Water Gauges

First the safety 'talk'.
There is always a risk of glass shattering so good safety glasses are not just a luxury, they are required.
Anything that will keep angle grinder sparks and cinders out of your eyes are adequate.
Gloves, if used, should be of thin leather for sensitivity of touch, and to protect against accidentally picking up hot glass.

Cutting:

There are two common ways to cut glass tubing. The techniques are the same whether you are cutting Schellbach (white/colour striped) tube or clear glass.

The one most people use for smaller diameters (up to about 7mm OD) is the score and snap method. The technique is quite simple in principle, but needs practice to achieve good results. A notch (just a scratch is enough) is made with a suitable tool on the tube at the point where you want to cut the tube, and the tube is then snapped at the scratched point. Suitable tools are window glass cutters, so-called glass knives [Steelite(R) blade], diamond knives, or even a new three cornered file.

Then holding your forefingers on the tube at the points indicated on the drawing, gently apply pressure opposite the notch with both thumbs (thumbs should be touching ); the glass will snap very cleanly. As you snap the tube do this by applying a VERY slight bending action away from your body. The tube should snap quite easily. Bare hands are fine, but the use of thin leather gloves may be a wise safety precaution, as well as protective eye glasses or goggles.

Tips for a good result:

• The tube should be supported so that absolutely no bending forces affect the tube while it is being scored.
• When snapping the tube it should be held so that your thumbs meet exactly opposite the scored mark. Refer to the diagram at above left showing the score, your thumbs should be where the arrows in the picture are.
• As you snap the tube do this by applying a VERY slight bending action away from your body. The tube should snap quite easily.

The other common way is to use some sort of cut-off wheel. This can be used on any diameter tube up to about 35mm. Any wheel containing an abrasive harder than glass will do. The finer the abrasive and the thinner the wheel the better the job you will do. If the wheel is too small to transect the tube, then the tube should be rotated against the direction of the spinning wheel. Some people have suggested putting the job in a lathe to hold the tube. This will work but the glass MUST be protected from direct contact with the metal jaws of the lathe. Anything with a bit of 'give' will do as long as it is held concentrically. Glass blower's lathes have a modified three jawed chuck with the jaws wrapped in fibreglass cloth.

Another technique that is used by glassblowers for large (over 20mm) diameter tube is a combination of both these methods. It is very difficult with a high failure rate while you develop your technique. The scratch is made around the full circumference of the tube using a very fine edge. Use a diamond pen held in a drilled block at right angles to a short length of brass angle. A spare piece of glass is heated strongly with a torch flame and the hot glass pressed against the scratch. This should cause a stress fracture to 'run' along the scratch and cut the tube. I am NOT advocating you try this but merely include the information for completeness. The temperature of the heated glass is critical. Get it wrong and you could shatter the tube you are trying to cut. Flying glass is a common hazard so don't try it unless you are confident AND have safety equipment (safety glasses especially). [And some spare tube to practice with first!]

Once you have cut the tube it still will be almost certainly not be square enough for some purposes. You could use an abrasive wheel (180 grit or finer) using v-e-r-y gentle and careful pressure, but you can get a better result with a piece of carbide paper/cloth on a flat surface. Simply hold the tube and lightly rub the glass against the abrasive surface. A little water will help prevent clogging.

Click here for some photos of cutting and annealing a glass tube.

Annealing:

After the tube is cut, whether by score and snap or by using a cutting wheel, there's still one very important thing to do: Fire polishing (or annealing) the finished tube is extremely important. This removes any sharp edges, and stress relieves the glass which may otherwise lead to a fracture during use at a later time. It also slightly rounds the ends, which prevents damage to seals such as O-rings while inserting the tube into the gauge assembly.

This is easily done with a clear flame (no yellowness at all to it, oxy-gas or air-acetylene) by v-e-r-y gently passing the cut end through the flame until you see the sharp edge just start to round over. Rotating the glass whist in the flame helps an even heat. Glass has a very narrow solidus-liquidus range so it really shifts once it starts moving. Leave in the flame for too long and the tube will slump and bend or collapse. The glass edge melts ever so slightly, leaving a nice, smooth, rounded edge. A few tries will get it perfect.

Work on a firm surface that will resist burns from hot glass. Metal bench tops are a problem because tiny bits of swarf can scratch glass and weaken it. After all cutting glass is controlled scratching.

Sometimes, it can be difficult to see the water level in the glass, or be able to tell if it is completely full, or completely empty (a potentially boiler damaging condition!), and a visual aid is used to assist in determining the water level. This can be a coloured line (usually blue or red) painted along the glass during manufacture (Schellbach tube), like in a thermometer where the line appears thicker when covered by liquid, or something more elaborate.

Here is a little visual example of how to make the water level more visible in the glass, or tell the difference between full and empty.

Place a diagonally striped background, 1 to 2 tube diameters behind the glass, and you'll be able to see the water level much more clearly than without this aid!

These are actual, un-retouched photos of a 6mm (1/4") dia glass tube half filled with water, shot against a diagonal pattern. The water level shows still more clearly. For installing in the cab, make the pattern just about as wide at the tube, there will not be a distracting background (as here, for demonstration only).

• The lowest visible part of the tube (the so-called "bottom nut") MUST be more than 10mm above the crown sheet or flue tops within the boiler
• One water gauge is the minimum compulsory requirement, but often, if cab or back head space permits, 2 are installed.
• The top connection may be via a tube to the boiler if space is small, it only carries steam with very little flow. However, it should have its own dedicated boiler fitting and not use a spare outlet on the turret, otherwise a false reading will be obtained due to pressure change when other accessories are used.
• It is desirable to have a gauge blow down valve at the bottom of the assembly. Occasionally bubbles form inside the glass tube preventing an accurate level reading, and can be cleared using this valve.
• Use a piece of steel rod, the same diameter as the glass, during assembly to adjust the alignment etc of the fittings. When satisfied, remove the dummy metal rod and carefully insert the real glass as the final step.