Hi Hsv
First up what you want to do can be done in xml

Its going to be hard to explain in a simple message here.I would suggest that you have a look at the xml gauge tuts in the tut section here
http://www.fs2x.com/ next
have a look at this
http://forums.avsim.net/dcboard.php?az= ... pic_page=2whilst it deals with oxygen system it shows how to use L:vars to empty/refill
if thats all to daunting do a search for some fs9 gliders that have a ballast dump gauge, most I have seen are C++ gauge files(.gau) but you might find a sample xml code one
and finally do a search for crop dusting effects there may be a controller gauge that you could use learn from
cheers
Wozza