Chapter 6

Instrumentation

When operating any gas turbine, instrumentation is always useful and in some cases essential. Perhaps the two most important operating parameters of a gas turbine are speed and temperature, that is, how fast is it going? and how hot is it getting? The rotational speed of an engine must be considered during different stages of its operation and a maximum speed should never be exceeded. The exhaust temperature indicates the health of an engine and when it has reached its maximum permitted load.

There are many other useful operating conditions which can be monitored such as lubricating oil pressure and temperature, compressor delivery pressure, fuel pressure, and air intake temperature. In the case of electrical generators, volts, amps and power output may also be monitored.

Devices may also be fitted to gas turbines which measure cyclic events, the most common are total running time and the number of starts.

Engine Speed
Many gas turbine engines are fitted with some sort of speed measuring or indicating device, normally this consists of either an electrically operated tachometer or a mechanically driven speed indicator.

Mechanical Tachometer
Many stationary engines employ a mechanically driven tachometer to indicate the engine speed. A cable drive is used which is derived from the engine accessory gearbox. The cable connects to a simple automotive type indicator which works in the same way as a piston engine tachometer or a car speedometer. Inside the indicator a magnet is driven by the connecting cable and rotates inside an aluminium drum. Due to electrical currents which are induced into the drum, the drum turns in the same direction as the rotating magnet but it is restricted in movement by a spring. The faster the magnet turns the further around the drum turns which flexes the spring, the rotation of the drum is proportional to speed and so is used to move a pointer next to a calibrated scale.

Certain versions of the Rover 1S60 engine use a mechanically driven tachometer. The tachometer is also fitted with an "Odometer" type counter which records engine running time. The correct governed speed is marked on the tachometer dial and the dial is calibrated from 0 to 50,000 rpm.

Electrical Tachometers

There are many electrical systems which are used to indicate engine speed, the most common system uses a tacho-generator/indicator system. A small alternator is driven by the engine and normally mounted on a part of the accessory gearbox. The alternator provides a three phase electrical signal, the frequency of the signal is proportional to the engine speed. A three phase generator is chosen as it provides a current which will rotate a motor in a particular direction. The generator is connected to an indicator instrument which reads RPM. This arrangement is very common on older aircraft and can also be found on ground based stationary equipment.

A tacho-generator consists of a magnet which rotates inside a number of coils. Care should be exercised never to create an electrical short across the coils when the generator is turning, as this can damage the generator by reducing the strength of the magnet inside. A weakened magnet will result in less electrical output which may lead to false or unreliable readings on the indicator instrument connected to it.

The three phase electrical output from the tacho-generator is used to drive a small electric motor, this motor is mounted inside the speed indicator instrument. The instrument is constructed in a similar way to the mechanical tacho, the motor drives a "speedometer" type mechanism which moves a pointer to reveal engine speed. The combination of the electrical generator and the instrument mounted motor replaces the mechanical cable linkage.

The instrument dial face is calibrated in RPM or in many cases %RPM. Gas turbine engines unlike piston engines vary enormously in terms of operating speed, for instance a RR Derwent idles at around 3500 rpm a Saurer GT15 APU idles at about 50,000 rpm! Percent is an easier way of quantifying engine speed for all sizes of gas turbine. A common %RPM indicator consists of two dials, a large one for x10% and a small dial for x1%. The small dial is sensitive and rotates rapidly when an engine accelerates or decelerates.

%RPM indicator instruments usually conform to a common standard. The instrument indicates 100% when fed with an AC signal measuring 70 Hz in frequency. A two pole tacho-generator turning at 4,200 rpm will produce this signal. It is normally arranged inside the engine gearbox so that the tacho-generator is driven at 4,200 rpm when the engine is operating at maximum speed i.e. 100%.

Certain older engines may use other gear ratios and instrument calibrations, it is important to always be clear about the actual and indicated speeds of any running gas turbine. It is useful if possible to check a tachometer-generator and indicator instrument combination when removed from the engine. An electric motor can be used to rotate the tacho-generator at a particular know speed and the indicator reading noted. The drive to the tacho-generator on the engine can be examined and the speed ratio determined, this is achieved by rotating the main engine shaft slowly and watching for movement of the tacho-generator drive.

There are many other electrical tachometer systems, one simple system consists of a single phase tacho-generator and an AC volt meter. The EMF (electro motive force) produced by the tacho-generator is directly proportional to the rotational speed of the generator, as the speed doubles the voltage doubles. The generator output voltage can be measured with a simple low current moving coil meter and rectifier, the meter is calibrated in RPM.

