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27 Cards in this Set
- Front
- Back
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What does quality control of products do? |
Increases the confidence that a product is the same as previous batches. |
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How do you measure the size of an irregular particle? |
Give it an equivalent diameter to that of a sphere, by using volume. |
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What is the volume equivalent diameter, dv? |
The diameter of a sphere that has the same volume as the irregular particle- unambiguous, as particles have a well-defined volume. |
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What can you construct from distributions of particle size? |
A frequency histogram |
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What can you determine from a frequency histogram? |
Mode, median, mean and standard deviation. |
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What is a cumulative frequency representation? |
The percentage of particles above or below a given size rather than those at a given size. |
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What are the five different techniques for measuring particle size? |
-Particle Counting -Sieving -Sedimentation -Microscopy -Light Scattering |
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What does choice of technique for measuring particle size depend on? |
-Applicable size range -Cost -Time taken -Skill reqiuired -Precision -Quantity of material needed -How much data is provided |
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What is sieving? |
-A method of determining particle size -Oldest method -Separates fine material from course material by perforated surfaces > proportion of different size particles recorded -Sieve is shaken -Smallest size = 50 micrometres |
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What can the sieving method determine? |
A 'sieve equivalent diameter'- the size of the sphere which will pass through the square hole |
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What are the 7 errors in sieving? |
-Sieve holes may vary in size due to manufacture or damage -Powder may coat wires > leading to sieve apertures being reduced -Particles cohesive > won't pass through sieve -Vibration from shaking may damage particles > erroneous 'fines' -Stack might not be shaken for long enough to get particles to their final sieve -Sieve may be overloaded -Particle shape can cause problems |
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What is the theory behind sedimentation? |
Stoke's Law: v=2r²(ρ2-ρ1)g -------------------- 9η Where v= sedimentation velocity |
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Which methods can you use to measure sedimentation? |
-Settling velocity not measured directly -Can meausre amount of material settled in a particular time: "sedimentation balance" -Can measure the amount remaining in suspension vs time by passing a beam of light/x-rays through sample -Usually use centrifugal sedimentation to speed things up |
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How do you use an Andreasen Pipette? |
-Remove samples over time -Analayse for particle content -Calculate size distribution |
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How do you use a sedigraph III? |
Insert sample and push button |
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How does microscopy/image analysis work? |
-Uses very small volume of sample -<easures 1nm-mm -Computer thresholds image counts pixels in each region, constructs a histogram and computes statistics as required |
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What are the disadvantages of microscopy? |
-Measures relatively few particles |
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What is the main advantage of microscopy? |
-One of the few methods of getting shape information |
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What are the advantages of light scattering? |
-Rapid -Easy to use -Wide applicability -Wide size range |
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How does light scattering work? |
-Light hits a particle and becomes scattered in all directions -Place screen opposite particles and see a scattering/diffraction pattern |
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What is a diffraction pattern determined by? |
Particle size and shape |
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What happens if there are loads of particles in suspension during light scattering? |
You see just one scattering from each one added up- the various patterns "smear out" |
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What are the two problems with light scattering? |
-Measuring the diffraction pattern -Finding the particle size distribution from it |
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How does a laser diffraction sizer work? |
-Particles in dilute suspension> scattering measured -Laser light source of high intensity, single colour, unidirected light shone through -Array detectir measures light intensity at each point |
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Which intruments are usually used for particle counting? |
-Coulter Counter -Electrical Zone Sensing (EZS) technique |
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What is electrical zone sensing? |
-Tube containing a single small hole is immersed into a beaker containing a dilute suspension of particles -Suction applied to tube, suspension streams through hole, bringing some particles with it -Volume of suspension drawn through aperture is determined by the suction potential created -Electrode added to each chamber -If particle sucked through aperture, briefly occludes the hole and stops part of the current, reducing it -Can suck a known amount of suspension through the aperture and tine intrsument will count the number of times the current is blocked -Amount of blockage tells us size of each particle and size distribution |
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What is optical particle counting? |
-Particles in dilute suspension are passed through a narrow beam of light -As they pass, they cast a shadow, which is measured by a photodetector -Principle of the Hiac counter, used in tests for particles in injections |
