Operation

This unit is designed to operate with four possible read-out devices:

1. A suitable oscilloscope
2. A digital voltmeter
3. Accu-Gold digitizer (AGDN+ or AGDM+) running Accu-Dyn+ software
4. The Dynalyzer Digital Display (obsolete)

Measurements can be made with the optional Accu-Dyn+ system, oscilloscope (requiring interpretation), RMS Voltmeter or Dynalyzer Digital Display.

 

CAUTION: As 5 foot long high voltage cables are normally used to connect the Dynalyzer High Voltage Unit to the x-ray tube (necessitating that the HVU be placed on the table) the possibility exists that the HVU may be inadvertently left in the primary beam. If this occurs, the SF6 gas used to insulate the internal components in the tank section of the HVU may become sufficiently ionized to cause gross measurement errors, e.g., 20 to 30 kV. In order to guard against this possibility, the user must be careful to angle the tube away from the HVU or close the collimator shutters.

 

Readout Oscilloscope

• Measurement of kVp and exposure duration for both single-phase and three-phase machines may be read directly from the oscilloscope trace.
• Current measurements for radiographic exposures may be read from the oscilloscope trace.

Measurement procedure for kVp and Exposure Time Using Oscilloscope

• Connect the oscilloscope to the High Voltage Unit as shown in Figure 7.
• Adjust the gain of the preamp of the oscilloscope for 2-volts per division.
• Adjust the horizontal time base for 10 milliseconds per division.
• Connect the scope to the Anode + Cathode output or to the separate outputs if switching between individual and summed outputs is desired. The exact switching will vary due to differences in x-ray high-voltage generator controllers. Refer to specific instructions for the model being calibrated.

Anode + Cathode Output

It is often necessary to view kVp and mA on the same dual trace oscilloscope. As this unit features a composite kV output with a separate BNC connector, the Anode + Cathode kV waveform may be viewed on a conventional scope. The scale factor is 0.5 V/10 kVp (20000:1 ratio). The reduced scale factor is needed to prevent saturation of the amplifier.

The individual outputs are provided to allow checking transformer balance, or for use where multi-channel plug-in scopes are available for viewing all available data, or where the operator wishes to switch between dual trace and the add/invert mode (allowing the summation to occur at the scope).

 

Figure 9: + X-RAY TUBE Series Connection with Oscilloscope or Digital Voltmeter

Measurement Procedures for mA and mAs using Oscilloscope

This unit provides a unique means for viewing and measuring the current waveform in an x-ray generator. Interpretation may require some analysis on the part of the user. In general it will be found that three-pulse (constant potential) current measurements can be easily obtained from the oscilloscope trace while single-phase measurements will require some calculation.

Three-Phase Current Measurements (using Oscilloscope)

Current measurements in three-phase equipment may be read directly from the oscilloscope trace. Figure 8 shows a typical three-phase current waveform. By visual inspection, the average value may be read from the oscilloscope trace. The scale factor is 1 millivolt per milliampere. Through practice, the average area of ripple above and below the estimated current value should be equal. The x-ray tube is an emission limited device. Normally, the peaks of the current waveform have a lower slope than the voltage waveform and therefore the ripple in the current will be smaller than the ripple in the three-phase voltage.

 

Figure 10: Typical Three-Phase Current Waveform

 

Fluoroscopic Current Measurements (using Oscilloscope)

For these measurements, the front panel switch should be set to the FLUORO position. Threephase fluoroscopic current measurements can be evaluated per the Three-Phase Current Measurements section above. Single-phase fluoroscopic measurements may fall into two categories: 1) filtered and 2) unfiltered. The scale factor is 20 millivolt per milliampere. At low current levels (below 5 mA) in systems with sufficient cable, or external capacitor banks, the voltage applied to the x-ray tube may appear to be purely direct current. The current waveform will depend on the location of the Dynalyzer High Voltage Unit with respect to the x-ray tube. Use the FLUORO position if there is significant low level noise filtering by the amplifier circuitry in the High Voltage Unit.

