Correct way to calibrate and check Rain Gauge:
The tipping-bucket mechanism is a simple and highly reliable device.
The rain gauge must be calibrated with a controlled rate of flow of water through the tipping-bucket mechanism. The maximum rainfall rate that most tipping bucket rain gauges can accurately measure is 25mm of rain per hour (approx. 36 seconds between bucket tips). Therefore, the rain gauge is calibrated using a water flow rate equivalent to, or less than, 25mm of rain per hour (more than 36 seconds between bucket tips). If the flow rate is increased beyond 25mm / hr, a properly calibrated instrument will read low. Decreasing the rate of flow will not materially affect the calibration. The reason for this is obvious. Watch the tipping bucket assembly in operation. With water falling into one side of the tipping bucket, there comes a point when the mass of the water starts to tip the bucket. Some time is required for the bucket to tip (a few milliseconds). During the first 50% of this tipping time water continues to flow into the filled bucket; the last 50% of this tipping time water flows into the empty bucket. The amount of water flowing during the first 50% of time is error, the faster the flow rate the greater the error. At flow rates of 25 mm/hr or less, the water actually drips into the buckets rather than flowing. Under this condition, the bucket tips between drips, and no error water is added to a full moving bucket.
To Calibrate:
NB: Use a vernier caliper accurate to 0.02mm.
1) First work out the cross sectional area of your rain gauge. Measure the internal diameter of your rain gauge receiver if it is round use: A=πr^2. Assume a for arguments sake our rain gauge has a diameter of 6” or 152.4mm.
i) 152.4 / 2 = 76.2
ii) 76.2 x 76.2 x π = 18241.469 mm2
iii) 182.41 x 2 = 365 ml of water will represent 20mm of rain falling on our gauge.
Ellipse rain Gauges are slightly more complex; Area = π x a x b;
Where semi-major axis of length A and semi-minor axis of length B. (These semi-major axes are half the lengths of, respectively, the largest and smallest diameters of the ellipse.) Type area of ellipse into google for the calculation and do the same arithmetic as iii) above ie: divide by 100 and multiply by 2 to get the amount of water equivalent to 20mm of rain.
2) Obtain a plastic or metal container of at least one litre capacity. Make a very small hole (a pinhole) in the bottom of the container.
3) Place the container in the top funnel of the Rain Gauge. The pinhole should be positioned so that the water does not drip directly down the funnel orifice.
4) Pour exactly 365ml ( or the amount you have calculated for your rain gauge) of water into the container. Assume each tip of the bucket represents 0.2 mm of rainfall. Then your gauge should have tipped 100 times. (this will vary according to make but should be in the Specification Sheet, some are 0.3mm per tip in which case the bucket will tip 67 times ) But you should see 20mm of rain register on your readout. If you are unsure about the bucket size manually tip your bucket until 20mm shows up on the display; if it is after 67 tips it is a 0.3mm bucket, if it is 100 tips it is a 0.2mm bucket etc.
5) If it takes less than one hour for this water to run out, then the hole (from step 1) is too large. Repeat the test with a smaller hole.
(Adjusting screws on Davis and the better quality gauges are located on the base of the Rain Gauge housing underneath the buckets. Turning the screws clockwise increases the number of tips per measured amount of water. Turning the screws counterclockwise decreases the number of tips per measured amount of water. A 1/4 turn on both screws either clockwise or counter-clockwise increases or decreases the number of tips by approximately one tip. Adjust both screws equally; if you turn one a half turn, then turn the other a half turn.)
6) Repeat Steps 3–6 as necessary until the Rain Gauge has been successfully calibrated.
Some interesting numbers; 1mm of rain falling over 1 hectare is 10,000 litres of water or 10 tonnes of water.
The tipping-bucket mechanism is a simple and highly reliable device.
The rain gauge must be calibrated with a controlled rate of flow of water through the tipping-bucket mechanism. The maximum rainfall rate that most tipping bucket rain gauges can accurately measure is 25mm of rain per hour (approx. 36 seconds between bucket tips). Therefore, the rain gauge is calibrated using a water flow rate equivalent to, or less than, 25mm of rain per hour (more than 36 seconds between bucket tips). If the flow rate is increased beyond 25mm / hr, a properly calibrated instrument will read low. Decreasing the rate of flow will not materially affect the calibration. The reason for this is obvious. Watch the tipping bucket assembly in operation. With water falling into one side of the tipping bucket, there comes a point when the mass of the water starts to tip the bucket. Some time is required for the bucket to tip (a few milliseconds). During the first 50% of this tipping time water continues to flow into the filled bucket; the last 50% of this tipping time water flows into the empty bucket. The amount of water flowing during the first 50% of time is error, the faster the flow rate the greater the error. At flow rates of 25 mm/hr or less, the water actually drips into the buckets rather than flowing. Under this condition, the bucket tips between drips, and no error water is added to a full moving bucket.
To Calibrate:
NB: Use a vernier caliper accurate to 0.02mm.
1) First work out the cross sectional area of your rain gauge. Measure the internal diameter of your rain gauge receiver if it is round use: A=πr^2. Assume a for arguments sake our rain gauge has a diameter of 6” or 152.4mm.
i) 152.4 / 2 = 76.2
ii) 76.2 x 76.2 x π = 18241.469 mm2
iii) 182.41 x 2 = 365 ml of water will represent 20mm of rain falling on our gauge.
Ellipse rain Gauges are slightly more complex; Area = π x a x b;
Where semi-major axis of length A and semi-minor axis of length B. (These semi-major axes are half the lengths of, respectively, the largest and smallest diameters of the ellipse.) Type area of ellipse into google for the calculation and do the same arithmetic as iii) above ie: divide by 100 and multiply by 2 to get the amount of water equivalent to 20mm of rain.
2) Obtain a plastic or metal container of at least one litre capacity. Make a very small hole (a pinhole) in the bottom of the container.
3) Place the container in the top funnel of the Rain Gauge. The pinhole should be positioned so that the water does not drip directly down the funnel orifice.
4) Pour exactly 365ml ( or the amount you have calculated for your rain gauge) of water into the container. Assume each tip of the bucket represents 0.2 mm of rainfall. Then your gauge should have tipped 100 times. (this will vary according to make but should be in the Specification Sheet, some are 0.3mm per tip in which case the bucket will tip 67 times ) But you should see 20mm of rain register on your readout. If you are unsure about the bucket size manually tip your bucket until 20mm shows up on the display; if it is after 67 tips it is a 0.3mm bucket, if it is 100 tips it is a 0.2mm bucket etc.
5) If it takes less than one hour for this water to run out, then the hole (from step 1) is too large. Repeat the test with a smaller hole.
(Adjusting screws on Davis and the better quality gauges are located on the base of the Rain Gauge housing underneath the buckets. Turning the screws clockwise increases the number of tips per measured amount of water. Turning the screws counterclockwise decreases the number of tips per measured amount of water. A 1/4 turn on both screws either clockwise or counter-clockwise increases or decreases the number of tips by approximately one tip. Adjust both screws equally; if you turn one a half turn, then turn the other a half turn.)
6) Repeat Steps 3–6 as necessary until the Rain Gauge has been successfully calibrated.
Some interesting numbers; 1mm of rain falling over 1 hectare is 10,000 litres of water or 10 tonnes of water.