelworks water gauges are sealed with PTFE seals, and the drillings through the valves are quite small diameter.  What happens is that when warm the PTFE creeps, and can block these small holes, which causes effects such as you describe.

I am not sure if the 5 in gauge water gauges are the same, but I would not be surprised, so I suggest you have a look.

Regards

Brian B

Before something nasty happens you MUST resolve this issue ! High when opening the regulator and low when closing sounds like surging ! if you have only done 1.5 miles then you lack experience. You do not make it clear as to if this applies when on a rolling road – if not then surging ! Something you will learn to live with !   If not surging then, Others have commented on pipe work and different pressures under working conditions. The construction of the gauge glass fittings should allow you to remove the glass, then remove the fittings to check that all the portways are clear and look for packings, be they O rings or other types that have somehow closed over the tube end. When reassembling use steel rod to align the fittings true before refitting the glass. Good Luck Noel.

Hello John

I think you have now made it clear that the rise and fall of water level is seen when you are driving on a section of track – as you accelerate and decelerate this is quite normal. We normally note the water level when at a steady speed. Similarly, you will see the water level rise and fall when going up and down hill. The Britannia has a long boiler and levels change quite visibly.

If you have an early 5″ gauge Winson you will need to check the firebox height with reference to the bottom of the gauge glass as suggested. If however it is a later Modelworks boiler as you say then this height error was corrected.

You must ensure that you can blow down both the glasses to visually ensure the passageways are clear, and this will be a part of the steam test.

Norm

I would agree, having looked at the pictures.  The length of actual glass is very short and you would expect to see that much variation due to the uneveness of the track and surging.  Is there any way you could make new gauge top fittings to increase the length of the glass?  I would like to see double that length of actual tube.