IRIS 2000 ATE PITOT-STATIC SYSTEM

  resources. Two wing cabinets provide additional work surface area, storage space for cables and accessories and

additional room for additional measurement and stimulus resources. The equipment layout gives easy and convenient access to all equipment during use as well as for maintenance. In addition, space is available for specific LRU hardware expansions and future expansions.

IRIS 2000 ATE PITOT-STATIC SYSTEM

There is no specific test for pitot systems as there is for static systems other than the normal inspections of the entire aircraft. If a problem is reported or suspected with a pitot system, there is a general leak test procedure in AC 43.13-1A, as well as some general guidelines for pitot-stattc system maintenance.

The procedure for leak testing the pitot system is: Apply pressure to the pitot tube to cause the airspeed indicator to show 150 knots. Seal off or 1 minute and the maximum loss of indicated airspeed should not exceed 10 knots.

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  PRECAUTIONS IN TESTING PITOT-STATIC SYSTEM

1. Perform all maintenance and inspections before leak testing.

2. Use a system diagram.

3. Check the test unit for leaks before beginning the test.

4. Run full range tests only if you are thoroughly familiar with both the aircraft and the test equipment.

5. Pressure in the pitot system must always be equal to or greater than the pressure in the static system.

6. The rate of change of pressure during testing should not exceed the limits for any installed instrument.

7. After testing make sure that the system is returned to flying condition, such as removing tape from ports and drain holes.

There is an FAR that concerns the altimeter setting which is set by the pilot in the Kollsman window.

ADAPTERS

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  METERS AND MEASURING INSTRUMENTS

INTRODUCTION: The Meters are analog or digital types, each with its own applications. A multimeter with AC and DC volt, ampere and ohm scales is tool commonly used in Avionics workshop. Megaohmmeter, Bonding Tester, Hi-pot Tester,

Oscilloscope, VSWR meter and Time Domain Reflectometer augment the capability of the shop. For some applications analog is better, since indications are steadier. Digital types react too quickly, causing digits to change wildly under standard test conditions. An accuracy of less than 5 % is usually sufficient for most tests. The faces should be large enough to read easily with illumination, if used in dim areas. Most generally available analog meters are based on DC ammeter

movements in the micro to milliamp range. External circuitry (resistors, rectifiers) determines their usage. With modified scale numbers, they can be setup for a wide range of voltages or currents. AC meters usually have a modified rectifier bridge to convert to DC required by the meter movement.

Moving Coil Meter Essential Parts

DC MOVEMENT

MULTIMETER

MIRROR TO REMOVE PARALLAX ERROR

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  OHM’S LAW

Definition

In mathematical problems, EMF is expressed in volts, and the symbol E is used to indicate the EMF until the actual number of volts is determined. R is the symbol for resistance in ohms, and I is the symbol for current, or amperage. The letter I may be said to

represent the intensity of current. The letter symbols E, R, and I have an exact relationship in electricity given by Ohm's law. This law may be stated as follows: The currrent in an electric circuit is directly proportional to the EMF (voltage) and inversely proportional to the resistance. Ohm's law is further expressed by the statement; 1 Volt causes 1 Ampere to flow through a resistance of 1 ohm.

The equation for Ohm's law is then I = E / R This indicates that current in a circuit is directly proportional to the source voltage and is inversely proportional to the resistance in the circuit. The meters are based on this law for measurements of current, voltage and resistance.

The different forms for the Ohm's law equation are derived by either multiplication or division.

OHM’s law diagram

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  Operation of the Moving-Coil DC Meter (D’Arsonval Movement/Galvanometer) as a Current Meter

Although the moving-coil DC meter is used to measure voltage and resistance, as well as current, it is basically a current-operated device. The current in the moving coil develops the magnetic field that reacts with the stationary magnetic field of the permanent magnet (produces rotary motion of the coil) and moves the meter pointer across the scale of the instrument. The direction of rotation of the coil is from the strengthened portion to the weakened portion of the resultant magnetic field. Depending on the particular design of a current meter, a certain maximum current in the coil causes maximum turning force, or full-scale deflection. Exceeding the maximum current rating "slams" the pointer off scale against the end stop. For example, passing 2 amperes through a 0.5 ampere (500 milliamperes) meter movement can easily damage the meter by burning out the Coil or by bending the thin pointer. Less than full-scale current flowing in the coil produces a proportionately reduced deflection. Thus, 50 mllljamperes of current flowing through a 100- milliampere full-scale meter causes half-scale deflection.

