2 Temperature measurements

2.1 Temperature sensing

2.1.1 State the points on an aircraft or aircraft component where temperature sensing is required for proper aircraft operation. (1)

Change in temperature is known as Delta T and it can be measured electrically (through electrical resistance), or monitored through the expansion of different materials such as the expansion and contraction of various solids, liquids or gases. Sensors measure temperatures of windshields, brakes, cabin, air ducts, hydraulic lines, and interstage turbine temperatures. The sensor products are designed to provide accurate and repeatable operation while maintaining configuration conformity to aircraft specifications.

1. Air temperature is one of the basic parameters used to establish data vital for monitoring of the aircraft and engines (as in the measurement of true airspeed, temperature control, thrust settings, fuel/air ratio settings) and it is therefore necessary to provide a means of in-flight measurement.

2. EGT and CHT multi-probes on reciprocating engines help provide troubleshooting on individual cylinders.

3. Engine instruments that indicate oil pressure, oil temperature, engine speed, exhaust gas temperature, and fuel flow are common to both turbine and reciprocating engines. However, there are some instruments that are unique to turbine engines. These instruments provide indications of engine pressure ratio, turbine discharge pressure, and torque. In addition, most gas turbine engines have multiple temperature-sensing instruments, called thermocouples, which provide pilots with temperature readings in and around the turbine section.

4. Variations of EGT systems bear different names based on the location of the temperature sensors. Common turbine temperature sensing gauges include the turbine inlet temperature (TIT) gauge, turbine outlet temperature (TOT) gauge, interstage turbine temperature (ITT) gauge, and turbine gas temperature (TGT) gauge.

5. Other applications where heat sensors might be applied in aircraft are: Heated Windshields and canopy Brakes  Cabin Environment  Air Ducts  Engine Control Modules  Hydraulic Lines  Interstage turbine temperatures  Fuel and Oil temperatures

2.1.2 State the type of device that would be used in each area. (1)

1. OAT uses a bimetallic strip made of iron and brass. The brass expands faster and bends the strip which moves the pointer. OAT indicator is used in conjunction with a TAS indicator. A knob on the TAS is turned to the temperature indicated on the OAT to obtain the aircraft’s true airspeed.

2. Oil temperature systems use Wheatstone bridge or a ratiometer circuit (a ratiometer is more reliable). Older types of oil temperature gauges use a vapour pressure or Bourdon tube type instrument.

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3. CHT and EGT gauges use thermocouples which produce a voltage as a result of increased temperatures.

4. In certain types of turbojet, radiation pyrometry – the radiation by any body at any wavelength is a function of the temperature of that body and is known as the body’s emissivity. If the emissivity is known, then by measuring the radiation the temperature of the body can be determined.

5. Resistance change instruments: Electrical resistance of some metals changes with their temperature. This property is used to measure relatively low temperatures as in oil temperature, OAT, and carburetor air temperature.

6. Wheatstone bridges and Ratiometers can be used in OAT, Carb air gauges, oil coolant an oil temperature measurement.

2.2 Non-electrical Types of Temperature-measuring Instruments

2.2.1 Describe the construction and operation of the following measuring instruments: 1. Solids expansion (bi-metallic strip) 2. Gas expansion (bourdon tube) (2)

Bi-metallic StripA bi-metallic strip converts temperature changes into mechanical displacement as an on/off measurement. The device consists of two strips of different metals with different coefficients of thermal expansion. The strips or elements are joined together by rivets, by brazing or by welding (see Fig. 1). Differential expansion causes the element to bend one way when heated, and in the opposite direction when cooled below its nominal temperature. The metal with the higher coefficient of expansion is on the outer side of the curve whilst the element is heated and on the inner side when cooled. The element can be formed as a switch contact, called a thermostat, to open or close a circuit when temperature limits are exceeded. Alternatively the element is formed in a spiral shape so that temperature changes cause a shaft to rotate; the typical application for this is an outside air temperature indicator. The sensing portion of the indicator projects through windscreen into the ambient air.

