A sight glass or water gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.
Advice from CAP
From The Miniature Locomotive, July/August 1953
Charles A. Purinton
2 Longfellow Road, Marblehead, Mass.
A few words of caution might well be in order. This time I want to preach a little about water glasses. Needless to say that they are one of the two most important safety devices on any locomotive, big or little. The second device, but of equal importance, are the safety valves. Before getting up steam, one should be absolutely sure that the pipes and fittings are 100% free of any obstruction and eposits of scale. Beware of a glass that looks full, it could look the same if it were bone dry! If you know for sure that the passages are free, roll your loco back and forth a bit and take particular notice if the water can be seen to move up and down in the glass.
I remember a case that happened several years ago. Two Live Steamers, one experienced and the other new, undertook to steam up the new man's engine. The glass looked full and they neglected to roll the engine back and forth to see if the water in the glass moved and also did not blow the glass out. I don't recall now whether or not the glass had a blow down valve, but if not it should have had one.
To make a long sotry short, they were unable to raise any steam and before they discovered the boiler was dry, the damage had been done.
The very first thing my favorite engineer never failed to do when taking over his engine was to check the water glass and then the gauge cocks. It is a matter of life and death to any engine crew.
In checking the top shut off valve where it was tapped into the boiler, he was not content to see if the handle on the extension turned and seemed to be open, but he always felt of the valve stem where it came out of the packing gland. He told me that he once got into trouble on an engine on which the handle was loose on the extension.
I once got on an engine in the engine house and looked at the glass. It showed more than half a glass. On checking the shut off valves, I found the top one shut. Luckily for everyone involved, there was still an inch of water showing after the valves were opened and the glass compared with the gauge cocks.
Another thing to consider is the location of the glass. I always try to locate mine so that as long as there is water in the glass there will be a good fat quarter inch of water over the crown sheet. They should also be located so one can see them easily. As I say this I refer to one beautiful running loco that comes occasionally to my track. I like to run and handle it as it runs exceedingly well hooked up and is also a very free steaming job. But I can't see the glass unless I stop and get off, which I do very often.