On a tube rating chart, kVp runs up the side, time runs along the bottom, and each curve is one mA

On the radiographic rating chart, kVp is on the vertical axis, maximum exposure time is on the horizontal axis, and each curve is one mA station. Below your mA curve is safe. Above it is unsafe.

These are the axes on the charts in Bushong (11th ed., Figure 6-26, PDF p.139). Read the labels on any chart you’re given before using it.

How do you read it?

1. Pick the right chart. Each one applies to a set of conditions: large or small focal spot, rotor speed, target angle and generator type.
2. Find your mA curve. The curves are labeled, for example 200 mA or 400 mA.
3. Find your time on the horizontal axis. The scale isn’t even: it stretches from thousandths of a second to about 20 seconds in the book’s charts.
4. Find your kVp on the vertical axis.
5. Mark where they cross. Below your mA curve is safe. Above it is unsafe.

Here is an example with made-up numbers. Say the 400 mA curve on your chart sits at 90 kVp at 0.1 s. Then 85 kVp, 400 mA, 0.1 s is below the curve and safe. At 95 kVp, the same mA and time is above the curve and unsafe.

The curves slope down: a longer time leaves less room for kVp. Higher mA curves sit lower on the chart.

Why are there several charts for one tube?

The conditions change the limit. In Bushong’s four sample panels, the 0.6 mm spot allows lower mA than the 1.0 mm spot, and the 10,000 rpm three-phase panels allow more than the 3400 rpm single-phase panels.

A small focal spot packs the heat onto a smaller area of the target. Bushong says that at about 400 mA and up only the large spot is allowed, because the anode’s heat capacity could be exceeded with the small one (PDF p.129). Its representative 10,000 rpm chart for the 0.6 mm spot still shows a 400 mA curve (PDF p.139), so go by the chart for your tube. Its anode cooling chart doesn’t depend on filament size (PDF p.140), so the difference shows on the rating chart, not the cooling chart.

A replacement tube can come with a different chart from the old one, so check after a tube change.

Which chart answers which question?

Chart	Question it answers
Radiographic rating chart	Is this single exposure safe?
Anode cooling chart	How much heat can the anode store, and how long does it take to cool?
Housing cooling chart	The same for the whole housing, with far more capacity and a cooling time of 1 to 2 hours

Bushong’s sample anode stores 350,000 heat units and takes about 15 minutes to cool fully. Cooling is fastest at the start. To use a cooling chart, you first need the heat unit formula, with Bushong’s 1.4 factor on three-phase or high-frequency units.

What happens if you exceed the rating?

The tube may fail abruptly. Too much heat in one exposure can melt and pit the anode surface, and heating it too quickly can crack it. Most units now block an unsafe exposure with a microprocessor control. Our page on tungsten anodes has more.

Is it on the registry?

ARRT®‘s outline lists “tube loading” under the x-ray generator. It doesn’t name rating charts, so we can’t say how they would be asked.

For more, read the Image Production guide and the Equipment Operation and QA chapter, then try the equipment practice questions.