A B N S N S N S R Sensitive element (bulb) Figure 10-70.  A ratiometer temperature measuring indicator has two  coils. As the sensor bulb resistance varies with temperature, different  amounts of current flow through the coils. This produces varying  magnetic fields. These fields interact with the magnetic field of a  large permanent magnet, resulting in an indication of temperature.  10-39 The hot junction of a thermocouple varies in shape depending  on its application. Two common types are the gasket and the  bayonet. In the gasket type, two rings of the dissimilar metals  are pressed together to form a gasket that can be installed  under a spark plug or cylinder hold down nut. In the bayonet  type, the metals come together inside a perforated protective  sheath. Bayonet thermocouples fit into a hole or well in a  cylinder head. On turbine engines, they are found mounted  on the turbine inlet or outlet case and extend through the case  into the gas stream. Note that for CHT indication, the cylinder  For example, if the resistance of the temperature bulb is  equal to the value of the fixed resistance (R), equal values  of current flow through the coils. The torques, caused by  the magnetic field each coil creates, are the same and cancel  any movement in the larger magnetic field. The indicator  pointer will be in the vertical position. If the bulb temperature  increases, its resistance also increases. This causes the current  flow through coil A circuit branch to increase. This creates a  stronger magnetic field at coil A than at coil B. Consequently,  the torque on coil A increases, and it is pulled downward  into the weaker part of the large magnetic field. At the same  time, less current flows through the sensor bulb resistor and  coil B, causing coil B to form a weaker magnetic field that  is pulled upward into the stronger flux area of the permanent  magnet’s magnetic field. The pointer stops rotating when the  fields reach a new balance point that is directly related to the  resistance in the sensing bulb. The opposite of this action  would take place if the temperature of the heat-sensitive  bulb should decrease. The magnetic pole ends of the permanent magnet are closer at  the top than they are at the bottom. This causes the magnetic  field lines of flux between the poles to be more concentrated  at the top. As the two coils produce their magnetic fields, the  stronger field interacts and pivots downward into the weaker,  less concentrated part of the permanent magnet field, while  the weaker coil magnetic field shifts upward toward the more  concentrated flux field of the large magnet. This provides a  balancing effect that changes but stays in balance as the coil  field strengths vary with temperature and the resultant current  flowing through the coils. The amount of voltage produced by the dissimilar metals when  heated is measured in millivolts. Therefore, thermocouple  leads are designed to provide a specific amount of resistance  in the thermocouple circuit (usually very little). Their  material, length, or cross-sectional size cannot be altered  without compensation for the change in total resistance that  would result. Each lead that makes a connection back to the  voltmeter must be made of the same metal as the part of the  thermocouple to which it is connected. For example, a copper  wire is connected to the copper portion of the hot junction  and a constantan wire is connected to the constantan part. Thermocouples are used to measure high temperatures. Two  common applications are the measurement of cylinder head  temperature (CHT) in reciprocating engines and exhaust gas  temperature (EGT) in turbine engines. Thermocouple leads  are made from a variety of metals, depending on the maximum  temperature to which they are exposed. Iron and constantan,  or copper and constantan, are common for CHT measurement.  Chromel and alumel are used for turbine EGT thermocouples. A thermocouple is a circuit or connection of two unlike  metals. The metals are touching at two separate junctions.  If one of the junctions is heated to a higher temperature than  the other, an electromotive force is produced in the circuit.  This voltage is directly proportional to the temperature. So,  by measuring the amount of electromotive force, temperature  can be determined. A voltmeter is placed across the colder  of the two junctions of the thermocouple. It is calibrated in  degrees Fahrenheit or Celsius, as needed. The hotter the high-  temperature junction (hot junction) becomes, the greater the  electromotive force produced, and the higher the temperature  indication on the meter.  [Figure 10-71]  Ratiometer temperature measuring systems are used to  measure engine oil, outside air, carburetor air, and other  temperatures in many types of aircraft. They are especially  in demand to measure temperature conditions where  accuracy is important, or large variations of supply voltages are encountered. Thermocouple Temperature Indicators