In this instructable I discuss a simple rain gauge that is is essentially made from trash.
This is a wonderful project for any group of people. We, possibly, cannot claim that this would be a substitute for professional instrument but as you will see that it is quite accurate. And at places where there are no weather instruments this will give some reasonable indication of rain fall.
Having said this this instrument has lots of educational value. Children learn to be disciplined observers, take readings and learn about the rainfall at their own location.
Step 1: Understanding How Rain Gauge Works
A rain gauge is an instrument to measure rainfall, which is measured in millimeters. Occasionally the rain fall is also reported in centimeters or in inches.
What the rainfall measure is telling us is the height of rain water collected in a vessel that has vertical walls and has uniform cross section from bottom to the top.
Therefore, in principle any vessel that has a uniform cross section can be used as a rain gauge. All the vessels will collect rain water to the same height 'h' as shown below. For example you can keep an aquarium in open with a scale pasted to it as a rain gauge. However, from an open rain gauge collected water will evaporate quickly when the rain stops affecting the final reading. Also a lot of dirt, like leafs etc. can fall into it.
The rain fall can be measured in different ways. As said above one can simply measure height 'h' of collected water in a vessel of uniform cross section. One can measure volume of collected water and convert this value into rain fall. More sophisticated rain gauges have electronic devices to automatically record rain fall.
Step 2: Volume Measure Method
If we collect rain water by placing a funnel over some vessel then volume V of rain water collected in that vessel would be the volume of a cylinder whose radius is r, height is h,
Then V= pi r2 h ........ (1)
or h = V / pi r2 ....... (2)
(see the next image)
we can easily measure the diameter of the the funnel mouth and get the radius by dividing it by 2. This value is not going to change, value of pi is also constant (you may take is a 3.142). The denominator in the equation is, therefore, a constant. Let us call it dividing constant 'dc'
Therefore, all we have to do is to collect rain water using a funnel, measure the volume of of water collected in it and then divide by the constant above to get the rain fall measure.
Step 3: Trap
We must remember to convert the units properly, else we will get very absurd results.
Volume is measured in milliliter and the rain fall is given in millimeter.
1 milliliter (or 1 ml) = 1 cm3 = 1* cm * cm * cm,
this is to be divided by (r cm)2 = r * r * cm* cm
we then have value of h in cm as (measured volume) * cm divided by the constant value.
Multiplying this by 10 gets us value in millimeter or mm
We used a funnel whose diameter is 4.5 inch = 4.5*2.54 cm = 11.43 cm
or radius r = 5.715 cm, giving the value of dividing constant as 102.608 cm
Now if the volume measured is 245 ml
then: h= (243/102.608)cm = 2.368 cm = 23.68 mm or rounding it to 23.7 mm
Step 4: Error Factor
Suppose in the example given in the last step we made a mistake in measuring volume by 1 ml. In our case where diameter of the funnel is 11.43 cm, 1 ml rain water collection would give h = 0.1 mm.
In other words if we make an error of +/- 1 ml in noting the volume of collected rain water our percentage error in reporting rain fall would be +/- 0.4 percent.
On 1st of Juy 2007 we measured volume to be 367 ml. This gives us value of 'h' to be equal to 25.7672 or rounding it to 35.8 mm. Now suppose we read the volume to be 366 or 368 then we would get 'h' to be equal to 35.6697 (~ 35.7) or 35.8646 (~35.9).
(If you are a bit of a mathematically minded you would notice that in the example above the dividing constant is very nearly equal to 100, which tells us that for light rain fall we can directly divide the volume by 100 and we will not be too off the correct reading.)
Step 5: Placement of the Rain Gauge
One must keep in mind that the rain gauges, like most meteorological instruments, should be placed far enough away from structures and trees to ensure that any effects caused are minimized. Rain gauge must be placed clear of obstacles so that raindrops do not splash into, or out of, the collecting funnel.
On ground one should have at least 10 meter clearance from all the sides. This could be a kind of luxury for schools in metropolitan cities. One may then place the the rain gauge on top of a building then it might be sufficient to to see that it is about 50 cm above the ground or rather terrace level and about 2 m away from near by railing walls.
Rain showers can be quite windy at times and you'll want to fasten your rain gauge somewhere so that it doesn't thrown off by wind. Locate a good place for your gauge. There should be nothing overhead, like trees, electric wires, or the edge of a roof. These obstructions can direct rainwater into or away from your gauge, creating a false reading. The edge of a fence, away from the building, is often a good place for your gauge.
In next two steps we will see two models of rain gauge.
Step 6: 100 CD/DVD Cake Box Rain Gauge
The basic material need is a funnel and a measuring cylinder. A good quality plastic funnel can be purchased from a grocery shop. We would require a measuring cylinder that can read at least 1 milliliter of volume. This can be purchased from a laboratory suppliers or science teacher of a near by school should be able to tell you where to buy one. For rest of the material you may follow the instructions below and/or improvise on it.
For our rain gauge we used 100 CD/DVD cake box with its spindle removed. On the top the cake box cover we cut a hole and inserted neck of funnel through it and glued it water resistant glue such as M-seal. A vessel to collect the rain water is placed on the base of the cake box. The cover can be locked to its black base.
This unit turned out to be quite light and could easily fly away with moderately strong wind. We found a 40 x 40 cm and 18 mm thick water proof wooden board in our carpentry workshop. So we screwed the base of the CD cake box to the board. Underneath this board we also fixed four caps of mineral or soft drink water bottles. One of the bottle could be cut to fit inside the cake box to serve as vessel for collecting rain water.
We simplified the above design by removing the wooden board and putting lead weights and used a pet bottle as a vessel for collecting the rain water. We also found a tall stool to place this rain gauge.
Step 7: Pet Bottle Design
This is an interesting design for collecting rain water. In this design we have used one small bottle (1 litre) and a thermal cover bag for it. This is kind of cover is available in the stores where material for school going children is sold. In this design we have used a discarded bag. This bag (with bottle inside it) can slide into 2 liter pet bottle with its bottom part cut. We then drilled hole in the cap of the small bottle to fit in the funnel. We sealed the funnel to the cap using M-seal.
We found a long (waterproof) wooden plank (one can use a rod or similar material). The plank was about 8 mm thick, 4 inch (10 cm) wide and about 5 feet (1.5 m) long (tall). We used the other 2 liter pet bottle and cut off its bottom part and fixed it upside down to the this plank at one end.To do this we made two matching holes in the plank and the bottle and tied the bottle using nylon rope. Then the first bottle is slide into this bottle.
Our rain gauge is now ready. We then placed it at some convenient place. The rain water collecting bottle easily comes out of the bigger bottle. We can now uncap the funnel and either collect water in a baker and measure volume of the water collected or measure it directly.
Step 8: A Suggestion:
If you do not have a good measuring cylinder at hand you can use two identical small bottles. A funnel is attached to one cap. And the other cap is used on the second bottle.
Students can use one bottle for collecting the rain water and other transporting.
This idea can be used for collective class room exercise. Students can be asked to set up their own rain gauges at home and bring the collected rain water to the school for measuring volume.