Aircraft technical Basics: TM 1-413, Aircraft Instruments, 1942: 33. Instrument Test Equipment

TM 1-413, TECHNICAL MANUAL,  AIRCRAFT INSTRUMENTS, Prepared under direction of the Chief of the Air Corps, WAR DEPARTMENT, WASHINGTON February 2, 1942. (This manual supersedes TM 1-413, November 7, 1940)

SECTION XXXIII - Instrument  Test Equipment

155. General.-The scope of tests which may be performed with type C-1 test set is limited. Various types of test equipment are used for other tests which need to be performed on aircraft instruments and related units. This test equipment and methods of application are outlined below.

156. Manometer.-a. Purpose and use.-(1) The purpose of a manometer is to provide means for measuring pressures of gases and vapors. The manometer is used as a standard for calibrating and testing differential and absolute pressure instruments.

(2) The manifold pressure gage and the altimeter assembly are absolute pressure instruments which are tested with a manometer. The rate of climb indicator, air speed indicator, and the suction gage are differential pressure instruments which are tested with a manometer. When performing tests of the above-mentioned instruments, it is necessary to provide a source of suction or pressure, and in some operations a vacuum chamber is essential. A detailed description of the vacuum chamber is given in paragraph 157b.

b. Description.-(1) Manometers may be classed as U-tube or straight tube types. In order to provide a large range of pressure with the manometer, various liquids are used including, among others, water (H₂O), kerosene, seal oil, and mercury (Hg.). The range of a manometer depends upon the over-all length of the tube and the specific gravity of the liquid. As the specific gravity of the liquid increases, the sensitivity of the instrument decreases. Therefore if small pressures are to be measured, it is desirable to use water or kerosene because of their lesser densities.

(2) The standard test manometer used in the Air Corps is a straight tube unit (fig. 122). It is equipped with a well which serves as a reservoir for the liquid. The liquid may be mercury, water, or kerosene. The well is constructed integrally with the tube or separately and connected to the tube. When a water manometer is used. an aniline dye (usually red) is placed in the water to facilitate quick and accurate reading of the column. The test manometer contains a multiple scale arrangement with a scale selector and zero adjustment knob. The various scales are shown in figure 123.

(3) Care must be exercised when reading a manometer as the shape or curvature (known as "meniscus") of the liquid level varies for water and mercury. Figure 124 shows the meniscuses for water and mercury and levels at which the readings are taken.

(4) When working with manometers it is advisable to remember some conversion factors commonly used :

29.92 inches Hg.=760 mm Hg.=34 feet H₂O=14.7 pounds per square inch.

e. Application,-(1) Inasmuch as the scale error test of an air speed indicator necessitates the use of a multiple scale manometer, this test will be used as an example. Many instruments may be tested and calibrated with the manometer. The Air Corps Technical Order for the particular instrument should be consulted for details.

(2) Figure 122 shows the connections for the manometer and the pressure supply; the panel of the vacuum chamber serves as a mounting base for the air speed indicator (connected to source of supply). The test is merely one of comparison, that is, the readings of the instrument under test are compared with those of the manometer. The air speed indicator is calibrated in accordance with the adiabatic formula which expresses the relationship between differential pressure and air speed. A table is used which is computed for air speed against inches of water or inches of mercury. Mercury is also used in the test because of the required pressure in the higher range which under practical conditions could not be attained by using water. The air speed scale of the mercury manometer starts at 250 mph. To perform the test, progressive readings are obtained (by gradually increasing the pressure) and the errors in the indicator readings compared with the tolerances as specified in the Air Corps Technical Order. If the error exceeds the tolerance, the instrument should be sent to the depot for overhaul or recalibration.

FIGURE 122.-Manometer and vacuum chamber.

FIGURE 123.- Manometer scales.

FIGURE 124. -Manometer reading levels.

d. Maintenance.-To provide accurate manometer readings, it is essential that the mercury and the tube are cleaned properly.

(1) Cleaning mercury.-(a) Mercury may be cleaned by stirring with a dilute solution of nitric acid (10 parts acid and 90 parts distilled water) or a solution of sulphuric acid (80 parts acid and 20 parts distilled water). After the mercury has been thoroughly washed and the acid solution removed by filtering, the mercury should be washed with distilled water, the water filtered off, and the mercury dried.

b) The preferred method utilizes the arrangement shown in figure 125. The mercury is slowly poured into the funnel. The filter paper (pinholed) removes some of the impurities. The mercury sprays through the small funnel tip and passes through the acid solution. Other impurities are thus removed and the mercury collects in the bottom of the tube. When a sufficient amount of mercury accumulates, the pinch cock is opened and the clean mercury allowed to flow into the beaker. The flow of filtered mercury should be con-trolled carefully to prevent the flow of acid solution into the beaker: this may be accomplished by allowing a small portion of the mercury to remain in the bottom of the glass tube at all times. The mercury collected in the beaker is next passed through distilled water. If drying is essential, the beaker containing mercury is placed on asbestos over an open flame and heated (not above 110° C.) until all moisture is removed. Alcohol may also be used as a drying agent. If the mercury to be filtered has accumulated grease, it should first he passed through a 10-percent solution of potassium hydroxide and the above procedure followed, using nitric acid (10 percent).

