sight glass has a valve at the bottom, in order to shut this end off
when replacement is needed.

There should be a shutoff valve at the bottom and top of a sight glass.
The upper attachment point for a sight glass is never right at the top
of the tank--- it's always an inch or two or more lower.  If the tank is
full and there is no closed valve at the top of the tube, and if the
tube splits or starts to leak, all the fuel in the tank down to the
level of the upper sight tube attachment will leak out of the tube into
the bilge and may get pumped overboard by the bilge pump.  In the case
of a large tank, this could be many gallons of fuel.

Our boat is equipped with sight tubes on the four main tanks, all with
valves on the bottom only.  The surveyor who did the initial survey on
the boat when we were contemplating buying it pointed out the danger of
not having an upper valve on each sight tube and said (I think) that
having both upper and lower valves is at least a "recommendation" of the
ABYC.  However he did not write up the lack of upper shutoff valves on
the tubes, nor did the surveyor we had survey the boat several years
later.  But lower valves only is not a good practice.

C. Marin Faure
GB36-403 "La Perouse"
Bellingham, Washington

Comment
It has come to my attention that some electronic vessel monitoring systems
have a pressure transducer for reading tank levels by inserting this
pressure transducer in the bottom fitting of the sight glass column.

To my mind this is not a good practice. Why bother with an electronic sensor
for remote readout unless you intend to leave the valve open all the time?
And that means you have the increased risk of the sight glass breakeing and
dumping fule into th ebilge.
If you have to go to the tank and open the valve each time in order for the
remote display to read tank level you are already there to read the sight
glass. Which makes the remote readout rather pointless.

Am I missing something?

Arild