How to read a tube rating chart: Y-axis is kVp, X-axis is time, curves are mA

What a tube rating chart actually is

Every x-ray tube has a limit. Heat it too much, and the anode cracks or pits. Send too much current for too long, and the target material itself can fail. A tube rating chart is the manufacturer’s instruction manual for staying under that limit.

It is a graph. The graph plots all the combinations of exposure time, tube current (mA), and peak voltage (kVp) that the tube can handle without overheating. Below the curve is safe. At or above the curve is damage.

The chart is specific to the tube model and focal spot size. A large focal spot tube has a higher rating than a small focal spot tube because the large focal spot spreads the heat over a larger area. Same heat, more area, lower temperature.

Radiographers check tube rating charts before planning high-heat exposures: fluoroscopy, rapid serial angiography, or computed radiography with high-mAs techniques. It is a critical safety check that the ARRT expects you to understand.

The common wrong answer (axes confused)

Many study materials show tube rating charts but label the axes incorrectly. The classic mistakes:

• “X-axis is kVp”, wrong. X-axis is time.
• “Y-axis is mA”, wrong. Y-axis is kVp. mA values are labeled on the curves themselves.
• “The curves are time”, wrong. The curves are constant-mA lines.

The reason these mistakes happen is that students sometimes confuse tube rating charts with load-time curves or heat-unit accumulation graphs, which have different axis meanings.

On an ARRT question, if you misread the axes, you will reverse the safe zone and fail the question. Memorize the axes now.

Standard tube rating chart:

• Y-axis: kVp (peak voltage)
• X-axis: exposure time in seconds
• Curves: each curve = one mA value

How to read the chart, step by step

You have a planned technique: 85 kVp, 400 mA, 0.2 seconds. You have the tube rating chart for your tube. Is this technique safe?

Step 1: Find your exposure time on the X-axis. In this case, 0.2 seconds.

Step 2: Move vertically upward from 0.2 seconds until you reach the curve labeled 400 mA.

Step 3: From that point on the curve, move horizontally (to the left or right) until you hit the Y-axis. Read the kVp value.

Let’s say the curve at 0.2 seconds and 400 mA gives a maximum kVp of 90. That is the highest kVp you can use at 400 mA for 0.2 seconds.

Step 4: Compare your planned kVp to the maximum:

• Planned: 85 kVp
• Maximum (from chart): 90 kVp
• 85 is below 90, so the technique is SAFE.

Step 5: If your planned kVp had been 95 kVp instead, it would exceed the maximum. The technique would be UNSAFE and would risk anode damage.

This five-step process is the standard method taught in radiography programs and tested on the ARRT.

Small vs large focal spot ratings

The same tube model comes with multiple focal spot options: 0.6 mm (small) and 1.2 mm (large) are common sizes.

Each focal spot has its own rating curve because heat capacity is proportional to area.

A large focal spot (1.2 mm) concentrates the electron beam over a larger area of the target. The heat dissipates over more material. Result: higher heat capacity and a higher rating curve.

A small focal spot (0.6 mm) concentrates the same electron current into a smaller area. The same heat density in less area means higher peak temperature. Result: lower heat capacity and a lower rating curve.

This is why small focal spot techniques are more constrained than large focal spot techniques on the same tube. If you switch from large to small focal spot, you must use the corresponding smaller rating chart.

The ARRT will sometimes ask you to predict what happens when you switch focal spot sizes and hold mA and time constant. If you go from large to small, you move to a lower rating curve, which means a lower maximum kVp. If you exceed the new limit, you risk damage.

What happens when you exceed the rating

Exceeding the tube rating means the anode accumulates heat faster than it can cool. The target material reaches a critical temperature.

Short-term effect: thermal stress. The anode expands. Stresses build in the ceramic and rotor bearings.

Medium-term effect: pitting and crazing of the anode surface. The surface develops small cracks and pits where electrons no longer hit smoothly. This degrades the x-ray output and image quality.

Long-term effect: complete anode failure. The target material can crack, the rotor can seize, or the tube envelope can fail. The tube becomes unusable and must be replaced. Replacement tubes cost $3,000 to $20,000+ depending on the model, and installation can take weeks, affecting clinical operations.

This is why exceeding the rating is not a minor transgression. It is a serious equipment failure risk that radiographers are trained to prevent.

Heat unit formula and tube cooling curves

A related concept is heat units: a measure of the total thermal energy deposited in the anode over time. One heat unit is generated by 1 mA × 1 sec × 1 kVp (with a correction factor depending on rectification mode).

Heat units accumulate. If you run multiple exposures in quick succession, the heat from all of them adds up. The anode cools slowly (minutes to hours, depending on the cooler rating). If the next exposure starts before the anode has cooled, the new heat is added to the old heat.

Tube rating charts show instantaneous safe combinations (single exposures). Tube cooling curves show how fast heat dissipates over time so you can plan back-to-back exposures without overload.

For a deeper look at the x-ray tube and circuit design, see our chapter on x-ray circuit and fluoroscopy.

The tube rating chart is the frontline safety check. The cooling curve is the secondary check for repeated use.

Why this matters on the ARRT

The ARRT Equipment section (part of Image Production) tests tube rating charts in multiple question types:

1. Axis questions: “Which axis represents kVp on a tube rating chart?” Answer: Y-axis.
2. Safety questions: “Is this technique below the curve?” Answer: use the five-step method.
3. Focal spot questions: “What happens to the rating if you switch from large to small focal spot?” Answer: the rating is lower (smaller maximum kVp).
4. Damage questions: “Exceeding the tube rating risks what?” Answer: anode damage, pitting, tube failure.

Many students fail these questions because they confuse the axes or misunderstand what “below the curve” means. Get the axis assignment right first, then practice the lookup method until it is automatic.

Quick reference table

Axis or Label	What it represents	Unit or Value
Y-axis (vertical)	kVp (peak voltage)	40 to 150 kVp range
X-axis (horizontal)	Exposure time	0.01 to 10+ seconds
Curves on the chart	Constant mA values	One curve per mA station
Below the curve	Safe technique	No anode damage risk
On or above the curve	Unsafe technique	Risk of anode overheating
Different focal spot	Different rating curves	Smaller spot = lower rating

ARRT exam tip

The tube rating chart is a straightforward tool once you know the axis assignment. Y-axis = kVp. X-axis = time. Curves = mA. Below the curve is safe. Use the five-step lookup method on every practice question until you can do it in 20 seconds.

Many exam questions pair this with other equipment concepts like heat units, cooling rates, or focal spot properties. If you master the basic chart reading, the paired questions become much easier.

For a comprehensive walkthrough of x-ray equipment and circuit design, see our chapter on x-ray equipment and photon interactions. For the image-production side, the post on ARRT Image Production connects tube rating limits to the broader exposure strategy.

If you are building an ARRT prep plan from the ground up, our Curriculum covers all four ARRT domains with plain-English explanations and free practice questions in every category.