Electrical zone sensing

• Mie diffraction

• Centrifugal sedimentation

• Packed bed permeability (Fisher Subsieve Analyzer)

• Photon correlation spectroscopy

• Elutriation

• Field-flow fractionation

• Size exclusion chromatography

This list is not exhaustive; however, it does represent the breadth of available techniques. Several of the methods listed, such as field-flow fraction, are specific to only a few industries and would not be encountered in companies producing components from technical metal and ceramic powders. Typical methods and effective size ranges are listed in Table 1.

Table 1 Typical methods of particle size and size distribution measurement

Measuring

Method

Approximate

principle

useful

size range, /'m

Mechanical or ultrasonic agitation

Sieving

5 ■■ 125 mm

Microscopy

Optical

0.5 -■ 100

Electron

0.001 —50

Electrical resistivity

Coulter counter

0.5 -■ 800

Electrozone

0.1 —2000

Sedimentation

Sedigraph

0.1 —100

Roller Air Analyzer

5 -■ 40

Micromerograph

2 —300

Light scattering

Microtrac

2 -■ 100

Light obscuration

HIAC

1 -■ 9000

Permeability

Fisher subsieve sizer

0.2 —50

Surface area

Gas adsorption (BET)

0.01 —20

With these different methods, it is important to understand that measurement methods are based on different analytical models (see, for example, the grouping of "measurement diameters" in Table 2 for a wide variety of methods. In reality no method measures the "true" particle size, because it is impossible to describe the size of a three-dimensional particle with a single number, therefore, all analytical methods measure an index of particle size such as geometric diameter, aerodynamic diameter, equivalent circle diameter, and so on. Because of this, even if the mathematical algorithm is accurate and the hardware is properly developed, one can obtain different "correct" particle size data on two different instruments.

Table 2 Methods of particle size analysis and nominal size ranges

Method

Nominal particle size

Measurement diameter

m

mil

Sieving

Dry

>10

>0.4

Geometrical

Wet

>2

>0.08

Field scanning

Optical microscopy

0.5-1000

0.02 to 40

Image

Electron microscopy

0.01-10

4x10-4 to 0.4

Stream scanning

Resistivity

0.05-500

2x10-3 to 20

Dynamic/Stokes

Optical

1-500

0.04 to 20

Ultrasonic attenuation

100-1000

4 to 40

Column hydrodynamic chromatography

0.1-1.0

4x10-3 to 0.04

Sedimentation field-flow fractionation

0.01-1.0

4x10-4 to 0.04

Laser Doppler velocimetry

0.01-3.0

4x10-4 to 0.12

Gravity sedimentation

Pipette

1-100

0.04 to 4

Stokes

Photoextinction

0.5-100

0.02 to 4

X-ray absorption

0.1-130

4x10-3 to 52

Centrifugal sedimentation

Photoextinction

0.05-100

2x10-3 to 4

Dynamic/Stokes

Mass accumulation

0.05-25

2x10-3 to 1

X-ray absorption

0.1-5

4x10-3 to 0.2

Other

Gas absorption

0.005-50

2x10-4 to 2

Equivalent spherical

Mercury intrusion

0.01-200

4x10-4 to 8

Gas permeability

0.1-40

4x10-3 to 16

Cascade impaction

0.05-30

2x10-3 to 12

Brownian motion

0.01-3

4x10-4 to 0.12

Geometrical

Source: Ref 5

With the large number of available size measuring methods and instruments, the challenge is to select the proper instrument for a required measurement. One method does not provide more accurate results than another method but one method may indeed be more appropriate or discriminatory in a given application.

0 0

Post a comment