Aperture tubes

Blocked aperture. If the aperture is blocked with particulate matter, it can be removed by applying a slight back-pressure, by gently brushing the aperture, by soaking the aperture tube in acid, or by application of mild sonica-tion. 

A comparison between the TF system, the edit system and the standard system has been given by Lines [49], who also discusses the effect of using a long aperture tube. 

D is the orifice diameter in micrometers and v the volume of the suspension in microliters monitored for each count. The factor 2.5 was determined experimentally using a 100 pm diameter aperture tube 75 pm long with V = 500 pi. If it is assumed that the sensing zone comprises the volume of the aperture plus a hemisphere at the entrance and exit, s = 1.11Z)3 so that s/v = 2.22x10 which is not greatly different to the value of 2.5x10 found experimentally. 

If the size range of the powder is too wide to be covered by a single orifice, two or more aperture tubes can be used. As a general rule, if there is more than 2% of the distribution in the smallest size interval it is advisable to use a smaller orifice to determine the fine end of the distribution. 

This procedure should be used as routine since it indicates whether all the powder is accounted for and allows for correction for powder outside the measuring range of the aperture used. In an extreme case, it has been found that less than 5% of the total distribution was being measured and decisions were made based on these incorrect distributions. Alternatively, if the whole size range cannot be covered using a single aperture tube, a two-tube technique is required. 

Beckman Coulter Z2 uses the same technology as the Zl. It performs channelyzation of particle data into 256 channels while displaying size distribution data. The advanced user interface allows the operator to view the data in a variety of ways. Sample volumes as small as 10 ml can be handled using Accuvette II vials or less than 2 ml using ampule insertable aperture tubes. 

Aperture tubes are available in 22 sizes ranging from 12 to 1900 pm in diameter. The orifice is drilled in a synthetic jewel that is permanently sealed into the wall of the tube. For diameters greater than 480 pm the holes are drilled through a ceramic insert. The effective range of particle sizes a typical orifice can cover is 3% to 70% of the diameter. 

A second class of measurement techniques, based on particle number, overcomes some of the problems associated with mass-based techniques. Electronic zone sensing, sometimes referred to as the Coulter principle, permits simultaneous particle size and number measurement. Particles are sized by suspending them in an electrolyte and drawing them through an aperture tube across... 

The shape of the aperture tube can vary according to application - for example a very narrow design, which can be inserted into glass ampoules as small as 1 ml capacity, allows the particle contamination of injectable solutions to be measured. 

Particle size range detectable depends on the aperture tube used. Each aperture tube is effective over a size range of about 2-40% of its nominal diameter. Apertures of sizes from 15 to 4000 pm are available. Before use, it is necessary to calibrate the equipment with a standard latex containing monosize spherical particles of mean size within 5-20% of the aperture diameter. 

Guthrie and Doutre have dipped an aperture tube made of boron nitride into molten metals, such as aluminium, zinc and lead, applied a high current across the resulting sensing zone by way of iron electrodes, and measured the insoluble particles, or inclusions, within the molten metal. Currents of 20-60A were used, along with apertures of up to 2mm diameter. 

The instrument consists of an electronic cabinet and a sampling stand. The former contains a digital counter, an oscilloscope screen, upper and lower threshold controls, aperture current and amplification switches, and a zero reset switch. The sampling stand consists of an aperture tube, a mercury manometer, a control stopcock, two electrodes, a vacuum pump, and a 35x microscope for viewing the aperture. 

Providing the coincidence correction is less than 5% see Table XVII), the precision of counts made with the Coulter Counter is a function of the number of particles counted. The precision decreases with the total number of particles counted but is independent of the size of the aperture tube employed. The following data will serve as a guide to the precision of count data. (Confidence limits given are two standard deviations.)... 

Select an aperture tube such that the particles being examined have a diameter between 2 and 40% of the aperture size. 

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