Modern portable x-ray units. criteria for selection

X-ray technique occupies, probably, the biggest niche in the industrial non-destructive testing. This ensures the continuous appearance of new developments. The basis for the majority of radiographic testing is the X-ray unit. Huge variety of different X-ray units is available on the market, significantly differing by many parameters. Such variety of the types of the units, their parameters and not always correct advertising can nonplus even the experts in the field of radiation methods of testing. In this article I’d like to make an attempt to tell to the readers about main types of X-ray units, their parameters and criteria, according to which the expert can select the necessary X-ray unit.
X-ray testing is carried out by X-raying of the detail under examination with fixation of the image on the X-ray film, reusable phosphorous plates or digital image detector.

Main axioms at the X-raying on the film or on PHOSPHOMATIC in field conditions:

1. Higher is the maximum voltage of X-ray unit, thicker steel it could x-ray.
2. Lower is the voltage of X-ray tube and higher is its current at the actual exposure, better are the contrast and the quality of the image obtained.  It means, to X-ray, for example, steel 20mm thick it would be more correct to set up the parameters of 160kV/5mA/2min., than 190kV/2mA/2min.
3. Higher is the X-ray efficiency of the unit, better quality of the image could be obtained. For example, half-cycle unit is much more effective than the pulsed one. And the unit of constant potential is much more effective than the half-cycle one.
4. Lower is the sensitivity of X-ray film, higher is the quality of the image.  
5. Fluorescent intensifying screens deteriorate drastically the image quality.

Now, after getting you acquainted with main axioms, pass to the detailed description of the types of portable X-ray units.

In the beginning of the article I classify X-ray units by types of X-ray tube power.

According to this classification the units could be divided into pulsed, half-cycle, constant potential of low frequency (500Hz and less), constant potential of medium frequency (1-10kHz) and constant potential of high frequency (more than 10kHz).

In pulsed units the X-ray tube is powered with short voltage pulses of small duration, fed with low and medium frequency.  The efficient power of these units is the lowest which provides the low quality of the image. The design of pulsed unit could be compared with photographic flash that generates big instantaneous light intensity but practically zero average efficiency. The cathode in most part of these units doesn’t have any filament and is based on the principle of explosive emission and fails rather quickly. Thus, the tube has to be often changed. So, the low efficiency of pulsed X-ray units forces to use the high-sensitivity film and intensifying screens which results in the worsening of the image quality. Due to this in EC countries the pulsed units are practically not in use. 

Very small weight and dimensions as well as low price could be considered as advantages of pulsed units. These advantages ensure rather widespread of pulsed units in Russia, but I hope that with the lapse of time more attention would be paid to the quality of obtained image and pulsed units would go aloft without any legislative prohibitions. The typical representative of pulsed X-ray units is the unit “Arina”.

Half-cycle X-ray units are the next step after the pulsed ones. They integrate the step-up transformer, but don’t have a multiplier. The X-ray tube itself which is the diode serves as voltage rectifier. Half-cycle units, as it follows from their name, use only half of high voltage sinusoid, and their efficiency is, at least, two times worse than that of units with constant potential, operating at similar frequency. In addition, the big voltage ratio doesn't practically allow the production of high-frequency X-ray unit. Therefore, half-cycle units operate at low or medium frequencies. In average, the efficiency of half-cycle unit is inferior to the high-frequency unit of constant potential in 4 times. Known in Russia units Balto and ICM relate to the half-cycle units.

Low and mid-frequency units of constant potential.
These units already integrate the voltage multiplier and provide the tube with voltage closed to the DC voltage. Unfortunately, the efficiency of the multiplier operation depends on the frequency. The low frequency increases the pulsations of output voltage and decreases the efficiency of the unit. The increase of the capacity of multiplier capacitors could compensate this problem but it would result in sharp increase of the weight and overall dimensions of the unit. Therefore, the low- and medium-frequency units are inferior to the high-frequency ones by efficiency. The portable unit РАП is an example of low-frequency units.

High-frequency units of constant potential.
They are the summit of the development. The frequency in such units usually exceeds 20 kHz. The voltage of the tube could be considered as almost constant. The efficiency of X-ray radiation yield is the highest and is limited only by physics of X-ray tube. The nomographic charts at high-frequency units of constant potential approximately coincide. The representatives of this family are units Eresco, Smart, MCT.