Figure 1: Bi-metallic strip principles (a) before heating, (b) after heating.

Bourdon TubeThe Bourdon tube was invented by Eugene Bourdon (1808–84), a French watchmaker and engineer. The pressure-sensing element is a tube with either a flat or elliptical section; it is formed as a spiral or curve, see Fig. 2.

Figure 2: Bourdon tube principles

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One end of the tube is sealed and connected to a pointer mechanism; the open end is connected to the fluid system via a pipe. As the applied pressure from the fluid system increases, the tube will tend to straighten out, while a reduced pressure will cause the tube to return to its original shape. This movement is transferred via the gear mechanism to move a pointer. The pointer moves across a scale thereby providing a direct reading of pressure. Materials used for the tube are selected for the pressure range being measured; these include phosphor bronze (0–1000 psi) and beryllium copper (0–10,000 psi). The Bourdon tube principle can also be used to remotely measure pressure, see Fig. 3. Used in fuel pressure gauges.

Figure 3: Bourdon tube/remote pressure sensing

2.3 Thermocouples

2.3.1 Outline the thermocouple system principle (Seebeck effect) (1)

The thermocouple principle is based on a thermoelectric effect; this is a generic expression used for temperature-dependent electrical properties of matter. Thermocouples use the potential difference that results from the difference in temperature between two junctions of dissimilar metals. This thermoelectric potential difference is called the Seebeck effect, named after the German physicist Thomas Seebeck (1770– 1831). Two metal conductors made out of different materials are welded at each end to form junctions, see Fig. 4(a).

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Figure 4: Thermocouple principles: (a) The Seebeck effect, (b) A simple thermocouple circuit, (c) A practical thermocouple circuit.

The thermocouple junction is housed within a stainless steel tube to provide mechanical support; the wires are terminated as a junction in the thermocouple head. When a temperature gradient between each junction is created, electrons will diffuse from the higher temperature junction to the colder junction. This creates an electric field within the conductors, which causes a current to flow, and an electromotive force (e.m.f.) to be generated. In a simple circuit, Fig. 4(b), the hot junction is exposed to an unknown temperature (T1). The cold junction is contained with the indicator (measuring current); this is at a known temperature (T2). The practical thermocouple circuit is shown in Fig. 4(c). The hot junction is located in the thermocouple head, or probe. Connecting cables back to the meter are either built into the probe as flying leads, or they are attached onto external connectors.

Since a thermocouple generates its own electrical current, it is capable of operating independent of aircraft power – this was in a test question.

2.3.2 List typical thermocouple applications. (1)

Sensors are of the two basic types:a. Surface contact type: This sensor is designed to measure the temperature of a solid

component and is used as the sensor in air-cooled cylinder head temperature indicating systems.

b. Immersion type or probe type of thermocouple is designed for the measurement of gas temperatures as in the EGT probe for sensing turbine exhaust gases. The thermocouple is mounted in the exhaust pipe, usually within 6 inches of the cylinder.

2.3.3 State the metal combinations used for low and high temperature thermocouples TEST QUESTION (1)

Positive Wire Negative wire Max temp 0C continuous

Application

Copper Constantan (Ni 40% Cu 60%) 400 CHTIron Constantan (Ni 40% Cu 60%) 850 CHTChromel (Ni 90% Cr 10%) Alumel (Ni 90% Al 2%) 1100 EGTPlatinum Rhodium-Platinum 1400 EGT*

* Pallett’s book says this type of thermocouple is not used in aircraft while other references say they are used.

Junction Materials Voltage (μV/◦C) Min Temp (◦C) Max Temp (◦C) Iron and Constantan 41 -40 750Chromel and Alumel 41 -270 790Chromel & Constantan 68 -270 790Platinum & Rhodium 10 100 1800

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The relationship between the temperature gradient and EMF is linear; a typical thermocouple will generate approximately 4 mV per 100ºC of temperature difference. There are many different combinations of materials used for the two thermocouple wires; the choice of materials depends mainly on the operational range of temperatures to be measured.

