CAPÍTULO 1: DISEÑO DE LA INVESTIGACIÓN
1.5. Caracterización del frente marítimo de San Miguel
It should be remembered that adding the active ingredient to the electrolyte of a discharged battery does not recharge the battery. Adding the active ingredient only increases the specific gravity of the electrolyte and does not convert the plates back to active material, and so does not bring the battery back to a charged condition. A charging current must be passed through the battery to recharge it.
Batteries are usually charged in battery shops. Each shop will have specific charging procedures for the types of batteries to be charged. The following discussion will introduce you to the types of battery charges.
The following types of charges may be given to a storage battery, depending upon the condition of the battery:
When a new battery is shipped dry, the plates are in an uncharged condition. After the electrolyte has been added, it is necessary to charge the battery. This is accomplished by giving the battery a long low-rate initial charge. The charge is given in accordance with the manufacturer's instructions, which are shipped with each battery.
• Normal Charge
A normal charge is a routine charge that is given in accordance with the nameplate data during the ordinary cycle of operation to restore the battery to its charged condition.
• Equalizing Charge
An equalizing charge is a special extended normal charge that is given periodically to batteries as part of a maintenance routine. It ensures that all the sulphate is driven from the plates and that all the cells are restored to a maximum specific gravity. The
equalizing charge is continued until the specific gravity of all cells, corrected for temperature, shows no change for a 4-hour period.
• Floating Charge
In a floating charge, the charging rate is determined by the battery voltage rather than by a definite current value. The floating charge is used to keep a battery at full charge while
Module 3.5 DC Sources of Electricity 5-33
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the battery is idle or in light duty. It is sometimes referred to as a trickle charge and is n accomplished with low current.
• Fast Charge
A fast charge is used when a battery must be recharged in the shortest possible time. The charge starts at a much higher rate than is normally used for charging. It should be used only in an emergency, as this type charge may be harmful to the battery.
• Charging Rate
Normally, the charging rate of aircraft storage batteries is given on the battery nameplate. If the available charging equipment does not have the desired charging rates, the nearest available rates should be used. However, the rate should never be so high that violent gassing (explained later in this text) occurs.
• Charging Time
The charge must be continued until the battery is fully charged. Frequent readings of specific gravity should be taken during the charge and compared with the reading taken before the battery was placed on charge.
Gassing
When a battery is being charged, a portion of the energy breaks down the water in the electrolyte. Hydrogen is released at the negative plates and oxygen at the positive plates.
These gases bubble up through the electrolyte and collect in the air space at the top of the cell. If violent gassing occurs when the battery is first placed on charge, the charging rate is too high. If the rate is not too high, steady gassing develops as the charging proceeds, indicating that the battery is nearing a fully charged condition.
Warning: A mixture of hydrogen and air can be dangerously explosive. No smoking, electric L) sparks, or open flames should be permitted near charging batteries.
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Fortunately, the magnitude of the effect depends on the metal in use. Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate
different voltages, leaving a small difference in voltage available for measurement. That
difference increases with temperature, and can typically be between one and seventy microvolts per degree Celsius (µV/t) for the modern range of available metal combinations. Certain
combinations have become popular as industry standards, driven by cost, availability,
convenience, melting point, chemical properties, stability, and output. This coupling of two metals gives the thermocouple its name.
It is important to note that thermocouples measure the temperature difference between two points, not absolute temperature. In traditional applications, one of the junctions-the cold junction-was maintained at a known (reference) temperature, while the other end was
attached to a probe.
Having available a known temperature cold junction, while useful for laboratory calibrations, is simply not convenient for most directly connected indicating and control instruments. They
incorporate into their circuits an artificial cold junction using some other thermally sensitive device, such as a thermistor or diode, to measure the temperature of the input connections at the
instrument, with special care being taken to minimize any temperature gradient between terminals.
Hence, the voltage from a known cold junction can be simulated, and the appropriate correction applied. This is known as cold junction compensation.
Additionally, a device can perform cold junction compensation by computation. It can translate device voltages to temperatures by either of two methods. It can use values from look-up tables or approximate using polynomial interpolation.
A thermocouple can produce current, which means it can be used to drive some processes directly, without the need for extra circuitry and power sources. For example, the power from a thermocouple can activate a valve when a temperature difference arises. The electric power
generated by a thermocouple is a conversion of the heat energy that one must continuously supply to the hot side of the thermocouple to maintain the electric potential. The flow of heat is necessary because the current flowing through the thermocouple tends to cause the hot side to cool down and the cold side to heat up (the Peltier effect).
Operation
If two dissimilar metals are joined together a contact potential, which is independent of any external electrical supply, will appear at the junction.
In a thermocouple two dissimilar metals are joined at both ends to form a hot junction and a cold junction.
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In the simplest arrangement the thermocouple would be connected directly to a meter, the meter terminals being the cold junction.
In an aircraft, however, the hot junction is in the engine and the meter indicator on the flight deck.