(2) Cleaning manometer tubes.-Following prolonged periods of operation, the manometer tube often becomes clouded as a result of impurities deposited on the glass. To remove these impurities, a piece of medium stiff piano wire with a swab attached to one end is used. Select a material which will shed a minimum of lint during the swabbing process.

FIGURE 125.- Apparatus for cleaning mercury.

(a) Swab the tube with a strong soap solution to which a little ammonia has been added. Next swab the tube with a dilute solution of nitric acid (25 parts nitric acid and 75 parts distilled water). Place the tube against a white background and subject it to close examination for cleanliness. The clean tube is then rinsed with distilled water and dried with alcohol.

(b) When refilling the manometer well, the fluid used must not contain any foreign matter. Pour the liquid into the well until it coincides with the index mark on the manometer.

157. Vacuum chamber. a. Purpose and use.-The vacuum chamber serves to maintain a condition of reduced pressure. The chamber is used for testing various differential pressure and absolute pressure instruments.

b. Description.-The chamber (fig. 122) is equipped with a heavy dual safety glass window through which instruments may be observed ; the chamber door is sealed with a rubber gland. Channels within the chamber provide a sliding mount for the instrument panel.

Several openings in the panel provide a means for testing various instruments simultaneously. On the rear of the panel is a small 110-volt, 60-cycle motor used to produce vibration by means of an eccentric weight attached to the end of the motor shaft. The electrical connection to the motor is completed when the panel has been entirely inserted in the chamber. A connection for the motor circuit is provided on the outside of the vacuum chamber. Suitable vents to the inside of the chamber are provided for the attachment of valves and a manometer.

c. Application.-(1) When testing an altimeter, the instrument is secured to the panel by two spring-loaded clamps. It is essential that a rate of climb indicator be installed with the altimeter on the panel to provide a means of regulating the rate of pressure change. When the two instruments have been correctly installed and adjusted, slide the panel into the vacuum chamber, close the door. place the safety latch in the locked position, and rotate the large wheel until an airtight seal is established. The motor is then placed in operation and the instruments are ready for testing.

(2) As the pressure in the vacuum chamber is decreased, comparative readings are taken between the instrument under test and the manometer. The table in the Air Corps Technical Order for the particular instrument with reference to the relationship between the manometer and the instrument readings should be consulted. The errors in the instrument readings should be within the specified tolerance.

(3) Detailed instruction pertaining to the use of the vacuum chamber when refilling and testing an aircraft. compass is set forth in the Air Corps Technical Order for the particular compass.

d. Maintenance.-(1) It is essential that the seal around the door of the chamber be airtight. To maintain an airtight seal, the contacting surfaces of the chamber and door must be thoroughly clean. The gland should be closely examined for defects; the application of a thin film of glycerin may provide a seal for slight defects.

(2) To avoid damage to the latch on the chamber door, the chamber should never be subjected to more than atmospheric pressure.

158. Dead weight tester.-a. Purpose and use.-The tester is furnished in two ranges : a low range of 0 to 200 pounds per square inch, and a high range of 0 to 3,000 pounds per square inch. The low range tester (fig. 126(1)) is used for testing comparatively low range pressure gages whereas the high range tester (fig. 126(2)) is used for testing comparatively high range pressure gages. The tester is a master gage used for conducting a scale error test. The tests are performed by comparing the readings on the instrument under test with the pressures developed in the tester. The following gages are calibrated with the dead weight tester:

(1) Automatic pilot oil pressure.

(2) Fuel pressure.

(3) Oil pressure.

(4) Landing gear pressure.

(1): Low range

(2) High range.
FIGURE 126. Dead weight tester.

b. Description.-Figure 126 shows the testers with gages attached for testing. Fundamentally, both the high and low pressure testers utilize the same principle of operation. Each unit consists of a manually operated piston, reservoir, weight table with piston, attaching fixture, and valves. The weights are numbered 1, 2, 5, 10, etc., which numbers designate respective pressures in pounds per square inch produced when the weights are used with the tester. The pressure produced in the tester depends upon two factors : the weight used and the cross section area of the piston. Thus, if the weight table piston has a cross section area of 1/5 square inch, a 1-pound weight would produce a pressure of 5 pounds per square inch.

c. Application.-The instrument subject to test is connected to the tester. The cylinder is filled with specified oil, weights placed upon the table, and the manual piston control rotated to increase the hydraulic pressure. When the pressure reaches a value which is sufficient to lift the table from the support, a reading is recorded on the gage under test. The test procedure and the allowable error in the gage reading must be as specified in the Air Corps Technical Order for the specific instrument. During the time the gage is subjected to pressure no weights should be removed from the tester table.

d. Maintenance.-The following rules of maintenance should be observed:

(1) Inspect for and repair leaks.

