Air Temperature Indicator in Aircraft – PART 3

Ratiometer Electrical Resistance Thermometer

One more method to determine the temperatures using electrical-resistance thermometers is to do it by utilizing a ratiometer. In the Wheatstone bridge detectors, there are failures due to fluctuations in the mains voltage. The ratiometers tend to be more stable, & they can provide much larger accuracy. As suggested by their name, ratiometer electrical-resistance thermometers help in measuring the ratio of current. 

The sensor part of the resistance bulb of such types of thermometers is primarily similar to the one explained above. The circuit consists of a variable resistance as well as a fixed resistance to give the reading. This consists of 2 power supplies; each having a coil that is mounted on one of the sides of that pointer mounted in the field of a large-sized permanent magnet. The changing current passing through the coils leads to varied fields reacting with the much larger permanent magnet’s field. Such an interaction turns the cursor against a dial that has been calibrated in degrees Fahrenheit/Celsius, providing the indication of the temperature being measured. 

The ends of the magnetic poles of such types of permanent magnets tend to be closer at the topmost point than at the bottom-point. It leads to the flux’s magnetic field lines among the poles to concentrate at the tip. When the 2 coils release their own magnetic field, the one that is stronger tends to interact & deflect down into the weaker one, which happens to be the less concentrated part of the permanent field of the magnet; however, the field of the weaker coils moves up into the more concentrated large magnet’s flux field. This leads to a sort of balancing effect, which alters but tends to remain balanced as the coil field becomes strengthened and varies with the temperatures & currents that are flowing in the coils. 

For instance, if the temp-bulb’s resistance is the same as the value of the fixed resistance (R), then the same values of current will flow through the coils. The torque(s) thus produced by the magnetic fields by each coil tends to be the same & cancels out any motion in the larger field. The pointer is in the vertical position. As the temperature of the bulb augments, so does its resistance. This leads to the current through one of the coils in the circuit-branch to augment. This leads to a stronger magnetic field being produced in one coil than in the other one. As a result, the torque on one of the coils increases, and it is pulled down to the weak point of the higher magnetic field. Simultaneously, lesser current flows through the resistance of the sensor bulb and the second coil, causing that coil to form a weaker magnetic field, which is pulled up into the stronger flux region of the permanent magnet’s magnetic field. The pointer halts turning when the fields tend to reach a new equilibrium point, which is directly related to the resistance of the sensing bulb. This would be the opposite if the temperature of the thermosensitive bulb were to drop. 

Ratiometer’s temperature measurement systems are being utilized for measuring outside air, engine oil, carburetor air as well as other temperatures in many of such types of aircrafts. These happen to be particularly useful for measuring temperature conditions wherein accuracy is more significant or where there are large fluctuations in the supply voltages.