| Reflex gauge | Transparent gauge | |
|---|---|---|
| Glass | One prismatic grooved glass | Two plain glasses, one each side |
| What you see | Liquid black, vapour silver | You look through the liquid |
| Readable from a distance | Very good | Moderate |
| Shows liquid colour | No | Yes |
| Shows liquid-liquid interface | No | Yes |
| Steam and caustic service | Glass etches — limited | With mica shield, good |
| Illuminator possible | Not usually needed | Yes, for dark liquids |
| Relative cost | Lower | Higher |
| Typical limit | 100 bar / 350 °C | 100 bar / 350 °C |
If the answer is "is it above or below this line", reflex wins. If it is "what colour is it" or "where is the interface", only transparent will do.
Saturated steam and caustic soda attack borosilicate glass. Over months the surface clouds and the gauge becomes unreadable. A transparent gauge with a mica shield puts a sacrificial sheet between the liquid and the glass.
Reflex prismatic glass produces a hard black-silver boundary visible from twenty feet. Transparent glass does not. On a boiler house platform that decides it.
Measure between the tapping centres on the vessel, choose side/side, side/bottom or top/bottom, and state the flange rating or thread. Gauges are made to that dimension.
Gauge valves with ball check, frost extension for cold service, illuminator for dark liquids, and a protective guard. Retro-fitting these afterwards usually means a shutdown.
A gauge that cannot be reached with a spanner will not be maintained. Check that the glass, the gaskets and the gauge valves can all be reached from a platform or a ladder, and that the vessel can be isolated without a full plant shutdown. On tall vessels, two shorter sections with an overlap are often easier to service than one long one.
Clean single-phase liquid, ordinary chemistry, reading from a distance, budget matters.
You need colour or an interface, or the service is steam or caustic and you want a mica shield.
The vessel is very tall or the liquid is dirty — a magnetic level indicator or a level transmitter may serve better.
Glass, gaskets and mica shields are wear parts, not permanent fittings. On steam and caustic service plan on replacing the shield annually and the glass every two to three years. Buying the gauge without a spare glass and gasket set is a false economy — the spare costs a fraction of the downtime.
The glass has 90-degree prismatic grooves. Where vapour touches the grooves light is totally internally reflected and appears silver; where liquid touches them it is not, and appears black.
It is a thin sheet placed between the process and the glass. Steam and caustic attack glass; the mica takes that attack instead and can be replaced at a fraction of the cost.
Yes — that is exactly the case where it is preferred, with a mica shield. A bare reflex gauge on saturated steam will cloud.
Up to about 1750 mm centre distance per section. Taller vessels use two or more sections in series with an overlap.
Forged chamber gauges are commonly rated to 100 bar and 350 °C. Higher ratings are made to order.
On clean, ordinary service a borosilicate glass lasts years. On saturated steam or caustic it clouds much faster — with a mica shield fitted, plan on replacing the shield annually and the glass every two to three years. Always replace the gaskets with the glass.
Tell us the medium, the pressure and temperature, and the tapping centre distance. We will confirm reflex or transparent and quote it made to your vessel.
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