Further we shall consider (and recommend to the users) the high-frequency units of constant potential only.

Portable X-ray high-frequency tube sets of constant potential.
They are designed for the use in field conditions as a rule. They are notable for small weight, higher resistance to heavy duty operation conditions (including the impact of low temperatures). They represent an X-ray tube set, consisting of high-voltage transformer, multiplier and X-ray tube. The most up-to-date units integrate the power block, oscillating the voltage on the high-voltage transformer. Due to the power block integration into the X-ray tube set it is becomes possible to abandon the transfer of high-frequency AC voltage by cable, to reduce the level of high-frequency interferences and to have practically unlimited length of the cable to the radiator.

High-voltage transformer, multiplier and tube are immersed into high-voltage oil or electronegative gas. It is impossible to definitely tell what is better, oil or gas. Every type of insulation has its advantages and disadvantages. Gas allows the creation of lighter units, oil – more compact ones. Oil has higher temperature expansion coefficient which forces to use low reliable rubber bulb-expanders, gas .is available in the unit under high pressure and it has the property to leak through smallest cracks that are being often generated during the operation (particularly, if the unit often falls). Oil is good heat conductor and allows the better evacuation of the heat, being released in the multiplier, which allows the increase of the X-ray tube current. A date most part of the units with cermet X-ray tubes are based on gas technology, units with glass tubes – on oil technology.

The most important characteristic of X-ray unit is the maximum voltage on the tube and maximum thickness of steel that could be X-rayed by a given unit. In the leaflets many producers, both Russians and foreigners, lie in respect with this parameter. For example, many Russian units that are advertised as units designed for 200kV have in fact 180kV only. Same happens with steel thickness. From the leaflet we could learn extraordinary things. For example, the unit on 200kV x-rays steel of 80mm thick. May be it is possible when using powerful intensifying screens and exposures over 5-10 hours. Obviously, this is not indicated in the leaflet.
To exclude such improper advertising the nomographic charts are invented. They represent diagrams, on which it is possible to define the values of high voltage, tube current and exposure time, necessary for the X-raying of required thickness of steel. In the description of nomographic charts it is also indicated under what conditions and on what film or phosphorous plate these nomographic charts were done.

Further, everything is very simple. The quality high-frequency unit of constant potential with voltage of 200kV and power 900W should X-ray about 42mm of steel onto the film type D7 during the period of 10 minutes or about 50mm steel onto the PHOSPHOMATIC system. If it does not X-ray, it means that the real maximum voltage of the unit is less than 200 kV, or the amperage is less than declared etc. Before to buy any unit, ask to send to you the nomographic charts of the unit and compare them with those, given in this article. If the nomographic charts have a significant difference, then this is a reason to think once more about the quality of the unit selected. After receiving the unit it could be possible to check its compliance with nomographic charts sent. The modern units, type MTC, are capable to display on-line the nomographic chart directly on the built-in control desk monitor. These units are also equipped with the system of automatic exposure time calculation.

There is also another fine point in the selection of maximum unit voltage. Maybe, many people know that the usual bulb, designed for 220V, would have a longer service life if it would be connected with the network of 210V. In this sense the X-ray tube does not practically have any difference with the bulb. If according to the name plate the tube is designed for 250kV, its service life would be longer if connect it to the mains of 225kV. Same relates to the tube loading. If the limiting tube loading is equal to 900W I would not recommend using it at the power exceeding 750W. So, if the unit is designed for 200kV and the tube, installed in it, is designed for 200kV, this is not so good. It is always better to fall short of 10-20kV. For example, in units MCT-200 (200kV) and MCT-225 (225kV) the cermet tube, designed for 250kV is used. Besides the maximum high voltage the main characteristics of portable units consist of power time of continuous operation and weight. The power of most part of the portable units makes up 900-1000W. At lighter versions it makes up 300-450W.

