Water gauge

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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 Carl

From The Live Steamer, March-April 1951

Charles A. Purinton

Not so long ago the question of where to put the bottom fitting for the water gauge was asked by one of the live-steamers. This is a natural question and also a very important one.

The bottom fitting whereit is screwed into the backhead of the boiler should be so located that as along as the crownsheet is covered with at least one quarter inch of water, water will be visible in the waterglass. In other words, as long as you can see water in the waterglass, there will be a minimum of one quarter inch of water on top of the highest point of the crownsheet. needless to say, it is most important that the crownsheet never be dry.


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.

Effect of Slope

Jeffrey G. Hook wrote the following:

In C. A. Purinton’s writings the installation of a water glass, also known as a water gauge, is described as being mounted on a typical miniature steam locomotive at an elevation such that its lowest reading, or the lowest level of boiler water that may be seen in the glass, is not less than one-quarter inch above the elevation of the highest part of the crown sheet. The foregoing understood to be accomplished with the locomotive in working order and standing on level track. Given the time period of C. A. Purinton’s writings it may reasonably be assumed that the author was referring to miniature steam locomotives constructed to represent full scale practice standard gauge examples and no larger than that which would result from employing a miniature to full scale practice reduction ratio of three-quarter inches equals one foot or possibly one inch equals one foot.

Given that a typical steam locomotive water gauge is not mounted laterally at a point directly to the left or right of the highest part of the crown sheet and instead is mounted near the back head of the boiler, any change in inclination of the locomotive will change the elevation relationship between the highest part of the crown sheet and the lowest reading of the water gauge. Thus if the smoke box end of a typical steam locomotive is raised in elevation relative to its firebox end, as when the locomotive is traversing forward with smoke box ahead on track described as an up grade, if the gradient of the track is continually increased an inclination of the locomotive will be reached that results in the highest part of the crown sheet being at the same elevation as the lowest reading of the water gauge. The engineer in charge of a steam locomotive, whether driving a short firebox 0-4-0 type or a long firebox with combustion chamber 4-8-8-4 type, regardless of the gauge of track, must be fully aware of what maximum track gradient the locomotive may be placed on with the smoke box up and the level of boiler water when seen at the lowest reading of the water gauge indicates that a sufficient depth of boiler water exists above the highest part of the crown sheet.

The relationships between the highest part of the crown sheet, the lowest reading of a water gauge, the center of the lowest boiler gauge cock and a given track gradient with the locomotive smoke box up in essence takes the form of a right triangle and is illustrated by Sketch No. 4 on Baldwin Locomotive Works (BLW) Standard Practice Sheet (SPS) 12-1C of January 3, 1936. The foregoing showing the elevation of the lowest gauge cock applied to the boiler and no water gauge being illustrated. On BLW SPS 12-4A of October 30, 1936 is illustrated the requirement that the “LOW WATER LEVEL” or lowest reading of a water gauge be at the same elevation as that determined for the center of the lowest gauge cock applied to the boiler. The following formulas and Deerfield and Roundabout Railway (DRRY) document DRMCH61-Eng-308 have been produced based on the principles illustrated by BLW SPS 12-1C Sketch No.4 and BLW SPS 12-4A.

E = ( L + T ) * ( G / 100 ) + D therefore G = ( E – D ) / ( L + T ) * 100

Where the following applies as specified and the locomotive is in working order and standing on level track:

Effect-of-Slope-Formula-Table-JGH 3-17-2019.png
Effect of slope on locomotive sight glass reading

Calculating Water Gauge Height

Jeffrey G. Hook has documented an old Lima trick for properly locating the height of the water gauge.

Calculating Water Level with Grade

Jeffrey G. Hook has provided a spreadsheet for calculating the maximum grade on which a locomotive can operate.

See also Fundamentals of Steam Locomotive Boiler Water Level Management.

Advice from Gerry Bowden

Jim Jackson wrote in HALS Stack Talk, October 1991:

From Southern Oregon Live Steamer, Gerry Bowden, who is visiting David Hannah this week, comes a way to reduce erratic water level bounce often seen in the boiler sight glass. Run the lowest backhead pipe connection a full quarter inch past the inside of the backhead. This buffers the unrestricted water movement in the boiler keeping the sight gauge level steadier.

Video Instruction

  • Dave Sclavi - Piping the Gauge Properly


  • Dave Sclavi - How to Cut Gauge Glass


  • Dave Sclavi - Installing a New Gauge Glass


See also

External Links