 

Measurements with Accu-Dyn+ system (see appendix A)

Measurements with Digital Display

The Dynalyzer Digital Display can be used in conjunction with the high voltage unit to provide the following five measurements simultaneously: 

1. 1. 1. mA and mAs
      2. Exposure time (triggered by kV, mA, external trigger or internal (line) trigger
      3. kVp of anode or cathode only or both
      4. Auxiliary readout of filament current

An oscilloscope can be connected in conjunction with the Digital Display to obtain a visual image of the waveform measured. Refer to the Digital Display Instructions for additional information.

Current Measurements with Dynalyzer III Digital Display (mA and mAs)

The Dynalyzer Digital Display contains all necessary signal processing required for accurate measurements of either mA or mAs in all modes described above. In operation, the incoming analog signal is digitized, and the resulting pulses are stored. Thus, the pulse count gives a true indication of the mAs of an exposure. Simultaneously with the current integration, the exposure time is being counted by a digital clock. The average current is displayed by electronically dividing the mAs by time.

Triggering for this measurement may be made with current, voltage, external trigger, or manually (auto mode).

Exposure Duration (Time)

The third line of the display is the exposure time that is triggered by the kV, mA, auto (internal), or an external signal.

Dynalyzer Digital Display of kVp Measurements

The Dynalyzer Digital Display is interconnected to the High Voltage Unit with a 20-foot long shielded cable. The Digital Display is powered from the 120/240 VAC, 50/60 Hz line. 

kVp measurements are made by selecting either the sum of Anode + Cathode or the individual terminals with respect to ground. The Digital Display is triggered by dialing in the fraction of the peak or preset (absolute) amount of kVp or mA that is considered the beginning of the exposure, i.e., 0-10% for single-phase and 75% for three-phase operation. A test exposure is made to set the gain factor of the display. Each subsequent exposure will trigger both the peak detector circuit and the electronic time clock. Both kVp and time in milliseconds will be displayed. (Current and filament will also be displayed.) A delay, adjustable up to 20 milliseconds, is provided to prevent measurement of the leading edge which may contain substantial overshoot of a three-phase exposure.

In both single and three-phase exposures, the kVp displayed will be the highest value within the exposure, with the exception of the kVp present during the delayed period.

Details on operation are supplied in the Digital Display Instruction Manual supplied with the Display Unit.

Filament Current Measurement with the Dynalyzer Digital Display

Pressing MANUAL TRIGGER after selecting AUTO at the TRIGGER SOURCE switch on the display causes the filament current to be displayed at an update rate of 0.5 second.

Measurements with Suitable RMS Voltmeter

An ac rms voltmeter, such as the Data Technology Model 31 may be used to obtain a readout of the filament current.

Rms conversion modules are available from Analog Devices, Burr-Brown, and Intronics, as well as other manufacturers. These modules require ± 15 VDC power to operate and a digital voltmeter to display their output.

Removing High Voltage Unit from X-ray Installation

CAUTION: Turn off the main power before removing the Dynalyzer. At the completion of testing                            remove the cables in the sequence provided to prevent any possible cross connection                            of anode and cathode cables.

1. 1. 1. Remove anode cable first. The jumper cable from the Dynalyzer to either the x-ray tube or high-voltage transformer, depending on test configuration, should be disconnected. Do not handle the insulator.
      2. Touch the pins of the cable to ground to remove any residual charge remaining in the cable.

NOTE: Grid bias operation often causes large residual cable voltages if no bleeder resistance             is present at the high tension transformer.

1. 1. 1. Clean off the oil or grease and place a protective cover over the cable ends.
      2. Disconnect the system cable from the other anode connector of the Dynalyzer.
      3. Reconnect the x-ray tube to the high-voltage transformer. Be sure sufficient oil or vapor-proofing compound remains in the receptacle or on the cable.
      4. Repeat the procedure for the cathode cables.

Interpreting Oscilloscope Measurement Waveforms

Three-phase current waveforms are generally easier to interpret than single-phase waveforms, however, following the procedure outlined, measurements may be made for both waveforms. Fluoroscopic exposure current values may be read directly on the DVM. The scale factor is 20 mV/mA. Filament current may be viewed on the scope, or may be measured with a digital voltmeter connected to the High Voltage Unit readout terminal. The waveform of the filament current will to some extent influence the accuracy of the reading of the peak reading as ac rectifier circuit used in most DVMs. Fluoro gain should be selected. The limiting parameter for measurements will be the response time of the DVM. In general, exposures having a duration of at least 3 seconds should be adequate for most DVMs to integrate and display.

