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CHAPTER 17. OPERATION OF EQUIPMENT SECTION 2. PRECIPITATION MEASURING EQUIPMENT The weighing-type recording gauge is designed to record the rate-of-fall as well as the depth of precipitation. It consists of a receiver of exactly 8 inches inside diameter through which precipitation is funneled into a bucket mounted on a weighing mechanism. The weight of the catch is recorded on a clock-driven chart as inches of precipitation. Most gauges in use have clocks which rotate the cylinder one revolution in 24 hours and a recording capacity of 12 inches of precipitation (6-inches single-traverse). 17-8. CHANGING WEIGHING RAIN GAUGE CHARTS In handling completed charts, avoid smearing damp portions of the trace. To change charts:
17-9. WEIGHING RAIN GAUGE - WINTER OPERATION To avoid damaging the mechanism, the weighing rain gauge should be winterized as soon as there is a danger of freezing, frozen precipitation, or freezing temperatures. Winterize the gauge at the beginning of the winter season. For gauges with funnels, remove the funnel from the bottom of the collector and if the gauge has a snow ring, install it in place of the funnel. Add propylene glycol and mineral oil to prevent damage from freezing and to retard evaporation. Instructions for using the antifreeze are printed on the label of each container. 17-10. WEIGHING RAIN GAUGE - SUMMER OPERATION At the beginning of the summer season, when it is believed no more freezing temperatures will occur, replace the funnel. 17-11. WEIGHING RAIN GAUGE - WINDING THE CLOCK Wind the clock each time the chart is changed. Do not overwind the clock. This is one of the leading causes for clock failures. 17-12. MAINTAINING THE WEIGHING RAIN GAUGE At least once a year:
17-13. WEIGHING RAIN GAUGE - DASHPOT If necessary to prevent the loss of ink from the pen and to minimize irregularities in the trace resulting from vibration or wind gusts, fill the dashpots with either propylene glycol, petroleum oil, or a similar fluid. Keep the dashpot filled to a level about 3/8 inch from the top. After filling the dashpots, move the weighing platform up and down several times to remove any air bubbles trapped under the dasher and check the fluid level again. Do not mix different types of dashpot fluids. 17-14. WEIGHING RAIN GAUGE - CLOCKS If the clock gains or loses more than 2 minutes a day, adjust the timing regulator. Open the small inspection plate in the top cover of the clock compartment for access to the adjusting lever. Move the lever toward "F" if the clock is losing time; toward "S" if the clock is gaining time. Make time adjustments when changing charts, if possible, to preserve the continuity of the record. When a clock gains or loses more than 10 minutes a day, the clock should be replaced unless it is practicable to have it repaired by a local watchmaker. Replacement clocks are available from the National Logistics Supply Center (NLSC).
17-15. WEIGHING RAIN GAUGE - PENS Clean the pen as often as necessary to maintain a legible trace. Clean the pen by washing it in water or, preferably, alcohol. Sharpen the pen, if necessary, by rubbing the edges gently over a fine-grain whetstone or a sheet of crocus cloth. After the pointer has been sharpened, rub it very lightly over the abrasive surface several times to remove any burrs that might catch on the chart and cause an erratic trace. A spare pen should be kept on hand for replacement purposes. 17-16. WEIGHING RAIN GAUGE - BUCKET The only bucket maintenance necessary is periodic cleaning to remove any accumulated dust or dirt. If at any time the contents of the bucket freezes, the bucket should be examined for any damage and replaced, if necessary, with a galvanized bucket of the same capacity. 17-17. WEIGHING RAIN GAUGE - EVALUATION PROCEDURE (See Figure 17-3, Evaluation of Precipitation on Weighing Rain Gauge Chart.) To determine the amount of precipitation during a designated period, subtract the reading at the beginning of the period from the reading at the end of the period. When the pen does not reverse exactly at the uppermost ordinate of the chart, readings from the 6- or 12-inch scale are in error. When this occurs, take the sum of the precipitation equivalent segments of the trace as follows:
In Figure 17-3, above, the total precipitation for the period, ABCD, is found as follows:
17-18. WEIGHING RAIN GAUGE - ANNOTATION OF CHARTS Before placing the chart on the gauge, enter in ink, the station name, date, time (to the nearest minute), and the local time zone designator. After removing the chart from the gauge, enter hourly precipitation amounts, in inches and hundredths, midway between the hour lines. Omit the entry if no precipitation occurred and enter "T" for amounts less than 0.005 inch. The total of the hour amounts should equal the total rainfall registered on the gauge. In addition the following annotations are required:
17-19. WEIGHING RAIN GAUGE - REPLACEMENT OF CHARTS Change charts at the time specified by the station supervisor. If at that time, sufficient precipitation is falling to cause a loss of record, change charts as soon after the designated time as possible. Change charts:
17-20. CLOCK INOPERATIVE ON WEIGHING GAUGE At each 6-hourly and midnight observation, when the clock is inoperative, turn the cylinder clockwise about one-half inch and mark the position of the pen. 17-21. ZERO ADJUSTMENT ON WEIGHING GAUGE A knurled, zero-adjustment nut above the frame that supports the linkage mechanism is turned clockwise to lower and counterclockwise to raise the pen. Except as specified in separate maintenance instructions, adjust the pen to the zero line on the form only when an empty bucket is on the weighing platform. Do not change the adjustment of the three platform-travel-limiting and shipping screws adjacent to the zero-adjustment nut, except as specified in separate instructions. If repetition of the trace over the same ordinate causes the pen line to appear too wide for accurate evaluation of small amounts of precipitation, adjust the pen upward approximately one-fourth of a linear inch and make a notation to this effect on the chart. 17-22. TIME ADJUSTMENT ON WEIGHING GAUGE To adjust the chart for time, turn the cylinder counterclockwise as viewed from above until the correct local standard time (to the nearest minute) is indicated by the position of the pen with respect to the labeled time lines. The heaviest lines are labeled to indicate the beginning of each hour (LST). Additional lines are provided at 15-minute intervals. Empty the weighing gauge whenever necessary to avoid exceeding the single-traverse capacity of the gauge, when charts are changed if the gauge is not changed with antifreeze, and when the mixture of precipitation and antifreeze is diluted to where there is a danger of freezing. 17-24. ICE ACCRETION INDICATORS Ice-accretion indicators are designed to indicate the occurrence of freezing precipitation. A freezing rain sensor on an ASOS is capable of reporting ice accretion amounts. Ice accretion is determined and reported automatically and there is no manual backup required. When an ASOS freezing rain sensor is not available, a wooden bracket with clamps mounted on each end to hold a strip of aluminum may be used to determine whether freezing precipitation is occurring. Expose a strip of aluminum whenever the possibility of freezing precipitation exists. When precipitation is occurring, exposing a second strip of aluminum (which is near the same temperature as the air) will help to avoid the possibility of reporting freezing rain as occurring when actually the ice on the indicator might have formed some time prior to the observation. When doing this, however, care must be taken to ensure that the strips are at or very close to the same temperature as the air. 17-25. MAINTAINING THE NON-RECORDING GAUGE At least once a year check the level of the rim of the gauge using a spirit level and check the condition of the supports. 17-26. MAINTAINING THE TIPPING BUCKET GAUGE
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