(2) Keep unit clean.

(3) Use specified oil.

(4) Store unit in a dry room.

(5) Carefully handle unit and all accessories.

(6) Place all accessories in their respective receptacles.

159. Voltmeters.-a. Purpose and use.-A voltmeter is an instrument for measuring the voltage or potential difference between two points of an electrical circuit.

b. Description.-There are many types of voltmeters. For test purposes the Air Corps uses d-c. alternating d-c, and a-c voltmeters. The range of each instrument depends upon its application.

(1) The mechanism of a d-c voltmeter consists essentially of a permanent magnet and a movable coil to which a pointer is attached. Actually, a d-c voltmeter is a D'Arsonval type galvanometer consisting of a high resistance connected in series with a movable coil (fig. 127). This type of instrument utilizes a scale of uniform graduations.

FIGURE 127.-D-c voltmeter.

FIGURE 128.-Dynamometer type voltmeter.

(2) The alternating d-c voltmeter may be of the dynamometer or rectifier type.

(a) The mechanism of a dynamometer type voltmeter (fig. 128) consists of stationary coils placed at right angles to a movable coil. A pointer is attached to the movable coil. A resistance is also connected in series with the movable coil. The scale on this instrument has nonuniform graduations.

(b) The rectifier type of voltmeter is basically a direct current instrument adaptable for use with alternating current by the addition of a rectifier. This type of instrument is equipped with a scale of uniform graduations.

FIGURE 129. Iron vane-type voltmeter.

(3) The most common a-c voltmeter is the iron vane type which is sometimes referred to as the movable iron type. A simplified diagram of this mechanism is shown in figure 129. It consists of a cylindrical coil with two laminated iron pole pieces, one fixed and the other movable. The pointer is attached to a movable pole piece. This type voltmeter is equipped with a scale having nonuniform graduations.

c. Application.-(1) Figure 130 shows a simple electrical circuit consisting of a source of supply, an ammeter, a resistor or load, and a voltmeter.

The ammeter is explained in paragraph 160. Before connecting a voltmeter into the circuit the following should be noted:

(a) Source of electrical supply, whether alternating current or direct current.

(b) Range of the voltmeter.

FIGURE 130.-Simple electrical circuit.

(2) When measuring voltage it is essential that the proper type of instrument is used. A majority of the a-c voltmeters (except those which use transformers) can be used to measure direct current. However, in such case the accuracy of the reading is impaired. For example, the dynamometer type of voltmeter requires about five times as much current as a d-c voltmeter of the same rating. Furthermore, a dynamometer type instrument consumes an appreciable amount of power which may also affect the reading. These factors should be taken into consideration when using voltmeters.

(3) When using a multiple range voltmeter always place the selector switch in a position to exceed the maximum value of the voltage to be measured. Figure 131(2) shows a multiple range voltmeter.

d. Maintenance.-(1) A voltmeter requires no lubrication.

(2) A voltmeter must always be handled so as to avoid jarring.

(3) Mechanical zero adjustment on the voltmeter is essential for correct indication of the instrument. If at any time (when not in use) the indication is not zero, adjust the pointer with a screw driver so that the pointer is in line with the zero mark.

(4) When doubt exists concerning the accuracy of a voltmeter, check the reading of the instrument against that of another voltmeter known to be correct; if in error the instrument should be sent to the depot for calibration (deflection potentiometers with volt and shunt boxes are used to calibrate voltmeters and ammeters).

(1) Ohmmeter (continuity meter). (2) Multiple range voltmeter. (3) Multiple range ammeter.
FIGURE 131. Electrical instruments for measuring resistance, voltage, and current.

160. Ammeters.-a. Purpose and use.-An ammeter is an instrument for measuring the rate of flow of current in an electrical circuit.

b. Description.-(1) Fundamentally, the ammeter is similar in construction to the voltmeter. The D'Arsonval mechanism is used for the d-c ammeter (fig. 132). The dynamometer and rectifier types of ammeters are used for small current measurement. The iron vane ammeter is used commonly for large a-c measurement.

