Sensing and Control for hobbyists and schools: Calibrating your sensors

This page was prompted by discussions of how a rain gauge might be calibrated. The gauge in question works by tipping, once for each time a certain amount of rain has fallen. It has bolts which can be adjusted to change the amount of rain needed for a tip to occur.

Some people spoke about their efforts to get the tips to occur after 0.01" of rain had fallen. To me, this is bad science.

Far better to be sure that the device is tipping reliably after ABOUT 0.01"... but always after the SAME amount. After you have achieved this, you determine how much that same amount is to the greatest possible accuracy. So what if your tipper is tipping after, on average, 0.01204"? You've got a computer to do the arithmetic that says that 62 (and a bit) tips means that 0.75" has fallen!

In any case... how much does it matter? Surely being able to compare what you are sensing from one day to the next with your prior observations is the main point? Of course, being able to relate your readings to someone else's is nice.

A word about "rainfall".... don't be too sure you are clear on this concept... I saw a long, confused discussion.

When we say "One inch of rain has fallen", what we mean is that if you had a container with vertical sides, e.g. a shoe box made of plastic, out in that rain, then you would have water an inch deep in the bottom of the container.

What seemed to confuse people was the effect of pouring that water into a narrower container.

It is quite usual to collect rain with a funnel, making it fall into a narrow cylinder. The advantage of this is that it "magnifies" the water, so that you can measure how much has fallen more accurately. Bear with me...

Suppose you didn't want ANY "magnification". Lets say you had a cylinder which is 10cm across. If 1 cm of rain falls, you will have water to a depth of 1cm in the bottom of the cylinder.

A 10 cm (radius, R, = 5cm) circle's area is 78 square centimeters. (pi x R x R, i.e. 3.14 x 5 x 5).

If you had another cylinder with half the cross section, and poured the water from the first one into it, the depth would double.

So how do you calibrate your tipper if you want to? Do most of this in metric units. You can change "cm of rainfall" to "inches of rainfall" at the end if you want to. (1cm = 0.3937 inches)

First you measure the diameter of the opening into which rain is falling. (Assuming it is circular!). Divide that by 2 to get the radius. Square that number. (I.e. multiply it by itself). Multiply THAT by pi (3.1416). The Dallas / AAG rain gauge's opening is 15.2 cm across, so the area is 181.4 square centimeters.

Once you know the area of your tipper's opening, multiply the number by 10.

Take that many cubic centimeters of water, pour it through the gauge. How ever many "tip" events you record will be the number of tip events when 10cm of rain falls. (Many kitchen measuring cups have metric markings. 1cubic centimeter = 1cc = 1ml. Filing that, a sensitive weighing machine might be to hand: 1814cc of water weighs 1.814 kg. Oh, I do like the metric system!)

How fast the "rain" falls can effect how the tipper responds. To get a reasonable rate of water flow into the gauge, I simply took a plastic milk jug, poked a small hole in the bottom, and put it in the top of the rain gauge while the water trickled out.

Want to calibrate an anemometer?

PLEASE take GREAT care with this approach... there would be no excuse for causing an accident....

If you were to mount your anemometer on a car, and drive it at various speeds on a windless day, that would do the job, wouldn't it!

Don't forget the Bernoulli effect. Air passing over the top of a typical car will be travelling faster than the car. Try to mount the anemometer slightly out in front of the car. BUT BE SURE IT cannot COME LOOSE ON THE HIGHWAY, PLEASE.

For a small group of readers:
In the first half of 2002, members of the Dalsemi weather-list had a joint project to produce barometers for the 1-Wire / MicroLan. If you don't have one of the kits, it may be too late, but you might still be lucky!. In October 2002, Simon Atkin still had a supply of the boards. Try him! Or post a request via the weatherlist. If you DO have one of the kits or devices, the following may be of use to you....Calibrators and Builders for Weatherlist 2002 barometers.
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