| Specification: | A sight glass or Level gauge is a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.
A reflex gauge is more complex in construction but can give a clearer distinction between gas ( steam) and liquid ( water) . Instead of containing the media in a glass tube, the gauge consists of a vertically-oriented slotted metal body with a strong glass plate mounted on the open side of the slot facing the operator. The rear of the glass, in contact with the media, has grooves moulded into its surface, running vertically. The grooves form a zig-zag pattern with 90° angles. Incident light entering the glass is refracted at the rear surface in contact with the media. In the region that is contact with the gas, most of the light is reflected from the surface of one groove to the next and back towards the operator, appearing silvery white. In the region that is in contact with the liquid, most of the light is refracted into the liquid causing this region to appear almost black to the operator. Well-known makes of reflex gauge are Cesare-Bonetti. Due to the caustic nature of boiler anti-scaling treatments ( " water softeners" ) , reflex gauges tend to become relatively rapidly etched by the water and lose their effectivess at displaying the liquid level. Therefore, bi-colour gauges are recommended for certain types of boiler, particularly those operating at pressure above 60 bar.
A bi-colour gauge is generally preferred for caustic media in order to afford protection to the glass. The gauge consists of a vertically-oriented slotted metal body with a strong plain glass to the front and the rear. The front and rear body surfaces are in non-parallel vertical planes. Behind the gauge body are light sources with two quite different wavelengths, typically red and green. Due to the different refraction of the red and green light, the liquid region appears green to the operator, while the gas region appears red. Unlike the reflex gauge, the glass has a plane surface which it does not need to be in direct contact with the media and can be protected with a layer of a caustic-resistant transparent material such as silica.
Magnetic Gauge a float on the surface of the liquid contains a permanent magnet. The liquid is contained in a chamber of strong, non-magnetic material avoiding the use of glass. The level indicator consists of a number of pivoting magnetic vanes arranged one above the other and placed close to the chamber containing the float. The two faces of the vanes are differently coloured. As the magnet passes up and down behind the vanes it cause them to rotate, displaying one colour for the region containing the liquid and another for the region containing gas. Magnetic gauges are stated in various manufacturers' literature to be most suitable for very high pressure and / or temperature and for aggressive liquids. |
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