If the thermocouple cold junction were to be connected to the meter by copper wires, as shown in Figure 5.25, the potential at the cold junction would be as if points "A" and "B" were joined together (provided that "A" and "B" were at the same temperature). This would still allow the meter to read the difference between V1 and V2.
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Figure 5.25 - Alternative thermocouple connections
If however, the hot and cold junctions were relatively close together, the temperature difference between them would not be so great as if they were far apart. The thermocouple EMF would, therefore, be reduced and, in Figure 5.25, there would also be a problem of fluctuations in the readings.
If the cold junction was in the meter itself there would be a greater temperature difference and hence a greater EMF and also less fluctuations.
To achieve this, the connecting leads from the thermocouple to the meter must be of the same material as the thermocouple or at least have the same thermoelectric characteristics.
They are called extension leads if they are of the same material and compensating leads if they are of the same characteristics.
The small EMF generated by the thermocouple is not only dependent upon the temperature but also upon the metals employed. Figure 5.26 shows a graph of voltage against temperature for several common thermocouples.
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nickel-ch iumlcopper-nickel iron/constantan
nickel-chromiLrrdnickel-aluminiumi
Figure 5.26 - Thermocouple Material Graph
Nickel/chromium and nickel/aluminium are normally chosen for aircraft thermocouples due to their near linear characteristics and their long operating life at temperature of up to 11001. The nickel/chromium is the positive connection and the nickel/aluminium the negative connection.
The thermocouple and its connections are housed in a protective metal sheath or probe which allows the hot junction to be exposed to the engine gases.
Thermocouples can be connected in series with each other to form a thermopile, where all the hot junctions are exposed to the higher temperature and all the cold junctions to a lower
temperature. Thus, the voltages of the individual thermocouple add up, which allows for a larger
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Thermocouple materials are available in several different metallurgical formulations per type, such as: (listed in decreasing levels of accuracy and cost) Special limits of error, Standard, and Extension grades. Extension grade wire is less costly than dedicated thermocouple junction
wire and is usually specified for accuracy over a more restricted temperature range. Extension grade wire is used when the point of measurement is farther from the measuring instrument than would be financially viable for standard or special limits materials, and has a very similar thermal coefficient of EMF for a narrow range (usually encompassing ambient). In this case, a standard or special limits wire junction is tied to the extension grade wire outside of the area of
Module 3.5 DC Sources of Electricity 5-37
temperature measurement for transit to the instrument. Since most modern temperature measuring instruments that utilize thermocouples are electronically buffered to prevent any
significant current draw from the thermocouple, the length of the thermocouple or extension wire is irrelevant.
Changes in metallurgy along the length of the thermocouple (such as termination strips or changes in thermocouple type wire) will introduce another thermocouple junction which affects measurement accuracy. Also, industry standards are that the thermocouple colour code is used for the insulation of the positive lead, and red is the negative lead.
Types
A variety of thermocouples are available, suitable for different measuring applications. They are usually selected based on the temperature range and sensitivity needed. Thermocouples with low sensitivities (B, R, and S types) have correspondingly lower resolutions. Other selection criteria include the inertness of the thermocouple material, and whether or not it is magnetic.
The thermocouple types are listed below with the positive electrode first, followed by the negative electrode.
Type K (chromel-alumel) is the most commonly used general purpose thermocouple. It is inexpensive and, owing to its popularity, available in a wide variety of probes. They are
available in the -200 CC to +1350 CC range. The typ e K was specified at a time when metallurgy was less advanced than it is today and, consequently, characteristics vary considerably
between examples. Another potential problem arises in some situations since one of the
constituent metals, nickel, is magnetic. The characteristic of the thermocouple undergoes a step change when a magnetic material reaches its Curie point. This occurs for this thermocouple at 354CC. Sensitivity is approximately 41 µV/ C.
Type E (chromel-constantan) has a high output (68 µV/t) which makes it well s uited to cryogenic use. Additionally, it is non-magnetic.
Type J (iron-constantan) is less popular than type K due to its limited range (-40 to +750 C).
The main application is with old equipment that cannot accept modern thermocouples. J types cannot be used above 760 CC as an abrupt magnetic transformation causes permanent
decalibration. The magnetic properties also prevent use in some applications. Type J thermocouples have a sensitivity of about 50 µV/CC.
Type N (nicrosil-nisil) thermocouples are suitable for use at high temperatures, exceeding 1200 CC, due to their stability and ability to resist high temperature oxidation. Sensitivity is about 39 µV/CC at 900CC, slightly lower than type K. Desi gned to be an improved type K, it is
becoming more popular.
Types B, R, and S thermocouples use platinum or a platinum-rhodium alloy for each conductor. These are among the most stable thermocouples, but have lower sensitivity, approximately 10 µV/CC, than other types. The high cost of these thermocouple types makes them unsuitable for general use. Generally, type B, R, and S thermocouples are used only for high temperature measurements.
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Type B thermocouples use a platinum-rhodium alloy for each conductor. One conductor contains 30% rhodium while the other conductor contains 6% rhodium. These thermocouples are suited for use at up to 1800 t. Type B thermoc ouples produce the same output at 0"C and 42 C, limiting their use below about 50 t.