COIL MOVES DUE TO THE INTERACTION OF TWO FIELDS

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  MOVEMENT SPECIFICATIONS

All moving coil meters have a rated current for FSD (Full Scale Deflection), and this parameter is of primary importance.

The FSD current determines how much load the meter will place on any drive circuitry, or for a voltmeter, how much current it will draw from the voltage source. This may or may not be important, depending on application.

Most commonly available meters are readily available with a sensitivity of between 50uA and 1mA FSD. More sensitive meters are available, but the cost goes up with increasing sensitivity.

All meter movements have resistance, because the coil uses many turns of fine wire. The resistance varies from perhaps 200 ohms or so (1mA movement) up to around 3.5k for a 50uA movement. These figures can vary quite widely though, depending on the exact technique used by the manufacturer.

Normally, moving coil meter movements are suitable for DC only. Some (such as VU meters for audio) have an internal rectifier so that AC may be measured, but accuracy is generally rather poor, especially with low voltages.

Some movements have a mirrored scale, where a band of highly polished metal is just behind the scale itself. This is used to eliminate parallax errors as you read the meter, and can improve reading accuracy dramatically

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  INTERNAL RESISTANCE AND SENSITIVITY

Every meter coil has a certain amount of DC resistance. The amount of resistance depends upon the number of turns on the coil .and the size of the wire used to wind the coil. The strength of the magnetic field about a coil increases as the number of turns on the coil increases. Therefore, if more windings are placed on a meter coil, a small current can create a magnetic field strong enough to cause the coil to deflect full scale.

The amount of current necessary to cause the meter pointer to deflect full scale is the meter sensitivity; it is an important characteristic of any meter. Typical current meter sensitivities vary from about 5 microampere, (0.000005 amperes) to about 10 mllliamperes (0.010 amperes). Some common values are 5, 50, and 100 microamperes; and 1, and 10 milliamperes. You have probably recognized that the sensitivity of a meter movement is the ninimum current that the movement can measure.

METER ACCURACY

The accuracy of a meter is specified as the percentage of error at full-scale deflection. For example, if the specified accuracy of a 100 milliampere meter is specified as ±2 percent, not only might the meter be off by ±2 milliamperes at a 100-milliampere reading, but it might be off by as much as ±2 milliamperes for any reading below full-scale deflection. Therefore; the accuracy of a meter becomes progressively poorer as the pointer moves farther and farther from full-scale deflection towards zero. For example, at a

STRENGTH OF MAG FIELD ABOUT THE MOVING COIL IS PROPORTIONAL TO 1) THE NUMBER OF TURNS IN COIL, 2)THE PERMEABILITY OF THE CORE ON WHICH THE COIL IS WOUND 3)THE AMOUNT OF CURRENT THROUGH THE COIL, AND

4) THE RATIO OF THE COIL LENGTH TO THE COIL WIDTH (REDUCING THE RADIUS OF THE COIL INCREASES THE STRENGTH OF THE FIELD}

Ammeter sensitivity is the amount of current necessary to cause full scale deflection (maximum reading) of the ammeter. The smaller the amount of current, the more "sensitive" the ammeter. For example, an ammeter with a maximum current reading of 1 milliampere would have a

sensitivity of 1 milliampere, and be more sensitive than an ammeter with a maximum reading of 1 ampere and a sensitivity of 1 ampere.

Sensitivity can be given for a meter movement, but the term "ammeter sensitivity" usually refers to the entire ammeter and not just the meter movement. An ammeter consists of more than just the meter movement.

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  meter reading of 50 milliamperes, the meter, since it could still be off by ±2 milliamperes, is only really accurate to ±4

percent. At a meter reading of 10 milliamperes, the meter could still be off by ±2 milliamperes, which is a true reading accuracy of ±20 percent.