Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 15. Vapor Pressure Thermometers

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XV - VAPOR PRESSURE THERMOMETERS

66. Purpose and use. a. Thermometers are used on the airplane to measure and indicate the temperature of

(1) Cooling liquid in liquid-cooled engines. This measurement is made at some point in the cooling system between the engine outlet and radiator inlet.

(2) Engine lubricating oil. This measurement is made at the oil inlet to the engine.

(3) Carburetor mixture. This measurement is made in the carburetor throat.

(4) Air inside and outside of cockpits and cabins. These measurements are made in the cockpit or cabin and at some external point on the airplane surface.

b. Some of the uses of the aircraft cooling liquid thermometer are to

(1) Enable the pilot to operate the engine within the temperature range at which best efficiency is obtained. Most liquid-cooled engines should be operated at a temperature of 70° to 160° C. To prevent rapid loss of liquid the temperature should always be held below the boiling point. While the boiling point of ethylene-glycol is 192° C. at sea level, it is only 175° C. at 20,000 feet altitude; consequently, at that altitude the engine must be operated at a correspondingly lower temperature. Shutters on the radiator enable the pilot to control the temperature in normal operation.

(2) Warn the pilot of engine overheating. Certain engine troubles are first indicated by overheating and if noticed in time by the pilot, the airplane may be landed with power before complete engine failure occurs.

(3) Indicate to the pilot or mechanic while warming up the engine on the ground when the engine has warmed up sufficiently for the take-off and flight. In general, the cooling liquid temperature should reach at least 70° C. before the take-off is attempted.

(4) Warn the pilot of the engine cooling too much during long glides. If the engine becomes too cool, it may backfire or stop when the throttle is opened suddenly.

c. Some of the uses of the aircraft oil thermometer are to

(1) Indicate in warming up the engine on the ground whether the oil has reached a temperature sufficient for the take-off. In general, the oil temperature should reach 30° C. before the take-off is permitted.

(2) Enable the pilot to adjust the oil coolant radiator control or check its operation when automatic control devices are employed.

d. Some of the uses of carburetor mixture thermometers are to   

(1) Indicate low temperatures in the carburetor throat which under certain humidity conditions would result in the formation of ice within the carburetor.

(2) Indicate high temperature in the carburetor when the heater is being used, which would cause detonation and loss of power.

e. Free air thermometers are used to indicate temperatures inside and outside the cabin and cockpits. Inside temperatures are necessary to provide comfort for the passengers. Outside temperatures are important in the general operation of the aircraft, especially when the moisture content of the air is such that there is danger of ice formations on the lifting surfaces of the airplane. Thermometers of this type are generally called ice-warning indicators. In construction and appearance they are the same as the distant reading types of engine thermometers, the exceptions being that they have a shorter range and the scales have characteristic markings at the critical range where ice formations occur, that is, at 0° C. or +32° F. Due to the dependence which is placed on free air thermometers in flight, they are built and calibrated very accurately and can be used at any time as a master to check other thermometers on the airplane, provided that at the time of comparison the bulbs of both thermometers are exposed to the same medium. These thermometers are also used when it is necessary to take the temperature of the free air surrounding the aircraft at various altitudes. With the aid of altitude correction computers, the true altitude may be obtained. This is especially important when the aircraft is performing a photographic or bombing mission, or where navigation problems are to be solved involving the use of air speed.

67. Description.-a. The vapor pressure thermometer consists of three units, the indicator which is mounted on the instrument panel, the bulb which is located at the point of temperature measurement, and the capillary tube which serves to connect the indicator with the bulb. The ranges of these instruments vary with their particular use and in general fall in the following classification :

(1) Oil thermometers, 0° to 100° C.

(2) Coolant thermometers, 0° to 200° C.

(3) Oil and coolant combination thermometers, 10° to 150° C. (fig. 25).

