http://www.crh.noaa.gov/iwx/program_...coop/8inch.php
Collector Funnel
"Two types of collector funnels are used with the SRG.
Older units are made of copper while newer funnels are made of fiberglass. Rainfall is caught in the collector funnel and conducted into the measuring tube. The funnel is
8 inches in diameter with a knife edged rim. It is designed to readily slide over the overflow can. Some lock into place with 2 slots fitting over corresponding screws in the overflow can. The collector funnel also serves as a deterrent to evaporation.
The cross-sectional area of the collector is 10 times that of the measuring tube. Therefore, the depth of the water standing in the measuring tube is exactly 10 times the depth of the precipitation that has fallen.
Caution should be taken when handling the metal collector funnels. Dents in the lower edge of the funnel can make it difficulty or impossible to fit over the overflow can. If necessary, a small hammer can be used to remove dents so it will fit over the can. Also, fiberglass collector funnels are easily cracked or broken, if dropped."
Collector Funnel and Measuring Tube
Is it 8" diameter?