Proper care and maintenance of the shelter is described in NAVAIR 50-30FR-518, Operation and Maintenance, Standard Air, Maximum and Minimum Thermometers, Townsend Support, Sling and Rotor Psychrometers, and Instrument Shelters. The routine care of the shelter includes keeping it free of dirt and debris and oiling the door hinges. Backup equipment usually kept inside the shelter includes the rotor, sling or electric psychrometers, and maximum and minimum thermometers mounted on a Townsend support. When the temperature is above freezing, a bottle of distilled water is also included for the psychrometers. REVIEW  QUESTIONS Q9. What is the purpose of the ML-41 instrument shelter? Q10. What tasks are included in the routine care for the instrument shelter? THERMOMETERS LEARNING OBJECTIVES: Describe the char- acteristics of liquid-in-glass thermometers. Explain how to properly read a thermometer. Identify   three   types   of   liquid-in-glass thermometers. Liquid-in-glass thermometers, such as alcohol or mercury thermometers, are found throughout the Navy and Marine Corps in various configurations. Some are simply closed glass tubes mounted on a graduated cardboard, plastic, or metal backing, and others have the graduations  etched  into  the  glass.  For  meteorological and  oceanographic  readings,  calibrated  thermometers with the graduations permanently etched into the glass are recommended, since they are considered the most accurate. In meteorology and oceanography, liquid-in-glass thermometers are used in the rotor psychrometer, the sling psychrometer, electric psychrometers, and as simple   thermometers   for   measuring   seawater temperature by the bucket method. The maximum and minimum thermometers found in the instrument shelter are special types of liquid-in-glass thermometers. Both NAVMETOCCOMINST  3141.2  and  NAVMET- OCCOMINST 3144.1 provide detailed instructions for use and care of the different types of thermometers. READING A THERMOMETER Accuracy  is  the  goal  when  making  any  temperature reading. To obtain an accurate reading, you must read the thermometer properly. Be sure the thermometer is clean, and allow the instrument to acclimatize prior to reading. Keep the following points in mind when reading temperatures: The  temperature  indicated  by  a  liquid-in-glass thermometer should be read to the nearest 1/10 of a degree at the top of the meniscus, with the eye looking straight at the fluid level (fig. 2-14) to avoid parallax error. Avoid obvious scale errors in reading. Errors of 5 or 10 degrees are more common than errors of 1 or 2 degrees. Do not allow any moisture to accumulate on a dry-bulb thermometer. Protect the thermometer from spray  and  precipitation. Do not hold a thermometer too close to your body or near a heat source, such as a light bulb or direct sunshine. Always stand downwind of a thermometer. When the air temperature is below freezing, wait at least 15 minutes after wetting the wet-bulb wick before using a wet-bulb thermometer. This allows the water to freeze on the wick to prevent false high readings. ROUTINE CARE Routine care of thermometers is limited to keeping the glass clean. When using a thermometer to measure wet-bulb temperature, keep the cotton wick clean. A weekly  wipe  down  of  glass  thermometers  with freshwater is usually sufficient. At sea, especially under windy conditions, minute salt crystals tend to accumulate  on  the  thermometers.  These  crystals  readily Figure 2-14.—Reading a thermometer to avoid parallax error. 2-9