Wednesday, September 11, 2019

The 8-inch Standard Rain Gauge

The 8-inch standard rain gauge hereafter referred to as SRG) is the workhorse of the National Weather Service Coop Program. In fact, this gauge is the world-wide standard for measuring precipitation. The gauge is your basic straight-sided cylinder and is the big brother of the 4-inch rain gauge we use in CoCoRaHS. The components are the same. Each has and outer cylinder which catches overflow from the inner measuring tube. Both rain gauges have funnels which direct the precipitation into the inner measuring tube. The SRG includes another component, a measuring stick graduated to hundredths of an inch.

For a long time the SRG outer cylinder (aka "the can") and funnel were made of copper, and the inner tube was made of brass. Better and less expensive materials have for the most part replaced copper. The outer cylinder of the SRG pictured here is stainless steel, the inner tube is made of poly carbonate plastic, and the funnel is made of fiberglass. The outer cylinders are also made of aluminum. The measuring stick is a laminated fiberglass that "wets" so that you can read the precipitation measurement. Even with these newer materials there are issues. The outer cylinders can develop leaks along the seams on the bottom, and those can be hard to detect. Both the brass and plastic inner measuring tubes can be damaged and develop leaks in freezing weather.


Funnel and inner measuring tube of 8-inch SRG. Credit: NWS
One of the advantages of the SRG is its capacity. The inner measuring tube holds two inches of rain, and the outer cylinder holds 20 inches of rain. However, that advantage is also a big disadvantage. The 8-inch outer cylinder holds about 4.3 gallons of water, and a gallon weighs 8.3 pounds. An observer would have a difficult time lifting 35 pounds of water and even a more difficult time trying to pour anything into the inner measuring tube. Four inches of water in the outer cylinder would be about a gallon, and that plus the weight of the cylinder itself can be cumbersome to handle. It is definitely not a one-person job. My personal experience with the 8-inch "can" and pouring led me to build a stand to hold the inner tube while pouring from the can. The funnel is placed on the tube before pouring.


Once I became a CoCoRaHS observer I designed and built a smaller version for the CoCoRaHS gauge. You can find the plans to build one at this link.


The 8-inch SRG is the standard at NWS primary Coop sites, but states "The four-inch plastic rain gauge is a suitable substitute for the eight-inch standard rain gauge because it meets the accuracy requirements". That accuracy is specified as ±0.02 inches. (National Weather Service Instruction 10-1302: Requirements and Standards for NWS Climate Observations, April 2018) 

1 comment:

  1. "That accuracy is specified as ±0.02 inches."

    I'll have to add that to http://wermenh.com/wx/pa-ob.html as both the Productive Alternatives "official" CoCoRaHS gauge and the Outback Blue gauge meet that standard for 1.00" of rain in the inner tube. The OB gauge reads a bit low, so it could be that rain sticking on the funnel makes the inner tube read too low.

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