(2) To measure large direct currents, an ammeter is connected in parallel with a shunt. A shunt is merely a low resistance which causes the larger portion of current to be diverted from the moving coil of the ammeter. By using shunts of different resistances, it is possible to utilize a number of ammeter scales and ranges. Figure 131(3) shows an ammeter which is provided with a selector switch for choice of any of three scale ranges.

c. Application.-A simple electric circuit with an ammeter and an external shunt is shown in figure 130. An ammeter must always be connected in series with the load. The resistance of an ammeter is low and if connected in parallel serious damage to the instrument would result.

FIGURE 132.-D'Arsonval type ammeter

d. Maintenance.-The maintenance outlined for voltmeters in paragraph 159 is also applicable to ammeters.

161. Ohmmeters.-a. Purpose and use.-An ohmmeter is an instrument for measuring the resistance of electric circuits or resistors.

b. Description.-The direct reading ohmmeter and precision ohm-meter are the two types used in the Air Corps.

(1) The direct. reading ohmmeter (continuity tester) illustrated in figure 131(1) is provided with a scale for reading the value of the resistance in ohms. The instrument consists of a sensitive volt-meter, a source of electrical supply (battery), and a rheostat. The instrument operates on the principle that when a resistance is connected in series with a voltmeter and a source of supply, the value of the resistance in ohms is indicated by the decrease in pointer deflection. The scale of the instrument is calibrated in ohms to show directly the value of the resistance.

(2) The precision ohmmeter (fig. 133) is basically a Kelvin double bridge. A simplified diagram is shown in figure 134. This bridge eliminates the effect of contact resistance (present in the Wheatstone bridge) and as a result provides a method of measuring very small resistances.

c. Application.-(1) The direct reading ohmmeter is used on aircraft to check the continuity of an electrical circuit and the approximate resistance of electric thermometer bulbs. Before proceeding to measure a resistance, select the desired range, short circuit the instrument prods, and adjust the rheostat for maximum deflection of the indicator (zero ohm reading). This adjustment is necessary to compensate for any variation in the battery voltage and must he performed as each range is selected. Next, place the prods across the resistor (or circuit) to be measured and read the indication on the instrument.

FIGURE 133.-Precision ohmmeter as used for measuring resistance.

(2) The precision ohmmeter is used accurately to measure resistance (not to exceed 11 ohms). The instrument is used frequently to determine the resistance of bonding.

(a) When using the precision ohmmeter to test the resistance of a resistor in the range between 1 and 11 ohms, it is necessary to provide the ohmmeter with two shunts : one between the terminals C1, and P1, and the second between the terminals C2, and P2. (fig. 133). To obtain accurate results, place the instrument in a level position and proceed by connecting the unknown resistance (for example, thermo-couple leads) between the terminals P1 and P2. Place the multiplier switch in the correct position. Release the galvanometer pointer by moving the clamp and check the mechanical zero setting. Next, lock the galvanometer key GA, press key BA, and note the galvanometer deflection. Adjust the bridge and when no deflection is noted on the galvanometer the instrument is balanced. The resistance is deter-mined by multiplying the scale reading by the ratio switch setting (indicated by multiplier).

FIGURE 134.-Simplified diagram of Kelvin double bridge.

(b) If it is necessary to use connecting leads between the instrument and the unknown resistance, the resistance of the connecting leads must be determined and subtracted from the total resistance measured by the instrument; the remainder will be the value of the unknown resistance.

(c) When checking the resistance or bonding, care should be exercised to remove any oxides from the bonding with emery cloth before proceeding with the check. With the clamps and prods in firm contact as shown in figure 135, the resistance of the bonding is determined as heretofore outlined.

FIGURE 135.-Method of determining resistance of a welded bond with precision ohmmeter.

d. Maintenance.-It is desirable periodically to inspect instruments of this type to insure accurate results.

(1) To check the precision ohmmeter, use a standard 2-ohm resistance connected between the terminals P1 and P2 and proceed in the manner as heretofore outlined. If the reading obtained is satisfactory, it is reasonable to assume that the instrument is accurate within the limits of the scale range.

(2) At times it will be necessary to clean the variable resistor, contact points, and switch points on the precision ohmmeter. This maintenance is required when erratic and unstable readings are notic-able on the galvanometer while performing adjustments. Never use an abrasive to clean the resistor or contact points. Such action will result in the removal of metal, thus changing the resistance of the resistor and impairing the accuracy of the instrument. Resistors and contact points should be wiped with a clean cloth dampened with benzine; this should be followed with application of a cloth greased with a small quantity of petroleum jelly. After cleaning any excess jelly should be removed.

(3) The precision ohmmeter is a very sensitive instrument and should be protected against jarring or bumping. When not in use, the galvanometer pointer should be locked with the clamp provided on the instrument. It is to be noted that the precision ohmmeter must not be lifted or carried by grasping the carrying case lid (the hinge pins slide out of the hinge with a lateral movement).

