20
Preventing Errors
Reference Test Pieces
To maintain high accuracy when taking measurements of different
materials, it is important to use a standard test piece that resembles the
material and conditions being measured. The ideal reference test pieces
should be a group of test pieces with different thicknesses made of the
same materials that is going to be measured. The test pieces can provide
calibrating factors for the meter (such as the microstructure of the material,
heat-treating condition, direction of particles, surface roughness, etc.).
To meet the highest requirements of accuracy a set of reference test
pieces are critical.
Under most situations satisfactory measuring accuracies can be met
with only one reference test piece. This should be the same material and
similar thickness to the material under test. Take an even-surfaced object,
measure it by using a micrometer, then use it as a test piece.
For thin material, when its thickness is near to the low limit of the sensor's
measuring range, you can use a test piece to determine the accurate low
limit. Never measure a material with a thickness lower than the low limit.
When the material under test is thick, especially an alloy with complex
internal structure, select a test piece similar to the object from a group of
test pieces, giving you an idea of calibration.
For most casting and forging, their internal structures have some direction.
In different directions, the sound velocity will experience some change. To
solve this problem, the test piece should have an internal structure with
same direction as that of the material under test, and the transmitting
direction of sound wave in it should also be same as that of the material.
Under certain circumstances, referring to a material speed-of-sound
table can replace reference test pieces. The value in the speed-of-sound
table may have some difference from the actual measured values due to
difference in the material's physical and chemical characteristics. This is
usually used for measuring low-carbon steel, and can only be taken as a
rough measurement.
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