Between a 200 kV X-ray tube and a 225 kV one there are 25 kV of difference and, in Italy, two different licensing routes. Legislative Decree 101/2020 requires prior authorization for machines in which the maximum energy of accelerated particles exceeds 200 keV (art. 50), while below that threshold the general rule is notification at least 30 days before start-up (art. 46). A figure that looks like catalog trivia can shift a project by months before it touches image quality.
An X-ray generator may be bound for an NDT lab, a fabrication shop or a field inspection company. Either way, the buyer is choosing four things at once: maximum voltage, beam shape, the ability to run for long stretches and the place the machine will work. The physics of focal spot, contrast and filtration is covered in our piece on X-ray tube focal spot and parameters. Here the view is the buyer's, starting from the hardest part the tube will have to penetrate.
Read the voltage off the standard's curve, starting from the thickest joint
For welds, ISO 17636-1:2022 (film) and ISO 17636-2:2022 (digital detectors) give a curve of maximum tube voltage as a function of penetrated thickness and material. It's a ceiling. Fewer kV means more contrast, so the standard shows how high you can go without losing too much, and the working setting is found below that limit. In the 2022 digital edition the figure, number 20, gained interpolation formulas, because the drafters found that shop floors use graphs far more than formulas.
With digital detectors the rule has extra room. The first compensation principle of ISO 17636-2 lets contrast lost at higher voltage be recovered with a higher signal-to-noise ratio, and the 2022 edition also rewards the opposite case. According to the summary the drafters presented at the 2023 DGZfP conference, if voltage stays below 80% of the figure's value, the required normalized SNR drops to 80% of the table value. A tube with some kV in reserve lets you run low on medium walls and go higher, with compensation, on the few thick joints.
What sizes maximum voltage is penetrated thickness, not nominal wall. In a double-wall pipe shot the beam crosses two walls plus reinforcement; on a casting it's the thickest section along the beam. Material moves the numbers a lot too: NIST attenuation data (the XCOM database) show that at the same energy iron attenuates X-rays far more than aluminum. So a thick light-alloy part is shot at kV settings that would already be marginal on thin steel. If thin parts or light alloys will come through, ask the supplier as well for the lowest voltage at which the tube runs stable.
When a tube is no longer enough
Beyond certain steel thicknesses a portable tube stops being practical. IAEA guide SSG-11 (2011) gives typical working ranges on steel of 8-30 mm for selenium-75, 12-70 mm for iridium-192 and 50-120 mm for cobalt-60, and above those sections linear accelerators come into play. The tube-versus-gamma comparison, with storage, decay and recurring costs, is covered in our guide to X-ray system costs and quotation. One conclusion follows for the tube purchase: if your most frequent thicknesses sit at the top of the range, check that a tube is really the right source before choosing kV.
Duty cycle: ask for it at your working voltage
Nearly all the electrical energy fed into an X-ray tube ends up as heat in the anode; only a small fraction leaves as X-rays. Duty cycle tells you how long the tube can emit before it has to stop and shed that heat. It's stated as the ratio of beam-on time to total cycle time, at a given voltage, current and ambient temperature. A manufacturer may quote continuous operation at half power and an intermittent cycle at full power. Both figures are true, and neither describes your work until you know the kV and mA you'll use on your most frequent joint.
So the request for quotation should ask for duty cycle at working voltage and current, along with the ambient temperature it refers to. On a summer job site, with the tube in direct sun, the same unit may need to rest more often than in an air-conditioned bay. Those pauses, added up over a day of joints, decide how many exposures a shift actually closes.
Waveform: why exposure charts don't transfer
Generators split into constant-potential units, today almost always high-frequency inverters, and pulsating-voltage units, which are simpler and lighter. The difference shows on the part before it shows on the datasheet. As early as 1932, Taylor, Singer and Stoneburner at the US Bureau of Standards measured very different outputs from different generator types set to the same peak voltage and the same average current. At the same RMS voltage, by contrast, the values came out similar. The kV figure on the nameplate isn't enough to compare two machines.
The buyer faces two consequences. Exposure charts built on one generator don't hold for another with a different waveform, so replacing the tube means rebuilding them, with the bay time that implies. And two offers with the same maximum kV but different waveforms won't deliver the same throughput. A fair comparison asks for exposure times on a reference thickness, at the same distance and with the same detector.
Directional or panoramic: the pipe decides
IAEA SSG-11 distinguishes generators for directional exposures from those for panoramic, radial-beam exposures, and describes crawlers that carry the tube inside a pipeline to radiograph girth welds from within. A panoramic tube makes sense when the source can sit at the center of the pipe: that takes enough diameter for the tube head or crawler and access from inside. In that position the whole circumference is exposed at once, single wall, and the 2022 edition of ISO 17636-2 added guidance on exactly this central exposure with the image quality indicator placed outside.
The directional tube remains the choice for plate, castings, cabinets and double-wall techniques from outside, which are standard when the pipe is small or closed. A collimated beam also helps radiation safety, because it concentrates radiation in a cone and shrinks the area to cordon off around the work point, while a panoramic tube radiates all around. A contractor working on both pipelines and structural steel may find the answer in both tubes on the same generator, if the supplier offers it.
Thicknesses and work site already defined?
Tell us your thickest joint, the material and where the tube will work: we'll point you to voltage, beam type and the likely licensing route, with a tailored quote within 24 hours and, if needed, a field demonstration.