162. Thwing potentiometer.-a. Purpose and use.-The potentiometer is an instrument used for accurately measuring and comparing voltages. As applied to aircraft, the primary use is in the performance of scale error tests upon a synchronism indicator and a cylinder head temperature indicator, provided the voltage to be measured does not exceed 0.1 volt (100 millivolts).

b. Description.-The Thwing potentiometer is entirely self-contained and includes necessary batteries and a standard cell. Figure 136 shows a simplified circuit of a potentiometer. The standard cell is the source of comparative voltage. At no time should the standard cell be used as a source of supply for testing an instrument. This cell is a vital part. of the instrument and if misused will terminate the useful application of the potentiometer until the cell is replaced. On the Thwing potentiometer the M1 rheostat dial reads in multiples of 10 millivolts; the M2 rheostat dial has a range of 11 millivolts with unit graduations of 0.05 millivolt.

c. Application.-(1) Figure 137 shows a Thwing potentiometer properly connected to perform a scale error test on a synchronism indicator. Before proceeding to conduct this type of test, it is advisable to consult the Air Corps Technical Order for the specific instrument to obtain tolerances and details (including the correct value of lead resistances to be used) . However, the maximum voltage required for full scale deflection of the instrument to be tested must not exceed 0.1 volt as this is the range limit of the Timing potentiometer. After checking the mechanical zero of the instrument, connect its positive (+ ) and negative (-) terminals to the correspondingly marked terminals of the potentiometer.

FIGURE 136.--Simplified wiring diagram of potentiometer.

(2) To operate the potentiometer, check the mechanical zero setting of the galvanometer. Place switch A in the "open" (up) position and perform the electrical zero adjustment by varying rheostats R1 (coarse adjustment) and R2 (fine adjustment), observing the galvanometer with the keys SC and GA in their "closed" positions. The galvanometer pointer will give a zero indication when the instrument is properly balanced.

(3) Set the couple lead resistance (selector switch) to the number nearest to the value of the lead resistance specified in the Air Corps Technical Order. Lock (by depressing and rotating) key E in the "on" position. The equipment is now ready for a scale error test.

FIGURE: 137.-Thwing potentiometer.

(4) Adjust rheostats A1 (coarse adjustment) and A2 (fine adjustment) to locate the pointer of the instrument being tested on the first test point on the scale.

(5) To determine the voltage across the terminals of the instrument, place switch A in the "closed" (down) position and balance the potentiometer by tapping galvanometer key GA, adjusting rheostats M2 and M1 until the galvanometer pointer shows no deflection when the key GA is held down. The sum of the readings of M1 and M2 indicates the applied voltage in millivolts. The difference between the sum of the readings of rheostat dials M1 and M2 and the number of millivolts specified in the Air Corps Technical Order should be within the limits of the specified tolerance. The above procedure is followed for all the test points on the instrument.

(6) As the pointer of the synchronism indicator may be deflected to the left or right, a recommended procedure is to set at one test point (either left or right) and reverse the meter terminals on the potentiometer to obtain the corresponding test point on the opposite side of the scale. This obviates two separate tests on the instrument.

(7) After completing a test with the Thwing potentiometer, release key E to avoid closing the instrument lid with this key in the "on" position.

d. Maintenance.-Maintenance as set forth for the precision ohmmeter (par. 161d(2)) is applicable to the Thwing potentiometer. It is to be noted that the potentiometer must not be lifted or carried by grasping the carrying case lid (the hinge pins slide out of the hinge with a lateral movement).

163. Decade box. a. Purpose and use.-A decade box provides a resistance which may be varied by tenths, units, tens, hundreds, and thousands of ohms. The decade box is suitable for use where precise resistance is required for testing electrical instruments.

b. Description.-Figure 138 shows a simplified wiring diagram of a decade box. A resistance of 0.1 to 9999.9 ohms may be obtained with this unit.

c. Application.-(1) The decade box (fig. 139) may be substituted for a resistance bulb when testing an electric thermometer. With the instrument attached to a source of voltage and the decade box set for resistance as specified the Air Corps Technical Order for the particular instrument, check the instrument readings. The readings should be within the tolerance specified in the order.

(2) It is to be noted that before application of voltage to the circuit in which the decade box is used, sufficient resistance must be set (in the box) to restrict the flow of current to a value which will not be excessive.

d. Maintenance.-Oxides and other impurities should be removed from contacts and contact arms to eliminate a source of error. Contacts and contact arms may be cleaned in accordance with the procedure as outlined for the precision ohmmeter (par. 161d(2)).

164. Vapor pressure thermometer tester. -a. Purpose and use.-The vapor pressure thermometer tester (model AVT-CHT-P) is used to perform a scale error test on a vapor pressure thermometer. The tester has a temperature range of from -40° C. to 200° C.

FIGURE 138.-Simplified diagram of decade box.

