Absolutely accurate calibration can be obtained only with laboratory equipment, but an approximate field check can easily be made. The tipping bucket mechanism is a simple and highly reliable device. The transmitter must be located in a clear area, away from trees, buildings, etc. To obtain accurate readings, the unit must be mounted level and be free of foreign material, dust, or other debris. The transmitter must be calibrated with the rate of flow of water through the tipping bucket mechanism under control. At least 36 seconds should be allowed to fill one side of the tipping bucket. This represents a maximum flow rate of one inch of rain per hour. If the flow exceeds that rate, then the instrument will read low even if properly calibrated. Decreasing the rate of flow, on the other hand, will not materially affect calibration. The reason for this is obvious if the tipping bucket assembly is observed when the weight of the water begins to tip the bucket. Some time is required for the bucket to tip (a few milliseconds). During a portion of this time, water flows into the empty bucket. This creates a small margin of error and the faster the flow rate, the greater the error. At flow rates of one inch per hour or less, water drips slowly into the bucket instead of flows, allowing the bucket to tip between drips, and eliminate the cause of the error.