Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 17. Cylinder Temperature Gages

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 XVII - CYLINDER TEMPERATURE GAGES

76. Purpose and use.-The purpose of cylinder temperature gages on airplanes is to measure and indicate the temperature of air-cooled engines at some point on one of the cylinders, usually the master rod cylinder.

77. Description.-a. The cylinder temperature thermometer consists essentially of an indicator, thermocouple, and the thermocouple leads. If more than one thermocouple is used, a multiple polar switch is employed for connecting the various thermocouples to the indicator as required.

b. The indicator is a sensitive D'Arsonval type mechanism having a coil arranged to move in an annular air gap of a permanent magnet.

(1)Rear view. (2) Front view.
FIGURE 34.-Cylinder temperature gage.

The coil carries the pointer and the control consists of two phosphor bronze springs which also serve to conduct the current into the coil. This mechanism is placed in a 2 3/4-inch molded bakelite case which is provided with a raintight seal and is covered with a static shield. Two binding posts protrude from the rear of the case and are marked with positive and negative signs, as shown in figure 34(1). Standard temperature gages are provided with a 3-volt light, the receptacle being integral with the instrument case. The indicator as shown in figure 34(2) has a range of 0 to 350° C. and is suitably calibrated and adjusted for use with iron constantin thermocouples and leads. It is adjusted to allow for an external circuit resistance of 2.00 ohms, which includes the thermocouple and leads, these always having the same value of resistance.

c. The thermocouple and thermocouple leads (fig. 35) are made of iron and constantin, the latter being an alloy of copper and nickel. They are selected so as to have a definite electromotive force per degree of temperature. The thermocouple consists of a solid copper gasket with the two extensions brazed to its surface. The gasket is the same in dimensions and is interchangeable with standard spark plug gaskets. The leads are insulated and have suitable terminals for connection at both ends. Switch leads, if used, are normally so short that their resistance is negligible and can be ignored.

FIGURE 35.-Diagram showing connection of cylinder temperature gage, thermocouple, and leads.

78. Operation.-a. The operation of this thermometer depends on the fact that when two dissimilar wires are joined together to form a junction ( generally called the hot junction) and the junction is heated, an electromotive force will be generated at the opposite ends of the two wires (generally called the cold junction). The magnitude of this electromotive force depends upon the difference in temperature between the hot and the cold junction and upon the composition of the metals used.

b. As the electromotive force generated depends upon the difference in temperature between the hot junction (the spark plug thermocouple washer) and the cold junction (the cold junction extends inside of the instrument case), it. follows that in order to indicate the true temperature at. the spark plug some means must be employed to compensate the indicator for cold junction temperature changes. This is done by means of a bimetallic spiral spring, the outer end of which is attached to one of the control springs of the instrument. This causes the instrument to be actuated not only by the voltage of the thermocouple, but also by the temperature surrounding the instrument itself. When the thermocouple lead is disconnected from the instrument, the instrument. will indicate the temperature of the location where it is installed. A negative temperature coefficient resistor is included in series with the moving coil of the instrument to compensate for the positive temperature coefficient of the moving coil.

79. Installation.-a. The general points on installation of instruments given in section III apply to the cylinder temperature gage. In addition, some specific points must be observed when service maintenance necessitates replacement installation. The binding posts of the indicator are short-circuited by a small piece of copper wire to close the electrical circuit for protection during shipment. This should be taken oft and discarded at the time of installation. The magnetic shield must not be removed as this would influence the magnetic system and cause an error in the readings. Sometimes a non-magnetic static shield and cable connector is also supplied. This will be found mounted on the rear of the magnetic shield covering the binding posts. This static shield should be loosened when connecting leads to binding posts. However, it has no influence on the readings or accuracy of the indicator and is used only to prevent interference with the radio equipment and the magnetic compass.

b. In making the installation, all connections should be clean and tight, otherwise resistance will be introduced and the readings incorrect. After the gasket thermocouple has been secured in position under the spark plug and connections made to the leads, the connectors should be anchored in some way either by insulation tape or by a small clamp to prevent breaking due to vibration. In any event., the connectors must be taped to prevent shorting to the engine. The thermocouple lead is attached to the thermocouple at the one end and the indicator at the other end. The indicator end of the lead has two eye terminals with holes of unlike diameter; the thermocouple end of the lead has one eye terminal and one terminal with a nut mounted on it. The plus and minus studs of the indicator are of unlike diameter, and the indicator studs and lead terminals are so arranged that connections can be made in one way only. Correct polarity is thus assured.

c. The thermocouple leads must not be lengthened or shortened as they are of a definite resistance and enter into the calibration of the indicator. When installing, it is often desirable to trace out thermocouple leads from the engine end. A convenient method of doing this is to use a spare thermocouple attaching it to the lead to be traced and then heating the couple with a soldering iron. In this way, enough voltage will be generated to obtain a reading on the indicator.

80. Maintenance.-The general points on maintenance given in section II are applicable to cylinder temperature gages. In checking the indicator for its zero position, one convenient method is to open the circuit at some point, for example, at the thermocouple, or by removing one of the eye terminals from one of the studs on the back of the indicator. The instrument should then indicate the temperature of the cockpit which can be checked by placing a mercury in-glass thermometer adjacent to the indicator allowing sufficient time for the mercury thermometer to attain this temperature. The zero position of the indicator can then be adjusted to agree with the thermometer reading by rotating the zero adjusting screw located on the glass face.