There are several major sources of error in milliampere readings with Dynalyzer systems. The most common errors are caused by these factors:
2) Connection of Dynalyzer II tank to Dynalyzer III display.
3) Failure to adjust the mA zero in the high voltage unit.
4) MAS limit exceeded in Dynalyzer III Display.
5) Reversing the polarity of the cables in the HVU.
6) Intermittent operation in Dynalyzer II displays.
7) Broken high voltage units.
8) X-ray damage to the light pipe.
9) Insertion losses due to cables and connectors.
1) Open Ground Wires:
The interconnect cable uses pin C as the master ground wire in Dynalyzer II systems. Stress in the cable often pulls out this connection. Check your cable with an ohmmeter. In addition pin G of the cable is the mA return ground wire in a Dynalyzer III.
2) Using a Dynalyzer II HVU with a Dynalyzer III display, and Visa Versa:
With one simple modification to the Bendix connector output on the Dynalyzer II HVU, it is easy to use the Dynalyzer II HVU with a III display. The III display is looking for a mA ground. An artificial ground is provided in the Dynalyzer III display, but there is some offset voltage associated with this circuit that can cause possibly a +/- 5 milliampere error. Open the front of the HVU and solder an insulated wire from pin G of the circular display output connector to pin C, the ground. This will provide a solid, loop free ground return. Ground wire C can have ground fault currents, especially if one connects the display and HVU to separate power outlets. By doing this, there is a real risk of blowing out some ground foil in the HVU! If you are not sure of the condition of your HVU, check continuity of grounds from the BNC's to the ground pin of the power receptacle. If it is open, the HVU needs repair.
When using a Dynalyzer II display with a Dynalyzer III HVU in the Fluoro mode, be sure to divide the indicated mA value on the display by 20. The scale factor of the HVU in the fluoro mode is 20 MV/mA. In the rad mode it is 1 MV/mA.
The Dynalyzer III display decodes a signal from the Dynalyzer III HVU and automatically displays the correct value.
3) Zero Adjustment of mA:
It is essential to check the Zero output of the mA circuit. It will drift with changes in temperature. The RAD and Fluoro mA both drift, and both have screwdriver adjustments. Check the BNC output with your DVM when starting a calibration. If your HVU is cold, check it again after it warms up. Best to connect the HVU to display or oscilloscope when adjusting.
4) MAS Limit Exceeded:
The Dynalyzer III Display first measures MAS and then divides by the exposure time. The maximum MAS in the RAD current mode is 2621 MAS. In the FLUORO mode the maximum is 131 MAS. Any exposures exceeding the MAS limit will cause the error light to come on and error 908 to be displayed. The exposure time and mA values will be in error beyond this limit. Do not exceed 50 mA on Fluoro ranges.
5) Reversing in and out on mA:
The mA circuit will provide outputs up to 2 amps (pulsed) input. It will not respond to negative inputs greater than 5-10 milliamps. Be sure to connect the anode transformer input to the transformer, and the anode tube connection properly. Do not use the anode circuit in the cathode. All three terminals in the anode circuit are shorted together, and connection in the cathode circuit will short circuit the filament.
6) Intermittent Dynalyzer II operation:
There are several types of intermittent operation in Dynalyzer II's. Some have been intermittent since their manufacture, where with certain mA values, the system begins to compute mA, and then all zero's appear. MAS can usually be displayed. This cannot be fixed, but the system is generally usable.
There is a second type of failure. This is caused by age and corrosion on the leads of the chips. Most corrosion occurs on the MOSTEK chips, which have not been manufactured for over 10 years. GiCi has a rebuild program for intermittent Dynalyzer II's that is approximately 80% effective. Call us for details.
7) Broken high voltage units:
The Dynalyzer HVU is a fragile instrument. Depending on how hard it was dropped, or how hard a tube arced, damage may occur. GiCi can make your HVU as good as new. Call us.
Do not operate a HVU with continuous mA value greater than 250 mA, or damage to resistors inside the current transmitter will result. It may be operated with pulses up to 2 amps peak. (Generally speaking, a Dynalyzer can handle any exposure an x-ray tube can!)
8) Influence of x-ray radiation on Dynalyzer
Extensive exposure of the HVU to x-rays will degrade the glass light pipe optic in the mA sensor. The light pipe will darken with radiation. This will make the mA appear to be low. We have seen units with 75% decrease in throughput from x-ray beams because of their exposure. Radiation can also ionize the SF6 insulation and cause the Dynalyzer to read high at higher KV values!
9) Insertion Loss of Dynalyzer and cables
The very act of inserting the Dynalyzer into the cathode circuit may influence the mA reading, The extra resistance of the cables and contacts will reduce the filament current to some extent. This is discussed in Application Note #2 in detail.