Electronics are used on certain engines and can be used to provide a speed indicating system when an engine was not originally fitted with such a system. Usually a rotating
shaft or gear can be used with a sensor and a train of electrical pulses provided which are proportional to speed. Magnetic reluctance probes are useful devices which may be used as engine speed pick-ups. A reluctance probe consists of a small magnet and a coil mounted on a common magnetic pole piece. The probe is placed near rotating objects such as gear teeth which are made of a magnetic material , the magnetic coupling then varies as the gear teeth pass by. The varying magnetic field leads to an induced electrical signal in the coil, this signal can then be amplified and processed to provide a speed indication.

LM2907 circuit/info........

Alternator speed sensor………

Exhaust Gas Temperature

Exhaust gas temperature or jet pipe temperature is the basic measurement of how hot a gas turbine is running. Exhaust temperature is one of the easiest parameters to measure and can be measured accurately. Exhaust temperatures are relatively high and can reach 750 degrees centigrade, changes are also quite large and so detecting this with simple electronics is easy.

The universal way of measuring exhaust temperature in a gas turbine is to place one or more thermocouple devices into the exhaust gas stream. A thermocouple consists of a junction of two dissimilar metals, when the junction is heated a small electric current is created, this current is proportional to the temperature of the junction. The electric current generated by the thermocouple can connected to an indicating device such as a moving coil meter or a digital readout.

The metals usually used for gas turbine thermocouples are chromium aluminium and nickel chromium alloys, these are often abbreviated to Cr and Al, a thermocouple of this type is also known as a "K" type device. When placed together the metals form a junction which develops a precise electrical signal proportional to temperature. Normally two junctions exist, a hot junction and a cold junction, the actual voltage produced is proportional to the temperature difference of the two junctions. The hot junction is obviously placed in the exhaust of the engine, the cold junction exists in an area of relatively stable temperature, in the case of a simple aircraft indicator instrument, the cold junction is placed inside it.

The thermocouple which is placed in the exhaust gas stream may consist of a simple bonded junction or the junction may be placed in a tube. Often the thermocouple is mounted in a tube and holes placed in the tube. The fast moving hot gases pass through the holes and into the tube, they then flow over the thermocouple junction and then passes out through more holes. This type of thermocouple system is referred to as a "stagnation thermocouple" and it exhibits a more accurate measurement as it measures the temperature at a much slower gas flow.

There two basic ways of indicating exhaust temperature, an analogue meter can be used or an electronic digital meter can be used. Simple analogue indicators consist of a sensitive moving coil meter, the cold junction and one or more calibration resistors. The cold junction and resistors are mounted inside the instrument. This type of instrument are common in older gas turbine powered aircraft and are also useful for stationary or ground applications. The beauty of the analogue meter is that it requires no source of power, the energy to drive it comes from the thermocouple signal.

Exhaust temperature indicators should be connected up in a particular way. Special cable should be used to connect the thermocouple junction in the engine exhaust to the exhaust gas temperature indicator. The cable used is called thermocouple compensating cable or extension cable. The cable is made of similar metals to that of the thermocouple junctions, this prevents unwanted electrical signals which are generated at the various electrical junctions from disturbing the accuracy of the indicator reading.

A consequence of using compensating cable is an electrical loss. The compensating cable unlike ordinary copper cable exhibits a significant electrical resistance, this resistance is compensated for in the calibration of the indicator instrument. The required resistance of the external cabling between an indicator instrument and thermocouple is referred to as the "External Resistance". The value of the external resistance is often written on the instrument dial face or on a specification plate attached to it. Common values are 8 or 25 ohms, the external resistance refers to the total resistance or loop resistance i.e. both conductors in the cable. When setting up an exhaust gas temperature indicator the connecting cable should be chosen to provide the required external resistance. Compensating cable is sold in various gauges, if a long run between the thermocouple and the instrument is required a heavier gauge cable should be selected. Typical resistance values for compensating/extension cables are
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The electrical signal produced by a thermocouple can also be amplified and displayed on a digital meter. Digital meters are complex devices and come in various different forms. Hand held meters can be purchased which are battery powered, or panel mounted modular units may be used in more permanent situations.

Digital meters are basically a form of digital volt meter which has had the calibration altered to make it read temperature. One of the simplest forms of digital meters consists of a counter device and a capacitor. The counter is incremented by a clock and at the same time the capacitor begins to charge up. The voltage across the capacitor is compared to an amplified version of the voltage which is to be measured, when the capacitor voltage equals the amplified voltage the counter is stopped. The reading on the counter relates to the time taken to charge the capacitor, it can be seen with a higher voltage to be measured the capacitor will take longer to charge up and so the counter reading will be higher. The reverse is true for a lower voltage and so the measured voltage is converted to a numerical value.

