Chapter 4: Calibration |
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The rain gauge is calibrated at the factory. Recalibration should not be necessary unless |
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damage during shipment or mishandling during installation has occurred. If the damage is |
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extensive, the gauge may need to be repaired or replaced before it can be used properly. |
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Verify the calibration by using a calibration bottle, a graduated cylinder, or a calibrated |
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burette. The calibration is verified by allowing a known volume of water to flow into the |
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funnel at a rate of 1 to 2 inches per hour producing a specific number of bucket tips. Before |
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putting the volume of water into the funnel run some water into the gauge to wet the funnel |
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and the buckets. The gauge calibration test must be made while the gauge is wet. Do not |
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wipe off any water especially from the buckets between tests and calibration screw | ’ |
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adjustments. Add the measured amount of water at the specified flow rate (refer to Table 1 | ). | |
The bucket should tip within the published gauge tolerances and should give the calculated number of tips within the gauge accuracy. If the number of counts is not within the gauge accuracy, then the bucket stops must be adjusted to increase or to decrease the amount of water needed to tip the buckets.
The individual buckets can be calibrated by placing an amount of water equal to one tip into the bucket and adjusting the bucket stop on the opposite side of the gauge until the bucket tips and empties its water. To move the bucket stop screw first loosen the hex nut on the outside of the rain gauge. The nut keeps the screw locked into place. With the nut loosened, the screw can be rotated by hand or by using a screw driver set into the head of the screw. The screw should only be rotated by a small amount between each test. Use a large rotation of the screw only if the bucket is out of calibration by a large number of counts.
Note that moving the bucket stop screw upward means that less water is needed to tip the bucket and moving the screw downward increases the amount of water needed to tip the bucket.
For inches of rain, the amount of water needed for a single tip equal to 0.01 inches is 8.24 milliliters. For a 1 millimeter tip the amount of water is 32.43 milliliters. Add the amount of water needed to each of the buckets separately and adjust the corresponding stop screws as needed to obtain a single tip form each bucket. Several adjustments of each calibration screw may be necessary as there will be some influence from each of the buckets upon the other as the adjustments are changed. If there is repeatability in the tip with the calibrated amount of water, then the adjustment screw has been correctly set and can be locked into place. Use the hex nut located on the adjustment screw at the outside the rain gauge base to lock the screw into position. Take care and do not allow the screw to rotate as the nut is being tightened. In case the screw has turned slightly, retest the tip of the bucket after the nut has been tightened.
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