Browse Prior Art Database

Adjustable Isokinetic Sampling Horn

IP.com Disclosure Number: IPCOM000046512D
Original Publication Date: 1983-Jul-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 36K

Publishing Venue

IBM

Related People

Spedden, SE: AUTHOR

Abstract

Described here is the design of an adjustable sampling horn for use with optical particle counters to allow for isokinetic sampling in laminar gas flow environments of differing velocities.

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Adjustable Isokinetic Sampling Horn

Described here is the design of an adjustable sampling horn for use with optical particle counters to allow for isokinetic sampling in laminar gas flow environments of differing velocities.

Optical particle counters sample an aerosol at set flow rates measured in the units of volume of gas per unit time they are designed to measure the concentration of particles in the aerosol sample. The concentration is read out as number of particles per unit volume of gas ( particles/volume gas).

In a laminar gas flow environment such as a Class l00 Clean Room or a ventilation duct, the gas is moving with a measurable velocity given in the units of distance per unit time. In order to accurately sample for particle concentration in such an environment, the sample must be taken isokinetically (at the same velocity as the gas in the laminar flow environment).

Flow rate and velocity are related by the equation Q = VA, where Q is flow rate in volume per unit time, V is velocity in distance per unit time, and A is area in the units of distance squared.

If the velocity into the sampling inlet (Vs) is lower than the velocity in the laminar gas flow environment being sampled (Ve), large particles (particles with enough inertia to prevent them from conforming to a change in streamlines of the gas caused by the difference in velocity) are oversampled and the concentration measured by the particle counter is higher than the true concentration of the aerosol being sampled. If the velocity into the sampling inlet is higher than the velocity in the laminar gas flow environment being sampled, large particles are undersampled and the concentration measured by the particle counter is lower than the true concentration of the aerosol being sampled.

As previously stated, to sample a laminar flow environment isokinetically, the velocity at the face of the sample inlet (Vs) must be the same as the velocity of the gas in the environment being sampled (Ve). This match in velocities (Vs = Ve) can be achieved by l) changing the flow (Q) into the sample inlet (sample flow rate) or 2) changing the area (A) of the face of the sample inlet.

On an optical particle counter, the sample flow rate is usually set at some optimal level for the instrument. So, in order to adjust the sample inlet velocity (Vs) to match the velocity of the environment being sampled (Ve), the area of the face of the sample inlet must be changed....