FIGURE 139.- Decade box as used for thermometer test.

FIGURE 140.-Vapor pressure thermometer tester.

b. Description.-The thermometer tester (fig. 140) is a compact device consisting of a complete refrigerating unit and a two-tube container (evaporator) in which the vapor, pressure thermometer bulbs are placed. The refrigerating unit consists of a two-cylinder, air-cooled, reciprocating compressor driven by a 1/3-hp, 110-volt, 60-cycle, single phase motor. The refrigerant (liquid) is nontoxic, noninflammable, and noncorrosive. The container houses the cooling coil, electrical heating element, and a liquid which will not freeze at -40° C. nor boil at 200° C. The liquid acts to transfer cold or heat from the cooling coil or heating element to the thermometer bulb. A thermostat, pressure control, and refrigerant safety control are provided for regulation of cold and heat cycles.

c. Application.-Insert the bulb of the thermometer to be tested in one well of the tester, and the bulb of a master thermometer in the other well.

(1) Cold cycle.-(a) Before operating the tester for a cold cycle, place all switches in the "off" position. Then, place the main switch in the "on" position. Next, place the cold switch in the "on" position. This starts the refrigerating unit. Note the reading on the compound gage. During the first cycle of operation of the refrigerating unit, a pressure between 20 and 70 pounds per square inch is registered on the compound gage. After the compressor has been in operation for a few minutes, the indicator pointer will drop below zero to the vacuum side. Next, place F-12 liquid switch to the "on" position. This allows refrigerant to enter the cooling coil in the evaporator. When the F-12 liquid switch is in the "on" position the left pilot light is illuminated. This light remains illuminated during the period the tester operates on a cold cycle. A pressure gage is located on the right side of the tester when viewed from the front. During operation, this gage will register from 120 to 150 pounds per square inch, and the indication will gradually recede to 90 pounds per square inch at -40° C.

(b) Readings maybe now taken on the instrument under test and compared with those of the master thermometer. In each case, the error of the instrument under test shall be within the tolerance specified in the Air Corps Technical Order for the particular instrument. The cooling rate is rapid at the beginning of the cold cycle and decreases slowly in the range from 0° C. to -40° C. If the cooling rate is too rapid to compare accurately readings of the instrument under test with the master thermometer, slightly open the refrigeration rate control valve to reduce the rate of cooling. When it is desired to stop operation of the cold cycle, all switches are placed in the "off" position.

(2) Heat cycle.-(a) Before operating the tester for a heat cycle place all switches in the "off" position. Then place the main switch in the "on" position. Next, turn the heat switch to the "high" position. The right pilot light will remain illuminated during the heat cycle. The temperature will rise until 200° C. is reached. If the rate of heating is too rapid to compare accurately readings between the master thermometer and the instrument under test, place the heat switch to the "medium" or "low" position. If any amount of refrigerant is left in the cooling coil when heat is applied, a high pressure will result (indicated on the compound gage). The F-12 dump switch should be left in the "on" position to prevent an excessive pressure from building up in the cooling coil and in the low side of the Compressor. After completion of the heat cycle, all switches should be placed in the "off" position.

(b) The usual procedure is to test a thermometer with the cold cycle and complete the test with the use of the heat cycle. If the heat cycle is accomplished first, the cold cycle must not be placed in operation until the liquid has cooled to room temperature to avoid damage to the tester.

(c) When doubt exists concerning any phase of operation of the tester, the manufacturer's operating instructions should be consulted.

d. Maintenance.-The following features are applicable to maintenance of the tester:

(1) Slippage of the V-belt may be minimized by application of castor oil to the sides of the belt.

(2) Condenser should be blown out with compressed air at intervals of three months.

(3) Do not permit lint or dust to collect in the fan.

(4) Motor bearings should be oiled at intervals of 6 months.

(5) Compressor need not be lubricated.

(6) A suitable inspection, lubrication, and maintenance record should be kept.

165. Analysis cell test box (SAAD-E-70).-a. Purpose and use .-This instrument is used to test the analysis cell of a fuel mixture indicator for electrical zero adjustment. In some instances it may be necessary to use the test box to heat the filaments of the analysis cell to remove the carbon residue.

b. Descciption.-The test box (fig. 141) consists of four 1 1/2-volt dry cells, a milliammeter, galvanometer (micromammeter), rheostat with switch, two toggle switches, four receptors (numbered 1, 2, 3, and 4). and four sets of leads (to accommodate the various types of analysis cell). All leads in each set are banded conveniently for guidance of the operator.

FIGURE 141.-Analysis cell test box (SAAD-E-70).

c. Application.-(1) Carefully read the operating instructions accompanying each test box and consult the Air Corps Technical Order for the particular analysis cell for details and wiring diagrams. To protect the galvanometer of the tester, it is recommended that a continuity meter be used to check the spirals (resistors) of the bridge in the analysis cell for open spirals.