(4) Free air thermometer, -40' to +50° C. (fig. 26).

b. The indicator consists of a conventional type Bourdon tube mechanism placed in a 1 7/8-inch bakelite case with a raintight seal. The instrument case is provided with an individual 3-volt light with the receptacle molded integral with the case. Each Bourdon tube is provided with a progressive restrainer to permit the use of a uniformly graduated scale because vapor pressure does not increase uniformly with temperature.

c. The bulb is a hollow brass cylinder about 1/2-inch outside diameter by 4 inches long. It contains a volatile liquid (usually methyl chloride) which actuates the instrument. The bulb of the free air thermometer is longer and formed in the shape of a helical tube in order to provide more surface for the air to contact.

d. The capillary tube enters directly into the stationary end of the Bourdon tube. It consists of a very small annealed copper tubing and is protected with braided copper wire armor throughout its length. In order to meet all requirements, the free air thermometers are provided with various lengths of capillary tubing, ranging from 5 to 34 feet. At each end of the capillary special reinforcing armor is placed around the tube to prevent breaking or kinking.

A. Bulb.                            B. Capillary.                          C. Indicator.
FIGURE 25. oil and coolant thermometer..

A. Indicator,   C Bulb.
B. Capillary.   D. Lamp.
FIGURE 26.-Free air thermometer.

68. Operation.-The operation of a vapor pressure thermometer is entirely automatic. The indicator, capillary, and bulb are integral with each other as shown in figure 27. As the temperature of the bulb increases the liquid inside the bulb, being highly volatile, is changed into a gas. This change of state causes an increase in pressure which is transmitted through the capillary tubing to the Bourdon tube b. The Bourdon tube tends to straighten out and this movement is transmitted through a linkage to the sector c which rotates around its fixed pivot. The teeth of the sector are meshed with a pinion on the pointer shaft which also rotates around a fixed axis and shifts the pointer a to various positions with reference to the graduations on the scale. Since the vapor pressure does not increase uniformly and because a uniform scale is most desirable in instruments of this type, the actual movements of the Bourdon tube are controlled by a progressive restrainer.

FIGURE 27.-Vapor pressure thermometer mechanism.

69. Installation.-a. In addition to the general points on installation covered in section III, some specific instructions should be considered for the vapor pressure thermometers. The units of these thermometers are integral and cannot be separated without a total loss of the assembly. Particular care is to be taken of the capillaries which should not be cut, broken, dented, or mashed, stretched or pulled taut. They are protected with friction tape shellacked on at all points of contact with other surfaces, and must not come in contact with exhaust stacks or other excessively hot parts of the engine. When replacements are made, longer capillaries may be used if the correct lengths are not available, provided the excess is neatly coiled and taped to some structural part of the nacelle or fuselage.

b. When installing free air thermometers on single-engine airplanes, it is desirable insofar as possible to install the capillary tubing inside the wing in an aluminum conduit, preferably near the leading edge in such a manner as to facilitate replacement of defective instruments by service activities without removal of the wing. This prevents the capillary tubing from disturbing the air flow over the wing as has been experienced when external installations have been made along the leading edge of the wing.

c. Where inside installations cannot be made, the capillary tubing should be securely fastened along the trailing edge of the wing and where wing strut installations outside the slip stream are not feasible, the coil or bulb is installed beyond the slip stream in a fore and aft direction attached to the rear spar and preferably on the lower side of the wing. If for any reason the tube must be placed along a spar, care must be taken that the spar flange and stiffeners are not drilled for attaching the fittings. If the airplane is equipped with flaps the capillary tubes should lie just even with the trailing edge of the wing and clear of the flaps. In any case on external installations the capillary tubing must be fastened at sufficient points to prevent whipping.

d. On two-engine airplanes, the coil or bulb is conveniently located on the outside of the fuselage.

70. Maintenance.-General information on maintenance of vapor pressure thermometers is contained in section II. In addition, each installed thermometer must be properly labeled, using name plates or aircraft enamel to indicated its use as listed below.

a. (1) Free air temperature.

(2) Carburetor air temperature.

(3) Carburetor mixture temperature.

(4) Cabin air temperature.

b. (1) Oil in temperature.

(2) Oil out temperature.

c. (1) Water temperature.

(2) Prestone temperature.

d. (1) Cylinder head temperature.

(2) Cylinder base temperature.