Testing the Hybrid Rain Gauge

After several iterations, we’ve narrowed down the hybrid gauge to two versions that we are going to subject to initial testing. The first version, modestly named the “Ultimate” is the largest version of the gauge that we can make in the confines of the 5” x 5” x 5” build volume that our current, preferred 3D printer allows. Other 3D printers have a larger build volume, but the model of printer we have selected we feel is more reliable for field projects.  The second version of the hybrid gauge, prosaically named the “4’’ Pipe” is designed to fit inside a standard 4” PVC pipe, so that the pipe acts as both the casing and the funnel rim, two of the largest and most time consuming parts to manufacture. This means that the bucket used in the 4” Pipe is significantly smaller than in the Ultimate, and if you’ve had a chance to read our fascinating analysis of the hybrid gauge design (see Hybrid Tipping Bucket Rain Gauge Design) you’ll know that one of our theories is that a larger bucket results in less tips, which reduces one of the sources of error that afflicts tipping bucket rain gauges; i.e. the tendency for rain to enter the bucket after it has begun to tip. All other things being equal, we think this gives the Ultimate a slight edge over the 4” Pipe, although the smaller bucket does allow us to use less gain on our signal amplifier, which means that we can get better reading resolution (more on this to come in a later article). To determine conclusively which is better, we will need to conduct some extensive field tests, which we hope to do in the coming months. Right now we just want to make sure that our concept is sound so that we aren’t laughed out of the testing center when we eventually get there, so here goes…….

Before we could start testing, we had to settle on how some of the electronic circuitry would be configured, at least temporarily, so that we could receive and process the data coming out of the gauge. Up until now, we have been using an Arduino board to read the output from the load cell and, for the moment, we’re going to continue with it, although we eventually expect to migrate to a Raspberry Pi platform for added flexibility and computing power. There’s an article elsewhere on the site about the amplifier circuit that we are using for the load cell (see 3D Printed Circuit Board) and we decided to house this in a central junction box that will also serve as the mounting platform for the rain gauge and the other gauges/sensors that we eventually hope to deploy, which should include an anemometer and a “Stevenson Box” to house multiple sensors, such as temperature, humidity, pressure, etc.  Also in the junction box are a couple of CAT 5e punch-down jacks that we will use to connect the various components together (for now, load cell to amplifier; amplifier to Arduino board – see Photo 1).

Photo 1: Inside the Junction Box
Photo 1: Inside the Junction Box

Until now, we hadn’t attempted to read more than a few seconds worth of data from the load cell, so storage for later evaluation hasn’t really been an issue, but for these tests to be meaningful, we needed to be able to collect several hours’ worth, which can really add up when you’re sampling as frequently as 10 times per second. To transfer the data from the Arduino to a laptop, we used PuTTY ( http://www.putty.org/) which is “a free and open-source terminal emulator, serial console and network file transfer application”.  Some initial fiddling with the load cell and our first test indicated that the output was a little jumpy, so I was very happy to find a simple little smoothing routine in a tutorial from our friends at Arduino (http://arduino.cc/en/Tutorial/Smoothing).

All we needed now was a rig on which to mount the gauge and a supply of simulated rainfall. It turns out that even torrential rain doesn’t look like a lot of water when it’s condensed into a stream that represents the amount falling within the collection orifice of a typical rain gauge, so the first requirement was a means of controlling water flow to a very slow drip. Rather than using a 1000 words to describe how we did it, take a look at photo 2. The mount for all this was cobbled together from an old lamp stand and various PVC and GI pipe pieces and fittings (photo 3).

Photo 2: Making a drip
Photo 2: Making a drip

Our initial testing has focused on pouring a measured amount of water through the gauge (measured by weighing the contents of the water flask before and after the test) and then totting up how much water the gauge has recorded passing through. So far the results have been pretty encouraging – all within 2% using a pretty rudimentary approach for data analysis – so we’re going ahead with the planning for some real field testing.

Testing the Hybrid Gauge in out secret testing facility not far from CIA HQ in Langley, VA
Photo 3: Testing in progress at our secret  facility —  not far from Langley, VA
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