(2) Before completing any connections, place all switches, including the rheostat switch, in the "off" position. Select the proper set of test leads, attach the cannon plug (or alligator clips in the event that the cannon plugs are not suitable) to the analysis cell, and place the banana plugs in their respective receptors. The banding ( used for numbering) on the leads shows continuity of the wire and should not be confused with the numbers on the analysis cell. Place the galvanometer switch in the "on" position and observe the pointer. If any deflection is noticeable, the bridge in the analysis cell is unbalanced and requires an adjustment. As the adjustment for each type analysis cell may vary, the Air Corps Technical Order for the particular cell should be consulted for details. When adjustment has been performed, the galvanometer should indicate zero.

(3) In the event accurate adjustment cannot he performed, it is possible that carbon residue has accumulated on the spirals in the analysis cell and must be removed. In many instances this residue may be removed by using the test box to heat the filaments of the analysis cell. The Air Corps Technical Order for the particular analysis cell and the operating instructions for the test box should he consulted for the correct value of current to be applied and the length of current application. Before applying the heating current to the analysis cell, place the galvanometer switch in the "off" position. This is a precaution which must be observed to prevent damage to the galvanometer. Then place the battery switch in the "on" position and by means of the rheostat adjust the current to the desired value. For an analysis cell which incorporates a ballast tube a special procedure must be followed. The methods are outlined in the operating instructions accompanying the test box.

d. Maintenance.-(1) The test box requires no periodic maintenance. If the voltage in the test box decreases to a value which does not permit sufficient operating current, the dry cells of the test box should be replaced.

(2) The instrument should be protected from dampness and jarring.

(3) When not in use, the switches and rheostat should be in the "off" position.

166. Tachometer test stand.-a. Purpose and use.-The tachometer test stand shown in figure 142 is available to perform scale error tests on centrifugal, chronometric, or electric tachometers.

b. Description.-(1) The test stand consists of a housing which contains the primary gear mechanism for the operation of five vertical spindles. A master tachometer, usually of the chronometric type, is attached to one of these spindles. The main drive shaft extends through the ends of the housing; an adapter protrudes from the rear side of the housing and is used as an auxiliary drive.

FIGURE 142.-Tachometer test stand.

(2) The tachometer test stand may be operated by a variable speed motor or a constant speed motor equipped with a variable speed drive.

c. Application.-Figure 142 shows the assembly with a generator-voltmeter tachometer installed for testing. To prevent damage to the instruments attached to the test stand, the variable drive (or variable speed motor, if used) must be placed in the "slow speed" position before starting. Adjust the speed so that the master tachometer indicates the rpm specified in the Air Corps Technical Order for the tachometer under test. Comparative readings of the master tachometer and the aircraft tachometer are observed at specified consecutive speeds. If the readings of the master and aircraft tachometers differ by more than the tolerance specified in the Technical Order, the instrument should be returned to the depot for repairs.

d. Maintenance.-Required maintenance is as follows:

(1) Keep the unit clean.

(2) Inspect for and repair leaks.

(3) Lubricate, as needed, the gear mechanism of the test stand and the drive unit.

167. Turntable assembly. a. Purpose and use.-The turntable assembly (fig. 143) is designed to rotate a plate about a vertical axis at a selected rate of speed. The turntable assembly is used primarily for testing a bank and turn indicator, but is also employed to conduct tests on a magnitude of turn indicator and an aircraft compass.

FIGURE 143.-Turntable assembly.

b. Description.-The turntable assembly consists of a gear train with gear ratios to govern the speed of the turntable. A selector gear shift is employed to obtain four distinct rates of rotation ranging from 36° to 1080° per minute. Optional direction for right or left turns is obtained by setting the selector gear shift to the right or left of the neutral position. An adapter plate assembly, complete with vacuum manifold, hose connections, valves, suction gage, and special clamps, is located on the plate of the unit. The plate is attached securely to the drive shaft to insure correct alignment.

When testing an aircraft compass, a special adapter plate for each type of compass is furnished to simulate aircraft installation.

c. Application.-(1) Bank and turn indicator.-Properly affix the instrument to be tested to the turntable assembly and complete the connections to a vacuum source. Next, adjust the suction setting to the value specified in the Air Corps Technical Order for the instrument, and allow the gyroscopic rotor to attain operating speed. Place the selector gear shift in position for the desired direction of rotation and turntable speed, and start the electric motor. Observe the pointer deflection with respect to the master rate index plate. The error in deflection must be within the tolerance specified in the Technical Order for the instrument. If a damping or sensitivity adjustment is required, slightly turn the adjusting screw, again check the instrument on the turntable assembly, and repeat the adjustment and check until the indication is within the tolerance.