Digital meters measure the precise voltage developed by the thermocouple junction, cold junction compensation is also built in to maintain good accuracy. Digital meters normally exhibit a high input resistance and so do not require a specific external resistance to be maintained. It is important to use the correct compensating cable when connecting up a digital thermometer.

Digital meters are useful for checking the calibration of analogue indicators. Analogue meters are useful and sometimes clearer to view in certain situations but accuracy of the reading is also important. Many models of digital meter have a maximum reading hold facility, this is useful for recording high temperatures during starting or the application of a high loads.

There are other methods and more sophisticated systems for measuring exhaust temperature. More complicated aircraft instruments exist which incorporate an electronic amplifier and may also employ an analogue dial and a digital readout all in one instrument. There are also electronic amplifier integrated circuits which can be used to convert the small thermocouple signal into a direct DC voltage calibrated in volts/per hundred degrees, such a chip is the AD595. The AD595 is useful if proper aircraft analogue instruments are not available and the user prefers not to use a digital readout and use a voltmeter instead. The AD595 is also useful for interfacing computer based data logging equipment which may expect or be calibrated for relatively large voltage changes. A temperature signal developed by a AD595 chip may be applied to a fuel control system and used to provide overtemperature protection.

Measuring Other Temperatures
Thermocouple type probes are useful for measuring other temperatures within a gas turbine engine such as oil temperature or air intake temperature. Research or test rig engines may employ many thermocouples placed at each stage of the working cycle.

There are alternatives to thermocouples for sensing temperatures particularly oil temperature. Oil temperature is sometimes measured on engines originating from aircraft, these often use a thermistor probe which is placed in the engine oil system. A thermistor probe consists of a special resistor which varies in resistance according to its temperature. A thermistor probe is normally wired to an indicator, the indicator consists of a moving iron type mechanism which with two coils forms part of a bridge circuit. The thermistor and a fixed resistor (inside the instrument) form the other part of the bridge. The whole system is powered from 24V, the use of a bridge circuit enables the system to be insensitive to power supply voltage changes.

There are two types of thermistor, these devices either have a positive temperature coefficient law (PT Law) or a negative temperature coefficient law (NT Law). A positive law is such that the resistance of the thermistor increases with temperature and a negative law is the opposite where resistance decreases with temperature. The thermistor law required may be marked on the indicator instrument.

Certain Rover gas turbine engines are fitted with a non electrical oil temperature indicator. Here a fluid filled capillary is connected to a pressure type gauge. The fluid at one end of the capillary is heated by the oil and expands moving the pointer on the pressure gauge which is calibrated for temperature.

A typical oil temperature for a running gas turbine may be around 80-100 degrees C.
In some cases an oil cooler system may be automatically controlled from the oil temperature.

Pressures
The oil pressure in a gas turbine is as important as it is in reciprocating engines, a loss in oil pressure can become catastrophic if unnoticed. Oil pressure may be measured using electrical devices or can be indicated on an ordinary pressure gauge.

A pressure gauge is the simplest method of measuring oil pressure. Engines originating from aircraft installations may have had electrical oil pressure senders fitted. The oil pressure sender should be identified and replaced with a gauge via a suitable hydraulic hose connection. For engines not equipped with oil pressure monitoring then the oil feed from the pump will need to be identified and intercepted.

Depending on the type of engine, oil pressure readings can be quite low compared to that off piston engines. Oil pressures as low as about 5 PSI/ XX Kg cm2 are acceptable in some installations, normally the acceptable oil pressure range is stated in the engine specification. Oil pressure is normally higher when an engine is cold.

Oil pressure can also be measured using an electrical system consisting of a sender and an indicator. Here an expanding bellows is pressurised with the oil and mechanically connected to a variable resistance. The variable resistance is connected to an indicator. The indicator consists of a moving iron assembly which is operated by two coils, the coils and the sender are connected in a bridge circuit and powered from a nominal 24V supply. As the oil pressure changes the variable resistance changes and causes the pointer in the indicator to move. The bridge circuit is used so that the system is not adversely effected by power supply voltage changes.

Oil pressure senders are calibrated to be operated at different pressure ranges, when constructing an instrumentation system from aircraft indicators the correct indicator for a particular sender should be used. On many British engines which use Weston senders, the indicators are electrically identical but use different scales to match different senders.

