Military roentgenology / War Department.

Date:
[1944]
Catalogue details

Licence: Public Domain Mark

Credit: Military roentgenology / War Department. Source: Wellcome Collection.

    78/470 (page 68)
    to anode) is known as the “useful” voltage. This useful voltage is the prime factor which determines the quality of the X-ray beam. Since the voltage drop in the secondary winding ts directly determined by the value of current flowing, it follows that, as the current through the tube is increased, the voltage drop will increase, and the voltage across the X-ray tube will decrease; hence, the importance of knowing the useful kilovoltage values at various current settings. The no-load voltage in self-rectifted units is always greater than the voltage under load. In a poorly designed and poorly built high tension trans- former this difference may be as high as 20 percent. The prereading kilovolt meter on most machines Is not calibrated in terms of useful voltage; mstead, the values are usually no-load voltages. Therefore, it Is necessary that one determine and know the useful voltage in terms of the no-load voltage. This may be accomplished by connecting two spheres (one movable and one fixed) in parallel with the X-ray tube. When the tube is energized, the movable sphere is approximated to the stationary sphere. At the point where the impressed voltage is sufficient to arc across the gap, a measure of the voltage Is obtamed from a scale. It can be seen that when the are occurs there is actually a short-circuit of the high tension transformer. In order to protect the transformer against the excess current drawn and the spheres against pitting, high resistances (at least 1.5 megohms) are placed in series with each sphere. The useful voltage is determined by this method for various milliamperage loads over the kilovoltage range. The procedure is relatively simple with non- shockproof equipment but it is not very easy with shockproof units. With shockproof units one needs a pair of extension sleeves—insulated sleeves that can be inserted into the sockets of the tube head or transformer terminals. The ends of the imsulated cables are inserted into the protruding ends of the sleeves. In a self-rectified circuit such as described in figure 26, the maximum peak voltage that would be determined would be the no-load voltage. In order to determine the useful voltage, It is necessary to connect a valve tube in the circuit, as shown in figure 66a. It must be borne in mind that current must flow through the valve tube during the same period of time that current is flowmg through the X-ray tube. Therefore, the filaments of both tubes should be connected to a common point. For a high degree of accuracy, it is essential that the tempera- ture of the surrounding atmosphere be considered, and that the barometric pressure be taken into account. For such correction factors, reference to any engineering handbook is suggested. With full- wave rectification (valve-tube) no-load voltages are of unimportance and it is not necessary to include a valve in the sphere gap circuit. The U. S. Army Field Unit is well designed and reasonable duplica- tion by one unit by another can be expected. Figure 68 65 shows an average kilovoltage calibration for various loads of a group of field units. This curve may be utilized for any field unit without appreciable error. The field unit, Item No. 96085, is so designed that, when the major autotransformer control is placed in position “6”? and the minor control is placed in position “‘4,” the prereading kilovolt meter should read “*70,” if one js to obtain standard results. If the kilovolt meter should read a value greater than “70,” the Ime voltage is greater in value than the setting of the line voltage adjustment, which is located on the panel board of the control stand. This means that, if the line voltage adjustment was set at, for instance, “112,” and the kilovolt meter reads greater than “70,” with the major control at “6” and the minor control at “4,” the adjustment strap should be moved to a higher value. The line voltage adjustment is varied in steps of 8 volts. One should adjust and readjust, if necessary, until the needle is as near to “70” as possible. The largest possible variation should not be greater than 3 scale divisions; if one obtains a reading of “73,” place the strap on the next point, and possibly obtain a value of “68,” which would be nearer the prescribed value and I CURRENT TENDS TO FLOW IN EITHER DIRECTION. Figure 66(b). Measuring inverse voltage.