The time of continuous operation (at limiting power) depends on the unit design, dimensions and efficiency of the cooler, power of the fan and environmental temperature. Sometimes it is called “work cycle” of the unit. For example, 100 % cycle at 20◦means that at the environmental temperature of 20◦ the unit could work continuously, at least one hour. The cycle of 50 % means that after, for example, 5 minutes of operation the unit should have 5 minutes of the rest. The cycle of the best units makes up 100 % at 20◦. Depending on the intensity of works the cycle of 50 % could be also considered as acceptable. Though to X-ray any big thicknesses it has to choose the units with maximum continuous operation time. This parameter is one of the most suppressed. Many producers either don’t mention it at all or openly lie in the leaflets. Especially this relates to the oil units with glass X-ray tube. These units have one trick. The time of continuous operation when the cooler is face down cardinally differs from the time when the cooler is face up. When the cooler in the unit in operation is face down, the first start up of the unit ensures the very long holding time as it is necessary to warm up rather big volume of the oil. But on the other hand, then it would be necessary to wait over very long time until this oil would get cold and the second holding period would be already much shorter than the first one. Improper producers suppress this feature of oil units. Therefore, when testing the oil unit on working cycle, it is necessary to test it when the cooler is face up. In this position there is practically no any oil convection and it is really possible to check the efficiency of the cooler and cooling fan.

Lighter units have usually smaller size of the cooler and shorter working cycle or lower power (often, both) accordingly. Some lighter units don’t have any fans at all and are designed for the oil cooling only; so their real working cycle could be less than 10%, i.e. in equal conditions it is almost impossible to use them. .
It has to take into consideration that the working cycle of half-cycle units is almost always equal to 100%. This is explained by that the average power of half-cycle units is almost in 4 times lower. It means, it is equivalent to high-frequency unit of constant potential operating at a quarter powers. So, it is impossible to compare these values and the working cycle in 100 % at the half-cycle units (type ICM) could not be considered as honest one.

Now it is turn of the most tricky but actual parameters – weight of the unit. The weight of the unit depends very much on maximum high voltage and power. In accordance with GOST and sanitary-epidemiological service the portable X-ray unit should provide the radiation dose not exceeding 1 Roentgen per minute at the distance of 1m when the inlet window is closed and the voltage and power are maxima. This is achieved by slipping the special protective coupling over the unit; the thickness of this coupling depending on the maximum admissible voltage of the tube and the coupling weight being comparable with the unit itself. This coupling protects against the retroactive and lateral radiation, and the use of the unit without this coupling is prohibited in accordance with GOST (and common sense). Western producers indicate the weight of the unit already together with the coupling, and at many Western producers this coupling is not removable.  Russian producers usually indicate the weight of the unit without coupling, and it gathers the wrong impression that the unit is lighter. Especially this relates to the advertising of lighter and diminutive units. The weight of the coupling mostly depends on the limiting voltage of the unit. For example, the unit of 200kV with power of 900W and the unit with power of 450W don’t p[practically differ by weight. Some improper producers obtain hygienic certificates with normal protection and supply the unit with lighter one. Unfortunately, this relates not only to Russian but also to some western producers. This is the case of the units designed for very high voltages that with normal coupling become extremely heavy. Using the unit with coupling removed or with lighter coupling you would violate the sanitary norms and jeopardize yourself to the radiation exposure. After receiving the X-ray unit it makes sense to test it on compliance with sanitary norms on the protection against the radiation, and in case of non-compliance claim the producers.

One of the main components of the unit is the X-ray tube; therefore we shall arrest our attention to this component. In most part of Russian portable units, available at Russian market, the glass X-ray tube is in use. The glass tube is also used in many foreign units, including Balto and ICM. The often application of glass tube is substantiated by its low cost price. However, the reliability and service life of glass tubes is in times lower than that of cermet ones. Especially, this is noticeable if take as example portable units where even under not strong impacts the tube would be often broken.

Type of the tube available on the world market is cermet tubes. Among the units being on sale in Russia the cermet tubes are installed in units of Eresco, Smart, MCT. The units MCT use the tubes of leading producer of the components for non-destructive testing – company THALES ELECTRON DEVICES (France).
Now this is the end of short overview of X-ray units. What I would like to tell more. When selecting the unit, pay attention to the control desk. In modern units tens of service functions, very convenient in routine work, are used. One of the main differences of up-to-date digital X-ray unit consists in the suitable graphic display and keyboard available. All service messages of the system, including the diagnostics of the failures are displayed. Modes and service functions could be programmed from the keyboard. The presence of these components is the sign that the development did not become obsolete 20 years ago and the engineers of the company-producer constantly up-to-date the unit. Unfortunately, many foreign units have the control desk that looks, to put it mildly, not so pleasant.

August 23, 2006.
B.Y. Kramer