WHAT ARE IMAGE QUALITY INDICATORS(IQIs), THEIR SELECTION & PLACEMENT?
For establishing the quality of an image, an Image Quality Indicator, or penetrameter, is used during Radiographic Testing. The idea is to place a Penetrameters or IQI on a specimen that is being radiographically tested, to evaluate the sensitivity of the radiograph.
Radiographic sensitivity is defined as the smallest or thinnest material change that a radiograph or RT film after exposure can reveals.
Again, sensitivity is a measure of the quality of an image, related to the smallest discontinuity it shows. For example, if a radiograph has 2% sensitivity for a material that is 12mm thick, then the smallest defect that can be found would be:
Types of IQI in Radiographic Testing
Two types of Image Quality Indicators or IQI used in industries are:
- Hole Type IQI
- Wire type IQI
Hole Type IQI
In the early times of radiographic inspection, you would put, in front of the piece to be inspected, a plate with a perforation the size of the smallest discontinuity you wanted to find in that piece. That was known as a “Hole Type” IQI. However, nowadays the most used type of IQI is the “Wire-type IQI”.
A step/hole type in which a step wedge of the same or similar material to the specimen has one or two drill holes in each step, the hole diameter being the same as the step thickness: the holes must not be chamfered. This type of IQI is shown in detail in EN 462-2:1994, AFNOR NFA 04 304, and BS 3971:1985. Typically, there are six steps, with two holes in the steps thinner than 0.8mm. The step thicknesses and hole diameters are from the same series given for wire diameters. IQI step/hole sensitivity is based on the smallest discernible hole diameter, not the step visibility: if there are two holes in the step, both must be discernible.
Wire Type IQI
Wire-type IQIs come in four sets (Set A, Set B, Set C, Set D) containing six wires each of different diameters. Sets A, B C, D are selected depending on the thickness of the piece to be evaluated. The thickness on which the IQI is based is the nominal single-wall thickness plus the estimated weld reinforcement not to exceed the maximum permitted by the referencing Code Section. If the piece has 2 walls, the IQI is the same (single wall) but exposure time is double. Wire type IQIs should be placed directly across the weld. The IQI shall be made up of the same material as that of the material to be radiographed or the IQI shall be of a radiographically similar material.
These consist of a series of straight wires of the same or similar material to the specimen, the wires being of different diameters taken from the series 0.10, 0.125, 0.16, 0.20, 0.25, 0.32, 0.40, 0.50, 0.63, 0.80, 1.00,… The wires may be 10, 20, 30, 50mm according to the various Standards and are held parallel to one another in a low-density plastic mount, with appropriate identification symbols.
How to make the selection of Image Quality Indicators ( IQI)?
Let´s say you need to select a penny for a piece 6,4mm thick with an 1/8″ reinforcement in each side (The picture of the radiographic film in this article corresponds to that situation). Then the single wall thickness would be 2/4″ = 12,7mm. If you need 2% sensitivity, then multiply 12,7*0,02 = 0,254mm. You go to T-233.2 in ASME V and read through the line of the thickness you need. In this case, the Wire-type essential wire designation is 6. This wire designation corresponds to the set A or B wire type IQI.