A friend asked me a while ago why the FTS Cesium tubes were so light. Since they were designed from the beginning to fit in a 5.25" high chassis that cut quite a bit of the weight. However a large amount of weight was saved by using a Ramsey cavity that was not milled from a massive block of copper like the HP tubes use. The HP cavities weighed about 3 1/2 pounds for a standard tube and 4 1/2 pounds for the high performance model. The FTS design weighs a mere 14 ounces. Attached is a PIX of the FTS cavity. Cheers, Corby