The Dynalyzer x-ray calibration system is a highly accurate tool for the measurement of x-ray generator voltages and currents. To use this instrument efficiently, it is necessary to understand some of the physics behind the measurements. This application note will outline the procedure that will produce the maximum accuracy, and then will describe the physical reasoning behind it.
The procedure outlined below will enable a technician to calibrate properly an x-ray generator, when combined with the protocol of the equipment manufacturer. This procedure must be modified for specific types of generators.
1) Equipment Installation:
Install the Dynalyzer between the x-ray tube and the x-ray generator according to Figure 5. Connect the Dynalyzer terminal exactly as shown. Connect both the anode and cathode cables.
1.1) Do not add more than 5 feet of cable between the Dynalyzer and x-ray tube. Extra cable length will increase the series resistance in the filament circuit. Extra filament circuit resistance will decrease the filament current in generators using constant voltage power supplies. The extra cable capacitance stores energy. In single and three phase generators the capacitance would mostly affect timing accuracy at low anode mA, where cable discharge time is significant. High frequency generators may not function properly with a Dynalyzer or voltage divider (bleeder, KV "Drop Box," etc.) in series with the transformer. Some of these generators are resonantly tuned to their cables and tubes, and introduction of the Dynalyzer will cause problems. Check with the manufacturer for specific recommendations.
1.2) Do not place the Dynalyzer under the
x-ray tube. The x-rays will ionize the SF6 gas, and at higher voltages cause measurement errors. It will, over time, darken the mA light pipe and reduce mA output.Following the machine specific instructions, adjust the anode current for each tube station during tube installation, or during preventive maintenance, first measure the KV and mA for each station.
2.1) If the all the KV's are correct or slightly high, and all the mA's are slightly low, then readjust the filament master resistor (there should be a separate adjustment for small and large filament) if the generator is so equipped. Decide if the small error is caused by the introduction of the extra cable and contact resistance of the measurement equipment. If this is the case, then the calibration of all stations may proceed at this point.
2.2) If there is no master filament adjustment, as may be the case in a solid state design, then adjust the mA of each station and check that the KV is correct. If the mA errors are both high and low, then complete calibration of the generator is required. 2.3) As a check, measure and record anode + cathode, anode only and cathode only. This applies whether a Dynalyzer or bleeder is used. In a single phase generator the sum of anode peak and cathode peak will equal anode + cathode. This is because the positive and negative peaks are in phase. A three phase generator may be built as a "6 pulse" or a "12 pulse" unit. Checking the anode only and cathode only will produce interesting results. In a 6 pulse, anode (only) and cathode (only) will be one half the peak KV. In a 12 pulse, the anode (only) and cathode (only) will be 30 degrees out of phase, and the anode (only) and cathode (only) will be 1.15 x 1/2 the sum or 57.7% of the A+C. See figure 6A for the anode and cathode waveforms. Figure 6B illustrates the 12 pulse ripple that is approximately 4% of the total voltage, p-p..
Sum of A+C
4% Ripple @
720 Hz
Figure 6B
In a 6 pulse generator, the secondary coils for the positive and negative voltages, with respect to ground, are symmetrical, and are usually both Wye connected. This symmetry results in the anode and cathode waveforms to ground being in phase. In a 12 pulse generator, there are essentially two transformers. One has a delta primary and one has a wye primary, the two secondaries may be wye and have a full wave rectifier output producing 87.5 KVP to ground. There is a 30 degree phase shift between the anode and cathode ripple waveform resulting in the need for vector addition of the voltages, not algebraic addition of A (only) and C (only). Therefore it is necessary to have the voltage divider in both the anode and cathode during the initial set up of a three phase 12 pulse generator.
3) Final Adjustments
Once the generator is set up, the KVP will be a function of the KV compensation circuitry and the expected mA.
The divider or Dynalyzer may be removed from the cathode circuit, and the x-ray transformer cable from the cathode is directly attached from tube to transformer. Then the mA may be verified. If the mA has changed and the generator has a " master" filament adjustment or a "cable compensation adjustment," then adjust it get the correct mA value on that filament, and verify that the other mA stations have been brought back into adjustment. During this procedure, it is not necessary to verify or reset the KVP, however if the anode (only) data was previously recorded, it could be monitored.
From the above discussion, the important points to remember when calibrating a x-ray generator are:
1) Introduction of extra resistance in the cathode circuit will often reduce filament current, causing a reduction in mA and increase in KVP.
2) Anode and cathode peak voltages do not always add algebraically, but especially in 12 pulse three phase, must be added vectorially.
3) A final check of the anode current can be made without a bleeder or Dynalyzer in the cathode circuit to reset the filament master or other appropriate mA adjustment.
4) When testing a high frequency generator with a Dynalyzer or bleeder, be certain that it does not upset the tuning of the high voltage transformer.
5) Keep the Dynalyzer out of the x-ray beam.
Figure 6
| Anode Voltage
Cathode Voltage |
 |
Figure 6a