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DILUTERS 

An aerosol (or gas) dilution device may be used, for example, to obtain a 10:1 dilution of a sample, perhaps to lower its concentration or to reduce the humidity or temperature of the sample. The basic elements are flow channels for sample and diluent, a mixing region, and a flow channel for the mixture. The flow rates for the channels need to be known to some extent, described next.

The sample volume flow is Q’ (e.g., mL/s) and the dilution gas flow is Q", so the total flow is Q = Q' + Q". The sample concentration is c' and the diluter concentration is c" = 0, so the diluted concentration is c = c' [Q' / (Q' + Q")]. For example if Q' = 1 and Q" = 9, the concentration will be c = c'(1/10).

Determining two of the three flow values (Q, Q', Q") determines the third one. Approaches commonly used to determine (or control) the flows are as follows:

1. Unmetered: a tube or orifice or filter with a known resistance is used along with a known (or common) pressure drop. Examples would include tubes of the same diameter but having lengths in the ratio of 9:1 or 9 identical flow paths combined for the dilution and one for the sample or flow paths with 9 identical orifices in one path and 1 such orifice in the other. Another example is the critical orifice, based on the speed of sound as the upper limit to orifice flow velocity.

2. Metered: pressure (using a monometer) or flow (using a float - in - tube rotameter) is measured and adjusted. The unmetered systems generally disturb the sample less but are more nearly constant (robust). The metered systems tend to disturb the sample more but can be adjusted to obtain the desired values (and can drift out of adjustment). Metered systems include those that measure pressure drop across an orifice and those that measure mass flow by heat transfer or yet another physical phenomenon.

The sample line needs to be designed to minimize losses for the species of interest. For aerosol particles, this means taking into account such effects as gravitational settling, diffusion, inertial impaction, and electrostatic precipitation. Generally, one avoids measuring the sample flow with a rotameter, for example, out of concern for losses.

Mixing needs to be "thorough but gentle," so that the sample is well mixed with the diluent without being lost to the walls. Co-axial, or nearly coaxial flow, followed by turbulent tube flow (Reynolds number greater than 2100, length of tube more than 10 diameters) is a common approach. Impinging flows or flows converging at a right angle are also used. More thorough mixing can be achieved with a "Stairmand disk," an obstruction that is centered in the tube and that has half the diameter of the tube, but this may not be gentle enough. [Mixing will be covered more fully subsequently.]

The line with the mixture can be monitored by pressure drop or by rotameter at its exhaust, thus not interfering with the sample concentration before it is measured or the sample captured.

A problem arises in determining the sample flow rate, Q'. Rotameters are to be avoided, as are orifices, if aerosols are being sampled. One could measure the pressure drop across a long, small tube and related it to flow, which is one good option. Another approach that should work is to filter the exhaust, perhaps drying it, too, and make it the source of dilution air, withdrawing Q' from it before recycling it.