Email us Message us on WhatsAppCooling and insulation: where the tube will spend its hours
Anode heat has to go somewhere, and how it leaves separates field machines from bay machines. Portable heads are usually gas- or oil-insulated in a sealed housing and air-cooled, sometimes with forced ventilation. Tubes for fixed installations more often use an oil or water circuit with an external heat exchanger, which lets them run for long periods close to rated power. In bipolar tubes the voltage is split between anode and cathode, each at half the potential to ground, so insulation needs drop for the same kV. In unipolar tubes, by contrast, the anode is grounded and easier to cool. These configurations differ in weight and size, and the choice follows where most beam-on hours will be spent.
The same logic applies to the stated focal spot. Ask which method measured it, today ISO 32543-1:2024 with the pinhole camera or ASTM E1165-20, and at what power. E1165 also uses the measurement to detect changes caused by tube overloading, and two values obtained with different methods can't be compared.
Portable or fixed: who carries the weight and who signs off the shielding
The first signature is the employer's, who under art. 128 of Legislative Decree 101/2020 appoints the radiation protection expert. For a portable tube in the field, what counts next is the weight of head, generator and cables, available power and the area to control at each exposure; for a fixed installation, the shielded bay or self-shielded cabinet, interlocks and warning signals. On licensing, the 200 keV threshold applies to both, but art. 50 also requires authorization for mobile sources used at locations that can't be determined in advance. How that interacts with the exclusion of generators up to 200 keV has to be read case by case with the radiation protection expert before ordering. Above the threshold, for category B industrial uses authorization comes from the Prefect (art. 52), for category A from the competent ministries (art. 51).
Buyers sourcing from a foreign manufacturer should know the threshold is Italian. Directive 2013/59/Euratom requires registration or licensing for radiation generators but sets no keV limit. The 200 keV line was drawn by the national legislator, and the supplier's documentation has to be supplemented with what the Italian procedure needs.
The lines to write into the request for quotation
| Item | What to ask for | Why it matters |
|---|---|---|
| Voltage | Maximum and minimum stable | Maximum follows the highest penetrated thickness; minimum serves thin parts and light alloys |
| Current and power | At working voltage, not only at maximum | They set real exposure times |
| Duty cycle | With reference voltage, current and temperature | It sets the pauses and so the exposures per shift |
| Waveform | Constant potential or pulsating | Output and exposure charts change at the same kV |
| Focal spot | Value, measurement standard and power at which it was measured | Without a stated method two values can't be compared |
| Beam | Directional or panoramic, emission angle | Follows joint type and access |
| Cooling | Type and permitted ambient conditions | Separates field use from continuous duty |
| Weights and power supply | Head, generator, cables, power draw | They decide transport, generator set and installation |
If every line holds a value tied to your conditions, two offers can really be compared. If a line stays blank, that's where a price gap hides a performance gap.
What to send us for an assessment
To propose the right X-ray tube and generator, we start from these data:
- materials and minimum and maximum penetrated thickness, with the exposure technique (single or double wall);
- part types: plate, pipe with diameter, castings, assemblies;
- technique standard and required class, detector in use or planned (film, CR, DR);
- place of use: shielded bay, cabinet, line or field, with available power;
- exposures per day and expected cycle time;
- sources already in use and current licensing status, if any.
You can send them by email to info@dipitech.com or on WhatsApp, with drawings or photos of the parts.
The choice comes down to a simple check. The right tube covers the highest thickness you'll penetrate while staying under the standard's curve, keeps up with your exposure rate at working voltage and comes with a licensing route you've already put on the calendar. With those numbers in hand, a tailored configuration comes with a quote within 24 hours, technical support in Italy and a loaner unit during service. How tube and generator then fit into a complete system, with detector and software, is shown on our industrial X-ray equipment page.
Frequently asked questions about industrial X-ray tubes and generators
How many kV are needed to radiograph a steel weld?
It depends on penetrated thickness, which in a double-wall exposure includes both walls. ISO 17636-1:2022 and ISO 17636-2:2022 give a curve of maximum voltage by thickness and material: the working voltage is found below that ceiling, and the tube is chosen with headroom over the thickest case.
Is authorization needed to use an industrial X-ray tube in Italy?
Yes, in different forms. Under Legislative Decree 101/2020, machines in which accelerated-particle energy exceeds 200 keV need prior authorization (art. 50), the others as a rule a notification at least 30 days before start-up (art. 46). For mobile field use art. 50 contains specific provisions, to be assessed with the radiation protection expert appointed under art. 128.
What's the difference between a directional and a panoramic X-ray tube?
A directional tube emits a cone-shaped beam and suits plate, castings, cabinets and double-wall techniques from outside. A panoramic tube emits a radial beam all around and pays off when the source can sit at the center of a pipe, exposing the whole circumference at once, single wall.
What is the duty cycle of an X-ray tube?
It's the ratio of beam-on time to total cycle time, stated at a given voltage, current and ambient temperature. Ask for it at your real working voltage, because duty cycle sets the pauses and therefore how many exposures a shift can close.
X-ray tube or gamma source?
A tube emits only when powered and lets you adjust voltage to the part, so it's preferable in shielded bays, cabinets and on medium and thin walls. Gamma sources cover what a tube can't reach: IAEA SSG-11 gives typical steel ranges of 8-30 mm for selenium-75, 12-70 mm for iridium-192 and 50-120 mm for cobalt-60.
How much does an X-ray tube with generator cost?
It depends on voltage, power, waveform, beam type, cooling and accessories, so a generic price would be misleading. DIPITECH prepares a tailored quote within 24 hours from materials, penetrated thicknesses, part types and place of use.