2.3.4 Describe the following: a. Cold junction compensation of various types of thermocouple TQb. Placement of cylinder head thermocouples c. Compensating and extension leads d. Typical thermocouple circuits e. Indicating devices f. Importance of lead length (2)

a. Cold junction compensation of various types of thermocouple: The e.m.f produced by a thermocouple is depends on the difference in temperature between the hot and cold junctions. If the ambient temperature of the indicator changes (cold junction), while the hot junction remains constant, a change in e.m.f will result causing an error in the reading. The reading should be of the hot junction only and so automatic detection and error correction is required. This is accomplished using the bi-metallic strip principle, in the shape of a flat spiral spring. This compensates for differences in cockpit temperature changes or for reference-junction temperature changes.

b. Placement of cylinder head thermocouples: Piston engines use thermocouples to monitor the cylinder head temperature (CHT). This indication is used to monitor the temperature of an air-cooled engine. The system can be arranged with either one CHT thermocouple per cylinder or just one CHT thermocouple per engine. In the latter case, the thermocouple is installed in the hottest running cylinder; on a horizontally opposed engine this would be the rear cylinder. The thermocouple can be formed to fit under the spark plug as in a spark plug gasket-type fitting or a bayonet-type connected directly into the head itself – it must be in good thermal contact with the hottest cylinder head. The advantage of monitoring each cylinder is that trend indications can be derived.

c. Compensating and extension leads: Using the example of a copper/constantan thermocouple connected to a CHT indicator. If a normal copper cable is connected to the thermocouple then one lead is copper to copper junction and the other will be a copper to constantan junction creating another hot junction. Similarly all terminal connections necessary for routing cables through the aircraft, including the connection at the indicator, will create further hot junctions. In order to eliminate hot junctions use extension leads made of the same material as the thermocouples. Sometimes leads of similar thermo-emf characteristics can be used, for example, a chromel/alumel thermocouple can be joined to its indicator by copper/constantan leads - these leads are known as compensating leads. Since the indicator is a current-measuring instrument, the resistance of the thermocouple leads must have a specific value. This is usually 2 or 8 ohms and so leads must not be cut to suit the installation.

d. Typical thermocouple circuits: A typical engine temperature measurement system is illustrated in Fig. 5 overleaf. On larger engines, it is possible to have a range of temperatures in the exhaust zone due to the turbulence of gases. Some installations feature a thermocouple that has two or even three hot junctions within the same outer

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tube. This arrangement provides an average of temperature within the zone to provide an average temperature at different immersion distances in the engine.

On piston engines, the EGT indication has a different application; it is used to adjust the fuel–air mixture for economical running of the engine. The thermocouple is located in located in the exhaust system, reducing the amount of fuel into the cylinders (called leaning the mixture); this results in a more efficient combustion and the exhaust temperature increases. When this temperature reaches a peak, maximum efficiency has been achieved. Using one thermocouple per cylinder averages out individual differences and provides enhanced fuel efficiencies.

Figure 5: Engine temperature system – exhaust gas system

e. Indicating devices: Resistance is an important factor in the external circuit, therefore, indicators calibrated for use with the external circuit are of a specific resistance value, for example, 8 or 25 ohms, and are identified accordingly. A turbine inlet temperature indicator provides a visual display of the temperature of gases entering the turbine. Dual-unit thermocouples installed in the inlet casing measure the temperature of each inlet. The indicator scale is calibrated in degrees Celsius (°C) from 0 to 12 (times 100). The digital indicator reads from 0 to 1,200°C, in 2-degree increments.