Single-Phase Current Waveform

Figure 9 illustrates the waveform of a typical single-phase generator for an exposure of 200 mA, 120 kVp, 1/20 sec. A properly timed single-phase generator will contain one or more complete pulses, each lasting 1/120 of a second on a 60 Hz power line, or 1/100 of a second on a 50 Hz system. Thus, a single-phase exposure time for normally operating equipment is N x 1/120, where N is the number of pulses. For longer exposures, the time may be read from the start of the first major pulse to the end of the last pulse. The trailing off of the voltage, (region C of Figure 9), is due to cable capacitance. Cable capacitance is also responsible for the failure of the voltage to drop to zero at the end of each pulse.

In low-level fluoroscopic systems, or in systems with a capacitor bank, level B may be equal to the kVp. By convention, kVp is the greatest amplitude value observed, which may vary by amount D. Pulse A is due to the system of contacting which provides for the closing of the contactor during the end of the last pulse preceding the beginning of the exposure. This is done to minimize the overloading of the contactor and reduce the development of high-voltage transients. This pulse typically is 5-20% of the peak amplitude. A higher pulse indicates mis-adjustment of the contactor timing. (Refer to the manufacturer’s specifications before readjusting.) This pulse should not be counted in the total exposure time, if sufficiently small. At low voltages, the x-radiation produced by this pulse will either be filtered out, or will not produce x-rays. The intent of the exposure time measurements is to measure the effective x-ray generating period. 

Figure 11: Typical Single-Phase Waveform

Three-Phase Current Waveforms

Figure 10 illustrates the waveform of a typical three-phase generator for an exposure of 700 mA, 80 kVp, 1/20 second. The kVp for this exposure would be the amplitude of area J. The overshoot area, E, is not significant in this particular exposure because the dominant radiation is of the lower kVp value. The peak value must not, however, exceed the rating of the tube of generator, as it does contribute to the voltage stress on the system.

Although there is some deviation from this value, the effective exposure time of a three-phase generator should be measured between the intersection of 75% or the kVp value with time. This allows the start-up pulse area H (and the capacitance trailing effects) to be ignored.

In three-phase exposures, in the order of 1/120-1/60 second, the overshoot, area E, contributes a significant portion of the total radiation. This may present a problem in calibration were the effective kV is assumed to be independent of exposure time. Overshoot should be eliminated, if possible, by adjustment of the contractor or SCR phasing circuit.

Figure 12: Typical Three-Phase Waveform

kVp Calibration for X-Ray Generator

This should be done for each mA station. In general, voltage decreases with increasing current. If the converse is found, the system is overcompensated. Refer to the manufacturer’s specifications to determine if the system is operation properly.

Equipment Fault Diagnosis by Voltage Waveform Analysis

The High Voltage Unit is a useful service aid. Among the faults possible in an x-ray controller, the following are listed as an aid:

Transient Detection

Transient irregularities in the waveform seen on the oscilloscope may be due to a variety of causes. These transient conditions may produce voltages which are considerably in excess of the kVp setting and may damage the x-ray tube, high-voltage cables, or highvoltage transformer.

Transient voltages in the high-voltage circuit may occur at a very high frequency so that the trace on the oscilloscope is very low in intensity or may not show at all with normal adjustments. Whenever a pulse or portion of a pulse is missing from the trace, some improvement may be obtained if the exposure is repeated with the intensity control set to a higher level. The trace obtained should be examined closely for excessive voltages.

Defective X-ray Tube

Gas in any x-ray produces a waveform which is very irregular at high kVp. “Kicking” (arcover) of the x-ray tube is accompanied by rapid oscillations which may show up as a missing pulse. 

Defective Valve Tube

If a valve tube in a full-wave rectified circuit is defective, pulses may be unequal in amplitude or missing entirely. If a valve tube is gassy, it may produce irregularities in wave form which are similar to those produced by a gassy x-ray tube.

