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Aviation History
1941
1941 - 1060.PDF
MAY 8TH, 1941. CATMOOC OIL. COOLINS DUCT PROTECTORY MOOD Fig. 3. The arrangement of the various components of anX-ray tube. of the X-rays produced and the greater their penetrative power. The typical curve in Fig. 2 shows the relationship between tube voltage and penetration for iron (this metal representing a practical and convenient penetration standard). Tube Current.-—This term refers to the current flowing through the X-ray tube and is the factor controlling the density of the beam and hence the exposure time required to produce a radiograph of a given nature and thickness of material using a specified tube voltage. Exposure time.—This term is self-explanatory and the exposure time required for a given quality and thickness of material is dependent upon the tube voltage and tube current used. The three foregoing factors are, then, very closely related and the following typical example will make this clear: — Material . . .. lin. of iron Tube voltage .. 120 kV. ••^'.'/"(r.r.-J.r. Exposure constant .. 15 milliAmps. /mins. ...'.i. Target/film distance aSin. . • This means that in order to produce a satisfactory radiograph through iin. of iron we require a tube voltage of 120 kV. at a tube current of, say, 5 mA. for three minutes or 10 mA. for 1.5 minutes where the film (behind the specimen) is at a distance of 28in. from the X-ray tube. The degree to which X-rays can penetrate a material depends upon its atomic weight or (more accurately) its atomic number. Thus, with industrial metals, aluminium and magnesium are very easily penetrated, while lead and tungsten offer much greater impedance to an X-ray beam. Between these limits come copper, nickel, tin, iron and the various steels. A practical indication of the relative degrees of penetration can be given. With X-rays generated at 200 kV. the following approximate maxi- mum thicknesses of material can be penetrated : Aluminium . . . . 6in. to 8in. (actually limited by scatter effect) Iron . . . . . . 3Jin. Copper or brass . . 2^in. Lead .. . . . . r\in. Manufacturers of industrial X-ray apparatus provide very accurate curves showing the optimum kilovoltage, current and exposure time required for a very wide range of industrial metals'. Control In designing the control circuit we see that it is neces- sary to provide accurate control of the voltage delivered by the high-tension generator to the X-ray tube, and also accurate control of the current passing through the X-ray tube. Exposure time can be regulated by an electric timer or manually with exposure switches and a stop watch. The output voltage from the generator is usually con- trolled by one of two methods. In the first case the high- voltage transformer (comprising part of the generator cir- cuit) is supplied through an auto-transformer housed in the control unit. This auto-transformer is provided with tapped windings connected to a rotary switch representing voltage increases in fixed amounts. The second method employs an inductive regulator which Las the very great advantage of providing absolutely smooth (stepless) volt SEEING IT THROUGH (Continued) age control and also eliminates rotary switchgear. One well- known manufacturer using inductive regulators has pro- vided motor operation so that the many thousands oi volts supplied to the'X-ray tube can be accurately adjusted by simply holding down a press-button. The necessity for smooth and accurate voltage control becomes greater at the lower end of the voltage range. Such defects as segregation and inter-crystalline porosity necessitate, in many cases, the use of very low voltages in order that they be clearly denned. Tube current control is simpler and usually takes the form of a rotary resistance coupled to the transformer feed- ing the X-ray tube filament. Both electrical functions are indicated by suitably calibrated meters—one calibrated in tube voltage and the other in tube current. Modern equipment also incorporates various protectory devices which switch off the apparatus in the event of safe maximum values being exceeded, thereby protecting the costly components of the equipment such as trans- formers, condensers, rectifiers and X-ray tube. Generator. The design of high-voltage generators is a complicated business, and the finer points of high-voltage technique need not be referred to here. Broadly, three types of high- voltage generator are used. The first type consists simply of a step-up transformer which supplies the X-ray tube with alternating current. This type of generator has the advantage that it is cheap and simple to construct and is compact in design, since only one component is required for supplying the high voltage. For industrial use, however, the A.C. generator has cer- tain disadvantages. Under certain conditions of operation the tube may tend to pass current in the reverse direction, and this would result in its immediate destruction. The second type of generator delivers a rectified pulsat- ing output to the X-ray tube. A typical arrangement in called the Villard circuit. By an ingenious system ot coupling rectifying valves and condensers with the high- voltage transformer a method of " voltage doubling" is achieved. The output voltage is doubled by means of charged condensers which hold and then add their charge to the transformer output at intervals. The third type of generator supplies the X-ray tube with pure direct current. The circuit is, however, bulky and is unnecessary and rarely employed. To summarise, it can be said that for all-round efficiency the Villard generator delivering rectified output is the' best type for industrial work, and properlj- designed industito^ apparatus to-day incorporates this type of high-voltajs circuit. A modern X-ray installation for aircraft work operating at voltages up to 140,000 is illustrated X-Ray Tubes In considering the suitability of an X-ray tube for industrial work, two very important qualities are involved. Fig. 4. Diagrams showing the need for a small target areain order to produce a sharp image.
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