The exhaust gas temperature indicator provides a visual display of the engine's exhaust gases as they leave the turbine unit. A typical exhaust gas temperature indicating system for a modern naval jet aircraft is shown in figure 5.

f. Importance of lead length: Thermocouples, their leads, and harnesses where appropriate, are made up in fixed low-resistance lengths. Extension and compensating leads are also made up in lengths of uniform resistance to suit the varying distances between thermocouple hot junction and indicator locations. If resistances are too low, a special constantan-wire resistor may be installed in the negative lead.

2.3.5 State the precautions to be taken when installing or removing indicating gauges. (2)

Thermocouple cables are colour-coded to reduce the likelihood of different materials being cross-connected, or mixed in the same installation (note that these codes vary in some countries; always refer to the maintenance manual):• Nickel-chrome - Chromel (white)

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• Nickel-aluminium - Alumel (green)• Iron (black)• Constantan (yellow)• Copper (red)

Thermocouple instruments are polarized and extremely sensitive to resistance changes within their electrical circuits. There are a few precautions to be observed, they are:

a. Observe all colour coding and polarity markings because accidently reversing the wires causes the meter to move off scale on the zero side.

b. Ensure all electrical connections are clean and the correct torque value is applied.c. Thermocouple leads are typically supplied as matched pairs and secured together by common

braid. The leads are a specific length, matched to the system to provide accurate temperature indications. The lengths of the leads cannot be altered because it alters the resistance.

d. Since the indicator is a current-measuring instrument, the resistance of the thermocouple leads must have a specific value – usually either two or eight ohms. If the resistance is too low then a special constantan wire resistor may be installed in the negative lead.

e. For accurate temperature indication, the reference or cold junction must be held constant. f. In some cases the wiring leads are permanently attached to the thermocouple, warranting the

replacement of the entire harness and thermocouple should a wire break or becomes damaged.

g. Some competitor's probes use red and white coloured wires, which are the same as yellow and black wires. Do not confuse thermocouples with thermistors. Thermocouples produce millivolts and produce their own current, while thermistors change resistance and rely on an outside source of power.

2.3.6 Detail testing procedures for thermocouple systems. (2)

Perhaps the best way to evaluate a used thermocouple is to ‘probe’ the location by placing a new thermocouple that has a known output alongside the suspect one in an operating process and compare the readings. If it is not practical to have two sensors in place at the same time, the next best thing is to remove the suspect probe and replace it with another one known to be good. Then, as long as the good probe is located in the same place as the removed one had been, and the process has not changed during the exchange, the readings from the two probes can be compared.

Install the probe in the same location as factory-installed system, or rear cylinder on horizontally opposed engines, or #1 cylinder on radial engines. If unsure, make several similar test flights changing thermocouple locations between flights to select the hottest cylinder. That cylinder should remain the hottest unless airflow is altered because of cooling airflow problems such as air baffling leaks, etc. or cylinder runs excessively lean.

One very useful instrument for troubleshooting thermocouple systems is a portable temperature indicator. A number of these devices are capable of operating with two or more different thermocouple types, and some offer an ‘output’ function that will produce an electrical output to simulate a thermocouple operating at any temperature of choice. Can also test by applying a known value in millivolts and this reads a datum temperature

Inspect entire system for loose connections, broken wires/connectors, or mismatched colour-codes between lead/meter and probe. Disconnect meter from lead and check the loop resistance of the lead and probe and compare with value on meter label. Use cigarette lighter or soldering iron

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to heat probe and check for reading on meter. Use boiling water for accurate check at first meter mark. If you obtain no indication and lead/probe resistance and type are correct, then you may have a bad calibration potentiometer or damaged meter movement.

2.3.7 Explain the effect of ambient temperatures on thermocouple gauges (2)

For accurate temperature indication, the reference-junction temperature – the cold junction - must be held constant. It is not practical to do this in an aircraft instrument, so the indicator needle is mounted on a bi-metallic hairspring in such a way that it moves back as the cockpit temperature increases. This compensates for reference-junction temperature changes.