Improper Closing of Primary Contactor

If the contacts of the x-ray primary contactor close at an improper phase angle, the first pulse may be considerably higher than succeeding pulses and irregular in shape. If the contactor opens too early or too late, an irregularity will occur at the end of the trace accompanied by arcing of the contacts in mechanical contacting systems. 

Single-Phase Operation of Three-Phase X-ray Generators

A three-phase x-ray unit operating as a single-phase unit can readily be detected by observing that only two pulses are produced per cycle on the oscilloscope trace rather than six or twelve, and therefore the waveform looks identical to that of a single-phase generator.

Evaluating Fluoroscopic Current Measurements

The FLUORO mode should be selected for this test. Circuit analysis would show that the system is an RC filter circuit (Figure 11). If the High Voltage Unit is located near the transformer, the current waveform will include the charging current of the cables. With a capacitance of 30 pf/ft., a 50-foot cable will present a load of 1500 pf. The RC time constant of the system is 30 milliseconds. From circuit analysis, the expected voltage ripple factor would be 10%. A peak current of 35 milliamps could be observed if measurements were performed at the transformer.

Thus, measurement of the current at the transformer will show peaks in the order of 35 milliamperes, while measurement at or near the x-ray tube will show a relatively constant current in the order of 5 milliamperes.

As fluoroscopic exposures may be of an extended period, a digital voltmeter may be used to measure the average voltage output of the High Voltage Unit. The DVM will read current at a scale factor of 20 millivolts per millampere.

Figure 13: Schematic Diagram of a Typical X-ray Generator Circuit

Single-Phase Radiographic Current Measurements using Oscilloscope

When the current level is sufficiently high, the effect of cable capacitance may be neglected. If the load is resistive, the current waveform would follow the voltage. For a full-wave rectified sine wave, the average current should be: 

l_o = 2l_m/ = 0.636 x l_m

However, close inspection will reveal that this is not the case. Because the current is emission limited, the current will increase with the lower kV values, but at some point, will saturate (see Figure 12). The formula above is thus no longer valid and graphical integration of similar areas will yield a better value. Figure 13 illustrates the sectioning of the current waveform into triangular and rectangular areas for integration. Division of the waveform into equal width segments is a second approach to graphical integration as shown in Figure 14. The easiest approach would be to take 10 values of the current, equally spaced in time in the period of one cycle, and add them together and divide by ten.

The peak current is different in each pulse, while the form factor or waveshape remains the same. If graphical integrations are used at maximum accuracy, the ratio of peak to average value of one pulse will be obtained, and this ratio used to determine the average value of each pulse. Then the average value of the entire exposure can be approximated.

Under certain conditions it may be possible to make extended radiographic exposures without exceeding the instantaneous loading and the target heat storage capacities. In general this procedure would be limited to exposures under 200 mA, and under 6 seconds. The rating sheet of the tube unit should be consulted.

Under certain conditions it may be possible to make extended radiographic exposures without exceeding the instantaneous loading and the target heat storage capacities. In general this procedure would be limited to exposures under 200 mA, and under 6 seconds. The rating sheet of the tube unit should be consulted.

Figure 14: Typical Single-Phase Waveform

Figure 15: Current Waveform Analysis by Triangulation

Figure 16: Current Waveform Analysis by Integration

Filament Current Measurements using Oscilloscope

Connection of an oscilloscope to the filament output terminals will allow viewing of the amplitude and waveform of the filament current. With an oscilloscope one may observe the sequencing of the filament boost circuit, the operation of the current stabilization circuit, if any, and the type of waveform present.

Many x-ray generators use saturable reactors, or other non-linear control means to control the current in the x-ray tube filament. This results in distortion of the input sine wave into a waveform approximating a square wave. If this occurs, the rms current would be equal to ½ the peak current, assuming a 100% conduction angle. If the wave form is sinusoidal, the rms current is 0.353 x the peak-to-peak current viewed on an oscilloscope. Figure 15 is a graph of the rms and average values of a square wave as a function of conduction angle. The scale factor of the current waveform is 0.1 volt per ampere.