Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 16. Electrically operated 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 XVI - ELECTRICALLY OPERATED THERMOMETERS

71. Purpose and use.-The uses of electrically operated thermometers are the same as for vapor pressure thermometers as given in paragraph 66. (Section 15)

72. Description.-a. The electrically operated thermometer consists of three main parts, the indicating instrument, the temperature sensitive element (bulb), and the connecting lead wires between the indicating instrument and the bulb. The ranges of the various electrically operated thermometers depend on their particular use and in general may be classified as follows :

(1) Oil temperature thermometers, 0° to 100° C. and 20° to 120° C.

(2) Fuel mixture thermometers, -10° to +50° C.

(1) Oil thermometer (20° to 120°).    (2) Oil thermometer (0° to 100°)

(3) Fuel mixture thermometer. (4) Free air thermometer.
FIGURE 28.-Electrically- operated thermometers.

(3) Free air thermometers, -45° to +45° C. Typical electrically operated thermometers for temperature measurement of oil, fuel mixture, and free air are shown in figure 28. The oil and fuel mixture thermometers have 1 7/8-inch dials, while the free air thermometer is housed in a case with a 2 3/4-inch dial.

b. Electrically operated thermometer indicators are of two general types : those having a sensitive D'Arsonval mechanism and those having a ratio meter mechanism.

FIGURE 29.-Wiring diagram of electrically operated thermometer.

(1) The indicators of the first type have a permanent magnet and a moving coil. This coil carries an aluminum radium-treated pointer and two phosphor bronze springs, and is equipped with two steel pivots. The pivots are ground so as to have a slight roundness at their point in order to withstand the vibration encountered in aircraft. This assembly is arranged to rotate in sapphire bearings which are mounted in screws secured by means of lock nuts and arranged to allow adjustment of end play of the coil. The instrument mechanism is mounted on a bakelite backplate and housed in a bakelite case over which a soft iron shield is placed to minimize the effect on the magnetic compass. Mounted on the backplate are three spools which are wound with manganin wire to have a resistance of 100 ohms each. These spools are connected to form three arms Of a bridge designated as A, B, and C in figure 29, the fourth arm being the sensitive element (bulb) . The instrument mechanism is connected across two opposite corners of the bridge and the source of current supply is connected across the two remaining corners, the instrument mechanism serving as the galvanometer. On the back of the instrument three connections are brought out in a bakelite block for the purpose of connecting the indicator to the sensitive bulb and to the source of current supply. Over the terminals is also a static shield arranged to take standard shielding material for shielding the conductors in order to prevent any interference with radio. The indicators of the later types of these thermometers are provided with an individual 3-volt light, the receptacle being molded integral with the case.

(2) The principal feature of the ratio meter type indicator is that its accuracy is maintained independent of voltage variations. The mechanism of the ratio meter type thermometer has both a magnetic and coil system as shown in figure 30. The magnet is a heavy alloy steel structure which is mounted on a bracket which also holds the pole pieces. The pole pieces and core are both circular but the relation between the core and the pole pieces is one of eccentricity, that is, the core is set above the center. The moving coil consists of two individual coils cemented together. The common terminals, which are connected together and are for temperature measurement purposes are brought out as a common connection to the batteries. One coil is connected through a manganin resistance and back to the battery, thus carrying a current proportional to the voltage and resistance. The other coil is connected through the resistance bulb and back to the battery.

(1) Side view.

(2) Rear view.
FIGURE 30.-Ratio type thermometer internal mechanism

c. The temperature sensitive element or bulb (fig. 31), made up of a winding of specially selected pure nickel wire wound on an anode-treated aluminum tube, changes its resistance with temperature and is adjusted to be exactly 100 ohms when subjected to a temperature of 0° C. The protection tube is made up of monel which is silver-soldered into a hexagon head. The two ends of the nickel winding are soldered to a twin plug connector, which is made of bakelite having molded into it two silver-plated brass female inserts arranged to receive a bakelite two-prong plug with silver-plated brass split pins. In addition, an adapter which is designed to cover the connecting plug is also threaded to receive the nut on the end of the static shield over the conductors. A gasket is provided between the adapter and the hexagon head in order to seal against moisture. In the case of the free air thermometer, the resistance bulb is placed in a specially designed housing for attachment to the skin of the fuselage or wing.

1) Free air thermometer bulb.

(2) oil or fuel mixture thermometer bulb.   (3) Connecting lead.
FIGURE 31.-Resistance bulbs and lead.