(2) Magnitude of turn indicator.-Affix the instrument to a proper adapter (par. 168) and mount on the adapter plate assembly. Connect the instrument to a source of vacuum, adjust the suction to the desired value, and allow the gyroscopic rotor to attain full speed. Set and uncage the instrument knob, place the selector gear shift of the turntable assembly to desired turntable speed of rotation, and start the motor. The error in the indication must be within the tolerance specified in the Technical Order for the instrument.

(3) Compasses.-When conducting a damping test, mount the instrument as shown in figure 144 or 145 (depending upon the type of compass) and allow the compass needle to cease oscillating. With the selector gear shift placed in position for desired turntable speed of rotation, permit the turntable to move through one complete revolution and note the position of the card in degrees at the completion of the turn. The error in the reading must be within the tolerance specified in the Technical Order for the instrument.

d. Maintenance.-The following rules of maintenance should be observed :

(1) Keep the unit clean.

(2) Use and store the unit in a clean, dry room.

(3) Generally inspect unit and accessories.

(4) Lubricate the unit as needed.

(5) Carefully handle the unit and all accessories.

(6) Keep a record of inspection, lubrication, and maintenance.

168. Gyroscopic instrument test fixture.-a. Purpose and use.-This fixture (fig. 146) is used for performing tests on the magnitude of turn indicator and the flight indicator.

FIGURE  144.-Damping test of pilot's compass.

FIGURE 145.-Damping test of navigator's compass.

FIGURE 146.-Gyroscopic instrument  test fixture.

b. Description.-The fixture consists of a swivel frame with provision for two individual mounting plate adapters which may be secured in a sliding groove by means of knurled thumbscrews. The instrument affixed to the mounting plate adapter may be rotated about the lateral, vertical, and longitudinal axes. A vacuum manifold with suitable valves and attachment fittings is affixed to the rear of the fixture. A suction gage is located on the left side of the fixture.

c. Application.-(1) The fixture must be mounted in a level position and firmly attached to the test bench. Fasten the instruments to the mounting plate adapters, complete all necessary connections to the vacuum source, and adjust the suction to a valve as specified in the Air Corps Technical Orders for the particular instruments under test.

(2) Perform the test procedure for each instrument as outlined in the Technical Order. The error should be within the specified tolerance.

d. Maintenance.-Carefully handle and clean the fixture and accessories.

169. Portable vacuum pump assembly.-a. Purpose and use.-The portable vacuum pump assembly (fig. 147) provides a source of vacuum for testing gyroscopic instruments in the shop or in the hangar.

FIGURE 147.-Portable vacuum pump.

b. Description.-The portable vacuum pump assembly consists of a single phase, 110-volt, 60-cycle electric motor (1/3-hp), coupled to a vane type vacuum pump which in turn is connected to an oil-air separator. Located between the vacuum side of the pump and the oil-air separator is a sight glass connector which serves to indicate the flow of oil for pump lubrication. Suitable valves and nipple connections (for the attachment of rubber hose) are provided.

c. Application.-Connect rubber tubing from the vacuum inlet to the fixture or instrument line. When the vacuum relief valve on the aircraft is used to adjust the volumetric flow, regulate the valve to secure the maximum rate of flow, thereby reducing the load on the pump. Observe the suction gage to determine whether vacuum as specified in the Air Corps Technical Order for the particular instrument is being obtained.

d. Maintenance.-(1) Consult the Technical Order for the port-able vacuum pump to obtain information concerning the grade of oil to use under various operating conditions.

(2) Inspect the motor brushes for correct length, and check the commutator for cleanliness. Supply proper lubricant as needed to the motor.

(3) Maintain sufficient oil in the reservoir of the oil-air separator.

170. Portable oil pump assembly.-a. Purpose and use.-The portable oil pump provides a source of oil pressure when performing various inspections and adjustments on the hydraulic system of the automatic pilot. The pump makes ground operation of the aircraft engine unnecessary.

b. Description.-The pump assembly (fig. 148) consists of a single phase, 110-volt, 60-cycle motor (1/3-hp), coupled to a vane type oil pump. Two flexible sections of rubber tubing are provided for unnecessary connections.

FIGURE 148.-Portable oil pump.

c. Application.-(1) When inspecting the hydraulic system of an automatic pilot in an aircraft installation, disconnect the hydraulic line connections at the hydraulic power pump and substitute the portable oil pump for the power pump. Care must be exercised to connect the pressure side of the portable oil pump to the pressure side of the hydraulic system.

(2) The unit should be securely mounted on a suitable stand.

d. Maintenance.-( 1) The pump is self-lubricating and requires no periodic maintenance.

(2) Inspect the motor brushes for correct length, and the commutator for cleanliness. Supply proper lubricant as needed to the motor bearings.