CHAPTER 17.   OPERATION OF EQUIPMENT

SECTION 10.   MEASURING EQUIPMENT - OTHER

17-81.   PSYCHROMETRIC CALCULATOR

The observer shall use the scale based on the barometric pressure nearest the normal station pressure; e.g., observers at sea-level stations shall use a 30-inch scale for all observations. At stations where the normal station pressure is unknown, the appropriate scale, based on the elevation of the station, shall be determined using Figure 17-12, Selection of Psychrometric Calculator on Basis of Standard Atmosphere.

17-82.   PSYCHROMETRIC DATA

The method of obtaining dew-point and relative humidity values with a psychrometric calculator varies with temperature conditions affecting the wet-bulb at the time of observation. At wet-bulb temperatures near freezing, the observer shall determine visually that the wet-bulb wick is not frozen before using the wet-bulb depression data.

Figure 17-12. Selection of Psychrometric Calculator on Basis of Standard Atmosphere

Instructions for using the calculator are printed on it. Note that different temperature scales of the calculator will be used depending on whether the wet-bulb is covered with ice or water at the time of observation. When the wet-bulb temperature is 0°C or more, the observer shall use the high range of the calculator; when the wet-bulb temperature is less than 0°C, the observer shall use the low range printed on the reverse side of the calculator. At wet-bulb temperatures below 0°C the wet-bulb should be frozen. If not, the observer shall follow the instructions on the low range side of the calculator, relating to a wet wick on the wet-bulb at a temperature less than 0°C.

17-84.   MAINTENANCE OF PSYCHROMETRIC CALCULATOR

When not in use, the calculator shall be kept in an envelope or drawer, or otherwise protected. It shall be kept out of direct sunlight and away from radiators or other objects with relatively high temperatures.

a.  Cleaning. Once a month, the observer shall clean the calculator with a damp blotter. If a more thorough cleaning is necessary, the observer shall disassemble it and wash the parts with soap and water. The observer shall not use acetone, benzene, lacquer, paint thinner, or other solvents. Care should be taken to reassemble the calculator with spacing washers between each moving part and with the rotor disks on the proper faces of the base. The observer shall not lubricate the pivot assembly since oil may cause discoloration of the vinylite.

b.  Alignment Checks. The observer shall check the alignment of the disks of the calculator:

(1)  Upon receipt of the calculator.

(2)  After reassembly.

(3)  Periodically, to prevent misalignment due to pivot wear.

The check shall be made by using the four sets of reference marks printed on each side of the calculator. Each set of marks consists of three lines on the moveable, center disk and one line on the outer disk. To make the check, the observer shall line up the inside mark on the outer disk with the center line of one of the sets on the inner disk; and then with this alignment, check to see if the other three marks on the outer disk fall within the outside lines of each of the corresponding positions on the inner disk. The observer shall repeat the procedure for each of the other seven sets of marks. If, for any of the checks, the single reference mark falls outside the reference marks on the inner disk, the observer shall not use the calculator for official psychrometric calculations.

17-85.   MAINTENANCE OF LIQUID-IN-GLASS THERMOMETERS

The observer shall maintain clear, legible graduations by cleaning the thermometers and renewing the ivory black as often as necessary. The observer shall apply the ivory black with a matchstick, or equivalent, to the full length of the graduated portion of the stem. The observer shall remove excess pigment by rubbing the surface of the stem lightly with blotting paper. A black, china-marking pencil may be used instead of ivory black if relatively few markings require renewing.

17-86.   MAINTENANCE OF PSYCHROMETERS - WEEKLY CHECKS

The observer shall check the condition of the thermometers, change the wick on the wet-bulb, and clean the thermometer if foreign matter has accumulated. In some localities, it may be necessary to replace wicks more often to prevent their becoming excessively soiled. A solution of vinegar and water will usually prove effective in removing mineral deposits on the glass bulbs. The observer shall clean the gear teeth and exposed surfaces of the whirling psychrometer by removing dirty grease, oil, and loose dirt. The observer shall lubricate the bearings with a few drops of motor oil (SAE 30 or higher) or, in the case of gear teeth and worn bearings, with a thin film of light grease.

17-87.   MAINTENANCE OF PSYCHROMETERS - MONTHLY CHECKS

The observer shall oil the bearings of psychrometer fans with one or two drops of light oil, such as anemometer oil, if the bearings are only slightly worn. The observer shall use SAE 10 or 20 oil if the bearings are in poorer condition.

17-88.   INSTRUMENT SHELTER - ILLUMINATION

The observer shall use electric lamps rated at a total of not more than 30 watts in large or medium shelters. The lamps shall be mounted as far as possible (at least 10 inches) from the closest temperature-sensing element. The observer shall not leave the lamps turned on any longer than is necessary to read the instruments. The observer shall use a flashlight for illumination in small shelters.

17-89.   INSTRUMENT SHELTER - MAINTENANCE

All surfaces of the shelter, both inside and outside, shall be painted with glossy-white. The outside shall be painted as often as necessary to maintain a uniformly covered, clean, white appearance. If the metal supports show evidence of rust or corrosion, the damaged areas shall be cleaned, primed and painted in accordance with best commercial practices. A top coat of aluminum paint shall be applied.