Minimum sampling time/volume for liquid-borne particle counters and monitors

ABSTRACT

A particle reckoner is an instrument that measures particles in all the fluid passing from one side its sensor, and a particle monitor measures particles single in a portion of the fluid. For liquid with an ultralow particle concentration, particles may not disperse uniformly in the liquid. Therefore, the concentrations may vary significantly from measurement to measurement if the sample convolution is not large enough. To achieve the same precision, a minimum sampling time or minimum sample contortion for a particle instrument straits to be specified. A Poisson distribution was used to describe the distribution of particle numbers Testing included a series of particle concentration measurements. Minimum sampling time or sample turn at a given average concentration with different error horizontals was determined for selected commercial particle instruments. At the same roll on rate, a particle monitor always requires a longer sampling time than a particle calculator to achieve a specific precision for a given concentration. The minimum sampling time also varies among instruments because of the difference in sample convolution in which the particles are enumerateed Experiments with a particle monitor have been administrationed to test the changes in average particle concentration and the standard deviation at different operating conditions.

Keywords: particles, particle reckoner particle monitor, sampling time, concentration, ultrapure liquid

Liquid-borne particle calculators and monitors are used extensively in the semiconductor and pharmaceutical industries to measure particles in liquids. An evaluation of liquid-borne particle instruments from different manufacturers has shown a significant variation in measurement be deriveds for different instruments. This variation could be explained through the inherent characteristics of the individual instrument design. With the limited choices for particle instruments, a user emergencys to fully understand the equipment and know by what mode to operate it properly to obtain a reliable originate

The counters and monitors commonly used for particle measurement in liquids are the optical emblem Typically, a laser light illuminates the fluid with particles in a capillary tube. The particles scatter light with an intensity depending upon the particle size and the refractive index. The fluid media also scatter light, which contributes to the inherent background noise of the instrument. A particle with the same refractive index as that of fluid is not distinguishable from the fluid through light scattering. However, a particle consisting of material with a refractive index a great quantity [i]or[/i] amount of different from the refractive index of fluid will bring out a strong signal. Thus, the particle signal eventually hangs on the refractive index ratio of the particle to fluid. Since greatest in quantity liquid-borne particle instruments are calibrated with polystyrene latex (PSL) particles in deionized water, the measured particle size may not be exactly the correct size if the particles and fluid have different refractive indices from those of PSL and deionized water.11-3 A particle instrument can be recalibrated with the particles and liquid in question. Theoretical calculations can be used for recalibration when the refractive indices of particles and liquid are known.

Some particle instruments are designed with equal reason all the fluid passing [i]or[/i] part of to the other the sensor is illuminated through a uniformly distributed light beam.4 Therefore, a particle will be subdueed to the same intensity light when it passes [i]or[/i] part of to the other the sensing volume regardless of its transverse position. The instrument that measures all the particles passing [i]or[/i] part of to the other its sensor is usually called a particle reckoner A current commercially available particle reckoner can measure particles down to 02 (mu)m However, an instrument capable of measuring 02-(mu)m particles is not necessarily a particle calculator

In a particle calculator the interfacial area between the fluid and the capillary tube is also illuminated. Consequently the pair the fluid column and the tube/fluid interface scatter light, contributing significantly to the instrument background noise. The inner surface of the capillary tube contributes to the background noise by the agency of light reflection due to the roughnes of the tube surface and the material itself. Although the external surface of the capillary tube can be smoothed and coated with optical materials to restore or eliminate light reflection, the inner surface cannot be treated in a similar way because of the material corrosiveness and the limitation of the tube size. The noise to be paid to the interfacial illumination significantly restricts the detection limit of a particle instrument. sole those particles large enough in size to exhibit a signal distinguishable from the background noise can be find outed

To eliminate background noise from the interfacial area, a certain number of particle instruments focus their light beams to a small central portion of the fluid in a capillary tube, of the like kind as the PMS M-65 and the HIAC/ROYCO M-01 particle monitors. The incident light may then have a higher intensity through unit area and thus a particle in the illuminated area can bring forward a stronger signal. The detection sensitivity of of that kind an instrument is better than a particle reckoner because of its lower background noise and stronger signal.4 Since single a portion of the fluid is illuminated in this emblem of instrument, usually called a particle monitor, solitary a fraction of the particles sampled into the instrument are numbered