This turned up on my Facebook page:

https://www.facebook.com/DurangoExposur ... =3&theater

Interesting info! Pyrex was also used to make insulators.

Pyrex Red Stripe is the best tubing for water glasses on smaller boilers.

Sammy, do you mean Red Line? (I thought that was just the applied stripe for easier reading, and that the actual 'grade' was something like high-pressure heavy-wall borosilicate).

There is a potentially useful guide for gauge glass selection downloadable from here:

http://boilerparts.boilersupplies.com/gauge-glass-selection-guide-0?__hstc=213249483.a5f40275cc8d70a54ec44849c56a2499.1443837841976.1443837841976.1443837841976.1&__hssc=213249483.1.1443837841983&__hsfp=2151590729

In my working days as an analytical chemist, I used lots and lots of Pyrex glassware (beakers, flasks, etc) in the lab.

Sorry, but I can't read the gauge glass guide on my prehistoric pc. I'm also the kind of engineer that goes forward/backup/stop, not a chemical or mechanical one, so I don't know diddley about glass formulas. I just make sure we don't run low on water.
It's been years since I bought water glass tubing, but I'm pretty sure Holmann carried this under the Pyrex Red Stripe brand. "Red Line" sounds just as good, it's probably the same thing only different.

Whatever you call it, and whatever it's made of, the stuff I like has a painted or baked on stripe, and I'd say it makes for a real easy read. I can usually watch the water level from the ground or from the cab of another engine on the next track with no problem, makes life easier when you're busy.

These later comments on gauge glasses got my curiousity up about something.

It's quite understandable how modern gauge glass material would be something like Pyrex with its ability to handle high temperatures. But gauge glasses have been around in some form since at least the 1860s. What sort of glass would have been in use then? If it was some sort of common glass, how was it made strong enough to handle what it did?

There are really two connected issues involved here: making the glass capable of handling heat expansion, and making glass (or gauges) resistant to corrosion from elements such as alkaline treatment in the water.

In the 'olden days' both the working pressure and the corresponding saturated-water temperature were lower. Strength of the glass was not as much a concern as the expansion of the glass against the seals and fittings holding it, especially in the days before high-temperature elastomers -- presumably some form of packing was used at either or both ends and the flat or tubular glass allowed to compress or slide past it.

The great innovation in borosilicate glass is that its coefficient of expansion is very slight. When used for baking dishes and the like, this is important because rapid temperature changes are significant as well as overall expansion and contraction during temperature swings -- think of pouring relatively cold liquid into a hot baking dish to make gravy. Glass cracks or shatters when there is a discontinuity in the structure, and if the coefficient of expansion is very slight there will be less tendency for differential expansion or contraction (particularly the latter!) to produce propagating cracking.

In a gauge, the situation is a little different, as it is when making something like a Geissler tube (think neon lighting) where a metal electrode goes through a glass envelope. Here your concern is to match the coefficient of expansion of the glass with the metal. In some formulations, the metal used is adjusted to suit the glass (that is a reason platinum was often used for electrode material).

There are many people here, Mr. King prominent among them, who can tell you far more about the practical construction and 'care and feeding' of these gauges than I can.


Meanwhile, the chemical etching and thickness reduction of gauge glass in a major concern. It is my understanding that the boron in borosilicate glass is subject to some of this effect, although much less than sodium, lithium, etc. in more conventional glass formulae. There will be experts here who know the precise formulation of low-expansion glass for high-pressure steam service that minimizes the practical "wasting" or etching that occurs, or what the best "tradeoff" between boiler-metal corrosion reduction and gauge-replacement interval due to wasting is.

Note that a very common method of dealing with this, with flat gauge glass, is to use a thin sheet of something like 'ruby mica' between the inner face of the glass and the water. I suspect that some other chemical treatment, like the method used to prestress "Gorilla Glass", might make glass surfaces more resistant to attack from effective boiler treatment.

Bob, we don't worry about those things and just change them frequently before they fail. That's one reason the thicker reflex glasses came about to replace the tubes - less frequent need for replacement.