Type R thermocouples use a platinum-rhodium alloy containing 13% rhodium for one conductor and pure platinum for the other conductor. Type R thermocouples are used up to 1600 `C.
Type S thermocouples use a platinum-rhodium alloy containing 10% rhodium for one
conductor and pure platinum for the other conductor. Like type R, type S thermocouples are used up to 1600 C. In particular, type S is used a s the standard of calibration for the melting point of gold (1064.43 C).
Type T (copper-constantan) thermocouples are suited for measurements in the -200 to 350 `C range. Often used as a differential measurem ent since only copper wire touches the probes. As both conductors are non-magnetic, type T thermocouples are a popular choice for applications such as electrical generators which contain strong magnetic fields. Type T thermocouples have a sensitivity of about 43 ltV/t.
Type C (tungsten 5% rhenium -- tungsten 26% rhenium) thermocouples are suited for
measurements in the 0 C to 23209C range. This the rmocouple is well-suited for vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at
temperatures above 260 C.
Type M thermocouples use a nickel alloy for each wire. The positive wire contains 18%
molybdenum while the negative wire contains 0.8% cobalt. These thermocouples are used in the vacuum furnaces for the same reasons as with type C. Upper temperature is limited to 1400 t.
Though it is a less common type of thermocouple, look-up tables to correlate temperature to EMF (milli-volt output) are available.
Module 3.5 DC Sources of Electricity 5-39
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Thermocouple Comparison and Identification
The table below describes properties of several different thermocouple types. Within the E.J tolerance columns, T represents the temperature of the hot junction, in degrees Celsius. For
example, a thermocouple with a tolerance of ±0.0025xT would have a tolerance of ±2.5 CC at
1000 1C. I L
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Table 5.1 -- Thermocouple comparison and wire identification
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[ 1 Applications
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A practical thermocouple is shown in Figure 5.27.L
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Figure 5.27 - A practical thermocouple
Two basic types of probe are employed for measuring exhaust gas temperatures in turbine engines. These are shown in Figure 5.28.
COUPLE COUPLE
SHEATH SHEATH
STAGNATION TYPE RAPID RESPONSE TYPE
Figure 5.28 - Turbine engine probes
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Figure 5.29 - Examples of thermocouple hot junction assemblies
The stagnation probe has a large entry port and a small exit port so that the gas is brought almost to rest, preventing errors caused by the kinetic energy of the gas flow. This type is designed for high velocity gas flow.
The rapid response probe is designed for slow exhaust gas velocity. The gas flows from the inlet port, over the junction, to the diametrically opposite outlet port.
Exhaust gas thermocouples are mounted radially around the engine tail pipe. There are usually a minimum of four. The RB 211 engine, however, has seventeen connected in a parallel
arrangement which has the advantage that the failure of one or more thermocouples does not cause complete failure of the output signal.
A typical thermocouple installation is shown in Figure 5.30.
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EXHAUST THERMOCOUPLE AND HARNESS
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THERMOCOUPLE INSTRUMENTATION
AND CONTROL SYSTEM Figure 5.30 - Thermocouple installation
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Photocells
l.JPhotocells undergo a change in their electrical parameters when exposed to light energy and
are known as photoelectric devices. They are affected by light in three different ways as follows. ] 7 Photo-emission:- Where the application of light causes the emission of electrons from a
prepared surface as discussed in Chapter 4, the construction of which is shown in Figure 5.31.
AIRTIGHT EVACUATED GLASS ENVELOPE
EXTERNAL CONNECTIONS
Figure 5.31 - The Photocell
With the positive potential of a supply connected to the anode of the cell and the negative to the cathode, the current in the circuit will depend upon the amount of light falling on the device: no light, no current; high intensity light, high current.
When the cell is used in an aircraft smoke detector, a projector lamp shines abeam of light past the detector cell. If no light reaches the cell, no current flows in the cell's external circuit and no warning is given.
When smoke appears in the detection chamber the projector lamp beam is refracted onto the detector cell by the smoke particles. The cell conducts activating the smoke warning circuit. This is shown in Figure 5.32.
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NORMAL
Figure 5.32 - Smoke Detector Operation
Solid state devices have now largely replaced this type of cell.
Photo-voltaic:- Where the application of light causes the production of a voltage.
The photo-voltaic (or solar cell), can be used to produce electrical energy for a variety of
purposes. If a large number of cells are connected together to form a solar panel the power
generated is limited only by the number of cells employed.
The silicon solar cell consists of a wafer of silicon which has been doped to make it a
semiconductor. A thin layer of boron is then diffused into it.
The wafer is reinforced with metal and Figure 5.33 - A photovoltaic cell panel provided with electrical contacts to enable it
to be connected to other cells.
Photons of light penetrating an atom of the cell forces electrons in the atom into the conduction band. This produces a voltage across the cell which can be used to drive a current around an externally connected circuit.
There are many uses of the solar cell, from the operation of light meters in cameras to powering
There are many uses of the solar cell, from the operation of light meters in cameras to powering