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Making sight tubes easier to read



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What is the issue?
Some older boats are fitted with transparent tubes, called sight glasses, to gauge the level of fuel or water stowed in a tank. These are by far the best systems to accurately assess the contents of a container but they have one critical downfall, sight tubes can be very difficult to read.

Mounted near to tanks they tend to be situated in awkward places that are as often as not subject to poor lighting. Also, as in the case of water which is almost transparent, they are very difficult to see.

Why address this?
Keeping track of fuel and water availability is essential when running and provisioning a vessel. A good sight tube system is perhaps the most reliable indicator if it can be used.

How to address this?
Use the refracting property of liquids to make the fluid levels highly visible as presented.

Sight glass with diagonal lines
Drawing: Tony Gibson
A series of diagonal lines behind a sight tube will be refracted to the horizontal behind the fluid, marking the fluid level highly conspicuous.

Sight glass using the refracting property
Photo: Elsie esq. Les Chatfield via CC BY 2.0
Most yachts these days use sophisticated float switches instead of sight glasses. But they have to be calibrated correctly and cannot be totally relied upon until that is done. However, I got myself into a terrible mess on spring tides in the crowded Beaulieu River trying to figure out why my engine was running rough and cutting out when my gauge said I had somewhere between ¼ and a ⅓ of a tank of fuel remaining i.e. it was calibrated incorrectly. A sight glass may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other, but it cannot be calibrated wrong. The level of liquid in the sight glass will be the same as the level of liquid in the tank of course, after making allowances for the shape of the tank.


With thanks to:
Michael Harpur, Yacht Obsession.

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