In answer to Rob Buckingham who was looking for clear, rigid tubing.
If you are anywhere near an agricultural area ask a farm supply store
or fertilizer dealer for anhydrous ammonia tubing.  It is usually
slightly cloudy and, while not rigid, it is quite stiff.  Not perfect,
but readily available in agricultural areas.  My sight gauge is flexible
tubing fastened to a piece of scrap lumber, and it does the job.

John Dewey