For example, assume the reference or cold junction is at 0 0C and the hot junction is 500 0C. This produces an e.m.f in a chromel/alumel junction of about 20.64 mV (20.64/500 = 0.04128 microvolt per degree Celsius). If the ambient temperature rises to 20 0C and the hot junction remains at 500 0C, then the temperature difference is now 480 0C or an e.m.f of 19.85 mV. Thus the change in ambient temperature in this instance will cause the thermocouple indicator to read low by an amount equal to the change in the ambient temperature – by 20 0C.

2.3.8 Describe the principles of operation of the following thermocouple types: a. Bayonet b. Gasket (2)

A thermocouple used for measuring CHT is a loop made from two different types of wire. One wire is made of constantan, a copper-nickel alloy, and the other is made from iron. One end of each wire is embedded in a copper spark-plug gasket or is joined inside a bayonet. The end of the loop is called the hot or measuring junction.

Gasket style Cylinder Head Temperature thermocouples can read 50°F to100°F (normally about 60°F) more than bayonet style CHT’s. When replacing an existing CHT probe (thermocouple) the same cylinder should be used from where the old probe was removed. Some engines do not have thermowells to install bayonet probes so gasket types must be used. In cold climates, thermowells are sometimes used for heater installations. Spring loaded bayonet types are more accurate than simple screw in types because the probe touches the bottom of the thermowell and senses metal rather than air temperature.

The hot junction is always installed in the cylinder head in one of two ways: the 2 dissimilar wires may be joined inside a bayonet probe which is inserted into a special well in the top or rear of the hottest cylinder or the wires may be embedded in a special copper spark plug gasket. The cold junction or reference junction is typically located in the instrument case.

2.3.9 State the gauge reading if an open circuit occurs TEST QUESTION (1)

Due to the unchanging nature of thermocouple characteristics all that should ever be needed to check thermocouple probes, is a simple go/no go test; with the thermocouple disconnected from the gauge, connect an ohmmeter to the output pins of the thermocouple (the probe does not need to be heated for this test). The ohmmeter should be set to its lowest resistance range. The ohmmeter should read about 1.5 ohms for a CHT probe or about 2.5 ohms for an EGT probe. If the ohmmeter shows an open circuit (infinite resistance), the probe is bad.

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An open circuit in the thermocouple detector means that there is no path for current flow, thus it will cause a low (off-scale) temperature reading. A short circuit in the thermocouple detector will also cause a low temperature reading because it creates a leakage current path to the ground and a smaller measured voltage.

Compared with the erroneous readings seen when a thermocouple drifts, open circuit faults are immediately obvious. An open circuit is usually caused by breakage of a thermocouple limb, but always check for poor connections to the compensating or connecting leads. Limb breakage generally occurs close to the hot junction where the wire is weakest and where the effects of contamination are greatest.

2.4 Resistance circuits

2.4.1 Outline the circuit layout and components found in the following electrical resistance thermometers:

a. Wheatstone bridge b. Ratiometer system (1)

a. Wheatstone bridge

Figure 6: A Wheatstone bridge has 3 fixed resistors and one variable resistor – the temperature probe. The probe’s temperature varies with temperature of the substance flowing past. The bridge in the circuit consists of a galvanometer that is calibrated in degrees to indicate temperature.

Some resistance thermometers measure temperature changes by placing the bulb in one of the legs of a Wheatstone bridge. The bridge is balanced when no current flows through the indicator or galvanometer, i.e., when the ratio R1 : R3 is the same as the ratio R2 : RBULB. As the temperature sensed by the bulb decreases, the bulb resistance decreases and the bridge unbalances, sending current through the indicator that drives the needle towards the low side of the dial. An increase in temperature drives the indicator needle towards the high side of the dial.

b. Ratiometer System

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A ratiometer temperature measuring system operates with two circuit branches that balance electromagnetic forces. One branch contains a coil and fixed resistor, while the other contains a coil and variable resistor, located in the temperature sensing probe. The coils are wound on a rotor which pivots in the centre of a permanent magnet air gap.