73. Operation.-a. The operation of electric thermometers is dependent upon the current supply. The source of this supply is the battery generator system of the airplane which is turned "off" and "on" by use of individual control switches or a single master switch.

b. The D'Arsonval system (fig. 29) is essentially an unbalanced Wheatstone bridge. The bulb or sensitive element is installed at the point where the temperature is to be measured. For every temperature the bulb will have a definite resistance proportionate to that temperature, and the bridge of which the bulb forms one arm will assume an unbalanced condition in a direction and in magnitude corresponding to the particular temperature to which the bulb is exposed. The indicator acts as the galvanometer in the Wheatstone bridge circuit, and the pointer deflection will indicate temperature in accordance with the unbalanced condition of the bridge. When the sensitive element is exposed to a temperature of 0° C., its resistance will be 100 ohms and since the arms A, B, and C are 100 ohms each, it follows that under this condition the bridge is balanced. The indicating instrument will then have no current flowing through its moving coil and will therefore be in its normal zero position. When the sensitive element is exposed to a temperature higher than 0° C., its resistance will increase, thereby unbalancing the bridge in a direction which causes the indicating pointer of the instrument to deflect toward the right, thus indicating the increase in temperature. Likewise when the temperature sensitive bulb is exposed to a temperature lower than 0° C., the bridge will be unbalanced in a direction which causes the pointer to deflect toward the left part of the scale, thereby indicating a temperature lower than 0° C.

FIGURE 32.- Wiring diagram of ratio type thermometer.

c. The ratio meter type system consists of a circuit having two parallel branches as shown in figure 32. One has a fixed resistance in series with the coil C–2 and the other the resistance bulb in series with the coil C–1. The direction of current in each coil and the polarity of the magnet is such as to cause the coil carrying the greater current to move into the weaker field. When the resistance of the bulb is identical with the manganin resistance R, and the same value of current is flowing through each coil, the torques balance and the pointer remains in the vertical position. If the temperature of the resistance bulb is raised, its resistance is increased and the current through the movable coil C–1 is reduced. The torque of this coil is reduced as a result and the other coil C–2 pushes downward into a weaker field, whereas coil C–1 with its lower current goes into a stronger field and the torques again balance due to the differences in field strength at these new positions. The pointer will therefore move across the scale to the new position, which is calibrated in terms of temperature of the bulb. By the same token, a reduction in temperature of the bulb would increase the current through coil C–1 and cause the pointer to move to the left. At a definite value of temperature and hence definite bulb resistance, the instrument reading is independent of comparatively large variations of battery voltage as the position taken by the pointer depends only upon the ratio of current of the two coils.

74. Installation.    a. The general points on installation of instruments as discussed in section III apply to these thermometers. After unpacking the indicators, they should be inspected to make sure that the glass is not cracked or broken, care being taken that when the instrument is rotated gently the pointer swings freely. The bulbs should be examined to see that no parts of the connecting head are missing.

b. When connecting the wires to the bulb leading to the instrument, the insulation is removed for about 1/4 inch on the ends of the wires and the wires scraped clean. The wires are then connected to the bulb by removing the adapter and exposing the bayonet plug. The bayonet plug may then be pulled out from the bulb proper. The wires are slipped through the hole in the adapter and secured to the plug. The plug is then replaced into the bulb and the adapter put back in position.

c. Figure 33 shows two wiring diagrams. Figure 33(1) is grounded system, while (2) is insulated from the ground. Their functions, however, are identical. To take off the static shield the four hexagon nuts and lock washers in the back of the instrument are removed, after which the cover can be removed and the shield will slide off exposing the terminal block. One wire leading from the bulb is connected to the binding post marked X, the other wire from the bulb being connected to the frame of the airplane as shown in figure 33(1) or to the terminal marked XV as shown in figure 33(2). After connecting the wires to the instrument, the static shield and cover fitting are replaced over the terminal opening and secured by means of the hexagon nuts and lock washers.

d. Before turning on the battery current, make sure that the instrument is on its normal zero which is 0° C. If it is found that the instrument is not on zero, adjust to zero by means of the small screw on the front of the instrument. By means of a small screw driver, rotate the screw slowly toward right or left until the instrument is on zero and then turn on the current.

75. Maintenance.-a. The general points on maintenance of instruments given in section II apply to these thermometers. In checking for and adjusting the mechanical zero it should be ascertained that the current to the instrument is off.

(1) Grounded system.    (2) Insulated from ground.
FIGURE 33. Installation diagram of electrically operated thermometer.

b. No servicing of the bulb is required. Unless the bulb is damaged by excessive heat, it should function indefinitely. The bulb will withstand for a short period temperatures of 302° C. (575° F.) but temperatures in excess of this may seriously damage the bulb and make it inoperative. In that case, it must be replaced with a new one and the old one forwarded to the depot for repair. Before condemning or reporting either the indicator or bulb as being unserviceable, a careful check should be made to insure that the apparent failure is not due to loose or shorted connections especially at the bulb end.

c. When service maintenance necessitates replacement of electric thermometer units, part numbers must be checked carefully in order to match bulbs and indicators. For further information on the interchangeability of the electric thermometer units reference is made to tables in Air Corps Technical Orders.