Oil pressure switches are useful for warning of failing engine oil pressure. Many engines have these if not fitted with a gauge. The switch can be used to simply operate a warning light or can be incorporated into an interlock system. An interlock system can be used to stop the engine if the oil pressure fails, in this case the interlock may need to be overridden to allow the engine to be started.

Air pressure from a gas turbine engine compressor may be monitored during engine operation. Compressor delivery pressure is often referred to as P2 pressure (P1 being atmospheric at the engine intake). P2 is not commonly recorded in aircraft installations as it normally falls within close limits when an engine is running normally at a governed speed. P2 is recorded as part of engine test or research rigs and is a useful indicator when operating an engine of unknown characteristics. All gas turbine engines when started begin to self sustain when the P2 starts to build up, during the first 20% P2 will hardly register, beyond 30% a rapid rise in P2 is experienced. Once the engine is running with significant P2 it can be considered as self sustaining, this is useful when testing an unknown engine, the engine may be safely operated below governed speed but at a speed sufficient to maintain acceptably low temperatures. Operating the engine at too slower a speed may cause it to overheat due to a lack of cooling air flow.

P2 is measured by simply tapping off a fraction of the air delivered from the compressor. Many engines will have some sort of tapping on the main air casing and P2 air may be fed to the fuel control unit. Air bled from the compressor is connected to a standard pressure gauge, in some cases fluctuations may be seen on the gauge in which case a restriction should be placed in the line feeding the gauge to dampen them out.

Typical values for P2 pressures experienced with small gas turbines range from between 15 and 45 PSI.

Air pressure switches are employed in some engine installations. Some versions of the Rover 1S60 engine use a group of air pressure switches to control various functions during starting. As the engine speed rises so does the air pressure, these switches can be used to cancel such devices as the starter motor and ignition system.

Other air pressures are sometimes measured particularly in research or educational stationary engines. Low pressures i.e. fractions of a PSI are measured with liquid filled manometers, the slight pressure losses in air intake ducting may be recorded using this apparatus. The air intake mass flow if passed through a suitably calibrated venturi and bell-mouth can also be calculated by using a manometer.

Fuel pressure is sometimes useful to know, these exhibit pressures up to 500 PSI depending upon the engine type. An indication of fuel pressure at the pump inlet (LP Fuel) is useful in some installations, a pressure switch may be placed in the fuel line and will indicate satisfactory operation of any preceding priming pump or booster pump.

Events Counters
Gas turbines are often fitted with events counters, the two most common are hours run and number of starts. Normally an electric hours run meter is connected so that it is energised when the main engine fuel valve (HP Cock) is open, it will then increment all the time the engine is running. The meter effectively records the life of the engine and is used to indicate when servicing and inspections are necessary. When obtaining scrap or surplus gas turbine engines, the hours run is very useful to know. Many gas turbine engines have an hour meter mounted on them.

A common practice is to fit gas turbine engines with a starts counter. A starts counter simply records each time the engine is started or in some cases an attempt to start it has taken place. When an engine is started it experiences relatively high temperatures which may have an effect on the overall life of the unit. A starts counter is normally connected into the starting circuit of a particular engine installation. It may be operated when ever the starter motor is energised or when a combination of events takes place. A useful combination is the operation of the ignition and the starter motor simultaneously, this condition usually signifies a start attempt. It is often required that the engine is rotated on the starter motor without actually being started, also the ignition may need to be tested. Recording a combination of events ensures that the starts counter is not falsely incremented and only records "Real" starts.

Other events can be recorded, certain test rigs may record trip conditions such as over speed or over temperature. Lucas gas turbine starter units are fitted with a counter which records transitions from one mode of operation to another.

Instrument Panels
A comprehensive instrument panel puts a nice touch to any stationary small gas turbine engine. Instruments should always be closely monitored during engine operation and normal running values should be noted. Ideally indicators should be arranged so that normal operation is indicated by pointers and needles that reside half way around there respective dials. One exception to this is RPM, normally if an engine is running at governed speed it will be operating at 100%.

In addition to instruments, clearly marked controls are important and appropriate warning lights are also useful. A means of stopping an engine in an emergency is good practice, a clear "Stop" or "Abort" button should be placed where it can easily be reached. Consideration should be given to any electrical fault conditions, engines fitted with solenoid valves will normally shut down if electrical power is lost in a fail safe condition. Engines are sometimes fitted with motorised actuators and may continue to run in the event of a control power supply failure.

An instrument panel should be placed away from the hazard areas of an engine i.e. the rotation plane of the rotor, the air intakes and exhaust outlet.