Figure 7: A Ratiometer system

The shape of the permanent magnet provides a larger gap at the bottom (see figure above) than at the top, therefore the flux density is strongest at the top and gets progressively weaker as the gap gets wider. Current flow through each coil creates an electromagnet that reacts with the polarity of the permanent magnet, creating torque that turns the rotor until the forces are balanced. If the resistance of the probe and the fixed resistor are equal, then the current through both coils are the same and the pointer remains in the centre position. If the probe or sensitive element (bulb) temperature increases, its resistance increases which causes the current in coil B to decrease. This in turn causes the torque in coil B to decrease. This makes A the stronger coil and it pushes into the weaker magnetic field of B, while B is pushed into the stronger magnetic field of A until the forces are once again balanced.

Ratiometers are especially important when accuracy is critical or large variations of supply voltages are encountered and are therefore preferred over the Wheatstone bridge circuits.

2.4.2 Compare the difference between platinum and nickel resistance bulbs. (2)

Resistance temperature devices or detectors (RTDs) are the most accurate sensors for industrial applications and also offer the best long-term stability. A representative value for the accuracy of a platinum resistance is +0.5 percent of the measured temperature. After one year there may be a shift of +0.05 degrees Celsius through aging. Platinum resistance thermometers can cover temperature ranges from -200 to 800 degrees Celsius.

The principles of this type of temperature-measuring device are based on measuring the change of resistance of a metal element and interpreting this as temperature. Metal elements have a positive temperature coefficient; certain metals, e.g. nickel and platinum, have a very stable and linear relationship between resistance and temperature (see Fig. 9). The resistance in a nickel resistance bulb changes from approximately 30 ohms at -70 degrees F to 130 ohms at 300 degrees F.

Nickel is less accurate than platinum, but has lower cost. All RTDs used in precise temperature measurements are made of Platinum and they are sometimes called PRTs to distinguish them. RTDs employ the property that the electrical resistance of metals varies with temperature. They are positive temperature coefficient (PTC) sensors whose resistance increases with temperature.

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The main metals in use are platinum and nickel. The most widely used sensor is the 100 ohm or 1000 ohm RTD or platinum resistance thermometer.

Figure 8: Resistance temperature devices (RTD) features

The acronym “RTD” is derived from the term “Resistance Temperature Detector”. The most stable, linear and repeatable RTD is made of platinum metal. The temperature coefficient of the RTD element is positive and almost constant. Typical RTD elements are specified with 0°C values of 50, 100, 200, 500, 1000 or 2000Ω. Of these options, the 100Ω platinum RTD is the most stable over time and linear over temperature. The RTD element requires a current excitation. If the magnitude of the current source is too high, the element will dissipate power and start to self-heat.

Figure 9: RTD temperature and resistance chart comparing nickel and platinum

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2.4.3 State the effects of an open circuit and short circuit in the bulb. TQ (1)

In an open circuit there would be no output signal as the circuit is not closed and therefore the reading would be as if the bulb had been disconnected from the circuit. An open circuit in the RTD or in the wiring between the RTD and the bridge will cause a high temperature reading or full scale reading. A short circuit produces a low reading and the indicator gauge will read low.

A short circuit offers little or no resistance to the flow of electrons. A short makes the circuit behave as if the component wasn't there.  Loss of power or a short within the RTD will cause a low temperature reading.

Thermistors are different from RTDs in that the latter are metal and the former use metallic oxides.

There are two types of thermistors: Positive Temperature Coefficient (PTC) types have a resistance that increases with

increasing temperature. Negative Temperature Coefficient (NTC) types have a resistance that decreases with

increasing temperature.

For a small thermistor, put an ohmmeter on it and the heat it up with a blow dryer, heat gun, or the tip of a soldering iron - the resistance should change smoothly (up or down depending on whether it is PTC or NTC type). If the resistance changes erratically, or goes to infinity or zero, the device is bad. However, you will need specifications, temperature measuring sensors, etc. to really determine if it is operating correctly.

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